f 
COVER: 

Airbrush-enhanced image of OAK crater based on sidescan-sonar, bathymetry, and sea-floor observations. GZ=ground zero. 
Sea-Floor Observations and 

Subbottom Seismic Characteristics of 

OAK and KOA Craters, Enewetak Atoll, 

Marshall Islands 

Edited by D. W. FOLGER 

A study conducted by the U.S. Geological Survey for the Defense Nuclear Agency 

Introduction to the Volume, by D. W. FOLGER 

A. Bathymetry of OAK and KOA Craters, by D. W. FOLGER, j. C. HAMPSON, j. M. ROBB, R. A. 

WOELLNER, D. S. FOSTER, and L.A. TAVARES 

B. Sidescan-Sonar Survey of OAK and KOA Craters, by D. W. FOLGER, j. M. ROBB, j. C. HAMPSON, 

P. A. DAVIS, P.M. BRIDGES, and D. j. RODDY 

C. Single-Channel Seismic Survey of OAK and KOA Craters, by j. M. ROBB, D. S. FOSTER, D. W. 

FOLGER, j. C. HAMPSON, and R. A. WOELLNER 

D. Multichannel Seismic-Reflection Survey of KOA and OAK Craters, by J. A. GROW, M. W. LEE, J. j. 

MILLER, W. F. AGENA, j. C. HAMPSON, D. S. FOSTER, and R. A. WOELLNER 

E. Seismic-Refraction Survey of OAK Crater, by H. D. ACKERMANN, j. A. GROW, and J. M. 

WILLIAMS 

F. Observations of OAK and KOA Craters from the Submersible, by R. HALLEY, R. A. SLATER, E. A. 

SHINN, D. W. FOLGER, j. H. HUDSON, j. L. KINDINGER, and D. j. RODDY 

G. Preliminary Analyses of OAK Debris Samples, by R. B. HALLEY, R. P. MAJOR, K. R. LUDWIG, 

Z. L. PETERMAN, and R. K. MATTHEWS 

H. Scuba Observations of OAK and KOA Craters, by E. A. SHINN, j. L. KINDINGER, R. B. HALLEY, 

and j. H. HUDSON 

This volume is published as chapters A through H. These chapters are not available separately. 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1986 

For sale by the Book and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Sea-floor observations and subbottom seismic characteristics of OAK and KOA craters, Enewetak Atoll, 

Marshall Islands. 

"A Study conducted by the U.S. Geological Survey for 

the Defense Nuclear Agency." 

Bibliography : p. 7, A26, B18, C51, 046, E18, F17, G11, 

H39 

Supt. of Docs. no.: I 19.3:1678 

1. Nuclear weapons-Testing. 2. Ocean Bottom-Marshall 

Jslands-Enewetak Atoll. 3. Seismic reflection method. 

I. Folger, David W. II. Geological Survey (U.S.) 

Ill. United States. Defense Nuclear Agency. 

U264.S417 1986 623.4'5119'0287 86-600112 

Use of trade names is for identification only and does not constitute endorsement either by the U.S. Geological Survey or by the 

Defense Nuclear Agency. 
CONTENTS 

[Letters designate chapters] 

Introduction to the volume: by D. W. Folger 

(A) Bathymetry of OAK and KOA craters: by D. W. Folger, J. C. Hampson, J. M. 

Robb, R. A. Woellner, D. S. Foster, and L. A. Tavares 

(B) Sidescan-sonar survey of OAK and KOA craters: by D. W. Folger, J. M. Robb, 

J. C. Hampson, P. A. Davis, P.M. Bridges, and D. J. Roddy 

(C) Single-channel seismic survey of OAK and KOA craters: by J. M. Robb, D. S. 

Foster, D. W. Folger, J. C. Hampson, and R. A. Woellner 

(D) Multichannel seismic-reflection survey of KOA and OAK craters: by J. A. Grow, 

M. W. Lee, J. J. Miller, W. F. Agena, J. C. Hampson, D. S. Foster, and R. A. 

Woellner 

(E) Seismic-refraction survey of OAK crater: by H. D. Ackermann, J. A. Grow, and 

J. M. Williams 

(F) Observations of OAK and KOA craters from the submersible: by R. B. Halley, R. A. 

Slater, E. A. Shinn, D. W. Folger, J. H. Hudson, J. L. Kindinger, and D. J. Roddy 

(G) Preliminary analyses of OAK debris samples: by R. B. Halley, R. P. Major, 

K. R. Ludwig, Z. L. Peterman, and R. K. Matthews 

(H) Scuba observations of OAK and KOA craters: by E. A. Shinn, J. L. Kindinger, 

R. B. Halley, and J. H. Hudson 

Contents Ill 
GLOSSARY 

[Most definitions were taken from: Bates, R. L., and Jackson, J. A., 

Editors, 1980, Glossary of geology, 2d edition: American Geological Institute, Falls Church, Va. 749 p.] 

A-Frame. The "A"-shaped steel frame mounted on the stem of a 

vessel from which oceanographic instruments are towed. 

Acoustics. The study of sound and characteristics of its transmission through various media. 

Airgun. A sound source for seismic reflection and refraction profiling powered by compressed air. 

Anemone. A marine organism, often attached to the bottom, related to corals, jellyfish, and plant-like colonies of hydroids. 

Apparent crater. The locus of the zero-difference contour surrounding a crater; that is, the locus of points along which the 

effects of an explosion can no longer be detected when the preevent 

contours are compared with the postevent contours. 

Apparent velocity. In refraction seismic surveys, the velocity 

obtained with one unreversed profile. 

Arrival. Refers to the appearance of energy on a seismic record. 

Backreef. In this study, the side of the reef away from the sea and 

toward the lagoon. 

Bathymetry. In this volume, depth of the sea floor beneath the sea 

surface. 

Beachrock. A friable to well-cemented sedimentary rock, formed 

in the intertidal zone in a tropical or subtropical region, consisting 

of calcareous debris (from corals and coralline algae) cemented 

with calcium carbonate; often a thin, clearly stratified, seawarddipping calcareous sandstone found on sandy coral beaches. 

Bench. A long, narrow, relatively level or gently inclined strip or 

platform of land, earth, or rock, ~ounded by steeper slopes above 

and below. 

Check shots. In this volume, air-gun shots fired in the water above 

a hole drilled in the crater or lagoon bottom while a hydrophone 

was lowered down the borehole with the objective of obtaining the 

sound velocity of the drilled strata. 

Chronostratigraphy. The branch of stratigraphy (the study of 

sedimentary rock layers) that interprets the geologic record by 

determining the age and time sequence of the rock strata. 

Cobble. A rock fragment larger than a pebble and smaller than a 

boulder, having a diameter in the range of 64 to 256 mm (2 .5-

10 in.). 

Common depth point (CDP) shooting. A type of seismic profiling designed for multiple subsurface coverage to establish common 

depth points to key mappable horizons (reflectors) using several 

hydrophones in a long, linear array. The data acquired using this 

technique require computer processing and give better results in 

most cases than less sophisticated techniques. 

Coral knoll. In this volume, small patch reefs. 

Cuesta. A hill or ridge with a gentle slope on one side and steep 

slope on the other, created by the outcrop of a dipping sedimentary 

layer. 

Debris. Used in this volume for sedimentary material that has been 

moved from its original position and deposited elsewhere by a 

nuclear explosion. 

Decibel (dB). A unit of sound intensity equal to 20 times the 

common logarithm of the ratio of the pressure produced by the 

sound wave to a reference pressure. 

IV Glossary 

Deconvoiution. A data-processing technique applied to seismicreflection for the purpose of improving the visibility and resolution 

of reflections. 

Diffraction. A record of seismic data produced by diffracted energy. Such records commonly result from a termination of reflectors and are characterized on seismic records and sections by a 

distinctive curved alignment pattern of reflections. 

Disconformity. An unconformity in which the bedding planes 

above and below the break are essentially parallel, indicating a 

significant interruption in the orderly deposition of sedimentary 

rocks, generally by a considerable interval of erosion or nondeposition, and usually marked by a visible and irregular erosion surface of low relief. 

Distal. Sedimentary deposits formed farthest from the source area; 

in this volume debris derived from the explosion that is farther 

away from ground zero than the remainder of the debris. (See 

proximal.) 

Ejecta. Airborne debris from an explosion. 

Eocene. An epoch in the lower Tertiary Period, after the Paleocene 

and before the Oligocene, about 38 to 55 m.y. before the present. 

Event. A nuclear explosion. 

Fault. A fracture along which there has been displacement of the 

sides relative to one another parallel to the fracture. 

Fish. In this context an instrument containing electronic equipment, such as a sidescan sonar, usually towed in the water behind 

the ship. 

Fluted. Scalloped 

Fold. A bend in strata or any planar structure. 

Forereef. The seaward side of a reef. 

Frequency spectrum. A description of the distribution of energy 

within a frequency range; used to characterize acoustic or seismic 

devices. 

Galvo-camera. A device that records seismic signals; commonly 

used for evaluating performance during operations. 

Geometric center. In this volume, the point that is equidistant 

from points of equal elevation that lie on opposite sides of the 

crater. 

Geomorphology. The study of the shape of the sea floor or land 

forms. 

Grainstone. A mud-free (less than 1 percent of material with 

diameters less than 20 micrometers), grain-supported, carbonate 

sedimentary rock. 

Granule. A rock fragment larger than a very coarse sand grain and 

smaller than a pebble, having a diameter in the range of 2 to 4 mm. 

Grooves. See spur and groove structure. 

Ground zero. The point directly under an explosion. 

Gully. A small ravine. 

Hogback. A ridge produced by highly tilted strata. 

Holmes and Narver datum. Equivalent to mean low water spring 

minus 0.18 m, or 0.6 ft. See mean lower low water. 

Holocene. One of two epochs in the Quaternary Period of geologic 

time that dates from final retreat of the ice sheet, about 8,000 years 

ago, to the present. 

Horizon. An interface indicative of a particular position in a stratigraphic sequence, commonly a distinctive very thin bed. 

Hydrophone. A pressure-sensitive acoustic detector designed for 

use in liquid elastic wave-propagating media such as water. An 

underwater microphone. 
IVY grid. A geographic-rectangular coordinate system, devised in 

1951 , named after Project IVY of the nuclear test program and 

used for subsequent testing and surveys of the atoll. Coral head 

OSCAR, about 4 miles west of Runit Island in the Enewetak 

lagoon, was selected as the source of the IVY grid and was given 

a value of 100,000 N. and 100,000 E., in international feet. 

Isobath. A line on the bottom of equal depth below the sea surface. 

Julian Day. The day of the year in sequence to 365, or 366 in leap 

year. 

Laser theodolite. A surveying instrument that measures distances 

to a high level of precision. 

Line-of-sight tube. In this volume, a helium-filled tube used for 

photographing nuclear explosions from several kilometers away. 

Lithostratigraphy. Sequence of sedimentary rock layers that are 

defined by physical or mineralogic characteristics. 

Mean lower low water (MLLW). A reference level to which 

soundings are corrected. In this volume, MLLW is equivalent to 

mean low water spring minus 0.6 feet, which is the Holmes and 

Narver datum for Enewetak. 

Micrometer (Jlm, or micron). One millionth of a meter. 

Millisecond (ms). One thousandth of a second. 

Miocene. An epoch of the upper Tertiary Period, following the 

Oligocene and before the Pliocene; 5 to 22.5 m. y. ago. 

Morphology. The physical form of the sea floor or exposed lands. 

Multichannel seismic system. A seismic system that records returned seismic energy on several hydrophones spaced along a 

linear array. In this study, 12-channel and 24-channel systems 

were used. 

Multiple reflection. A seismic wave that has been reflected more 

than once, commonly used for the trace of a seismic echo that has 

reflected from the water surface and back from the sea bottom. 

Norwegian floats. In this study buoys to which the hydrophones 

were attached during the refraction seismic study. 

Nose. The projecting end of a hill. Usually, in this volume, a 

projection on the wall of a crater. 

Nuclear device. Any device capable of producing a nuclear explosion. 

Operation Crossroads. An extensive study of the physical characteristics of Bikini and Enewetak Atolls intiated in 1946 prior to 

testing of nuclear devices. 

OSCAR. A patch reef in the Enewetak lagoon assigned as the 

origin of the IVY grid used during operations and later mapping to 

locate most structures, vessels, and so on. 

Overburden. Loosely consolidated upper part of a sedimentary 

deposit. 

Overwash deposit. Sediment deposited by water landward of a 

berm crest that does not flow directly back into the sea. 

Packstone. A sedimentary carbonate rock whose granular material 

is arranged in a self-supporting framework but that also contains 

some matrix of calcareous mud. 

Patch reef. Small, thick, generally unbedded lenses of limestone 

and dolomite that rise from the lagoon floor up to several tens of 

meters and occasionally reach the sea surface. 

Pebble. A rock fragment larger than a granule and smaller than a 

cobble, having a diameter in the range of 4 to 64 mm. 

Physiography. The study of the genesis and evolution of land 

forms. 

Pleistocene. One of two epochs in the Quaternary Period of geologic time that dates from approximately 8,000 to 1.8 m.y. before 

the present. 

Profile. A cross-sectional representation. A subbottom profile, or 

seismic-reflection profile, is a cross-sectional image of subsurface 

sediments created by a series of acoustic echoes. The reflections of 

a sound pulse at the sea surface, or within the water column, are 

recorded and displayed next to each other. The sea bottom and 

reflecting surfaces within the bottom sediments are depicted as 

dark lines or areas that indicate subbottom stratigraphy and structure. Acoustic-, or seismic-reflection profiles are usually somewhat distorted because the vertical scale is a measure of reflection 

time rather than vertical distance and the speed of sound is variable 

in different media, varying with density and with porosity of sediments, for example. 

Promontory. In this volume, a bluff or prominent hill of sediment 

projecting from the crater wall. 

Proximal. Closer; in this context, debris from the explosion that 

is closer to ground zero than the remainder of the debris. (See 

distal.) 

Ray. Lineament of debris usually extending radially from the 

center of a crater. 

Raypath. The imaginary line along which wave energy travelsalways perpendicular to the wave front in an isotropic medium. 

Reef flat. A stony platform of dead reef-rock, commonly strewn 

with coral fragments and coral sand, generally dry at low tide and 

formed as the summit of the reef above low tide. 

Reef plate. Rock formed of cemented reef rubble that occurs in a 

band several hundred meters wide lagoonward of the living reef. 

Reef rock. A resistant massive unstratified rock consisting of the 

calcareous remains of reef-buiding organisms, often intermingled 

with with carbonate sand and shingle, the whole deposit cemented 

by calcium carbonate. 

Reentrants. Any indentation in a landform whether above or 

below the water surface. 

Reflector. A velocity discontinuity that reflects seismic energy. 

Riprap. A layer of large, durable fragments of broken rock specially selected and graded, thrown together irregularly or fitted 

together to prevent erosion by waves or currents. 

Scarp. An escarpment, cliff, or steep slope of some extent along 

the margin of plateau, terrace, or bench. 

Secondary crater. Small crater formed by falling ejecta from an 

explosion. 

Seismic. Commonly used as an adjective to describe sound in 

frequency ranges of about 20 Hertz to 500 Hertz. 

Seismic refraction shooting. A type of seismic survey based on 

the measurement of the travel times of seismic waves that have 

moved nearly parallel to the bedding in high-velocity rock layers, 

to obtain the depth to the layer and velocity of sound in the layer. 

Seismic streamer. A linear array of hydrophones towed behind a 

ship to receive the energy from a sound source that has been 

reflected from rock layers below the bottom. The geometry of the 

array suppresses noise relative to seismic signals. 

Seismogram. The record made by a seismograph whether recording natural earthquake waves or sound waves generated by an 

explosion or other sound source. 

Glossary V 
Seismology. The study of the structure of the Earth by both natural 

and artificially generated seismic waves. 

Side echoes. Sound echoes from a reflector off to the side rather 

than directly below a hydrophone. Side echoes are usually unwanted and confusing signals. 

Sidescan sonar. An acoustic device that sends signals out to each 

side and receives echoes from the sea floor that are then mapped. 

The oceanographic equivalent of airborne side-looking radar. 

Sideswipe. Spurious data occurring in seismic refraction shooting 

when sound energy that travels along a transmission path beside 

the source-receiver set-up arrives before signals that traveled directly beneath it. 

Single-channel seismic system. Usually a high-resolution seismic 

system that records only on one channel. 

Solution unconformity. A break in the sedimentary record produced by dissolution of carbonate strata during subaerial erosion. 

Sonobuoy. A buoy, usually free-floating, that contains a hydrophone and a radio transmitter that broadcasts acoustic signals received by the hydrophone. Used in seismic refraction surveying. 

Source signature. The wave form of the outgoing pulse from an 

acoustic source such as an air or water gun. 

Spur and groove structure. A comb-tooth structure common to 

many reef fronts, best developed on the windward side, and consisting of elongate channels or grooves a few meters wide and 

deep, separated by seaward-extending ridges or spurs. 

Stacking velocity. In seismic reflection surveys, the constant velocity for an overlying medium that would most nearly give the 

observed normal moveout for a horizontal reflector. It is determined from normal moveout values in velocity analysis. Also 

called normal moveout velocity. 

Strike. The direction or bearing of a horizontal line in the plane of 

an inclined stratum, joint, fault, or other structural plane. 

Selected Abbreviations Used in this Report 

em, centimeter 

crnls, centimeters per second 

dB, decibel 

ft, feet 

gal/min, gallons per minute 

G.m.t., Greenwich mean time 

Hz, hertz 

in., inch 

kg, kilogram 

kHz, kilohertz 

kJ, kilojoule 

km, kilometers 

kn, knots 

kW, kilowatt 

VI Abbreviations 

Structure-contour map. A map that portrays subsurface configuration by means of structure-contour lines (lines of equal depth). 

Subcrop. The areal extent of a truncated subsurface rock unit that 

succeeds an unconformity. 

Swale. A slight depression in a generally level surface. 

Syncline. Generally concave-upward fold of which the core contains stratigraphically younger rocks. 

Talus slope. A steep slope formed by accumulation of loose rock 

fragments; especially such a slope at the base of a cliff, formed by 

the coalescence of several rockfalls or alluvial cones. 

Terrace. Relatively flat, horizontal or gently inclined surface, 

sometimes long and narrow, bounded by a steeper ascending slope 

on one side and by a steeper descending slope on the other. 

Thalweg. A line joining the deepest points of a channel. 

Track lines. The representation on a map of the path of the survey 

vessel. 

Transient crater. That part of the crater removed during the first 

few milliseconds of an explosion. 

Transparent. Used to describe rocks that contain insufficient velocity discontinuities to cause seismic energy to be reflected. 

Transponder. A device which can transmit and receive signals. 

Unconformity. A break or gap in the sedimentary record where a 

rock unit is overlain by another that is not next in stratigraphic 

succession, that is, some strata are missing either due to nondeposition or erosion. 

Wackestone. A mud-supported carbonate sedimentary rock containing more than 10 percent grains (particles with diameters 

greater than 62 micrometers). 

Watergun. A source of acoustic energy for marine seismic surveying that creates a signal by rapidly forcing water under high pressure into the ocean. 

lb, pound 

Umin, liters per minute 

m, meters 

MLL W, mean lower low water 

mm, millimeter 

ms, millisecond 

rnls, meters per second 

nmi, nautical miles 

psi, pounds per square inch 

s, second 

V.E., vertical exaggeration 

J..Lm, micrometer 

UTM, Universal Transverse Mercator 
Introduction to the Volume 

By D. W. FOLGER 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Abstract 1 

Purpose 2 

Data aquisition 2 

Setting 2 

Previous work 3 

Participants 4 

Acknowledgments 6 

References 7 

FIGURES 

1. Map showing the location of Enewetak Atoll and the locations of craters and islands on the submerged reef 3 

2. Photographs showing vessels used in the study 4 

TABLES 

1. Comparison of names of islands and coral heads (site names) assigned before and 

during nuclear tests with those used by the natives 5 

2. List of nuclear test events cited in this volume 6 

Contents Ill 

Introduction to the Volume 

By D. W. Folger 

Abstract 

From June 17 to September 20, 1984, the physiographic and subsurface effects of thermonuclear explosions 

were studied in two submarine craters created during nuclear 

testing in 1958 at Enewetak Atoll, Marshall Islands. The first of 

a three-phase field program included echo-sounding, sidescansonar, and single-channel seismic-reflection profiles; the second included sea-floor observations and bottom sampling by 

scuba and research submersible; and the third included multichannel seismic-reflection profiles, seismic-refraction profiles, 

and additional sea-floor observations and sampling. In this 

report are the results of the 1984 field work supplemented with 

some preliminary data acquired during an extensive follow-up 

drilling program conducted during 1985. 

OAK Crater 

OAK crater is a large circular depression that has a maximum radius to the 5-m (16ft) isobath of about 600 m (1 ,970ft) 

and a maximum depth of 60 m (197ft). The geometric center 

of the crater lies about 100 m (328 ft) southeast (lagoonward) 

of ground zero where the 8.9-megaton device was exploded on 

a barge located at the lagoonward edge of the reef in 4 m (13ft) 

of water. Both bathymetric and sidescan imagery show that a 

flat, circular crater floor is surrounded by a complex of hummocks and terraces probably formed by slumps and mass 

movement that took place after the initial crater was formed. 

Along the reef, the crater is bordered by a series of scarps 

several meters high. Around much of the lagoonward side, a 

blanket of debris extends about 850 m (2,789 ft) from ground 

zero. An estimate of crater volume based on preblast and postblast bathymetry is 23.6X 106 m3 (833 x 106 ft3). Highresolution single-channel seismic-reflection profiles reveal the 

detailed geometry of two strongly reflecting layers that are 

shallower than about 60 m (197 ft) below sea level and less 

than 20m (66ft) subbottom in the area around the crater. The 

upper of these reflectors in most areas cannot be traced closer 

to ground zero than 600 m (1 ,970ft) except to the south where 

an inward-dipping flap extends as close as 400 m (1 ,310 ft). 

The lower reflector commonly can be traced about 550 m 

(1 ,804 ft) from ground zero but may extend as close as 440 m 

(1 ,444 ft). With multichannel seismic-reflection data a small 

segment of the lower reflector can be traced within 250 m 

(820 ft) of ground zero. 

The multichannel profiles define the distribution and deformation of deeper reflectors. These profiles show that an 

acoustically transparent pond of sediments, apparently chaotically mixed debris and postevent deposits, in the crater center 

extends to a depth of 110m (361 ft) below sea level or 50 m 

(164 ft) below bottom. In most of the crater the transparent 

sediments extend 400 to 500 m (1 ,312-1,640 ft) from ground 

zero and are inferred to outline the transient crater. A zone of 

intense fracturing extends 70 m (230ft) below the base of the 

transparent sediments to 180 m (591 ft) below sea level. A 

strong acoustic reflector that lies about 95 m (312ft) below sea 

level outside the crater is downwarped to 115 m (377ft) about 

300 m (984 ft) from ground zero, where it becomes too fractured to map. Downwarping of deeper reflectors decreases to 

about 10m (33ft) at a depth of 280m (919ft) below sea level. 

Seismic refraction data suggest that fracturing may extend as 

deep as 425 m (1 ,394 ft). 

Extensive diving by submersible and scuba revealed a 

transition from fine sediments in the center of the crater to 

boulder-sized detritus in talus slopes on the reefward side of 

the crater. In the hummocky area between the crater floor and 

rim are small hills consisting of coarse, dark-brown detritus 

which is similar to sediment cored about 230m (755ft) below 

the crater floor. This material may have been transported upward through fractures during dewatering due to the compressional effects of the blast and postevent subsidence. Evaluation 

of analytical methods for correlating debris samples with cored 

samples suggests that the original depths of the debris samples 

may be estimated to within 6 m (20ft) using strontium isotopes. 

Scuba observations in waters less than 30 m deep (98 ft) 

show that the reefward margin of the crater subsided as much 

as 21 m (69ft). Rock strata along the reefward rim of the crater 

most commonly dip toward ground zero. In one area west of 

ground zero they dip away from the crater and may be part of 

an upturned flap. This dip implies that the transient crater 

margin in that area may be as large as 550 m (1 ,804ft). 

KOA Crater 

The crater was excavated entirely from reef rock, reef 

plate, and beachrock by a 1.4-megaton device mounted in a 

water tank on Teteiripucchi Island. Five other nuclear tests 

were conducted nearby including the largest (1 0.4 megatons) 

exploded in the atoll. The radius of KOA crater from ground 

zero to the 5-m (16ft) isobath averages about 500 m (1 ,640ft), 

and its maximum depth is 33m (108ft). Bathymetric maps and 

sidescan-sonar imagery show that the crater floor is elliptical 

with the long axis oriented northeast. Between the crater floor 

and rim, the bottom is hummocky and comprises numerous 

slump masses. Some manmade objects that were emplaced as 

part of the original testing program are clearly visible in 

sidescan-sonar imagery on the southeastern side of the crater. 

Multichannel seismic-reflection profiles show that a 

transparent pond of sediments in the center of the crater extends to a depth of about 80 m (262 ft) below sea level or 

47 m (154 ft) below bottom. As at OAK, these sediments are 

interpreted to be chaotically mixed debris and postevent deposits that filled the transient crater. The profiles do not define 

the perimeter of these deposits. A zone of intense fracturing 

Introduction 
appears to extend to a depth of 140 m (459 ft) below sea level and 

laterally 300 to 500 m (984-1 ,640ft) from ground zero. A reflector 

at 75 m (246ft) below sea level warps downward at about 500 m 

(1 ,640ft) from ground zero to a depth of 100m (328ft) at about 200 

m (656 ft) from ground zero; beyond 200 m from ground zero the 

reflector is obscured by intense fracturing. Deeper-lying reflectors 

that correlate across the crater suggest that acoustically detectable 

fractures and downwarp are not present below 230m (755ft); cores 

indicate that fractures may persist as deep as 297 m (974 ft). 

Submersible and scuba observations revealed only a few 

scarps around the margin of KOA. The crater floor is covered with 

coarser sediment than at OAK because of the continuing influx of 

abundant detritus from the adjacent reef to windward. The dives 

PURPOSE 

The thermonuclear explosions, detonated in the Marshall Islands from 1952 to 1958, were the only events conducted at or near sea level before the nuclear test-ban treaty 

required that all further testing be conducted underground. 

Thus, these are the only sites available to assess the effects 

of such large explosions on surface and subsurface geology. 

This study was initiated at the request of the Defense 

Nuclear Agency (DNA) to acquire detailed information on 

the size, morphology, and deformation depth of two submarine nuclear craters, OAK and KOA, located on the north 

side of Enewetak Atoll (fig. 1). The report represents the 

first part of a two-part study that responded to the DNA 

request. It comprises a geological and geophysical assessment of the surface and subsurface geology based on data 

collected at Enewetak from June 17 to September 20, 1984. 

The second part comprises mainly a drilling and coring 

program carried out from January to August 1985. Only 

preliminary results from the second part are included in this 

report. 

OAT A ACQUISITION 

Sea-floor and subbottom characteristics discussed include: ( 1) bathymetry (depth below the sea surface), physiography, and the distribution and character of materials on 

the bottom or to a depth of about 10m (33ft) below bottom, 

and (2) the depth, distribution, extent, continuity, and seismic velocity of acoustic reflectors below the bottom. We 

acquired bathymetric data, sidescan-sonar imagery, and 

single-channel seismic profiles on the first leg (June 17 to 

July 4, 1984); sea-floor descriptions, bottom samples, and 

cores on the second leg (July 14 to August 12, 1984); and 

multichannel and refraction seismic profiles and additional 

sea-floor descriptions and bottom samples on the third leg 

(August 21 to September 20, 1984). 

Vessels used to support the program included 

Egabrag II, Delta, Halimeda, the "Mike" boat, and various 

whalers (fig. 2). 

also documented the locations of manmade objects on the southeastern side of the crater, including a concrete test platform, a 

line-of-sight tube support, vertically emplaced railroad rails that 

had been used to support causeways and a collimator station, and 

other debris such as cables and riprap. Two sets of rails, one found 

in water as deep as 18 m (59ft), and another in about 6 m (20ft) 

of water, apparently were originally continuous but were separated 

by postevent slumping. By moving the deep set eastward and joining it to the shallow set, the effects of the postevent slump can be 

eliminated. This places riprap west of the last rail about 430 m 

(1 ,411 ft) from ground zero, which probably is a good estimate for 

the maximum transient crater radius in the southeast quadrant of 

the crater. 

Preliminary data from the second part of the study 

concerning the character and age of subsurface samples are 

integrated mainly into chapters C, D, E, and G. 

SETTING 

Part of the Marshall Islands group, Enewetak Atoll lies 

approximately 4,600 km (2,500 nmi) southwest of Honolulu; it is elliptical in shape with its long (40 km, or 22 nmi) 

axis oriented northwest and the short (28 km, or 15 nmi) 

axis northeast. Most islands in the atoll are located along the 

northeast side, which faces the prevailing trade winds 

(fig. 1). More detailed discussions of settings that pertain 

specifically to acquisition of different data sets are included 

in each chapter. 

During this study, positions on Enewetak Atoll were 

determined using the IVY grid as well as geographic coordinates of latitude and longitude. The IVY grid is a planar 

coordinate system established during the postwar nuclear 

testing program, which has been used subsequently to locate 

structures and test sites. It is an orthogonal grid, measured 

in feet, based on site OSCAR, a coral head in Enewetak 

lagoon (fig. 1), which was assigned the position of 100,000 

ft N. and 100,000 ft E. 

As the result of satellite observations used to locate 

five transponder stations, we relocated the geodetic position 

of site OSCAR. (See table 1.) This position will be further 

refined by the Defense Mapping Agency (DMA) through 

analyses of the corrected satellite orbits. The former position of OSCAR is ll 0

32'20.254"N., and 162°17'10.944"E. 

The new position of OSCAR is 11°32'07.432" N., and 

162°16'49.408" E. or, in space, 762.21 m (2,500.7 ft) 

southwest of the old position. The latitude and longitude of 

the atoll and craters on our maps have been changed accordingly and do not agree with positions shown on older charts. 

We have used the names of islands after Tobin (1973) 

in Freisen (1982). (See table 1.) Of the 43 nuclear tests 

conducted on Enewetak Atoll, we include in table 2 a list of 

the 8 craters studied or referred to in this volume. 

2 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
SOUTHWEST fa 

PASSAGE 

Bokaidrik (Helen) ~ 

+ 

+ 

• (Oscar) 

SOUTH CHANNfl. 

0 10,000 

11111111111 

0 

0 

I 

8 

8 

~ 

+ 

8 

0 

0 

!f? 

+160000 

ENEWETAK ATOLL 

+ 

20,()00 

I 

I I 

/PREVAILir«i WIND 

+120000 

-<~ "Runit (Yvonne) 

'\z ~T•U' ·; +80000 

.!!. J.~aptan (David) 

EAST CHANNEL 

Medren (Elmer) 

+40000 

+IVY GRID (ft) 

30,000 40,000 50PQO 60,000 FEET 

I I I I 

10 NAUTICAL MILES 

10 15 20 KLOMETERS 

I I I I I I I I 

162°201 162°301 

Figure 1. Map showing the location of Enewetak Atoll, the locations of craters (shown by crosses), and of islands 

(in black) on submerged reef. Names in parentheses are site names, which are listed in table 1. Letters, such as A, 

B, and C, are the first initials of each site name. 

PREVIOUS WORK 

Operation Crossroads, initiated in 1946 prior to testing of nuclear devices on Bikini and Enewetak Atolls, included extensive geologic, geophysical, oceanographic, and 

biologic studies of the area. The principal products of these 

investigations were many papers assembled as U.S. Geological Survey Professional Paper 260. The first of these appeared in 1954 (Emery and others, 1954); others were published over the next 15 years. This enormous volume of 

papers and the references cited include most of the scientific 

work concerned with the area prior to 1970. Subsequently, 

in addition to many studies concerning radiation effects on 

biota (see Friesen, 1982, and references cited), geologic and 

geophysical surveys of the craters and crater areas were 

conducted during a series of projects designed to understand 

the mechanisms of cratering (Circeo and Nordyke, 1964; 

Henny and others, 1974; Couch and others, 1975; Ristvet 

and others, 1978; Tremba and others, 1982). Their work 

documented many characteristics of the craters vital to 

understanding cratering mechanisms, but it also left many 

questions unresolved concerning the geometry of craters. 

This study was designed and carried out to provide answers 

to these unresolved questions. 

Introduction 3 
B 

Figure 2. Vessels used during this study. A, Egabrag //-

41-m (135 ft) mud boat and principal platform for all 

phases of the study. 8, Mike boat-15-m (SO ft) landing 

craft used for shallow-water surveys and for transporting 

equipment and personnel. C, Delta-4.6-m (15 ft) submersible capable of diving to 366 m (1,200 ft). 0, Halimeda-a 7.6-m (25 ft) inboard-outboard which was the 

main platform for shallow drilling and scuba. E, Whaler6-m (19 ft) outboard, one of four small boats used for 

submersible tracking, shallow-water surveys, and transporting equipment and personnel. 

PARTICIPANTS 

On the first leg of the cruise, U.S. Geological Survey 

(USGS) personnel included D. W. Folger and J. M. Robb, 

cochief scientists, and G. K. Miller, K. F. Parolski, and 

D. S. Blackwood. Navigators from Meridian Ocean Systems; Inc. (MOS), included R. C. Hollar, party chief, and 

A. Pearce, S. L. Thoresen, and M. Milburn. E. L. Tremba 

and B. L. Ristvet were observers for DNA. H. V. Woodworth of the Defense Mapping Agency, with the assistance 

of E. L. Tremba, helped to calibrate the navigation system 

transponders, and he also relocated several critical positions 

on Enewetak and Lojwa Islands within the IVY grid at the 

outset of the first leg. On the second leg, U.S. Geological 

E 

Survey personnel included E. A. Shinn, chief scientist, and 

R. B. Halley, J. H. Hudson, J. L. Kindinger, M. W. 

Hempenius, D. W. Folger, and D. J. Roddy. Operators of 

the submersible included R. A. Slater of GEOCUBIC, Inc., 

and D. N. Privitt of MARF AB. Because of his experience 

in submarine carbonate terrains, Slater also served as a 

principal scientist. The Defense Nuclear Agency observer 

was B. L. Ristvet. Navigators from MOS included R. C. 

Hollar, R. F. Johnson, and S. L. Thoresen. Many observers 

from the Defense community made orientation dives on the 

submersible during this leg (see appendix, chap. F). Between legs 2 and 3, at DNA's request, additional sidescan 

and bathymetric surveys were conducted in nearshore areas 

with the Mid-Pacific Laboratory's Mike boat (landing 

craft). D. W. Folger and K. F. Parolski of USGS and S. L. 

4 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Table 1. Comparison of names of islands and coral heads (site names) assigned before and during nuclear tests with 

those used by the natives 

[Table is taken from Freisen (1982). Names used before nuclear testing are from Bryan (1971). The native names from Tobin (1973) are used 

in this report] 

Native Names From From From Tobin, 

U.S. H~drogra2hic Office Bryan 1973 

Site 1946 1968 1971 Native names a 

ALICE Bogallua Bogallua Peony BOKOLUO 

BELLE Bogombogo Bogombogo Petunia BOKOMBAKO 

CLARA Ruchi Eybbiyae Poinsettia KIRUNU 

DAISY b Lidilbut Primrose LOUJ 

EDNA* b b Rambler BOCINWOTMEc 

EDNA'S DAUGHTER b b b b 

FLORA* Elugelab b Sagebrush b 

GENE* Tei teiripucchi b Sunflower b 

HELEN* Bogairikk Bogeirik Violet BOKAIDRIK 

IRENE Bogon Bogon Zinnia BOREN 

JANET Engebi Engebi Fragile ENJEBI 

KATE Muzinbaarikku Mujinkarikku Arbutus MIJIKADREK 

LUCY Kirinian Billee Aster Blossom KIDRINEN 

PERCY -b b b TAIWEL 

MARY Bokonaarappu Bokonarppu Bitterroot BOKENELAB 

MARY'S DAUGHTER b b Bluebonnet b 

NANCY Yeiri Yeiri Buttercup ELLE 

OLIVE Aitsu Aitsu Camellia AEJ 

PEARL Rujoru Rujiyoru Canna LUJOR 

PEARL'S DAUGHTER b b Carnation b 

RUBY* Eberiru Eberiru Columbine ELELERON 

SALLY Aomon Aomon Clover AOMON 

SALLY'S CHILD b b Dandelion b 

TILDA Biijiri Biijire Daisy BIJILEc 

URSULA Rojoa Rojoa Delphinium LOJWA 

VERA Aaraanbiru Arambiru Gardenia ALEMBEL 

WILMA Piiraai Piirai Goldenrod BILLAE 

YVONNE Runit Runit Hawthorn RUNIT 

SAM b b b BOKO 

TOM b b b MUNJOR 

URIAH b b b INEDRAL 

VAN b b b b 

ALVIN Chinieero b b JINEDROL 

BRUCE Aniyaanii Japtan Jasmine ANANIJ 

CLYDE Chinimi Chinimi Lavender JINIMI 

DAVID Japtan Muti Ladyslipper JAPTAN 

REX Jieroru Bogen Lilac JEDROL 

ELMER Parry Parry Heartstrings MEDREN 

WALT b b b BOKANDRETOK 

FRED Eniwetok Eniwetok Privilege ENEWETAK 

GLENN Igurin Igurin Lantana IKUREN 

HENRY Mui Buganegan Mimosa MUT 

IRWIN Pokon Bogan Mistletoe BOKEN 

JAMES Ribaion Lib iron Oleander RIBEWON 

KEITH Giriinien Grinem Oca KIDRENEN 

LEROY Rigili Rigile Posy BIKEN 

OSCAR (coral head) b b b DREKATIMON 

MACK (coral head) b b b UNIBOR 

8As confirmed by the Enewetak people during the Ujelang field trip of July 1973. 

bNo name reported. 

cBOKINWOTME and BIJIRE are preferred according to current literature and are so spelled in this 

report. 

*Original island destroyed by nuclear tests except for small portions of EON A, HELEN, and RUBY. 

Introduction s 
Table 2. list of nuclear test events cited in this volume 

[Characteristics after Freisen, 1982; B. L. Ristvet and K. L. Mills III, written commun., July 1985; E. L. Tremba, oral commun., July 1985. --- , 

no information available] 

Event Date Location 

MIKE ----------Oct. 31, 1952 Sudace, Flora 

NECTAR---- ----May 13, 1954 Barge, Mike crater 

LACROSSE -----May 4, 1956 Surface, Yvonne 

SEMINOLE -----June 6, 1956 Sudace, Irene 

APACHE------- -July 8, 1956 Barge, Mike crater 

HURON --------July 21, 1956 Barge, Mike crater 

KOA -----------May 12, 1958 Sudace, Gene 

OAK -----------June 28, 1958 Barge, lagoon 

Thoresen of MOS carried out the work. On the third leg, 

USGS personnel included J. A. Grow, chief scientist, J. J. 

Miller, T. F. O'Brien, D. R. Nichols, and D. H. Mason 

and, from Seismic Data Systems, Inc., T. Owens. During 

the latter part of the leg, Shinn, Halley, Hudson, and 

Kindinger returned to complete the scuba observations. In 

addition, a bottom sampling program was carried out from 

a 6.4-m (21ft) whaler by USGS personnel T. W. Henry and 

B. R. Wardlaw. MOS navigators included R. F. Johnson 

and S. L. Thoresen. DNA observers included E. L. Tremba 

and R. F. Couch. Captain R. L. Wilson was master of the 

Egabrag II during the entire field program. 

Shore personnel who played large roles in the processing, analysis, and synthesis of data included H. D. Ackermann, J. C. Hampson, L.A. Tavares, D. S. Foster, R. A. 

Woellner, V. F. Paskevich, E. L. Wright, J. E. Dodd, B. H. 

Lidz, M. W. Lee, W. F. Agena, J. M. Williams, P. A. Davis, 

P.M. Bridges, R. P. Major, K. R. Ludwig, and Z. L. Peterman of USGS and R. A. Goldsmith, S. B. Volkmann, and 

J . R. McCullough of the Woods Hole Oceanographic Institution, and R. K. Matthews of Brown University. 

ACKNOWLEDGMENTS 

We are particularly indebted to W. J. Stanley, Director, Pacific Area Support Office (PASO), Department of 

Yield Average IVY grid co- Height of burst apparent (kilotons) radius ordinates 

3.1 m 10,400 887 m 147,754 N. 

(10ft) (2,910 ft) 67,789 E. 

1,700 147,750N. 

67,790 E. 

2.4m 40 61 m 106,885 N. 

(8.0 ft) (200ft) 124,515 E. 

2.1 m 13.7 99m 149,897 N. 

(7.0 ft) (324ft) 75,237 E. 

148,063 N. 

69,227 E. 

148,304 N. 

70,015 E. 

0.8 m 1,400 658 m 149,360N. 

(2.67 ft) (2,160 ft) 71,120 E. 

1.75 m 8,900 875 m 124,981 N. 

(5.75 ft) (2,870 ft) 36,108 E. 

Energy, in Honolulu, and to his staff, including especially 

H. U. Brown, PASO Deputy for this project. P. R. Haggerty of Holmes and Narver, Inc., and his staff at Honolulu, 

particularly J. H. Matthewman, were of invaluable assistance in arranging co_ntracts and transportation. S. S. 

Miyasato and his staff at Enewetak offered every assistance 

to solve the many on-site technical and logistical problems 

during the project. E. L. Tremba, B. L. Ristvet, and A. S. 

Melzer provided us with much information that was useful 

in the planning and execution of the work. C. E. Otterman, 

owner of Egabrag II, also provided us with valuable suggestions. H. V. Woodworth ofDMA has supplied us with much 

important information concerning the precise locations of 

our transponder stations and of submerged objects that we 

located with the submersible and scuba. V. E. Zadnik of 

USGS helped to write many contracts for equipment and 

services and shepherded them through the system. P. L. 

Forrestel, J. A. Zwinakis, and D. S. Blackwood drafted and 

photographed most of the illustrations. P. A. Davis prepared 

the three-dimensional computer images for both craters. 

M. W. Hempenius typed the entire manuscript and appendices. P. B. Worl typed drafts of chapters D and E. D. M. 

Morton, R. F. Couch, E. L. Tremba, B. L. Ristvet and 

B. R. Wardlaw kindly reviewed the manuscript. 

6 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
REFERENCES 

Bryan, E. H., Jr., 1971, Guide to place names in the Trust Territory of the Pacific Islands: Pacific 

Scientific Information Center, Bishop Museum, Honolulu, Hawaii, not paginated. 

Circeo, L. J., and Nordyke, M. D., 1964, Nuclear cratering experience at the Pacific proving 

grounds: Livermore, University of California, Lawrence Radiation Laboratory, TID-4500, 

UC-35, (UCRL)-12172, 88 p. 

Couch, R. F., Jr., Fetzer, J. A., Goter, E. R., Ristvet, B. L., Tremba, E. L., Walter, D.R., and 

Wendland, V. P., 1975, Drilling operations on Eniwetok Atoll during Project EXPOE: Air 

Force Weapons Laboratory Technical Report TR-75-216, Kirtland Air Force Base, New 

Mexico, 87117, 270 p. 

Emery, K. 0., Tracey, J. 1., Jr., and Ladd, H. S., 1954, Geology of Bikini and nearby atolls: U.S. 

Geological Survey Professional Paper 260-A, 259 p. 

Freisen, Bert, ed., 1982, Enewetak Radiological Support Project, final report: U.S. Department of 

Energy, Nevada Operations Office, Las Vegas, Nevada, NVO 213, 349 p. 

Henny, R. W., Mercer, J. W., Zbur, R. T., 1974, Near-surface geologic investigations at Eniwetok 

Atoll: Air Force Weapons Laboratory Technical Report TR-74-257, Kirtland Air Force Base, 

New Mexico 87117, 36 p. 

Ristvet, B. L., Tremba, E. L., Couch, R. F., Jr., Fetzer, J. A., Goter, E. R., Walter, D. R., and 

Wendland, V. P., 1978, Geologic and geophysical investigations of the Eniwetok nuclear 

craters: Air Force Weapons Laboratory Technical Report TR-77-242, Kirtland Air Force 

Base, New Mexico 87117. 

Tobin, J. A., 1973, The Enewetak Atoll people: A special report for the radiological survey of 

1972-73: Atomic Energy Commission, Washington, D.C., not paginated. 

Tremba, E. L., Couch, R. F., Ristvet, B. L., 1982, Enewetak Atoll Seismic Investigation (EASI): 

Phases I and II (final report): Air Force Weapons Laboratory Technical Report TR-82-20, 

Kirtland Air Force Base, New Mexico 87117. 

Introduction 7 

Chapter A 

Bathymetry of OAK and KOA Craters 

By D. W. FOLGER, J. C. HAMPSON, J. M. ROBB 

R. A. WOELLNER, D. S. FOSTER, and L. A. TAVARES 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction Al 

Purpose Al 

Setting Al 

Previous work Al 

Methods A2 

Results A7 

OAK crater A7 

Preevent bathymetry AS 

Present bathymetry AlO 

Crater floor A12 

Inner slope A13 

Inner terraces A13 

Outer slope A14 

Outer terraces A16 

Rim scarp A 16 

Rim slope A17 

Debris blanket A17 

Comparison of preevent and postevent bathymetry A17 

Three-dimensional compuier images of OAK crater AlS 

KOA crater A20 

Crater floor A20 

Inner slope A20 

Terraces A20 

Rim scarp A21 

Debris mounds A22 

Inter-crater channel A23 

Three-dimensional computer images of KOA crater A24 

Summary A24 

References A26 

Appendix 1. Depth-data contouring A30 

Appendix 2. NOAA tide tables for Enewetak A32 

FIGURES 

1. Map showing bathymetry of Enewetak Atoll A2 

2. Point plots of echo soundings along tracks of vessels used to acquire bathymetric 

data in OAK crater A3 

3. Point plots of echo soundings along tracks of vessels used to acquire bathymetric 

data in KOA crater A4 

4. Flow chart showing procedure used to process bathymetric data AS 

5-6. Graphs showing: 

5. Tide-gauge data acquired in OAK and KOA craters A6 

6. Tide-gauge data from KOA crater used to correct echo-sounding 

records A7 

7-12. Maps showing: 

7. Bathymetry of OAK crater AS 

8. The preblast bathymetry in the area of the OAK site A9 

9. The bathymetry of OAK crater, the locus of some isobaths used to locate the geometric center, and other physiographic features AlO 

10. Bathymetry of OAK crater with physiographic provinces outlined All 

11. Bathymetric profiles that pass through or close to ground zero in OAK 

crater, and ground zero location projected orthogonally into lines A12 

Contents Ill 

Chapter A 

Bathymetry of OAK and KOA Craters 

By D. W. Folger, j. C. Hampson, J. M. Robb, R. A. Woellner, D. S. Foster, and L.A. Tavares1 

INTRODUCTION 

Purpose 

Bathymetric data, collected as part of Project EASI 

(Enewetak Atoll Seismic Investigation) by Tremba and 

others (1982), provided definition of most geomorphological features in OAK and KOA craters. However, a bathymetric survey was requested with much closer line spacing 

so that greater data density could be exploited by sophisticated image-processing techniques developed at the U.S. 

Geological Survey's (USGS) Branch of Astrogeological 

Studies in Flagstaff, Ariz. These techniques have the potential to provide much more resolution to assess the extent, 

distribution, and thickness of debris, and to define the characteristics of the crater rims, walls, and floors in greater 

detail. Therefore, we acquired bathymetric data on 25-mspaced (82-ft-spaced) lines, which was the closest spacing 

practicable in view of the echo-sounder beam width, navigation errors, and ship maneuverability. Our purpose was to 

integrate bathymetry with sidescan-sonar data, so that more 

detailed information on the morphology of these submarine 

craters could be provided. The bathymetric data were used 

as guides for the acquisition of all other geophysical data, 

submersible and scuba observations, and siting of sample 

stations and drill sites. 

Setting 

The 48 small islands and coral heads that rise above 

the reef make up an area of only 7 km2 (2 nmi2), contrasting 

markedly with the area of the lagoon which totals about 

1,000 km2 (292 nmi2) (fig. 1). The bottom of the lagoon, 

sounded in 1944 by the U.S. Navy (Emery and others, 

1954), comprises three physiographic provinces: (1) a terrace, as wide as 4.8 km (2.6 nmi); (2) an almost perfectly 

elliptical, flat-floored basin that has a maximum depth of 

62 m (210ft) in its northwestern half, and (3) several thousand patch reefs or coral knolls. Some of these knolls come 

within a few meters of the lagoon's surface; most, however, 

1

All authors are with the U.S. Geological Survey, Branch of Atlantic 

Marine Geology, Woods Hole, Mass. 02543. 

lie below about 36m (120ft) of water (Emery and others, 

1954) (fig. 1). 

Both OAK and KOA craters lie on the north side of 

the atoll. The 8. 9-megaton OAK device formed the westernmost of the craters; it was detonated over the terrace close 

to the reef, on a barge that was anchored in water approximately 4 m (13 ft) deep. Present depth of ground zero is 

close to 60 m (197 ft). OAK was created in an area which 

has been little disturbed by other blasts. The nearest other 

crater, MIKE, which was not studied in detail, lies about 

12 km (6.5 nmi) to the northeast. The much smaller (1.4-

megaton) KOA device was fired on Teiteiripucchi (Gene) 

Island close to the sites where five other tests had taken 

place-of which MIKE was the largest (1 0.4 megatons) 

fired in the atoll. KOA obliterated the island, and ground 

zero now lies under slightly more than 33 m (108 ft) of 

water. 

Previous work 

The extent of echo sounding by the Germans, or subsequently by the Japanese when they were in possession of 

the atoll, is unknown. In 1944, the U.S.S. Bowditch conducted an extensive survey of the entire atoll, and more than 

180,000 sounding depths were plotted (Emery and others, 

1954) (fig. 1). Some additional data were added during 

Operation Crossroads in 1946. Lead line and echo-sounding 

surveys were conducted in the area of OAK crater prior to 

and after the test in 1958. This survey apparently consisted 

of at least 23 lines run normal to the reef at about 90-mspacing (300-ft-spacing) that were rerun subsequent to the 

detonation. At KOA, three lines were run through ground 

zero before and after the detonation, but apparently there 

was some problem with navigation as the before-and-after 

lines were not in the same locations (B. L. Ristvet, oral 

commun., October 1984). The most extensive and most 

recent survey conducted prior to this study took place during 

Project EASI, which started in 1979. Approximately 260 

km (140 nmi) of data in a grid with line spacing ranging 

from 100 to 140m (328-450 ft) were collected in the OAK 

and KOA crater areas and near Enjebi (Janet) Island 

(Tremba and others, 1982). 

Bathymetry of OAK and KOA Craters A 1 
+ 

W20' +IVY GRID (ft) 

zwroo ~ ~ 

5 

0 5 10 

I I I I I 

+ 

Bokaidrik (Helen) 

50.F 60ifX' FEET 

10 NAUTICAL MILES 

15 20 KILOMETERS 

I I I I 

+ + 

8 

8 

~ 

+160000 

+120000 

+80000 

+00000 

162000 162°10' 162°201 162°301 

Figure 1. Map showing bathymetry of Enewetak Atoll. Letters (A, B, and C) next to islands represent the first letters of 

site names. (See table 1, Introduction to the volume.) The map has been generalized and simplified after Emery and 

others (1954) on the basis of soundings collected by U .S.S. Bowditch. Contours in OAK, MIKE, and KOA are based on 

soundings collected in this study. 

METHODS 

Echo-sounding data (figs. 2, 3) were acquired in both 

craters from three different vessels: MS Egabrag II, Mike 

boat (landing craft), and a 6.4-m (21 ft) whaler (fig. 2, 

Introduction to the volume). Most data were collected from 

Egabrag II along 25-m-spaced (82-ft-spaced) lines during 

the first leg of the cruise. Data collected during the multichannel leg augmented the data base. Smaller vessels were 

used mainly nearshore in shallow water. Each vessel was 

positioned with a Motorola FALCON-IV Miniranger System (accuracy ±3 m, or 10ft) which was set up and operated by Meridian Ocean Systems of Ventura, Calif. The 

navigation net was controlled by five transponders located 

atop towers on Biken (Leroy), Bokoluo (Alice), Enjebi 

(Janet), Lojwa (Ursula), and Enewetak (Fred) Islands 

(fig. 1). The transponder towers were tied into the IVY grid 

net by H. V. Woodworth of the Defense Mapping Agency 

using a laser theodolite and were found to be consistently 

within 4.5 m (15 ft) of the positions previously established 

by Meridian personnel using Motorola Satellite Navigation 

Systems. 

We mounted an Atmospheric Instrumentation Research (Model AIR-DB-1A) digital barometer aboard 

Egabrag II to record variations in atmospheric pressure. To 

A2 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
IN 

1\l 

0 

0 

o 162°5'30•E 

11 °37'N + 

1 1 036 '30•N + 

IVY GRID(ft) 

11°36'N + 

-~· . . ·-:· .. 

··. 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

IN 

gJ 

0 

0 162°6 'E 

+ 

~ -~-

~ ...... 

... 

g 

0 

162°6 '30•E 0 

+ 

.·. 

... 

1\l 

g 162°7 'E 0 

1 3000( 

+ 11 °37' 

1 1 °36 '3 

11 °36'N 

+ 11 °35'30 

U.S. GEOLOGICAL 

SURVEY 

250 0 250 500 750 1000 1250 

COMPILED: Se~ 1985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 2. Point plots of echo soundings along tracks of all vessels used to acquire bathymetric data in OAK crater. Closer 

points in spacing most often reflect areas of more variable bathymetry. GZ=ground zero. 

monitor the 1.5-m (5 ft) tidal fluctuations, we deployed two 

Sea Data tide gauges, one in OAK crater and one in KOA 

crater. No atmospheric pressure correction was necessary 

because variations were too small to affect the level of the 

water in the lagoon significantly or to require a correction to 

the pressure-depth acquired with the tide gauges. We also 

found that measured values differed by as much as 0.6 m 

(2ft) from NOAA (National Oceanographic and Atmospheric Administration) tide table predictions for the Enewetak lagoon. 

To acquire bathymetric data, we used a Raytheon 

719B depth-recording system (200kHz, 534 pulses/minute, 

accuracy of ±2.5 em, with a beam width of 18° at -10dB). 

Echo-sounding data were processed in the USGS laboratory 

Bathymetry of OAK and KOA Craters A3 
0'1 

(Xl 

0 

0 

0 

162°11 '30'"E 

11 °40 '30'"N + 

•• + + • + 

+ + ••• 

11 °40'N 

" 0 

0 

0 

0 

+ 11 °40'30'"N 

IVY GRID (ft) 150000 

~ -JIIIJj 

+ 11 °40'N 

162~~1~1~'3~0~ .. =E----~~----~----~-L-16_2__1L2-'E~------------~-------1-6-2-o1_2_'-30-,.-E--~ 145000 

MAP PROJECTION: UTM 

250 0 250 500 750 1000 

U.S. GEOLOGICAL 

SURVEY COMPILED: SEPT 1985 METERS HHHHHI 

1000 0 1000 

FEET 

2000 3000 

Figure 3. Point plots of echo soundings along tracks of all vessels used to acquire bathymetric data in KOA crater. Closer 

point spacing most often reflects areas of more variable bathymetry. GZ=ground zero. 

at Woods Hole, Mass., to create a digital data base and 

detailed bathymetric maps. The procedure, as shown in 

figure 4, begins with three data sets: (1) a computer data 

base of time (G.m.t.) and depth pairs from the echosounding records (bathymetric data); (2) tide-gauge data 

(also in time and depth); and (3) navigation data (time and 

position). The separate data sets merge when the tide data 

are used to standardize the depths to mean lower low water 

(MLLW) (mean low water spring minus 0.18 m, or 0.6 ft) 

and when the navigation data are used to interpolate a position for each tide-corrected, time-depth pair. The resulting 

position-depth pairs were further edited on the basis of mapcrossings and computer contouring. Figure 4 shows a loop 

for this stage that can be repeated until all data have been 

processed and errors minimized. In the last phase, we used 

the corrected position-depth data to create a final contour 

map and a digital data base on magnetic tape for further 

processing. Appendix 1 describes the programs used in 

processing. 

The echo-sounding data were digitized on a magnetic 

graphics tablet linked to a Hewlett-Packard HP-1000 computer and were processed using an in-house package of 

seismic data analysis programs. The echo-sounding profiles 

collected in the field are printed in feet, based on a built-in 

A4 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
conversion using 4,800 ft/s (1,463 m/s) as the speed of 

sound in water. The conversion to meters was applied by 

recalculating travel time and then converting to meters on 

the basis of a sound velocity of 1,542 m/s (5,060 ft/s). This 

velocity was derived from tables published by Chen and 

Millero (1977) and from water temperature (isothermal at 

28.5 °C) measured aboard the submersible. 

The tidal variations in KOA and OAK craters, shown 

in figure 5, are within a few centimeters of each other. Most 

of the bathymetric data were collected within the time period 

Bathymetric Tide Navigation 

data: data data 

200-kHz echograms digital cassettes digital tapes 

Digitize Read tide data Read navigation tapes; 

Bathymetry, cassettes and Convert 'IVY coordinates 

Time (G.m.t.)& process to Time ( G.m.t.) to latitude and longitude 

Depth (ft) & Depth (em) files compatible with mapping 

routines. 

+ + + 

Plot each digitized I Establish mean lower Plot raw navigation, check 

profile for check low water data for each for completeness, errors. 

\ tide gauge {Appendix 2) ! / Correct Julian Day errors, 

Apply tide correction to time-series errors, bad 

bathymetry {baseline = mean fixes, etc. 

lower low water) 

+ 

+ Sort into subfiles by vessel 

Convert echogram feet to two- & data collection system 

way travel time (4,800 ft/s). ¥ JJ 

Convert two-way travel time to Convert to Track line plot, 

meters (1,542 m/s) binary/condensed for research 

~ format 

+ Merge tide-corrected depth 

and time with corrected lat-long 

and time 

+ Correct errors, 

if any l Extract and compile digital data 

t base of all lat-long-depth values 

+ t 

Check crossings and ~Plot crossings for I Grid and contour I preliminary contours for error check data base 

bathymetry and navigation I I errors 

~ 

[Reverse lat-long conversion ' I Final contour maps I t IVY & lat-long 

Final digital data base 

lat-long - depth (m) - IVY N (ft) - IVY E (ft)l 

I ~ 3-D projections I To USGS, Flagstaff, ITo archive and other I 

Ariz.: merge with DNA users 

sidescan mosaic 

Figure 4. Procedure used to process bathymetric data. 

Bathymetry of OAK and KOA Craters AS 
550 OAK CRATER 550 

525 525 

500 500 

u 

~475 475 ~ 

u 

450 450 

425 425 

400 400 1 1 13 15 17 19 (Gmt.) AUG 

1984 

550 KOA CRATER 550 

525 525 

500 500 

u 

~475 475 ~ 

u 

450 450 

425 

~ 425 

400 

1 1 1 3 1 5 

AUG 

+---.------~~-----+ 4 0 0 

1 7 1 9 (Grn.t.) 

1984 

Figure 5. Tide-gauge data acquired in OAK and KOA craters. Note the similarity of the two profiles. G.m.t.=Greenwich mean time. 

A6 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
of the tide record shown in figure 6. The maximum tide 

range is about 1.3 m (4.25 ft). The tide-gauge data were 

linked to MLLW by tide tables for Enewetak supplied by D. 

Simpson (written commun., October 1984) of NOAA (appendix 2). This is within 0.03 m (0.1 ft) of the Holmes and 

Narver datum. In figure 6, MLLW falls at 720 em, yielding 

a tide correction from -0.25 m ( -0.82 ft) to -1.55 m 

( -5.08 ft). 

The navigation data were converted from IVY grid 

coordinates to latitude and longitude to be compatible with 

the USGS processing and mapping programs. The results 

were identical to latitude and longitude conversions provided by the navigation contractor, Meridian Ocean Systems. The conversion was reversed in the final phase, and 

IVY coordinates were added to the position-depth pairs in 

the digital data base. 

900 

875 

850 

825 A 

~ 

~800 

u ~ 

775 

750 . 

A few maps in subsequent chapters contain generalized versions of 5-m (16ft) bathymetry for orientation only. 

These may differ in some areas by as much as 50 em (1.5 

ft) from the versions that appear throughout this chapter, 

which should be used as the reference for any questions 

regarding water depth. 

In OAK crater we collected 400 km (216 nmi) of 

bathymetric profiles, and in KOA we collected 150 km 

(82 nmi). Additional lines not in the crater areas both inside 

and outside the atoll totalled 717 km (388 nmi). 

RESULTS 

OAK Crater 

OAK crater is a large circular depression (fig. 7) with 

an average apparent diameter of 1,750 m (5,740 ft) and a 

~900 

~ 875 

~ 850 

~ 825 

~ 800 ~ 

u 

~ 775 

~750 

725 . MLLW (APPROX.) ~725 

700 I 700 I I I I I I I I I I I I I I I I 

17 19 21 23 25 27 29 1 3 (G.m.t.) 

JUN JUL 

1984 

Figure 6. Tide-gauge data from KOA crater used to correct all of the echo-sounding records collected during the first leg of the 

survey to approximate mean lower low water (MLLW), which is equivalent to mean low water spring minus 0.18 m (0.6ft). 

G.m.t.=Greenwich mean time. 

Bathymetry of OAK and KOA Craters A7 
n•36'30"N + 

11"36'N + 

DATUM: MLLW (APPROX.I 

CCJITOURS IN METERS 

162•5'30"E 

MAP PROJECTION: UTM 

162-r'E 

t __,..,__ 

l 

+ 

+ 

+ 

119000 

162•7'E 

U.S. GEOLOGICAL 

SURVEY 0 250 500 750 1000 1250 

COMP I LEO: Sept. 1 985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 7. Map showing the bathymetry of OAK crater. Contour interval is 1 m. GZ=ground zero. 

maximum depth of 60 m (197ft) (B. L. Ristvet and K. L. 

Mills, III, written commun., July 1985). The 8.9-megaton 

device that formed the crater was fired on a barge moored 

in 4 m (13 ft) of water (Circeo and Nordyke, 1964). The 

reported location of ground zero is at IVY grid coordinates 

124981.4 N. {11°36'14.9" N.) and 36108.02 E. 

(162°06'6.6" E.). However, the geometric center of the 

crater for all isobaths as shallow as 40 m (130ft) lies about 

100m (330ft) southeast of ground zero as noted previously 

by Tremba and others (1982). Examples of similar offsets 

between ground zero and the geometric center for highexplosive craters generated on slopes have been observed 

(R. F. Couch and B. L. Ristvet, oral commun., June 1985). 

Preevent Bathymetry 

Preevent and postevent bathymetric maps for the 

OAK area, constructed by Holmes and Narver, were based 

on lead-line and echo-sounding data apparently acquired 

along 90-m-spaced (300-ft-spaced) lines laid out at right 

angles to the reef. The preevent bathymetric map (fig. 8), 

digitized from a copy of the Holmes and Narver map provided by B. L. Ristvet, covers an area of 3.24 km2 (0.94 

nmi2), and includes all of the crater but little of the surrounding area. 

The northwest half (reef side) of the preevent map 

comprises reef plate with superimposed sand bars. At the 

breaker line was a 0.5-m (1.6 ft), almost linear scarp which 

A8 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
~ g: ~ • 

8162...,'30"£ 8 8 g 0 r=--.:.....:.."=--=..----------ro~16~206~'E:__ _______ ~1~62:.:•6~'~30C!•E:_ ___ _:o~----1!.!6~2•~~7:!'E~.130000 

11°37'N + 

IORJOOJNG: 

11°36'30~ + 

11°35'30~ + 

IORJDOED AT .25 CM ( 

IORIO= 156 X 188 CM. 

+ 

+ _,_,_ t 

l 

+ 

+ 11°37.'N 

+ 11 °36'30~ 

+ 11°36'N 

+ 11°35'30~ 

t------=-IV.:....:Y~G~R~IO:..!!.!!. ____ -+-------------------+-------~ 120000 

~1~6~2.~5~'30~"E~-------~ 16~2=6~'E~--------1-6~2.~6~'3-0·-E----~----16-2-•7-'E~ 11 ~00 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

U.S. GEOLOGICAL 

SURVEY 

250 0 250 500 750 1000 1250 

COMPILED: June 1985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 8. Map showing the preblast bathymetry in the area of the OAK site (taken from Holmes and Narver, Topography 

and Hydrology, Site 25, Alice Reef, July 31, 1956). Gridding parameters are described in appendix 1 of this chapter. 

GZ=ground zero. 

was indented by a small embayment (fig. 8) beside a 

promontory of similar size. From the 1.5-m to 6-m (5-20ft) 

isobaths, a narrow, gently sloping (1.6°) shelf ranged in 

width from 107m (350ft) to 290m (950ft). 

The southeast half of the map comprises the reef 

slope, lagoon floor, and numerous mounds, which were 

probably mostly patch reefs. The slope was steepest (15°) in 

the northeast part of the map area and more gentle ( 4 °) to the 

southwest where it descended to the lagoon floor as a series 

of terraces. According to the Holmes and Narver map 

(fig. 8), it was incised by a 15-m-deep (50-ft-deep) gully 

with a steep (23°) headwall; the gully flattened out over a 

Bathymetry of OAK and KOA Craters A9 
distance of about 305m (1 ,000 ft) to lose its identity in 38m 

(125ft) of water. At the foot of the slope, the bottom slopes 

gently (0.8°) toward the center of the lagoon where it is 

interrupted by 19 mounds, or patch reefs, that rise as high 

as 12m (40ft) above the bottom and range in diameter from 

about 60 m (200 ft) to 300 m (l ,000 ft). These appear to 

vary in shape from elliptical to triangular, a result which is 

doubtless in part due to the sampling density. 

DATUM: MLLW (APPROX.) 

CONTOURS IN METERS 

MAP PROJECTION: UTM 

I ~6'E 

Present Bathymetry 

We have contoured the crater on 1-m (3 ft) spacing, 

but we have thinned out some isobaths in the steeply dipping 

areas so that the reader can see details that otherwise would 

have been masked by the concentration of lines (fig. 7). 

However, on the annotated versions (figs. 9, 10), we have 

included all the contours to accentuate the variations in 

slope. Note that ground zero lies on the extreme northwest162-&'!0•E 16r7'E 

__,_,_ t 

l 

+ 

+ 

+ 

U.S. GEOLOGICAL 

SURVEY 

2!50 0 2!50 500 ~ 1000 12!50 

COMPILED: · Sept. 1 985 

1000 0 1000 

METERS 

2000 

FEET 

3000 4000 5000 

Figure 9. Map showing the bathymetry of OAK crater, the locus of some isobaths used to locate the geometric center (GC) 

of the crater, and other important physiographic features. Average perimeter of apparent crater is after B. L. Ristvet and K. L. 

Mills, written commun., July 1985. Letters A and B are localities referred to in the text. 

A 1 0 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
em edge of the crater floor. The crater is almost equally 

divided through its geometric center with half extending into 

the reef and half extending into the lagoon. This symmetry 

about the geometric center of the crater is worth highlighting 

and is illustrated in figure 9. 

The crater floor is almost perfectly circumscribed by 

the 59-m (194 ft) isobath which has a radius of 100 m 

(330ft) from the geometric center. Similarly, northwest and 

southeast of the geometric center, much of the 50-m (164ft) 

isobath, which outlines the top of a slope (inner slope, 

fig. 10) surrounding the crater floor, lies along a circle with 

a radius of 183m (600ft). The distance of the 50-m (164ft) 

DATUM: MLLW (APPROX.I 

CONTOURS IN METERS 

MAP PROJECTION: UTM 

isobath from the geometric center increases equally in both 

southwest and northeast directions (symmetrically) to 250m 

(820 ft) due to the presence of two broad terraces. 

Farther up the crater side, the approximate top of the 

outer slope (fig. 10) lies in waters 35 to 40 m (115-131 ft) 

deep and varies little from a circle with a radius of 390 m 

(1 ,280ft) from the geometric center. Another circle with a 

radius of 463 m (1 ,520 ft) lies close to the bottom of the 

main scarp that surrounds the reefward edge of the crater 

and also follows closely the crest of the lagoonal crater rim; 

and another with a radius of 573 m (1 ,880 ft) lies close to 

the top of the same scarp and follows a lower secondary 

+ 

+ 

U.S. GEOLOGICAL 

SURVEY 250 0 250 500 750 1000 1250 

COMPILED: Sept. 1 985 

1000 0 1000 

METERS 

2000 

FEET 3000 4000 5000 

Figure 10. Map showing bathymetry of OAK crater with physiographic provinces outlined. GZ=ground zero. 

GC =geometric center. 

Bathymetry of OAK and KOA Craters A 11 
II 0 37'N + 

11°36'3011 N + 

11°35'3011N + 

120000 

_,.,._ t 

j 

DATUM: MLLW (APPROX.) g 

162°5'3011E 

MAP PROJECTION: UTM 

COMPILED: SEPT 1985 250 

10 

1'0 

+ + 

0 250 500 750 1000 1250 METERS 

0 

0 

+ 

--t--

+ 

+ 

162°71E 

U.S. GEOLOGICAL SURVEY 

1000 0 1000 2000 3000 4000 5000 FEET 

Figure 11A. Map showing bathymetric profiles that pass through or close to ground zero (GZ) in OAK crater. Dashed 

where inferred from bathymetric map. 

ridge lagoon ward of the rim crest. These circles all illustrate 

that many of the physiographic characteristics of the crater 

are symmetrical about the geometric center. 

We also have shown on figure 9 the average perimeter 

of the apparent crater, with a radius of 875 m (2,870 ft), 

from ground zero as defined by B. L. Ristvet and K. L. 

Mills, III (written commun., July 1985). 

On the basis mainly of slope changes, we have divided the crater area into eight major physiographic 

provinces (fig. 10) and will discuss each in sequence from 

the crater center outward. 

Crater Floor 

The flat to gently undulating floor of the crater has a 

diameter of 200m (650ft) and reaches a maximum depth of 

60 m (197 ft) below MLLW, which is the Holmes and 

Narver datum (figs. 1 0; 11, lines 203, 204; 12, line 75). The 

crater is circumscribed by the nearly circular 59-m (194ft) 

A 12 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
NE 

~~ 60 

W LINE 209 f 

2~~--- j~ ~ 

I 

fQ_ 

:~~ ~ 460 

NW liNE 203 Sf 

2~~ j~ w 

0 

:~~ ~--~---~60 

N liNE 204 S 

2~~ j!~ :~~ ~--~60 

0 200 400 600 800 1000 1200 1400 1600 1800 METERS 

0 1000 2000 3000 4000 5000 6000 FEET 

V.E. = X6.2 

DATUM: MLLW (APPROX.) 

Figure 118. Ground zero location projected orthogonally into lines. Contour interval is 5 m. 

isobath (figs. 9, 10). As noted above, ground zero lies on 

the northwest edge of the crater floor, and the geometric 

center lies almost precisely at its center. A low ridge extends 

across the crater floor close to the geometric center (see 

figs. 7; 12, line 75) possibly representing a central crater 

uplift. 

Inner Slope 

Surrounding the crater floor is a slope that rises as 

steeply as 17° to a depth of 50 m (164ft) along much of the 

crater's northwest and southeast sides (figs. 7; 10; 11, line 

204; 12, line 75). The steepness of this slope is only exceeded by the steepness of the scarps that lie along the 

reefward margin of the crater edge in much shallower water 

(5-30 m, or 16-98 ft) and of a scarp on the crater\vard side 

of a debris mound that lies about 425 m ( 1 ,394 ft) southeast 

of the geometric center. The area ranges in width from 37 

to 95 m (120-310 ft) and is well defined on both its lower 

and upper boundaries. At the upper boundary, the steep 

slope flattens out abruptly into the area of inner terraces 

where slopes are as low as 1 o. 

Inner Terraces 

This area ranges in depth from about 40 to 50 m 

( 130-164 ft), and the distance from lower to upper 

boundary, or width of the province, ranges from 61 to 

220 m (200-720 ft). The long axis of the upper boundary is 

oriented northwest-southeast and lies about 427 m (1 ,400 ft) 

symmetrically on either side of the geometric center; the 

short, equally symmetrical axis lies about 340 m (1, 115 ft) 

on either side of the geometric center. 

The area is characterized by variable bathymetry that 

ranges from flat to reversed slopes, that is, slopes that dip 

away from the geometric center, to a few areas where slopes 

are as great as 10° into the crater (figs. 7; 10; 11, lines 203, 

204, 209; 12, lines 75, 77). The noses and reentrants all 

suggest that the area comprises overlapping slumped bodies 

of sediment. The finer details of the bathymetry observed 

from the submersible, and their significance to cratering 

mechanics, are discussed in chapter F of this volume. 

Bathymetry of OAK and KOA Craters A 13 
II 0 37'N --+-

_,.,._ ~ 

j 

II 0 36'30"N + 

125000 

II 0 35'30"N + 120000 

8 DATUM: MLLW (APPROX.) ~ !'(') 

162°5'30"E 

MAP PROJECTION: UTM 

COMPILED: SEPT. 1985 250 

1000 0 

-t- + 

J8J 

162°61E 162°6'30"E 

0 250 500 750 1000 

1000 2000 3000 4000 

0 

0 

8 

¢ 

0 

0 

+ 

-t-

+ 

+ 

162°71E 

U.S. GEOLOGICAL SURVEY 

1250 METERS 

5000 FEET 

Fi~ure 12A. Map of OAK crater showing location of bathymetric profiles in and around the crater that are 

onented parallel to the reef. GZ=ground zero. Contour interval is 5 m. 

Outer Slope 

This is a narrow area 10 to 128m (33-420 ft) wide in 

which average slopes are steeper (approx 5°) than in adjacent areas. It does not represent a major discontinuity in the 

physiography of the crater, but it clearly separates the hummocky terraced areas on either side of it (figs. 10; 11, 

line 204). It may represent a poorly developed or partially 

buried slump scarp. On the reefward side of ground zero, it 

appears to be truncated by the bottom of the rim scarp. The 

steepest slopes (28°) lie on the southeast side of the crater 

where a large debris mound (fig. 9, site B) rises abruptly 

from the terraced area. Adjacent to it, a flat-floored channel 

about 30 m (100ft) wide cuts through the outer slope and 

extends from the crater more than 500 m (1 ,640 ft) into the 

lagoon (figs. 9, 10). 

A 14 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
sw NE 

liNE 58 

oo 

liNE 75 

20 20 

40 40 

60 .. 

liNE 77 i !~~ 60~ -- ~ .--------------------~60 ~: 

00 

liNE 79 

20 20 

40 40 

20~ ~24000 40---------~ 

liNE 98 

!~~-------~--------------~-~~ liNE 82 

0 200 400 600 800 1000 1200 1400 1600 1800 METERS 

0 1000 2000 3000 4000 5000 6000 FEET 

Figure 128. Ground Zero (GZ) location projected orthogonally into lines. 

V.E.=X6.2 

DATUM: MLLW (APPROX.) 

Bathymetry of OAK and KOA Craters A 15 
0 

20 

40 

0 

~,:o+ 040 

119 60 

g DATUM= MLLW (APPROX.) 

0~ ,. + li 0 37'N 

20 

40 

e 

§o 

0.. 

lJ.J 

a 

20 

40 

0 

20 

40 

111A 60 125000 

IVY GRID (ft) 

L-..____..L ___ _L __ ___J___j_....l....J.L......L--L-....:::..... __ __:::::...__:-:::...._;_L.:::......'-l..-~...~...----....)...._L-..l.l-----'-------' II o 36' N 

162°5'30"E 162°6'E 162°6'30"E 162° 7'E 

MAP PROJECTION = UTM U.S. GEOLOGICAL SURVEY 

COMPILED= SEPT. 1985 250 0 250 500 750 1000 1250 METERS 

1000 0 1000 2000 3000 4000 5000 FEET 

Figure 13. Bathymetric profiles collected nearshore around the reef margin of OAK crater. GZ=ground zero. Contour 

interval is 5 m. Vertical exaggeration for all profiles is X6.2. 

Outer Terraces 

Much of this area is similar in bathymetric expression 

to the inner terraces. It lies at the foot of the steepest scarp 

in the crater and probably also represents a mehinge of 

slumped material. It comprises three areas that are as wide 

as 150m (500ft) in which slopes range from flat, to a few 

degrees away from the geometric center, to as much as 9° 

toward the geometric center (figs. 10; 11, line 204). The 

upper boundary of this area is the most abrupt in the crater 

where it abuts the steep rim scarp. However, the boundary 

between it and the adjacent rim slope on the lagoonward side 

of the crater is more arbitrary, and we have drawn it on the 

basis of both slope change and isobath orientation. 

About 12 poorly developed gullies incise the sides of 

the crater from the outer terraces to the crater floor (fig. 9). 

Most extend into waters that are no shallower than 40 m 

(130ft). Their positions may be structurally controlled. (See 

chap. C, this volume.) 

Rim Scarp 

The steepest slopes in the crater, up to 30°, lie along 

its reefward side (figs. 9; 10; 11, lines 203, 

204; 12, lines 900, 58; 13, lines 111B-D, 113). In some 

areas, submersible and scuba observations revealed that the 

A 16 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
slope comprises vertical to overhanging ledges and talus 

slopes. (See chap. F and H, this volume.) 

The inner margin of the scarp ranges from 400 to 

520 m (1,300-1,700 ft) from the geometric center, and the 

outer margin ranges from 590 to 640 m (1 ,930-2,100 ft). 

However, the width is misleading because the scarp is separated on the western side of the crater by a terrace that is 

about 60 m (200ft) wide. The large, southwestern segment 

of the crater rim appears to have moved toward the geometric center, presumably in at least two parts. A similar terrace 

broadens the rim scarp on the northwestern side of the crater 

(fig. 10). 

Rim Slope 

These two areas are bounded on the upper or outward 

sides by a line connecting the highest points along the 

crater's lagoonward rim, except along the base of the rim 

scarp on its western side (fig. 10; 11, line 209). Its outer 

margin lies about 400 to 500 m (1 ,300-1 ,640 ft) from the 

geometric center, which is close to the distance (550 m, or 

1 ,800 ft) that Tremba and others (1982) found it from 

ground zero. On the east side of the crater, the highest point 

on the rim is at or slightly shallower than 35 m (115 ft); on 

the southeast side, the top of a large debris mound lies at 

32m (105 ft), and on the south-southwest side the crest lies 

at the same depth. 

The two arcs thus form the lagoonward crater rim and 

doubtless consist of abundant debris from the crater possibly 

uplifted in part by thrusting. (See chap. C and D, this volume.) 

Debris Blanket 

This large area of irregular bathymetry or rough bottom is physiographically little different from the rim slope; 

thus, both areas are probably underlain by abundant debris 

from the crater (figs. 10; 11, lines 203, 204, 209; 12, lines 

79, 89). The outer perimeter of much of this material lies on 

a circle about 750 m (2,460 ft) from the geometric center 

and 850 m (2,800 ft) from ground zero. On the southeastern 

side, two lobes that border the 90-m-wide (300-ft-wide) 

channel (figs. 9, 10) extend to the southeast about 1,000 m 

(3,300 ft) from the geometric center. 

The physiography of the debris blanket suggests that 

much debris was blown out into the lagoon directly opposite 

from the reef. This is not surprising, considering the relative 

elevation and resistivity of the two sides. Delineating the 

debris on the basis of bathymetry alone in the lagoon is 

difficult because of the 80-or-so patch reefs that are present 

in the area (figs. 7, 9); however, the physiographic analyses 

are supported by the subbottom data. (See chap. C, this 

volume.) 

The circular to elliptically shaped patch reefs range in 

maximum diameter from 46 m to 243 m (150-800 ft) and 

range in height from a few meters to 19m (62ft) above the 

lagoon floor. The patch reef or debris mound (fig. 9, site A) 

nearest the crater center lies about 390 m (1 ,280 ft) from it 

and rises about 9 m (30ft) above the adjacent channel floor. 

It may be capped by debris. East of it, on the other side of 

the channel, lies a larger complex of debris (fig. 9, site B) 

and patch reefs. 

Comparison of Preevent and Postevent Bathymetry 

Features in the preevent Holmes and Narver bathymetric map correlate well with those on our map outside the 

crater area. For example, the contours on the slope northeast 

and southwest of the crater are in close alignment on both 

maps, and eight of the patch reefs are close to the same 

location and height, although their shapes differ significantly (fig. 8). The character of the crater area shows many 

differences. 

The blast removed most of the slope area, including 

the deep gully shown on the preevent map near ground zero. 

Almost 600 m (1 ,970 ft) of reef or reef plate northwest of 

the crater center were disrupted, blown away or downfaulted. The only manifestation of the gully in the present 

bathymetry may be at the southwest comer of the boxshaped valley on the southeast side of the crater where the 

flat-floored channel cuts through the crater rim. Although 

that channel may show some structural control (see chap. C, 

this volume), it probably developed because two large patch 

reefs (305 m (1,000 ft) long, 10-12 m (35-40 ft) high) 

(fig. 8) funnelled some of the blast between them. The westernmost of the two patch reefs appears to have been mostly 

destroyed or buried, whereas part of the eastern one appears 

to be intact. The large (150m long by 90 m wide, or 490ft 

by 300 ft) mound of material noted above (see fig. 9, site B), 

now lies adjacent to it on its craterward side and rises 9 m 

(30 ft) above the preblast surface. Neither this additional 

material nor the mound (debris or patch reef) at site A, 

figure 9, appear on the preevent map. 

The preevent map does not extend far enough to the 

southeast to evaluate the two long ridges (rays?) that appear 

to terminate about 1,200 m (3,937 ft) from the crater center. 

On the opposite side of the crater (reef side), the box-shaped 

valley at 40 m (131 ft) coincides almost precisely with the 

peculiar reentrant on the preblast reef. Why, or if, this 

occurrence influenced the crater geometry is not clear. 

We have constructed an isopach map (fig. 14) that 

shows the difference in terrain between the Holmes and 

Narver digitized preevent bathymetric map (fig. 8) and our 

bathymetric map (fig. 7). In the unstippled area, our depths 

are greater than the values shown by Holmes and Narver; 

material has been removed and( or) the bottom depressed. In 

the stippled area, depths are less than those shown by 

Holmes and Narver; material has been added and( or) the 

bottom has been uplifted or bowed up outside the crater 

area. Our computer-calculated value of 23.6 x 106 m3 for the 

total volume of the crater is close to the crater volume 

estimated by Mills and Ristvet (in Ristvet and others, 1978). 

Bathymetry of OAK and KOA Craters A 17 
"' "' .. .. N g: 0 N 0 g 0 0 0 0 

0 16205'30"E 0 162°6'E 162°6'30"E 0 162•7'E 0 

130000 

11 °37'N ~RE -OAK POST -OAK DIFFE + + 11°37'N 

CONTOURING: 

PIXEL: 

DATA POINTS REDUCED TO .1 ~ GRIDDING: -filii- GRIDDED AT • 25 CH (HAP l 15 H CREAL SPACEl 

j GRID= 156 X 188 CH 

X-OFFSET 13 CH, Y-OFFSET 

SEARCH RADIUS = 1. 4 CH 

NPHAX = 1 , SH = 9 [)-'"" 

11 °36'30•N + + 11°36'30"N 

11°36'N + + 11°36'N 

11°35'30"N + + + + 11 °3!5'30"N 

r-----~I~VY~GR~ID~~---------+--------------------------------------~--------------~ 120000 

~1~~~05~'3~o•=E~---------------L-1~6~2°~6-'E--------------------16-2°-6-'3_0_"E ________ ~----------16-2-•7-,E--~ 11 00 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

U.S. GEOLOGICAL 

SURVEY 250 0 250 500 750 1000 1250 

COMPILED: June 1985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 14. An isopach map showing the difference in terrain between the Holmes and Narver preshot bathymetry (fig. 8) 

and the postshot bathymetry (fig. 7) acquired during this study. In the area not stippled, our values are deeper than those 

of Holmes and Narver, whereas in the area stippled, our values are shallower than those of Holmes and Narver. Gridding 

parameters are described in appendix 1 of this chapter. GZ=ground zero. 

Three-Dimensional Computer Images of OAK Crater 

Perspective views of OAK crater are shown in figure 

15. To accentuate the relief of the saucer-shaped crater, 

we have used a vertical exaggeration (5 X). 

The views from the southwest and northeast clearly 

show the large volume of material removed from the reef. 

They also show the difference in the reef slope on either side 

of the crater-steep to the northeast, more gradual to the 

southwest, which, based on the Holmes and Narver map 

(fig. 8), is close to the precrater physiography of the slope. 

Views from the southwest, and particularly from the 

southeast (fig. 15B), highlight the terrace, which has the 

A 18 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Figure 15. Three-dimensional images of OAK crater: A, from the southwest, 8, from the southeast, and C, 

from the northeast. Vertical exaggeration is x 5. 
appearance of slumped material on the reefward side of 

the crater. 

All four views show the thickest part of the debris 

blanket around the lagoonward side of the crater and the 

physiography of the apparent inner and outer rim crests 

(fig. 9). The extent of thinner debris accumulation may coincide with the 40- to 45-m (131-148 ft) depth range outside 

the thick debris at the crater edge. The channel through the 

crater rim opposite the reef that crosses this area also is well 

depicted on all four images. The integrated sidescan-sonarbathymetry data in chapter B (this volume) clearly show 

greater detail and texture of the crater, debris, and patch 

reefs. 

KOA Crater 

This nearly circular crater was formed in 1958 by a 

1.4-megaton device detonated in a water tank on Teiteiripucchi (Gene) Island. The crater, now covered by a 

maximum of 33m (108ft) of water, was excavated entirely 

in the lagoon terrace and has an average apparent radius of 

658 m (2, 160 ft) (B. L. Ristvet and K. L. Mills, III, written 

commun., July 1985) (fig. 17). The adjacent MIKE crater, 

southwest of KOA, was formed by a 10.4-megaton device 

6 years previously. Three smaller devices, NECTAR in 

1954 and APACHE and HURON in 1956, were detonated 

subsequently in or at the edge of MIKE crater (fig. 17). 

SEMINOLE, also detonated in 1956, lies just east of the 

map area (fig. 17). 

KOA ground zero is located at IVY grid coordinates 

N. 149360; E. 71120 (11°40'17" N.; 162°11'59" W.). The 

bathymetry of the crater is less complex than that at OAK 

crater because the blast excavated the KOA crater entirely 

from reef rock instead of from reef and lagoon sediments. 

Much of the debris blanket feU on the reef and nearby 

islands. Some of it, especially on the shallowly submerged 

southeast margin, probably was swept back into the crater 

immediately after the blast by rushing water and subsequently by waves associated with storms. Little information 

can be drawn from the debris blocks because their source in 

the area could have been from any of five events. 

The radius from ground zero to the 5-m (16ft) isobath 

along the northeast crater rim averages about 500 m 

(1 ,640ft). The crater is fairly symmetrical about ground 

zero, but isobaths deeper than 25m (82ft) are elliptical with 

the long axis oriented parallel to the reef trend (northeast). 

The 30-m (98 ft) isobath, for example, is about 210 m 

(690ft) from ground zero in the northeast and southwest 

directions but only about 110 m (360 ft) from ground zero 

in the southeast and northwest directions, whereas the 15-m 

(50ft) isobath is almost circular about ground zero. Bathymetry is complex at the southwest margin of the crater 

where it abuts MIKE crater (figs. 16, 17). 

We have divided the crater into four major physiographic provinces, from the crater center outward. These 

include: (1) the crater floor; (2) the inner slope; (3) terraces; 

and ( 4) rim scarp. In addition, other scarps, debris mounds, 

and an intercrater channel have been outlined (fig. 18). 

Crater Floor 

We have included the gently sloping area below the 

30-m (100ft) isobath as the crater floor. The flattest part of 

the floor lies mostly southwest of ground zero. To the northeast, it rises to the 30-m (100ft) isobath in a 0.9° slope from 

ground zero over a distance of about 200 m (656 ft). The 

crater floor is elliptical in shape and symmetrical about 

ground zero, with the long axis extending about 200 m 

(656ft) to the northeast and southwest and the short axis 

extending about 100m (328ft) to the northwest and southeast (figs. 16; 18; 19A,B). 

Inner Slope 

The transition from the crater floor to the inner slope 

is easy to define. Slopes increase to as much as 9° southeast 

of ground zero and about 4-5° elsewhere (figs. 18; 19A,B: 

lines 116, 304). The slope change that characterizes the 

outer boundary of this area is well defined except northeast 

of ground zero where our choice of its position is somewhat 

arbitrary because it changes little from crater floor to rim 

scarp (figs. 19A,B: line 507). The outer boundary is asymmetric about ground zero-335m (1, 100ft) to the northeast 

and only about 235 m (770 ft) to the southwest. The width 

of the area varies from about 120 m (400 ft) northeast of 

ground zero to zero southeast of ground zero where the toe 

of a probable slump impinges on the crater floor (fig. 18). 

Terraces 

Most of the crater bottom lies within the area designated as terraces. It is complex physiographically, having 

slopes as steep as 6° ranging to slopes in the more common 

terraces and low mounds where the bottom is flat and sometimes even slopes away from ground zero (figs. 18; 19A,B: 

lines 116, 304; 20A,B: lines 541, 539, 538, 507). These 

terraces are similar to those in OAK crater and, as there, 

appear to make up a complex of slumped material. 

Most of the flatter areas lie northwest and southeast of 

ground zero. These broaden the crater in this direction so 

that much of the upper boundary is almost equidistant from 

ground zero. The area thus is as narrow as 20m (66ft) to 

the southwest and as wide as 300 m (985 ft) to the northwest 

(fig. 18). The outer perimeter of the northwest, northeast, 

and southeast quadrants lies between 400 and 475 m 

(1 ,300-1,550 ft) from ground zero. However, it is interrupted by small scarps and offset by the position of the 

broken rim scarp above it. About 10 poorly developed gullies incise the area more or less evenly around the crater 

(fig. 17). 

A20 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
(J'\ 

CD 

0 

0 

0 

162°11 '30•E 

11 °40'30•N 

11 °40'N 

+ 

+ 

DATUM: 'MLLW (APPROX.) 

CONTOURS IN METERS 

"" 0 

0 

0 

0 

~+ 11 °40'30•N 

ISLANDS 

IVY GRIO(ft) 150000 

\\ 

_,...._ t 

j 

+ 11 °40'N 

16~2o~171~'3=o~.E~----------~--------16~2-o-12~'~E---------------------1-6-2-o1-2-'3-0-.E--~ 145000 

MAP PROJECTION: UTM 

COMPILED: SEPT 1985 

250 0 250 500 750 1000 

U.S. GEOLOGICAL 

SURVEY METERS 

AAHHHI 

1000 0 1000 

FEET 

2000 3000 

Figure 16. Map showing the bathymetry of KOA crater. GZ=ground zero. Contour interval is 1 m. 

Rim Scarp 

The change in slope between the terraces and the rim 

scarp is the most abrupt in the crater, ranging up to a maximum of 18° and commonly about 12° (figs. 16; 18; 19A,B: 

lines 115, 116, 120). The area is mostly between 30 and 

60 m (100-200 ft) wide, although broader areas are present 

along the transition into MIKE crater. The scarp is continuous around the northern margin of the crater where its outer 

perimeter lies consistently close to 500 m (1 ,640 ft) from 

ground zero. It appears to have deepened about 15m (50ft) 

on the western side of the crater. On the southeast and south 

sides of the crater, the scarp is broken and offset into the 

crater to within about 365 m (1 ,200 ft) of ground zero; due 

south of ground zero, it is apparently buried or undeveloped. 

The terrace between the broken part of the scarp is 

about 75 m (246 ft) wide, which is close to the distance 

(60 m, or 197ft) that the rim scarp is offset on the west side 

of OAK crater. Presumably, it was continuous in both 

craters and has been separated by faulting. Of particular 

relevance to this interpretation is the separation of railroad 

Bathymetry of OAK and KOA Craters A21 
11 040 '30-'N + 

MIKE, 

NECTAR 

11°40tN 

DATUM: MLLW (APPROX.) 

CONTOURS IN METERS 

"' 0 

0 

0 

0 

APPROXIMATE PERIMETER 

INNER RIM SCARP 

+ 

"' Ul 

0 

0 

0 

152000 

11 °40 '30-'N 

1~000 

11°40'N 

~----------------~--------------------------------------------~ 145000 162°11'30~ 162°12'E 162°12'30~ 

MAP PROJECT I ON: UTM EH3:JHE3:::lH3::::======E::::::::=::=3::::====:::::JE::::::::==:=3 U.S. GEOLOGICAL 

SURVEY 250 0 250 500 1000 

COMPILED: SEPT 1985 METERS 

1000 0 1000 2000 3000 

FEET 

Figure 17. Map showing the bathymetry of KOA crater, the circularity of some isobaths, and some important physiographic 

features. Crosses show ground zero (GZ) locations of KOA and other events. The average perimeter of the apparent crater 

is after B. L. Ristvet and K. L. Mills, Ill, written commun., july 1985. Contour interval is 1 m. 

rails discovered and mapped on the bottom of KOA crater 

by the submersible and by scuba divers. (See chap. H, this 

volume.) These rails appear to have been in a continuous 

line when they were set prior to the blast. They now are 

separated by a distance of about 50 m (164ft) on the bottom 

of the crater in the same sense and in the same area that the 

rim scarp apparently is offset. 

Two smaller scarps, one outlined in the northern part 

and one outlined in the southwestern part of the terraces area 

(fig. 18), may have been part of the rim scarp complex. 

Debris Mounds 

Two large debris mounds project into the southwest 

side of the area of terraces (fig. 18). They both rise to about 

15 m (50 ft) above the floor of KOA and 8 m (26 ft) above 

the floor of MIKE. Within the 25-m (82 ft) isobath, the two 

mounds are about 300 m (985 ft) wide in the northeast 

direction and about 130 m ( 425 ft) wide in the southwest 

direction (fig. 18). The site of the test known as HURON 

(fig. 17; see also table 2 of the Introduction to the volume) 

A22 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
CJ'I 

CX) 

0 

0 

0 

162°11 '30•E 

11 040 '30-'N 

11 °40'N 

+ 

+ 

DATUM: MLLW (APPROX.) 

CONTOURS IN METERS 

" 0 

0 

0 

0 

+ 

_,..._ t 

I 

+ 

" Ul 

0 

0 

0 

152000 

11 °40'30•N 

150000 

11 °40'N 

~----------------~------------------------------------------~ 145000 

MAP PROJECTION: UTM 

250 0 250 500 750 1000 

U.S. GEOLOG'ICAL 

SURVEY COMPILED: SEPT 1985 METERS 

HAHHH I I I 

1 000 0 1 000 2000 3000 

FEET 

Figure 18. Map showing bathymetry of KOA crater with physiographic provinces outlined. GZ=ground zero. Contour 

interval is 1 m. 

which was fired in 1956, 2 years prior to the KOA event, 

lies only 60 m (200 ft) southeast of the smaller of the two 

debris mounds. If a crater was formed, it apparently has 

been obliterated by KOA's blast and debris. The top of this 

mound lies 500 m (1 ,640ft) from KOA's ground zero, the 

same distance as does most of the outer edge of the rim scarp 

(figs. 17, 18). Southeast of the mounds, the bottom rises at 

about 2-3° in the broad expanse of reef plate that forms the 

southern margin of the crater. One part of it that rises more 

steeply (8-9°) may be part of the rim scarp between KOA 

and MIKE (fig. 18). 

Inter-Crater Channel 

This narrow (21 m, or 70 ft) channel rises 6 m (20 ft) 

above KOA' s crater floor to a depth of between 25 and 26 m 

(82-85 ft) which is only a meter or two above the flat floor 

of MIKE crater (figs. 16; 18; 20A,B: line 507) to the southwest. Why it has not filled with sediment is not clear; perhaps it is kept open by tidal currents. To the northwest, the 

steep (9-1 0°) slope appears to be part of the deepened rim 

scarp, above which the bottom flattens out in the gradual 

(2°) ascent to the exposed reef and islands. 

Bathymetry of OAK and KOA Craters A23 
307 306 

' I 118 

(ft) 150000 

f>1 _,..._ t + 

~ 

DATUM: MLLW (APPROX.) ~ 145000 

162°II'30"E 162°12' 30"E 

MAP PROJECTION· UTM U S GEOLOGICAL SURVEY 

COMPILED: SEPT 1985 250 0 250 500 750 1000 METERS 

1000 0 1000 2000 3000 FEET 

Figure 19~. Map showing !?cations of bathymetric profiles passing through or close to ground 

zero (GZ) m KOA crater. Profiles are dashed where inferred from bathymetric map based on other 

data. Contour interval is 5 m. 

Three-Dimensional Computer Images of KOA Crater 

Perspective images of KOA are shown in figures 

21A -C. A 5 X vertical exaggeration was used to accentuate 

the details of the physiography. 

The view from the southwest (fig. 21A) depicts the 

relation of MIKE to KOA and shows more features on the 

crater walls than the others. The terraces are perhaps the 

most striking features. The impression that the terraces are 

related to slumps, particularly those on the northeast and 

southwest sides of the crater, is clearer in the perspective 

images. 

SUMMARY 

The radii of the two craters from their ground zero 

(KOA) or geometric center (OAK) to the 5-m (16ft) isobath 

differ by about 90 m (295 ft); OAK's radius is 590 m 

(1,935 ft), and KOA's 500 m (1,640 ft). OAK is almost 

twice as deep (60 m, or 197 ft) as KOA (33 m, or 108 ft). 

Mainly because of the depth difference, the average slope 

from the floor of OAK to the 5-m (16 ft) isobath is 5.2°, 

whereas in KOA it is only 3°. However, on the southeast 

side of OAK, where water was 40 m (131 ft) deep before the 

blast, the average slope is only about 4°. Both crater walls 

are steepest around the outer parts, but in OAK, the slope is 

steeper and extends deeper around the reefward side. 

Much of OAK is symmetrical about a point that is 

100m (330 ft) southeast of ground zero. KOA's ground 

zero is the symmetrical center of the crater, but below 15 m 

(49 ft) of depth, isobaths are elliptical with the long axis 

oriented northeast -southwest. The scarps and terraces on the 

walls of KOA and OAK suggest that faulting and slumping 

are common and have affected the shape and size of both 

craters. 

A24 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
NNE liNE 116 GZ 

40t 2~~. '-----..... ~ 

ssw 

~~ 40 

N 

0

UNE 306 s 

4oE 20~.~ ~ l~ 40 

NNW SSE liNE 307 

40t 2~~._,---/ ~ ,~ 40 

~ NW liNE 115 2~r:;;::------.-----::;;=~j~o 

SE 

I 

1--

0... 

w 

0 4oE ~ J4o 

NW liNE 120 SE 

2~~ ~. __!l~ 40t ~ ~ ]40 

W E liNE 304 

4oE ~F ~ -__,;.,_--~ .~~ J4o 

SW liNE 507 NE 

!~~~---~--·--===1~ 0 200 400 600 800 1000 1200 1400 METERS 

0 1000 2000 3000 4000 FEET 

V.E.=X6.2 

DATUM: MLLW (APPROX.) 

figure 198. Ground zero location projected orthogonally into lines. Vertical 

exaggeration is x 6.2 

Bathymetry of OAK and KOA Craters A25 
+ 

(ft) 150000 

539 

II 040'N f07 

-filii- ~ + 

538 

j 

DATUM MLLW (APPROX.l ~ 145000 

162° II' 30" E 162°12'30"E 

MAP PROJECTION UTM US GEOLOGICAL SURVEY 

COMPILED SEPT 1985 250 250 500 750 1000 METERS 

A ELB H H RI _ 

1000 0 1000 2000 3000 FEET 

Figure 20. A (above), Map showing locations of bathymetric profiles of KOA crater that are 

oriented parallel to the trend of the reef. Profiles are dashed where inferred from bathymetric map 

based on other data. Contour interval is 5 m. 8 (right), Ground zero (GZ) location projected 

orthogonally into lines. 

REFERENCES 

Chen, C.-T., and Millero, F. J., 1977, Speed of sound in sea 

water at high pressures: Journal of the Acoustic Society of 

America, v. 62, no. 6, p. 1129-1135. 

Circeo, L. J., Jr., and Nordyke, M. D., 1964, Nuclear cratering 

experience at the Pacific proving ground Livermore, University of California, Lawrence Radiation Laboratory, TID4500, UC-35, (UCRL)-12172, 88 p. 

Emery, K. 0., Tracey, J. 1., Jr., and Ladd, H. S., 1954, Geology 

of Bikini and nearby atolls: U.S. Geological Survey Professional Paper 260-A, 259 p. 

Freisen, Bert, ed., 1982, Enewetak Radiological Support Project, 

final report: U.S. Department of Energy, Nevada Operations 

Office, Las Vegas, Nevada, NVO 213, 349 p. 

Ristvet, B. L., Tremba, E. L., Couch, R. F., Jr., Fetzer, 

J. A., Goter, E. R., Walter, D .. R., and Wendland, 

V. P., 1978, Geologic and geophysical investigations of the 

Eniwetok nuclear craters: Air Force Weapons Laboratory 

Technical Report TR-77 -242, Kirtland Air Force Base, New 

Mexico 87117, 298 p. 

Tremba, E. L., Couch, R. F., and Ristvet, B. L., 1982, Enewetak 

Atoll Seismic Investigation (EASI): Phases I and II (final 

report): Air Force Weapons Laboratory Technical Report 

TR-82-20, Kirtland Air Force Base, New Mexico 87117, 

124 p. 

A26 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
~ 

I 

fQ_ 

w 

0 

sw 0

UNE 804 G .. z 

0 

!~r ~ 1!~ NE 

0

liNE 800 

!~F ~~ liNE 541 

2~~ ~ • --A~o 40 ~ ]40 

0

l1NE 539 

4oE 

20[ ~~0 /'V ~ j40 

liNE 507 

40 

2~~~---~----~~ 40 

liNE 538 

2~'='-[ --- :=;1~0 4ot ~ j4o 

0 

LINE 544 

20~[ ___ ~ ~~0 4ot ~ ~ j4o 

LINE 803 

2~p 40 ~~0 40 

8 0 200 400 600 800 1000 1200 1400 METERS 

0 1000 2000 3000 4000 FEET 

V.E.=X6.2 

DATUM: MLLW (APPROX.) 

Bathymetry of OAK and KOA Craters A27 
Figure 21. Three-dimensional images of KOA crater: A, from the southwest, 8, from the southeast, and C, from the 

northeast. Vertical exaggeration is x 5. 

A28 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
APPENDICES 

Appendix 1. Annotated flow chart showing procedure used to process bathymetric data and 

description of depth-data contouring 

Appendix 2. NOAA tide tables for Enewetak 
> w 

Q 

l(l 

., 

~ 

~ 0 

i" 

I'D 

~ s· 

Ill = 

Ill 

= Q. 

tl'l 

c 

Cl"' 

Q 

~ 

3 

n 

Ill =-

~ ;-

.. 

~: ;:::;· 

~ 

~ 

Ill "' 

= Q. 

~ n 

~ I'D 

il 

APPENDIX 1: Annotated Flow Chart 

Key: upper case annotations ~ programs on USGS, Woods Hole HP-1000 computers 

lower case annotations = programs on USGS, Woods Hole Pacifies computers 

comments in parentheses 

Bathymetric 

data: 

200-kHz echograms 

Digitize 

Bathymetry, 

Time (Gm.t.)& 

Depth (ft) 

BDIG (digitize) 

:SEISS (reformat) 

TRNSC (create 

plot) 

PDEFP (instruct 

specific plotter) 

correction to 

Tide 

data 

digital cassettes 

Read tide data 

cassettes and 

process to Time ( G.m.t.) 

& Depth (em) files 

CTIDE = mean 1... I 

Convert echogram feet to twoway travel time (4,800 ft/s). 

Convert two-way travel time to 

meters (1,500 m/s) MOSTS 

SCNV4 

RDAGS 

Navigation 

data 

digital tapes 

Read navigation tapes; 

Convert IVY coordinates 

to latitude and longitude 

compatible with mapping 

routines. 

Plot raw navigation, check 

for completeness, errors. 

Correct Julian day errors, 

time-series errors, bad 

fixes, etc. 

system 

PREKR 

Check crossings and 

preliminary contours for 

bathymetry and navigation~~~~============================~~ errors I 

ivforshort 

Final digital data base 

lat-long - depth (m) - IVY N (ft) - IVY E (ft) 

PIXLR or SORT 

GRIDS 

CONMG (subroutine of MAPGN) 

~jd/enewetak/ivinverse 

MAPGN 

EDIT 

EDIT 

MAPGN 
APPENDIX 1: DEPTH-DATA CONTOURING 

Computer gridding and contouring have been the primary 

techniques used in creating maps of depth data; this includes 

bathymetry, structure contours and unit thickness. Gridding encompasses a body of statistical techniques used for interpolating 

depths between track lines in order to create a "grid" of evenly 

spaced values. Contouring is the mathematical technique for examining groups of values in the "grid" and "fitting" a surface, represented by contours, to those values. Note that even a single contour 

represents a surface, because it has an upslope and a downslope 

side. 

GRIDS, a program written at the U.S. Geological Survey, 

Woods Hole, Mass., is used for gridding depth data. The user must 

first select and create a specific map base (that is, one with scale, 

projection, boundaries, and margin widths). The map base is 

created by the mapping program, MAPGN. The depth data are 

then projected onto the map base using the program PREKR. In 

addition, the user must decide on grid parameters including (1) the 

size of grid matrix squares in centimeters (appropriate for the map 

scale being used), (2) the overall size of the grid, as number of 

squares in the X and Y directions, and (3) the positioning of the 

grid on the map base, as an offset in centimeters from the bottom 

(Y -offset) and from the left side (X-offset) of the map (including 

margin widths). 

GRIDS must handle large arrays of depths in order to interpolate values for any given square in the grid. Because of limitations in the HP-1000 computer's working memory, the data is 

sorted in theY direction and read in strips, moving up theY scale 

by adding a row at the top of the strip and dropping a row from the 

bottom. The HP-1000 memory capacity determines how many 

rows may be searched at once, on the basis of data density within 

each row. In cases where some sections of the data are very dense, 

the number of rows that can be searched may be of insufficient 

width to achieve smooth interpolation. This situation is improved 

using the program PIXLR which averages redundant or duplicate 

points, thinning out the densest portions of the data, and sorting. 

GRIDS permits varying the interpolation parameters. These 

are: (1) RMAX, (2) IOCT, (3) NPMAX, (4) SM. (1) RMAX is the 

maximum radius to search for points to interpolate a depth. RMAX 

also defines half the number of rows to be loaded into computer 

memory, and hence it is limited by the memory capacity. RMAX 

was always at least 6 times the size of the grid squares. (2) IOCT 

is a 0/1 off/on switch that determines whether GRIDS searches in 

a simple circle (=0) or searches each octant (eighth circle) individually (=1). We always use a 1. (3) NPMAX, with IOCT=1, 

represents the maximum number of points to find from each octant 

for the interpolation. (4) SM is the smoothing factor, indicating 

how to weight points according to distance from center of the 

search radius. "0" is no weighting for distance; that is, simply an 

average of all the points gathered based on RMAX and NPMAX. 

This translates into maximum smoothing, averaging out small details of terrain. "10" is a steep weighting for distance, where points 

found within approximately 0. 1 RMAX completely outweigh distal values, and smoothing is minimized. We used SM=8 for 

bathymetry, which translates to: any soundings found within a grid 

square override soundings from outside that grid square. 

With the grid parameters and interpolation control values 

set, GRIDS proceeds to create what is in essence a piece of graph 

paper with numbers (depths) in all squares located within RMAX 

of a sounding. These maps of depth data created by GRIDS can be 

contoured or compared with other grids. Specifically, two grids 

created with the same grid parameters can be subtracted, square for 

square (using the program SUBGD::3302). This technique was 

applied to arrive at a map showing the difference between preblast 

and postblast bathymetry and the maps of unit thickness above 

reflectors 10 and 20. 

CONMG, a subroutine of the mapping program MAPGN, 

creates the contour maps. CONMG reads the output of GRIDS or 

SUBGD and creates a MAPGN file of contours. CONMG permits 

selection of the contour interval, the contour labeling interval, the 

label size and format, and the angularity of contours connecting 

points. In addition, contouring can be restricted to selected portions of the grid or selected depth ranges. With the exception of the 

contour angularity control, called DEL, the variables are straightforward. DEL varies the curvature put on a contour connecting 

three or more points that are not in a straight line. Setting DEL 

equal to 2 yielded smooth curves without causing interference 

between adjacent contours. 

Based on the above information, the precision of computer 

contours on a map is seen to depend on the size of the grid squares 

that were contoured and on the distribution of the data used to 

interpolate values for these squares. This information is derived 

from the gridding parameters and by examination of the point plot 

(that is, data distribution) for the specific region and data set in 

question. 

Bathymetry of OAK and KOA Craters A31 
APPENDIX 2: NOAA Tide Tables for Enewetak 

ENrWETAK (MARSHALL ISLANDS) T.M. 180 E. DATUM = MLLW 

TIDE P~EDICTIONS (HIGH AND LOW WATERS) JUNE, 198'J 

REFERENCE STATION IS: KWAJALEIN ATOLL• MARSHALL LS. T • ~~ • 180 E. 

HWTIME : -7 HWHT = .oo HWR = +.77 

LWTIME = -3 LWHT = .oo LWR = +.77 

CONS : .30 

NOAA, NATIONAL OCEA~' SERVICE 

STANDARD TIME 

DAY TT~1r HT. TIME HT. TIME HT. TIME HT. 

1 F 451 H LJ.9 1123 L .a 1713 H 3.9 2304 L 1•0 

2 SA 52P H 4.9 1204 L .9 1753 H 3.7 2341 L 1.2 

3 su 606 H 4.7 1247 L 1.1 1836 H 3.5 

4 M' 24 L 1.4 651 H 4.5 1338 L 1.3 1931 H 3.3 

5 TU 1.11 L 1. EJ 744 H 'f-.2 1438 L 1.5 2037 H 3.2 

6 w 217 L 1.9 851 H 4.0 1553 L 1.6 2207 H 3.2 

7 TH 353 L 2.1 1015 H 3.7 1713 L 1.6 2338 H 3.4 

8 F 534 L 2.0 1143 H 3.7 1819 L 1.5 

9 SA 44 H 3.7 656 L 1.7 1253 H 3.7 ]915 L 1.3 

10 su 137 H 4.1 757 L 1.4 1353 H 3.8 2000 L 1.2 

11 M 222 H 4 • L~ 848 L 1.2 1441 H 3.9 2042 L 1.1 

12 TU 303 H 4.7 933 L 1.0 1523 H 3.9 2119 L 1.0 

13 w ~42 H 4.9 1012 L .9 1603 H 3.9 2156 L 1.0 

14 TH 417 H tt.9 1051 L .9 161.f.O H 3.8 2232 L 1.0 

15 F 454 H 4.9 112~ L .9 1715 H 3.7 2305 L 1.1 

16 SA 52~ H 4.8 1208 L 1.1 1753 H 3.6 2339 L 1.3 

17 su 604 H 4. f, 1242 L 1.2 1830 H 3.4 

18 M 16 L 1.5 641 H 4.3 1322 L 1.4 1910 H 3.3 

19 TU 55 L 1.7 71B n 4.0 1404 L 1. 6 19~;8 H 3.2 

20 w 13P L 1.9 803 H 3.8 1455 L 1.7 20!;8 H 3.1 

21 Trl ~3Q L 2.1 900 H 3.5 1555 L 1.8 2217 H 3.1 

22 F 40~ L 2.3 1012 H 3.3 1703 L 1.9 2336 H 3.2 

23 SA 54£' L 2.2 1133 H 3.3 1806 L 1.8 

24 su 3q H 3.5 656 L 2..0 1244 H 3.3 1859 L 1.7 

25 M 12q H 3.7 752 L 1.8 1340 H 3.4 1944 L 1.5 

26 TU 209 H 4.1 8:37 L 1.5 1428 H 3.5 20~6 L 1.4 

27 w ~49 H 4.3 919 L 1.2 1510 H 3.7 2111~ L 1..2 

28 TH ~26 H 4.(, 959 L 1.0 1550 H 3.8 2143 L 1.1 

29 F 40~ H 4 • P. 1039 L .a 1631 H 3.8 22~1 L l.o 

30 SA 44~ H 4.9 1120 L .a 1711 H 3.8 2302 L 1.0 

A32 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
ENEWETAK ( ~,ARSHALL ISLANDS) r.M. 180 E. DATUM = MLLW 

TIOE PREDICTIONS (HIGH AND LOW WATERS) JULY, 198~ 

REFERF.:NCE STATION IS: KWAJALEIN ATOLL, MARSHALL IS. T.M. 180 E. 

HWTIME = -7 HWHT = .oo HWR = +.77 

LWTIME :: -3 LWHT :: .oo LWR = +.77 

C0r.-1S :: .30 

NOAA, NATIONAL OCEAN SERVICE 

STANDAR'1 TIME 

DAY TIME HT. TIME HT. TIME HT. TIME HT. 

1 su 523 H 5.0 1200 L .8 1753 H 3.8 2344 L 1.o 

2 M 604 H 4.9 1242 L .a 1836 H 3.7 

3 TU 2l=. L 1.2 649 H 4.7 1'327 L 1.0 1925 H 3,7 

4 w 116 L 1.4 734 H 4.4 1415 L 1.2 201~ H 3.6 

5 TH 21? L 1.6 829 H 4.1 1510 L 1.~ 2122 H 3.5 

6 F 3('4 L 1.8 933 l~ 3.7 1614 L 1.5 2239 H 3.5 

7 SA 452 L 1.9 1054 H 3.4 1724 L 1.6 23'59 H 3,7 

8 su 627 L 1.9 1218 H 3.3 1830 L 1.6 

9 M lOP H 3.9 744 L 1.7 1329 H 3.3 19~1 L 1.5 

10 TU 204 H 4.2 842 L 1.4 1430 H 3.4 2021 L 1,4 

11 vJ 24q H 4.4 928 L 1.2 1515 H 3.5 210h L 1.3 

12 TH 331 H 4.6 1010 L 1.1 1558 H 3.6 2146 L 1.2 

13 F 40P H 4.7 1045 L 1.0 1632 H 3.7 2222 L 1.1 

14 SA 44!5 H 4.A 1120 L .9 1708 H "3.7 22~A L 1.1 

15 su 518 H 4.7 1153 L 1.0 1741 H 3.7 2333 L 1.1 

16 M 55? H 4. (..> 1224 L 1.0 1814 H 3.7 

17 TU 7 L 1.2 62.5 H 4.5 1255 L 1.1 1848 H 3.7 

18 w 4l L 1.4 656 H 4.2 1325 L 1.3 1923 H 3.6 

19 TH 117 L 1.6 72A ri 4.0 1359 '- 1.5 2002 H 3.5 

20 F 159 L 1.8 806 H 3.7 1'+36 L 1.6 2047 H 3,4 

21 SA 247 L 2.0 851 H 3.4 1524 L 1.8 2151 H 3.3 

22 su 405 L 2.2 957 rl 3.2 1628 L 1.9 2315 H 3.3 

23 M 54P. L 2.2 1134 H 3.0 1751 L 1.9 

2'+ TU 39 H 3.'5 723 L 2.0 1308 H 3.1 1904 L 1.a 

25 w 14r H 3.9 824 L 1.7 1414 H 3.3 2005 L 1.6 

26 TH 230 H 4.? 911 L 1.3 1502 H 3,5 2051 L 1.3 

21 F 315 H 4.6 952 L 1.0 1545 H 3.7 213'i L 1.1 

28 SA ~56 H 4.9 1031 L .7 1625 H 3.9 2218 L .9 

29 su 437 H s.o 1110 L .6 1703 H 4.1 2257 L ,A 

30 M 515 H 5.l 1147 L .s 1742 H 4.2 2339 L .a 

31 TU ~~" ._- <J ~ H 5.0 1224 L .6 1822 H 4.2 

Bathymetry of OAK and KOA Craters A33 
ENEWETAK O~ARSHALL ISLANDS) T.M. 180 E. DATUM : MLLW 

TIDE PREDICTIONS (HIGH AND LOW WATERS) AUGUST, 1984 

REFERENCE STATION IS: KWAJALEIN ATOLL• MARSHALL IS. T.M. 180 E. 

HWTIME = -7 HWHT = .oo HWR = +.77 

LWTIME = -3 LWHT = .oo LWR = +.77 

COtJS = .30 

NOAA, NATIONAL OCEAN SERVICE 

STANDARn TIME 

DAY TI~P.E HT. TIME HT. TIME HT. TIME HT. 

1 w 2] L .9 633 H 4.8 1301 L .7 1902 H 4.1 

2 TH 103 L 1.1 715 H 4.5 1340 L 1.0 1947 H 4.0 

3 F 151 L 1.4 758 y 4.0 1423 L 1.3 2037 H 3.A 

4 SA 249 L 1.7 848 H 3.6 1510 L 1.6 2141 H 3.6 

5 su L~Oq L 2.0 959 H 3.2 1618 L 1.9 2309 H 3.6 

6 M 60(!. L 2.1 1148 H 2.9 1750 L 2.0 

7 TU 47 H 3.7 747 L 1.9 1335 H 3.0 1918 L 1.9 

8 w 1.56 H 3.9 850 L 1.6 1438 H 3.2 2018 L 1.7 

9 TH ~47 H 4.2 930 L 1.3 1521 H 3.4 2104 L 1.5 

10 F 326 f-J 4.5 1002 L 1.1 1553 H 3.6 2143 L 1.2 

11 ~A 401 H t.t.G 1033 L 1.0 1625 H 3.8 2216 L 1.1 

12 su 432 H 4.7 1059 L .9 1653 H 4.0 2248 L 1.0 

13 M ~02 h 4.8 1127 L .a 1722 H 4.1 2320 L .9 

14 TU 530 H 4.7 1152 L .a 1747 H 4.1 2348 L 1.0 

15 w 55,:l ,... 4.6 1218 L .9 1816 H 4.1 

16 TH 1~ l 1.1 625 H 4.4 1245 L 1.1 1843 H 4.0 

17 F 47 L 1.3 651 H 4.1 1309 L 1.3 1913 H 3.9 

18 SA 11'J L 1.5 721 H 3.t' 1338 L l.5 1947 H 3.7 

l9 su 159 L l.e. 750 H 3.5 1410 L 1.7 2032 H 3.5 

20 M 252 L 2.1 835 H 3.1 1455 L 2.0 2144 H 3.4 

21 TU 43'3 L 2.3 lOlA H 2.8 1627 L 2.2 2350 H 3.4 

22 w 712 L 2.1 1300 H 2.9 1841 L 2.1 

23 TH 124 H 3.7 818 L 1.7 1414 H 3.2 19'57 L 1.8 

24 F 222 H 4.2 903 L 1.3 1459 H ~.6 2045 L 1.4 

25 SA 305 H 4.6 938 L .9 1534 H 3.9 2128 L 1.0 

26 su 344 H 4.9 1015 L .6 1611 H 4.2 2210 L .7 

27 M 422 H 5.1 1049 l .4 1647 H 4.5 2246 L .s 

28 TU 500 H 5.2 1122 L .3 1721 H l.f..6 2326 L .s 

29 w 53~ ~ 5.1 1155 L .4 1758 H 4.6 

30 TH 5 L .6 612 H 4.8 1229 l .6 1833 H 4.5 

31 F 44 L .9 64q H 4.4 1301 L .9 1910 H 4.3 

A34 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
ENEWETAK (MARSHALL ISLANDS) T.M. 180 E. DATUM : MLLW 

TinE PREDICTIONS (HIGH AND LOW WATERS) SEPTEMRE:Rt 1984 

REFERENCE" STATION IS: KWAJALEIN ATOLL, MARSHALL IS. T.M. 180 E. 

HWTIME = -7 HWI--'T = .oo HWR = +.77 

LWTIME : -3 LWHT = .oo LWR = +.77 

CONS = .30 

NOAA, NATIONAL OCEAN SERVICE 

STANDARD TIME 

DAY TI~~E HT. TIME HT. TIME HT. TIME HT. 

1 SA 12~ L 1.2 725 H 3.9 1335 L 1.3 1950 H 4.0 

2 su 215 L 1.7 803 H 3.4 1410 L 1.7 2045 H 3 •. , 

3 M ~26 L 2.1 901 H 2.9 1500 L 2.1 2217 H 3.4 

4 TU 611 L 2.2 1159 H 2.6 .1715 L 2.3 

5 w 3q H 3. c;-.l 808 L 1.9 1404 H 2.9 1933 L 2.2 

6 TH 156 H 3.n 853 L 1.6 1446 H 3.2 2024 L 1.8 

7 F 241 H 4.1 919 L 1.3 1515 H 3.5 2103 L 1.5 

8 SA 315 H 4.4 946 L 1.1 1541 H 3.8 2134 L 1.2 

9 su ~44 H 4.6 1008 L .9 1603 H 4.1 2204 L 1.0 

10 i"1 411 H 4.7 1032 l .e 1628 H 4.3 223l. L .Pt 

11 TU 437 H 4.7 1055 L .7 1653 H 4.4 2258 L .a 

12 w 504 H 4.7 1117 L .7 1718 H 4.5 2325 L .a 

13 TH 527 H 4.6 1141 L .8 1743 H 4.5 2352 L .9 

14 F 55~ H 4.4 1202 L 1.0 1806 H 4.4 

15 Sl\ lP L 1.1 617 H 4.l 1224 L 1.1 1.833 H 4.2 

16 su 47 L 1.3 643 H 3.8 1247 L 1.4 1902 H 4.0 

17 M 12;> L 1.6 710 H 3.5 1314 L 1.7 1939 H 3.8 

18 TU 210 L 2.0 750 H 3.1 1346 L 2.0 2043 H 3.5 

19 w 35A L 2.3 928 H 2.7 1513 L 2.3 2309 H 3.4 

20 TH 704 L 2.1 1308 H 2.8 1830 L 2.3 

21 F lOA H 3.7 803 L 1.6 1407 H 3.3 1947 L 1.8 

22 SA 205 H 4.(' 842 L 1.2 1444 H 3.8 2034 L 1.3 

23 su 246 H 4. ~} 917 L .a 1518 H 4.2 2117 L .9 

24 M ~26 H 4 .. Cl 948 L .s 1550 H 4.6 2154 L .6 

25 TU '+o~ H 5.1 1021 L .3 1624 H 4.9 2232 L .'+ 

26 w 43B H 5.1 1053 L .3 1657 H s.o 2309 L .3 

27 TH 513 ~ 4.9 1123 L .4 1729 H 5.0 2346 L .5 

28 F 54 A H 4.6 1155 L .6 1804 H 4.8 

29 SA 24 L .B 620 H 4.2 1223 L 1.0 183A H 4.6 

30 su 103 L 1.2 654 H 3.7 1250 L 1.4 1915 H 4.2 

Bathymetry of OAK and KOA Craters A35 

Chapter B 

Sidescan-Sonar Survey of OAK and 

KOA Craters 

By D. W. FOLGER, J. M. ROBB, J. C. HAMPSON, 

P. A. DAVIS, P. M. BRIDGES, and D. J. RODDY 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction B 1 

Purpose B1 

Previous work B1 

Field methods B 1 

Image processing B 1 

Results B4 

OAK crater B4 

Crater floor B9 

Inner slope B9 

Inner terraces B9 

Outer slope B9 

Outer terraces B9 

Rim slope B9 

Rim scarp B 13 

Debris blanket B13 

Lagoon floor B13 

KOA crater B13 

Crater floor B 14 

Inner slope B16 

Terraces B17 

Rim scarp B 17 

Debris mounds B17 

Channel B17 

Manmade structures B 17 

Conclusions B18 

References B18 

FIGURES 

1-2. Maps showing tracks of vessels along which sidescan-sonar data were acquired: 

1. In OAK crater B2 

2. In KOA crater B3 

3-4. Sidescan-sonar mosaics of: 

3. OAK crater BS 

4. KOA crater B7 

5. Airbrush-enhanced images of OAK crater based on sidescan-sonar, bathymetry, and 

sea-floor observations B10 

6. Airbrush-enhanced images of KOA crater with bathymetry overlay and locations of 

sea-floor observations B14 

Contents Ill 

Chapter B 

Sidescan-Sonar Survey of OAK and KOA Craters 

By D. W. Folger1, J. M. Robb1, j. C. Hampson 1, P. A. Davis2, P.M. Bridges2, and D. j. Roddy2 

INTRODUCTION 

Purpose 

The sidescan-sonar survey provided critical information on the distribution, trends, and texture of scarps, fractures, slumps, and bottom sediments associated with the 

craters and the debris accumulation around OAK crater. The 

~idescan-sonar survey also provided an on-site map-quality 

Image (a field-prepared mosaic) to guide submersible and 

scuba surveys. 

Echo soundings provide quantitative measurements of 

the surface of a submarine crater, but such data are limited 

to points directly beneath the tracks of a ship. Information 

needed to fill in details between ship tracks requires another 

technique. Side-looking radar or even oblique aerial photographs are appropriate for subaerial craters. Radar cannot be 

used for submarine features, however, and sophisticated 

submarine optical systems such as Angus (Ballard and 

others, 1984) could not be used in this study due to poor 

visibility (as reported by Tremba and others, 1982). Therefore, we used a sidescan-sonar system, the acoustic equivalent of side-looking radar. 

Previous Work 

The sidescan-sonar survey that was undertaken as part 

of the EASI (Enewetak Atoll Seismic Investigation) project 

provided no useful data. To our knowledge, no other 

sidescan-sonar observations in the craters have been made. 

Field Methods 

Most of the sidescan-sonar data were collected aboard 

Egabrag II on tracks spaced 75 m (246 ft) apart. We acquired data along a total distance of75 km (40 nmi) in OAK 

and 19.6 km (10 nmi) in KOA (figs. 1, 2). Additional tie 

lines were run in the lagoon between the craters, and short 

extensions of lines were run into shallow water by the landing craft (Mike boat) in both craters. Most data were acquired with a Klein Model 422S-101 AF transducer unit 

mounted in a 545-kg (1 ,200 lb) fish. Data from the landing 

craft were acquired with similar transducers, but they were 

1U.S. Geological Survey, Branch of Atlantic Marine Geology, Woods 

Hole, Mass. 02543 2U.S. Geological Survey, Branch of Astrogeological Studies, Flagstaff, 

Ariz. 86001 

mounted on a smaller fish. The port and starboard transducers have a horizontal beam width of 1 o, a vertical beam 

width of 40°, an output frequency of 100 kHz, a 0 .1-ms 

pulse length, and a resolution of 0.15 m. The stability of the 

large transducer vehicle in the water was a major contributor 

to the excellent record quality that allowed us to construct 

usable mosaics in the field. We used 100m (330ft) of range 

on either side of the fish, giving us about 30 percent overlap 

in each direction from line to line. Data were displayed on 

a Klein 531T three-channel wet-paper recorder. Analog signals were recorded on magnetic tape using a Honeywell 

14-track recorder and l-inch (2.54 em) magnetic tape. 

Although the best detail is generally obtained by towing the transducer fish close to the bottom at 10 percent of 

the instrument's range, we were forced to run it at shallower 

than optimal depths in the confined space of KOA and 

among the many patch reefs surrounding OAK. During 

cloudy weather, coral heads were difficult to see, maneuvering room for Egabrag II was constrained, and we had to 

terminate lines short of some critical nearshore areas in both 

craters. To overcome this problem, we subsequently 

mounted the sidescan-sonar system on the Mike boat (fig. 2, 

Introduction to the volume), and used a smaller, less stable 

fish that could be controlled quickly by hand. However, 

rapid changes in depth produced noisier records in some 

areas, especially within KOA. 

Following the Egabrag phase of the sidescan-sonar 

survey, the analog paper records were photographed at the 

Mid-Pacific Laboratory on Enewetak by D. S. Blackwood, 

and photographs were assembled into mosaics. These were 

then rephotographed and printed at Enewetak at scales appropriate for immediate field use aboard Egabrag, Delta, 

and Halimeda. 

Image Processing 

Subsequent to field operations, to enhance the 

sidescan-sonar images and to create map-quality mosaics 

using processed navigation and bathymetric data, the analog 

recordings of sidescan-sonar data on magnetic tape were 

digitized using a MASSCOMP MC-500 computer. A 30-

kHz digitizing rate was used over 180 ms of the 200-ms scan 

interval, with every third point recorded, giving about 1 ,800 

points per scan, for an average lateral pixel dimension of 

about 7.5 em (3 in.). At the usual ship speed of 4 kn (about 

Sidescan-Sonar Survey of OAK and KOA Craters 81 
II 0

37'N + + 

SIDESCAN -SONAR 

TRACK LINES 

125000 

II0 35.3011N + 

120000 IVY GRID(ft) 

DATUM: MLLW (APPROX.) 

MAP PROJECTION' UTM 

SPHEROID' CLARK 1866 

COMPILED' MAY 1985 

1000 

250 

0 

0 

0 

L{) 

r() 

0 

0 250 

1000 

+ 

500 750 1000 

2000 3000 4000 

8 

8 

v 

+ 

_,..._ t 

j 

+ 

U.S. GEOLOGICAL SURVEY 

1250 METERS 

5000 FEET 

Figure 1. Tracks of vessels along which sidescan-sonar data were acquired in OAK crater. GZ=ground zero. Contour 

interval is 5 m. 

2 m per second), the vertical, or along-track, pixel dimension was about 40 em (16 in.). 

The digital sidescan-sonar data were processed at the 

U.S. Geological Survey's (USGS) laboratories in Flagstaff, 

Ariz., using a VAX-111750-based computer system and a 

Ramtek interactive display unit. The computer programs 

used have been developed primarily within the USGS. 

Processing steps included noise removal, slant-range 

correction, aspect-ratio correction, and application of a constant contrast stretch so the data could be assembled in a 

mosaic. Film transparencies of each sonar track were prepared at scales appropriate for airbrushing. 

The noise removed took the form of ( 1) random electronic noise introduced by various hardware systems, and 

(2) speckle noise that is inherent in all sonar and radar data. 

Most of the random noise was visible as solid white or black 

horizontal lines and, with the Mike-boat data, some vertical 

lines. The position of either line or sample noise was deter82 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
+ 

DATUM: MLLW (APPROX.) o 

0 

0 

162°II 13011E 

MAP PROJECTION: UTM 

SPHEROID: CLARK 1866 

~ 

804 

538 

542 

803 

----!Om_r--

!5m~ 

+ 

IVY GRID (ft) 150000 

+ 

(

20m 

zom\ SIDESCAN- SONAR 

TRACK LINES 

162°1213011E 

US GEOLOGICAL SURVEY 

COMPILED= MAY 1985 250 0 250 500 750 1000 METERS 

1000 0 1000 2000 3000 FEET 

Figure 2. Tracks of vessels along which sidescan-sonar data were acquired in KOA crater. GZ=ground zero. Contour interval is 

5 m. 

mined and recorded by examination using an interactive 

display screen coupled to the computer. The line noise was 

removed from the digital data by duplication of adjacent 

lines (either vertical or horizontal). The duplication is unnoticeable in the image because of the high resolution of the 

data, but the duplication must be done prior to the slantrange correction because the latter would distort the noise 

and make its later removal extremely difficult. 

The speckle noise was removed by applying a median 

digital filter to the data. Preliminary tests showed that the 

minimum-sized boxcar filter (2-by-2) was sufficient to remove this noise. A 2-by-2 boxcar refers to the imaginary 

box configuration of the filter that runs laterally along each 

line of image data from the top to the bottom of the image. 

During this moving filter process, the value of each pixel 

(known as the digital number, or DN) in the four-pixel box 

Sidescan-Sonar Survey of OAK and KOA Craters 83 
is taken from the original image data base and accumulated 

in computer memory, and a median DN is selected. The 

resultant median value represents the new DN value for the 

upper left comer value of the box. This value is then placed 

in a blank image database at the original image position of 

the upper left pixel of the box. The advantage of the median 

filter over other digital filters is that it eliminates DN spikes, 

either as anomalously high or low values in the original 

data, but maintains as much of the information content of 

the original data as possible. 

The next step was slant-range correction, which requires knowledge of the height of the sidescan fish over 

bottom. Because the height over bottom varied and the 

built-in altimeter unit in the sidescan-sonar fish had failed 

near the start of the operation, we recovered these values by 

digitizing the first arrival of the sonar image. This procedure 

produced a file of across-track pixel positions for each line 

of image data, which was then used in the slant-range calculation to determine where pixels within a particular line of 

original image data should be located in an undistorted image. These positions were then used to extract pixels along 

a particular line of original image data and to place them at 

their true relative positions in a blank image file. The acrosstrack resolution of the resultant image data was degraded to 

about 30 em (12 in.) from the original 7.5 em (3 in.). 

RESULTS 

The sidescan data from OAK were of significantly 

better quality than those from KOA crater. This result probably was due to several factors that included: (1) restricted 

area in KOA for maneuvering the ship; (2) more reverberation noise in KOA data due to the crater's smaller size and 

enclosed geometry; and (3) more use in KOA of the smaller 

fish which had to be raised and lowered rapidly and was 

consequently more noisy. To illustrate the quality of the 

field sidescan-sonar data, we have included the mosaics of 

the two craters that were constructed at Enewetak from the 

analog records (figs. 3, 4). The mosaics include 5-m bathymetric overlays for orientation. The field mosaics show 

many characteristics that make interpretation of sidescan 

data difficult, such as the boundaries between strips of imagery recorded along each track, variations in directions of 

shadows depending on fish location, and complex patterns 

of noise. Digitizing and processing has minimized these 

characteristics and enhanced the definition of targets, especially in such areas as the inner crater. The processed, 

digitized sidescan data provided the principal input to the 

airbrush-enhanced sea-floor images of the two craters 

(figs. 5A-C; 6A-C). 

OAK Crater 

For the field mosaic, photographs of sidescan-sonar 

images were glued up with the look direction toward the 

reef. Accordingly, more details are visible on the northwest 

side of the crater than on the southeast side (fig. 3A ). Most 

major features are present on this mosaic, but many smaller 

features are not. The steep slopes on the reefward side of the 

crater show as the most intense reflections in the area. Few 

details of the numerous scarps that make up the slope are 

resolved. The debris blanket surrounding the lagoonward 

side of the crater is defined by a texture that is unique and 

appears nowhere else in the image. The flat-floored valley 

that cuts through the crater wall and broadens as it crosses 

the debris blanket is clearly apparent southeast of ground 

zero. Outside the debris blanket the ubiquitous patch reefs 

appear as conical mounds. In contrast to these features, the 

crater floor and crater walls out to a distance of about 260 m 

(850 ft) from the geometric center of the crater show few 

targets or reflectors. 

The airbrush-enhanced version of the sidescan-sonar 

data (fig. 5A) was also set up with lighting from the southeast. Light areas thus represent, for the most part, surfaces 

that slope toward the viewer, and dark areas represent surfaces that slope away from the viewer. Intermediate between the two are areas with relatively flat, smooth bottom. 

The third step was the correction for aspect ratio. 

Because the speed of the ship varies, the along-track spatial 

resolution of pixels in the original data set also varies and 

does not match the across-track resolution. This discrepancy 

gives the effect of different vertical and horizontal scales, 

and the slant-range correction alone will not produce an 

undistorted image. The aspect-ratio correction was calculated using the navigation data associated with each scan 

line to compress or expand the along-track data, setting the 

output-image data to an aspect ratio of 1.0, so that alongtrack and across-track scales were equal. 

After noise, slant-range, and aspect-ratio corrections, 

the resultant data were interactively examined to determine 

the digital stretches necessary to make images of overlapping strips appear to be taken under the same electronic 

"lighting" conditions. These stretch values were then employed by the electronic photowrite device to make film 

transparencies of each sonar strip at the appropriate scale for 

preparation of airbrush maps. 

Airbrush maps of the digitally processed data were 

constructed at Flagstaff, Ariz. The maps were created by 

overlaying the film transparencies of the digitally processed 

data and the navigation track lines and the 1-m contour 

bathymetric map, and reproducing the locations and shadings of features shown on the digitally processed data strips 

as a single mosaic having a constant lighting direction. To 

construct the airbrush maps, the relief shading of the grander 

features of the areas, the craters and the reef or reef plate, 

was derived from the bathymetric contours as well as from 

the sidescan-sonar images. The true position of individual 

features, such as the patch reefs around OAK crater, were 

taken from the bathymetric maps because the position of 

echo-soundings below the transducer mounted in the ship's 

84 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
til 

c: 

trl 

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1:11 

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US GEOLOGICAL SURVEY 

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JUNE, 1984 

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U S GEOLOGICAL SURVEY 

OFFICE OF ENERGY AND MARINE GEOLOGY 

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US. GEOLOGICAL SURVEY OFFICE OF ENERGY AND MARINE GEOLOGY 11 WOODS HOLE, MA 

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hull was more closely controlled than were the sidescansonar data, which were taken from a submerged fish at the 

end of a variable-length cable. The locations of features on 

the airbrush maps are probably within about twice the specified navigational error, or within about 10m (33ft) of their 

true positions. The images were reviewed and modified for 

compatibility with sea-floor observations made from the 

submersible and by scuba divers. Thus the airbrushenhanced images represent the most comprehensive integration of data in the report. 

The excellent quality of these products will greatly 

enhance the interpretation of the other data sets. They come 

too late to modify figures in other chapters prepared for this 

report, but they should provide valuable input to future 

analyses of these and other craters. 

We shall discuss the details of the airbrush-enhanced 

sidescan-sonar map of OAK in sequence from the crater 

floor outward, on the basis of the physiographic provinces 

defined by the bathymetry in chapter A of this volume. 

Those provinces include the crater floor, inner slope, inner 

terraces, outer slope, outer terraces, rim slope, rim scarp, 

debris blanket, and lagoon floor (fig. 5C). For purposes of 

this discussion, we have not altered the boundaries determined from the bathymetric map to fit the features depicted 

on the airbrush map except for the outer boundary of the 

debris blanket. Five-meter (16ft) bathymetric contours have 

been overlain on the airbrush map for orientation (fig. 5B). 

Crater Floor 

Inside the boundary of the crater floor, the bottom 

appears to be flat and featureless (fig. 5C). The small rise 

near its center, shown by the bathymetry, apparently is of 

insufficient relief to be detected by the sidescan-sonar. The 

perimeter of the crater, drawn on the basis of the bathymetric change in slope, lies slightly inside the steepest slopes 

interpreted from the sidescan data. 

Inner Slope 

The steep sides of this province are clearly defined on 

the airbrush map, particularly near its inside perimeter, 

where they have greater sonar expression than near its outer 

perimeter (fig. 5C). Valleys incise the slope, particularly on 

the southern and eastern sides. About half of the valleys 

revealed here are coincident with valley thalwegs shown in 

figure 9 of chapter A (this volume). The eastern side of the 

crater's inner slope appears to be breached, as if debris had 

flowed in that direction, whereas the western side is complicated by what appear to be downdropped areas. These areas 

probably reflect variations in the style of mass-wasting from 

the lagoon side to the reef side of the crater. 

Inner Terraces 

This broad area is characterized mostly by smooth 

bottom in the northeast, northwest, and southwest quadrants 

of the crater (fig. 5C). In the southeast quadrant the bottom 

texture more closely resembles that of the debris blanket 

farther from ground zero. The airbrush map shows three 

channels that cut through the eastern side of this area. The 

westernmost channel is obvious on the bathymetry, but the 

other two are not as clearly defined. All three join into the 

main channel that cuts through many of the major provinces 

on the southeastern side of the crater. A large mass 310 m 

(1 ,017 ft) long and 150m (490ft) wide of what probably is 

mostly debris separates the main channel from its neighbor 

to the north. In the other three quadrants of the area, probable debris mounds are mostly no larger than 120m (394ft) 

long and 60 m (197 ft) wide. 

Outer Slope 

The outer slope has a distinctive texture on the sidescan data (fig. 5C) that is surprising, considering the subtle 

change in slope that defines it on the bathymetry. Only in a 

small part of the area east of ground zero is the bottom 

smooth. Elsewhere, radially subparallel linear to arcuate 

features crenulate most of the bottom. These are commonly 

30 to 60 m (100-197 ft) long and are found around the 

southwest side of the outer slope. Similar but fewer features 

are present on the northeast and southeast sides. These features are probably the promontories discussed by Halley and 

others in chapter F (this volume) that give the slope a fluted 

appearance from the submersible. On the north side of the 

crater is an area about 140m (450ft) wide that appears to be 

incised by small, subradial ravines or gullies. 

Outer Terraces 

The outer terraces of the northern and western areas 

are characterized by a smooth bottom, like the inner terraces 

in those quadrants (fig. 5C). South of ground zero, however, the area is about 50 percent rougher, more like a debris 

surface, and contains several small, arcuate scarps up to 

130 m ( 426 ft) long. In the northwest quadrant of the crater, 

the natural reef slope projects into the area of inner terraces 

about 375m (1 ,230ft) from ground zero, and no outer terraces region is present. The western area appears to be 

complex, having reverse slopes and narrow scarps. The 

boundary between the outer terraces and the rim slope is 

marked by a change from smooth to rough relief and is clear 

on the northeast flank of the crater but less clear on the 

southwest flank. 

Rim Slope 

The two areas of the rim slope east and west of ground 

zero are similar in texture, each showing about 50 to 

70 percent smooth bottom. Both have scattered debris 

mounds, which are mostly less than 18m (60ft) in diameter 

(fig. 3C). However, in the southern part of the eastern area, 

a large debris or patch-reef complex about 100m (330ft) in 

diameter occupies most of the rim slope. A secondary crater 

Sidescan-Sonar Survey of OAK and KOA Craters 89 
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U.S. GEOLOGICAL SURVEY 

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U.S. GEOLOGICAL SURVEY 

162°5130"E 162°6 1

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30"E 162°7 1

E 

Figure 5C. Airbrush-enhanced image of OAK crater including a province map overlay derived mostly from bathymetric data. GZ=ground zero. 
about 20 m (65 ft) in diameter lies almost due south of 

ground zero. 

Rim Scarp 

The high reflectivity of the rim scarp clearly depicts 

the area of steepest slopes in the crater (fig. 5C). Alternating 

light and dark bands, commonly only 6 to 8 m (20-25 ft) 

wide, mark the scarps and terraces observed by submersible 

and by scuba divers. These smaller scarps and terraces contrast with the areas where major terraces, as large as 60 m 

(200ft) by 375m (1 ,230ft), break the scarp into steps west 

and northwest of ground zero. 

The uppermost scarp west of ground zero shows about 

40 small reflectors, less than 3m (10ft) in diameter, which 

may be pieces of reef plate that have slumped into the crater. 

Debris Blanket 

The debris blanket is made up mainly of two arcuate 

ridges rimming the lagoon ward side of the crater. These 

compose a broad debris complex that is more extensive to 

the northeast than to the southwest and is divided by the 

broad, flat-floored channel southeast of ground zero 

(fig. 5C). 

The boundaries of the debris blanket visible on the 

airbrush map coincide well with those we could define using 

the bathymetric map, except at the blanket's southern extremity. There, according to the interpretation of the sidescan data, the rough bottom typical of most of the debris 

blanket terminates as much as 250 m (820 ft) closer to 

ground zero than we drew it on the basis of bathymetry. 

Figure 5C shows the boundary that conforms with the 

airbrush-map interpretation. 

Farther to the southeast, about 1,150 m (3,770 ft) 

from ground zero, sea bottom having a similar texture to the 

debris blanket covers an isolated area 325 m ( 1 ,070 ft) long 

and 250 m (820 ft) wide. This area may be a mass of debris 

that swept through the main channel to be slowed down or 

stopped by several patch reefs, which it now mantles or 

abuts. 

Lagoon Floor 

The predominantly smooth lagoon floor within the 

study area is dotted by about 75 patch reefs (fig. 5C). The 

diameter of the reefs varies greatly from 10 to 20 m (33-

65 ft) to greater than 100m (330ft). The fine-textured bottom of the lagoon floor also comprises some complexes of 

mounds similar to those in the debris blanket that may consist of debris from the crater. 

Farther than about 800 m (2,600 ft) southeast of 

ground zero, the channel separates into two legs that extend 

to either side of the large mass of debris noted above. There, 

the channel floor is lineated with what appear to be erosional 

furrows and grooves. Because the subbottom profiles show 

the lagoon floor there to be underlain by chaotic reflectors, 

those features do not appear to represent truncations of horizontal bedding. 

Two circular depressions, which appear to be secondary craters about 25 m (83 ft) in diameter, lie about 900 m 

(3,000 ft) southwest of ground zero, on probable reef plate. 

KOA Crater 

Because KOA crater was blown out of an island, land 

or reef plate makes up much of its perimeter. Debris accumulations, similar to those at OAK, lie in shallow water or on 

land where they cannot be seen; therefore the sidescan-sonar 

mosaic only portrays the central part of the crater complex. 

The same kind of strong echoes associated with the 

scarps that are present along the reef side of OAK extend 

around the north, east, and south sides of KOA (fig. 4). On 

its southwest side KOA crater adjoins MIKE in an area of 

complex topography where a third test, HURON, was conducted in 1956 (Circeo and Nordyke, 1964; chap. A, 

fig. 17, this volume). 

In KOA only a few, small (less than 40 m, or 131 ft, 

in diameter) targets were recorded on the analog records 

deeper than the 25-m (82ft) isobath (fig. 4). This is probably due in part to sediment infilling of the crater center but 

also in part to the height off-bottom of the Egabrag -towed 

transponder. Targets are more common on the analog 

records especially to the southwest in the hummocky area 

between MIKE and KOA. There, however, even the two 

large conical mounds, which are so apparent on the bathymetric map, are not well defined. 

Targets are abundant in water shallower than 20 m 

(66ft). The increased frequency of targets is, in part, a 

result of having the transponder towed closer to the bottom 

from the MIKE boat, but it is also due to the presence of 

small scarps and debris from the main scarp that surrounds 

much of the crater between the 5- and 20-m (16- and 66-ft) 

isobaths. 

In the northeast quadrant, the upper edge of the main 

scarp is well defined near the 5-m (16ft) isobath. In the 

southeast quadrant, however, targets are complex and, except for a few segments, show little coherence. The 

airbrush-enhanced image was modified little with input 

from submersible and scuba observations in the deep areas 

of the crater because underwater visibility there was so 

limited. However, around the margins of the crater, seafloor observations were useful in identifying and defining 

some targets in detail, particularly the manmade structures. 

As for OAK, the airbrush-enhanced image for KOA 

(fig. 6A) was set up with lighting from the southeast. 

We shall discuss the features portrayed on the airbrush map in sequence from the crater floor outward, on the 

basis of physiographic provinces defined by the bathymetry 

in chapter A, figure 18, of this volume. These provinces 

include the crater floor, inner slope, terraces, rim scarp, 

debris mounds, and channel. None of the province 

Sidescan-Sonar Survey of OAK and KOA Craters 813 
ENHANCED SEAlFLOOR IM GE 

-KOA CRATER 

+ 

+ 

146000 IVY GRID (ff) 

0 

0 

500 METERS 

1500 FEET 

U. S. GEOLOGICAL SURVEY 

Figure 6A. Airbrush-enhanced image of KOA crater based on sidescan-sonar, bathymetry, and sea-floor observations. 

GZ=ground zero. 

boundaries in KOA crater have been altered for this discussion 

of the sidescan data, but the boundaries of the rim scarp and 

of the debris mounds have been modified slightly on the eastem margin of MIKE crater (fig. 6C). We show the airbrush 

map with a 5-m bathymetric-contour overlay for orientation 

and have labeled some of the target -echoes from manmade 

structures on the eastern margin of the crater (fig. 6B). 

Crater Floor 

The crater floor is smooth and subdued, doubtless 

owing in part to the influx of material continually being 

swept from the reef by the trade winds. The smoothest, most 

featureless area appears to be southwest of ground zero, 

suggesting that some material may be flowing from the 

814 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
ENHANCED SEA-FLOOR IM GE 

KOA CRATER 

DATUM: MLLW (APPROX.} 

+ 

/50000 

+ 

146000 IVY GRID (ft) 

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A COLLIMATOR STATION 

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500 METERS 

1500 FEET 

U. S. GEOLOGICAL SURVEY 

0 

0 

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I'() 

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Figure 68. Airbrush-enhanced image of KOA crater prepared with a 5-m bathymetric overlay and showing manmade 

structures. GZ=ground zero. LOS= line of sight. 

shallower floor of MIKE crater into KOA through the narrow channel that connects them (fig. 6C). 

Southeast and northwest of ground zero, the bottom 

relief is more variable. To the southeast this variability is 

apparently due to the toe of a slump which can be traced up 

the crater side to the south-southeast across the inner slope 

and terraces. Three possible blocks of debris about 10m 

(30ft) in diameter that may have been derived from the 

slump protrude above the crater floor in this area. To the 

northeast a small channel, which may be another conduit for 

the influx of detritus, debouches onto the crater floor. Relief 

is more subdued to the northeast than near ground zero and 

appears to be part of a sediment cone or slump associated 

with a scarp that cuts across the inner slope and terraces and 

Sidescan-Sonar Survey of OAK and KOA Craters 815 
+ 

..._MIKE 

~ GZ 

162°12•oo"E 

500 METERS 

1500 FEET 

U. S. GEOLOGICAL 

0 

0 

0 

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Figure 6C. Airbrush-enhanced image of KOA crater including a province map overlay derived mostly from bathymetric 

data. GZ=ground zero. 

joins with the scarp that forms the northwest inner slope. We 

might have drawn the perimeter of the crater floor farther to 

the northeast, along the base of that scarp, if it had shown 

better bathymetric expression. 

Inner Slope 

The province is clearly distinguishable on the sidescan images, and its boundaries correlate closely with those 

of the area as bathymetrically defined (fig. 6C). Only along 

the northwest side does the airbrush map show a steeper 

slope extending closer to ground zero. As noted above, the 

sidescan data suggest that the lower boundary could have 

been extended farther to the northeast than it was, on the 

basis of bathymetry. Northwest of ground zero, the inner 

slope shows low relief except for a small channel. East of 

ground zero, bathymetry is more complicated where slumping apparently has taken place. Southeast of ground zero, 

816 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
the province is apparently broken by a slide that impinges on 

the crater floor and obscures the slope. To the southwest, the 

province could be broken again by the narrow channel that 

connects the floor of KOA and MIKE, but the character of 

that channel, both on sidescan and bathymetry, is not distinctive close to the crater floor. 

Terraces 

The hummocky, complex nature of this large area is 

clearly apparent on the airbrush sidescan image (fig. 6C). 

West and northwest of ground zero, the inner half of the 

province is relatively smooth, having only a few, low-relief 

channels and mounds. The outer half is much rougher with 

numerous scarps and hills. These two areas of contrasting 

texture could be separated perhaps into inner and outer terraces by an outer slope as is the case in OAK. The large 

scarp which has been outlined due north of ground zero 

could be included within an outer slope province, but the 

continuity of such a province is not as clear as it is for OAK. 

East of that large scarp the relief smoothes out somewhat 

where the area of the terraces is narrower and marked only 

by sinuous features 30 to 60 m (100-200 ft) long. To the 

northeast are indications of possible grooves and furrows 

that trend northwest. East of ground zero, where the rim 

scarp is broken or separated, are two hummocky ridges 

oriented northwest, separated by a 10-m-wide (30-ft-wide) 

channel. The physiography of this area is apparently complicated by mass movement of sediment. Southeast of 

ground zero are broad terraces about 60 m (200ft) wide. 

Superimposed on them are widely scattered mounds most 

often less than 15m (50ft) in diameter. 

The shallow boundary of the province southwest of 

ground zero is not clear probably because the rim scarp 

either didn't develop or because it has been obscured by 

sediment. A significant scarp projects in a southeast direction from the mounds into the terraces; it may be part of the 

rim scarp. 

Rim Scarp 

West and northwest of ground zero, the bathymetry of 

the rim scarp is complex and comprises many hills and 

mounds that range from 3 to 40 m (10-130 ft) in diameter. 

To the north are three narrow (10m, or 30ft) scarps that 

make up the main scarp; east of them, details are lost in an 

area of high reflectivity. East of ground zero, the rim scarp 

appears to be offset by faulting or slumping. The sidescan 

data reveal that hummocky masses extend for almost 150 m 

(500ft) south of the offset area. This hummocky area is 

probably made up of slump blocks that moved toward 

ground zero into deeper water. Whether or not it should be 

included as the rim scarp is open to question. The dark 

shadow of a scarp lies along the crater margin in a fishhook 

shape, but the slope there is much less than within adjoining 

segments which have been included within the rim scarp 

province (fig. 6C). Southwest of the offset, the southern 

section of the scarp is coherent for about 330m (1,000 ft). 

Then it breaks up into about three noses and reentrants and 

loses its identity south of ground zero. The rim scarp based 

on bathymetry was continued around the southeastern margin of MIKE, but that picture doesn't fit the airbrush interpretation well (fig. 6C). To the east of MIKE, the adjacent 

flank of the large submarine ridge, which separates the 

craters from the lagoon, is characterized by northwesttrending furrows and ridges having a wavelength of about 

15m (50ft). 

Debris Mounds 

The airbrush image of the debris mounds between 

KOA and MIKE craters does not portray the mounds as 

clearly as they are defined by the bathymetric contours 

(fig. 6C; see also chap. A, fig. 18, this volume). The southwestern hill, which projects into MIKE crater, is better 

defined than the northeastern hill, which appears to be made 

up of a complex of smaller mounds. The mounds are flanked 

on the northwest and southeast sides by two channels. The 

southwestern hill terminates against a scarp on the southwest side of KOA crater, whereas the one on the northwest 

side connects the crater floors of KOA and MIKE. 

Channel 

The flat floor of the channel northwest of the mounds 

is well depicted on the airbrush image, narrowing from 

about 50 m (165ft) wide in MIKE to only about 10m (30ft) 

wide where it enters KOA (fig. 6C). A smaller tributary 

channel joins the larger channel at a right angle from the 

northwest and cuts through complex hummocky terrain that 

probably consists mostly of debris. 

Manmade Structures 

The digitally processed sidescan data clearly show 

echoes from several of the structures that were discovered 

by the submersible and the scuba divers along the eastern 

margin of KOA. These are identified in figure 6B. The 

clearest targets are the vertically emplaced railroad rails that 

retained the causeway leading to the collimator station. (See 

chap. H, figs. 33-36, this volume.) Though the rails on the 

northeastern side of the causeway were best documented by 

the scuba divers, enough material apparently was left on the 

southwestern side to produce sonar echoes. Thus the 

sidescan-sonar imagery appears to portray much of the 

causeway. Similarly, parts of the concrete base of the collimator station itself seem to be outlined (fig. 6B; see also 

chap. H, figs. 33, 36, this volume). A line-of-sight tube 

support is visible in shallow water (fig. 6B; see also 

chap. H, fig. 33, this volume) as are various parts of the 

concrete test station 360.01 (fig. 6B; see also chap. H, 

figs. 29-33, this volume). To the south, the railroad rails in 

about 6 m (20ft) of water are well depicted (shallow rails in 

Sidescan-Sonar Survey of OAK and KOA Craters 817 
figs. 6B; see also chap. H, fig. 33, this volume), but the 

rails in deeper water apparently are at least partly obscured 

by other echoes (chap. H, figs. 33, 38, this volume), some 

of which may be due to the abundant coral thickets surrounding them. In the same area two linear features trend 

roughly toward ground zero (fig. 6B); these features may be 

cables that were observed by the scuba divers but were not 

surveyed precisely. Several echoes similar to those derived 

from the rails appear in other areas on the airbrush image. 

Additional scuba observations must be carried out to identify them. 

CONCLUSIONS 

The airbrush-enhanced images based on sidescansonar surveys, bathymetry, and sea-floor observations have 

provided a comprehensive view of the physiography of the 

two submarine craters that could not have been obtained by 

any other combination of techniques. 

In OAK crater, for instance, bottom texture, visible 

on the sidescan imagery, clearly depicts the spatial distribution of the debris blanket. Similarly, the detailed characteristics of the main channel southeast of ground zero that cuts 

through the crater side provides important evidence that the 

significant amount of material was transported from the 

excavation to the adjacent lagoon floor. The clear portrayal 

of the distribution of scarps, not only where they are most 

common on the reefward side of the crater but within the 

inner and outer terraces, will provide good evidence for the 

mechanisms responsible for late-stage deformation of the 

crater. 

In KOA, the airbrush-enhanced image presents a 

good picture of the distribution of slumped debris especially 

in the southeast quadrant. It also suggests that the offset of 

the rim scarp east of ground zero may be a complex of slump 

blocks rather than a more simple offset as interpreted from 

the bathymetry alone. Finally, the image provides a comprehensive view of some of the important manmade structures 

that were surveyed by submersible and by scuba divers and 

indicates the locations of other objects, not identified, that 

might be equally useful in determining the absolute sense 

and magnitude of ground displacement during and following 

the KOA event. 

REFERENCES 

Ballard, R. D., Hekinian, Roger, Francheteau, Jean, 1984, Geological setting of hydrothermal activity at 12 degrees 50' 

North on the East Pacific Rise: a submersible study: Earth and 

Planetary Science Letters, v. 69, p. 176-186. 

Circeo, L. J., and Nordyke, M. D., 1964, Nuclear cratering experience at the Pacific proving grounds: Livermore, University 

of California, Lawrence Radiation Laboratory, TID-4500, 

UC-35, (UCRL)-12172, 88 p. 

Tremba, E. L. Couch, R. F., and Ristvet, B. L., 1982, Enewetak 

Atoll Seismic Investigation (EASI): Phases I and II (final 

report): Air Force Weapons Laboratory Technical Report 

TR-82-20, Kirtland Air Force Base, New Mexico 87117, 

124 p. 

818 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Chapter C 

Single-Channel Seismic Survey of OAK 

and KOA Craters 

By J. M. ROBB, D. S. FOSTER, D. W. FOLGER, 

j. C. HAMPSON, and R. A. WOELLNER 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction Cl 

Purpose Cl 

Previous work Cl 

Methods Cl 

Interpretation procedure Cl 

Results and discussion C2 

KOA crater C3 

OAK crater CS 

Reflecting surfaces CS 

Deformation of R I 0 and R20 surfaces CS 

Shallow subsurface character of the central part of OAK crater C39 

Surficial debris C40 

Patch reefs C43 

Physiography of R20 surface and geologic characteristics of the pre-R20 

deposits C44 

Geologic control of patch-reef locations C47 

Summary C51 

References C51 

FIGURES 

1. Photograph showing Huntec towfish C2 

2. Diagram showing the sequence of reflections that are received from the sea floor 

and water surface by using a midwater-towed sound source and hydrophone C3 

3. An example profile across OAK crater C4 

4. Location map of single-channel survey aboard MS Egabrag II, northern 

part of Enewetak Atoll, June and July I984 CS 

5. Map showing track lines and single-channel subbottom profiles, KOA crater 

area C6 

6. Subbottom profiles from KOA crater C7 

7. Map showing track lines and single-channel sub bottom profiles, OAK crater 

area ClO 

8. Profiles and basic interpretations of Huntec subbottom profiles for OAK 

crater Cll 

9. Subbottom profiles of lagoon areas C28 

I 0. Map showing locations and spot depths of subbottom reflectors shown by Huntec 

profiles in KOA crater C37 

1I. Reflector below southwest wall and floor, KOA crater C38 

I2-I8. Maps showing: 

I2. Locations of digitized points on reflector RIO surface, OAK 

crater, used to prepare structure maps and thickness contours C39 

13. Structure contours for OAK crater, reflector RIO surface C40 

I4. Thickness of overburden (isopach) over reflector RIO, OAK 

crater C41 

I5. Thickness (isopach) of sediment between reflectors RIO and 

R20, OAK crater C42 

16. Locations of digitized points on reflector R20 surface, OAK 

crater, used to prepare structure maps and thickness contours C43 

I7. Structure contours for reflector R20 surface, OAK crater C44 

I8. In-crater faulting and structural features of reflector RIO surface, 

OAK crater C45 

Contents Ill 
19-22. Profiles showing: 

19. Weak trace of reflector R20 along line 89, southwest quadrant C46 

20. Graben(?) forming valley floor on line 99, southwest quadrant C46 

21. Postevent fill and fault in the central part of OAK crater C47 

22. Fault in central part of OAK crater C48 

23. Map showing surface sediments of OAK crater area with distribution and thickness of debris and postevent fill in crater C49 

24. Photograph showing beachrock on lagoon side of Bokoluo (Alice) Island CSO 

IV Contents 
Chapter C 

Single-Channel Seismic Survey of OAK and KOA Craters 

By j. M. Robb, D. S. Foster, D. W. Folger, j. C. Hampson, and R. A. Woellner1 

INTRODUCTION 

Purpose 

In and around the craters, the upper 30 m (100 ft) 

of sediment includes a complex of debris distorted by folds, 

faults, and slumps. Distinguishing the debris from naturally 

deposited sediment and delineating the stratigraphy and geometry of both components is essential to understanding the 

cratering mechanics of the Enewetak nuclear events. For 

this purpose, we used a Huntec Hydrosonde Deep-Towed 

Seismic System. 

Previous Work 

Previous attempts to acquire useful shallow subbottom seismic profiles of OAK and KOA craters apparently 

were not successful. A 3.5-kHz high-resolution singlechannel seismic system was used during Project EXPOE 

(Exploratory Program on Enewetak), but Ristvet and others 

( 1978) included no profiles in their report. Subsequently, a 

3.5-kHz EDO-Westem Model-315 subbottom profiler with 

8 kW of power output was used during Project EASI 

(Enewetak Atoll Seismic Investigations). No profiles of 

these data were presented by Tremba and others (1982), 

although some illustrations of the profiles were included in 

a classified report (E. Tremba, oral commun., July 1985). 

METHODS 

The Huntec Hydrosonde DTS (Deep-Towed-Seismic) 

profiling system (fig. 1) produces a 500-Hz to 6-kHz wideband acoustic signal by using an air-backed, depthcorrected, impulse-driven piston as a sound source. A 10-

element hydrophone having a 3-m ( 10 ft) aperture is towed 

about 2m (6.5 ft) behind the sound source; both are towed 

at depth on a 545-kg (1 ,200 lb) fish. The signal insonifies 

an area of the bottom within about a 60° cone below the fish. 

Towing both the source and receiver close to the bottom 

achieves a better signal-to-noise ratio and horizontal resolution than conventional surface-towed profiling systems. The 

transducers for the Klein 100-kHz sidescan-sonar system are 

mounted on the same fish. The two systems can operate 

simultaneously, although we operated them independently 

1

All authors are with the U.S. Geological Survey, Branch of Atlantic 

Marine Geology, Woods Hole, Mass. 02543 . 

for the most part to eliminate crosstalk and because the 

optimum tow depths in the crater areas for the two systems 

were not the same. 

During operations, the Huntec fish was lowered over 

the stem A-frame of Egabrag II (fig. 1). Its depth was 

continuously monitored from a digital readout of a pressure 

sensor on the fish and from the incoming paper record and 

was controlled by the winch operator to avoid coral heads 

and patch reefs. 

The high-resolution seismic survey was carried out on 

the first leg of Egabrag II operations during June and July 

in 1984. Rain and clouds often reduced the visibility of 

underwater hazards, resulting in the shortening or elimination of some desirable nearshore profiles. Low visibility was 

less of a problem during subsequent legs when weather was 

better and the crew more familiar with the area. However, 

most of the time the wind was light and seas low, providing 

ideal conditions for towing the fish at slow speeds (150-

200 cm/s, or 3-4 kn). 

The Huntec boomer was fired at half-second intervals, corresponding to about 1 m (3 ft) of advance at the 

usual 200-cm/s (4 kn) speed. Data were electronically corrected for fish depth and for heave, filtered between 1 ,000 

and 2,500 Hz, and recorded on an EPC 3200 recorder at a 

0.125-s sweep and on lf2-inch analog magnetic tape. 

Interpretation Procedure 

The towed submarine source-receiver configuration 

of the Huntec system, while effective in acquiring shallow 

subbottom data, produced a complex pattern of watersurface and water-bottom multiples (figs. 2, 3). The sequence of signals includes a direct echo from the water 

surface, a direct echo from the bottom surface, subbottom 

reflections, then a second bottom and subbottom sequence 

from the sound wave that had first echoed from the water 

surface. These signals were followed by the more usually 

encountered water-surface and bottom-surface multiples. In 

most places, the second arrival (the pulse that had first 

reflected from the water surface) obscured deeper subbottom data. 

Subbottom profiles were interpreted by tracing reflectors on overlays that had been prepared using bathymetry 

from the echo-sounding data. The bottom profile was 

matched on the two data sets. This procedure corrected the 

single-channel data to the navigational reference point under 

Single-Channel Seismic Survey of OAK and KOA Craters Cl 
Figure 1. Huntec towfish suspended from stern A-frame aboard MS Egabrag II. San Diego harbor is in background. 

the echo-sounder and eliminated any water-depth discrepancies between the two systems due to minor differences in 

datum or artifacts of the electronic adjustments that were 

made to compensate for changes in Huntec fish depth. Multiples were identified first and marked on the overlays to 

avoid confusion with sub bottom reflections. 

We correlated reflectors from line to line by direct 

comparison of depths and sediment thicknesses at line crossings, and we closed correlation traverses to ensure consistent interpretations. We digitized reflectors for machine 

plotting using an automated digitizing table. For plots of 

depths and sediment thicknesses, sound velocity was taken 

as 1,542 rnls (5 ,060 ft/s) in seawater and as 1,650 rnls 

(5,413 ft/s) in the shallow subbottom sediments. 

RESULTS AND DISCUSSION 

Grids of single-channel profiles were acquired in both 

craters and on long tie lines between OAK and KOA crater 

areas and Enjebi (Janet) island (fig. 4). 

At KOA crater, because of the enclosing reef and 

limited maneuvering room, data were only acquired along 

eight lines, 75 m (250ft) apart, parallel to the axis between 

KOA and MIKE craters, and along two radial lines through 

KOA ground zero on other azimuths (fig. 5). Copies of the 

data are illustrated in figures 6A -F. 

At OAK crater, we acquired data along 25lines parallel to the reef (strike lines), spaced 75 m (250ft) apart; along 

7 lines perpendicular to the reef (dip lines), spaced 305 to 

C2 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
:r: 

1--

CL 

w 

0 

' 

To 

SEA 

SURFACE 

FISH 

DEPTH 

SEA 

BOTTOM 

TIME...,.. 

I FISH 

-..I DEPTH I._ 

1

EQUIVALENT 

~ MULTIPLE BOTTOM-BOUNCE 

ARRIVALS {GREATER SLOPES) 

' \ 

\ 

\ 

..,_ __ WATER-DEPTH __ ___.,.. 

EQUIVALENT 

Figure 2. Huntec mid-water boomer: ray path with time. Diagram explains the sequence of reflections that are received 

from the sea floor and water surface by using a midwatertowed sound source and hydrophone. The profiles have three 

parallel traces (8 1, 82, 83) and subsequent multiples (84, 85, 

and so on) that result from the illustrated ray paths. 82, which 

366 m ( 1 ,000-1 ,200 ft) apart; and on two lines through 

ground zero that lie at about 45° to the other two sets 

(fig. 7). These profiles are shown in figures 8A -HH, in 

sequence from reef to lagoon so that structure from line to 

line can be compared. Each profile is shown as recorded at 

sea, displayed by time along the ship's track; hence some 

are reversed in geographical orientation compared to others. 

Line drawings of digitized interpreted profiles, corrected to 

constant scale, are included with the photographs of the raw 

data from OAK crater area. 

The lagoon profiles, whose tracks are shown in figure 4, are illustrated in figures 9A -1. 

KOA Crater 

The event that created KOA crater was fired on a 

small island that was obliterated by the explosion. Because 

arrives simultaneously from two paths, commonly appears 

strongest and obscures subbottom reflectors of the 81 path. 83 

is usually weak. 84 and 85 are conventional multiples and 

show greater slopes than the first arrivals. 8=bottom reflecton; 

S=surface reflection. 

five other devices were fired in the area (MIKE, NECTAR, 

APACHE, HURON, and SEMINOLE-see chap. A, 

fig. 17, this volume), any debris that is mappable cannot be 

attributed uniquely to the KOA event. 

The profiles of KOA crater revealed only scattered 

subbottom reflections; none could be mapped as a surface. 

According to E. Tremba (oral commun., July 1985), data 

acquired during the EASI project show mappable reflectors 

below the shallow water outside KOA crater to the south, in 

an area where we did not acquire any subbottom profiles. 

Figure 10 shows the location of the fragmental reflections 

detected during this study and the depth below bottom of 

those reflections. Figure 11 shows line 701, which contains 

the only strong subbottom reflector. This reflector lies between 8 and 12 ms (about 6-9 m, or 20-30 ft) below the 

crater floor and apparently represents a change in the character of the postshot deposits, possibly from debris to natural 

sediments. The location of dipping acoustic patterns whose 

source is not clear are also plotted on figure 10. 

Single-Channel Seismic Survey of OAK and KOA Craters C3 
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·'~- .·.-L::·· : ~.. . : :' ... ' .. ., 

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Figure 3. An example profile across OAK crater (line 57). Labels point out features of the record, including reflections, multiples, field annotation conventions, time of 

day, and horizontal and vertical scales. This illustration shows the changes in the spacing of the multiples resulting from changes in the tow-depth of the fish. Note that 

changes in fish depth caused aberrant appearance of records during times that the fish was descending or ascending because the automatic adjustment mechanism took 

a few moments to reestablish equilibrium. B=reflection from sea floor; Rl 0(1-S)=reflections and multiple reflections from reflector Rl 0. R20(1 )=reflection from subsurface 

reflector R20. 
+IVY GRID (ft) 

0 0 30POO FEET 0 

..... , , ......... ....._!. I...LJ' l!u.... 1 _ ______.._ _ _____., Bokaidr ik (Helen) 8 0 5 NAUTICAL MILES CCI 

+ 

Boken (Irene) 

0 ·o 

0 

0 

C\J 

+ 

8 

0 

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~ 

+ 160000 

ENEWETAK ATOLL ~ Enjebi (Janet) K 

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+ 

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~ ... ~'*'i>i) 

-..,.-oc; ~v~ojwa (Ursula) 

~ ,Runit (Yvonne) 

• (Oscar) ~z 

~ 

+ 120000 

11°~ ~~------~~------------~----------------------------L------JL ________________ ~ 

162ooo' 162°10' 162°20' 162°30' 

Figure 4. Location map of single-channel subbottom survey aboard MS Egabrag II, northern part of Enewetak Atoll, June and July 

1984. For track lines within KOA and OAK crater areas, see figures 5 and 7. Subbottom profiles acquired within those areas are 

shown in figures 6 and 8. Subbottom profiles along tie lines in the atoll are shown in figure 9. Points a-1 locate points marked on 

subbottom profiles illustrated in figure 9. Uppercase letters are first letters of site names (table 1 of the Introduction of the volume). 

OAK Crater 

Reflecting Surfaces 

We mapped two reflectors that underlie the back reef 

and lagoon area surrounding the crater. They have been 

disrupted or blown away near ground zero. The two reflectors, termed RIO and R20, probably represent extensive, 

buried solution unconformities. RIO, the shallower of the 

two surfaces, is a strong reflector and can be picked with 

confidence. It is clearly traceable below sediments of the 

lagoon floor to distances greater than 4.5 km (I5,000 ft) 

from the crater and even below many large patch reefs. R l 0 

correlates with the Holocene-Pleistocene boundary (disconformity "I" between sediment packages I and 2; B. R. 

Wardlaw, written commun., July I5, I985). 

Figure I2 shows point locations, and figure 13 shows 

structure contours (elevation below sea level) on RIO. Figure I4 shows thickness of overburden above RIO. 

The R I 0 surface ranges in depth from 3 m to 25 m 

(10-82 ft) below bottom; most commonly, it lies about 4 to 

5 m (13-I6 ft) below bottom. It has subdued relief (see dip 

profiles, figures 8Z-HH) and slopes from the reef into the 

lagoon. It overlies a sequence of sediments marked by weak 

and discontinuous reflectors and locally incised by small 

channels. The thickness of sediment between RIO and R20 

ranges from 3 to I4 m (10-46 ft) (fig. I5). 

The deeper surface, R20, is a weak reflector, obscured in many places by multiples. It is best identified 

where it truncates lagoonward-dipping sedimentary units 

and is marked by apparent cuestas or hogbacks and depressions of I to 4 m (3-I3 ft), which appear to be parallel to 

the reef. (See dip profiles, figs. 8Z -HH, especially line 

20I, fig. 8AA ). Figure I6 is a point plot showing the locations of digitized picks. Figure I7 shows structure contours 

drawn on the top of the R20 reflector. R20 lies within upper 

Pleistocene sediments (disconformity "2" within the uppermost part of sediment package 2; B. R. Wardlaw, written 

commun., July I5, I985). 

Deformation of R1 0 and R20 Surfaces 

Figure I8 is a summary interpretive map showing the 

craterward extent of RIO and R20 and some structural characteristics of RIO. It can be compared with the structurecontour maps (figs. 13, I7). 

R I 0 and R20 are folded near the crater. In most 

places, they dip toward the crater, and terminate below the 

accumulation of surface debris that surrounds the crater on 

its lagoonward side; we also identified small upturned areas 

of RIO (for example, line 203, about 0046, fig. 8CC; 

line 2IO, about 2307, fig. 8DD; also shown by symbols on 

fig. I8). 

Single-Channel Seismic Survey of OAK and KOA Craters CS 
+ 

150000 

+ 

2000 FEET 

DATUM: K.LW (APPROX.) 

Figure 5. Track lines, single-channel subbottom profiles, KOA crater area. Tics indicate 1-minute intervals; circled 

tics indicate 5-minute intervals. Labels include: SL (start of line), line no., JD (Julian day/year); numerals (for example, 

0350:00) are time of day. Some of the bathymetric contours of the area outside the crater are adapted from Tremba 

and others, 1982. GZ=ground zero. Contour interval is 5 m. 

The R10 surface can be classified by seismic character as continuous (for example, line 89 from 0354-0359, 

fig. 8K), disturbed (for example, line 77, 0410-0414, 

fig. 8G), and fragmented (for example, line 79, 0578-

0520, fig. 81). We can identify some sites of probable 

fractures (for example, line 89, 0359-0400, fig. 8K; see 

also symbols on fig. 18). In most places, the R20 reflector 

is not recorded clearly enough to evaluate its deformation. 

Shallow water prevented us from profiling the reef 

and near-reef parts of the northwest quadrant of the crater, 

but we were able to define the shallow structure of the 

southwest, southeast, and northeast quadrants (fig. 18). The 

C6 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
(/) 

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-

w 

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wo 

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I-

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I 

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A 

KOA CRATER LINE 535 

JO /82, 1984 TIME OF DAY 

2357 0002 0007 

I . 

KOA CRATER LINE 533 

0 1500FT L...l __._.....___.J! 0 

I 500M 

I J 

0 

I 

8 

KOA CRATER LINE 531 

JO 182,1$4 TtME OF DAY 

2237 2242 2247 

r~--.~ 

KOA CRATER LINE 529 

1500FT 

I oL...., __.__--'--__.__~·L-5_.Jtp M N f 

Figure 6. A-F. Subbottom profiles from KOA crater. Horizontal scale is time along track. Vertical scale is 12.5 ms between scale 

lines. Note that absolute depth can have slight errors in correction for fish depth, and that for precise depths the reader should refer 

to chapter A of this volume. Subbottom thickness measurements are correct in time units. A, lines 535, 533; 8, lines 531, 529; 

C, lines 530, 532; 0, lines 534, 536; E, line 701; F, line 208. Date given is Julian day UD) and year. 

Single-Channel Seismic Survey of OAK and KOA Craters C7 
(f) 

-

w 

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KOA CRATER LINE 530 

JD 182,1984 TIME OF DAY 

2217 2222 2227 

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KOA CRATER LINE 532 

2257 2302 2307 

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KOA CRATER LINE 534 

JD 182, 1984 TIME OF DAY 

2332 2337 2342 23 4 7 

KOA CRATER LINE 536 

0 1500FT 0 500M D '--' --L_..._____., L-' ___.__........___...~....._.J...... ----~' NE 

Figure &-Continued 

C8 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
NE 

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JO 183, 1984 TIME OF DAY 

0157 0202 0207 0212 

E 

KOA CI1ATER LINE 208 

JO 185, 1984 TIME OF DAY 

0423 0428 0433 

0 15,QOFT 0 500 M L--~---'--J I I I I I I 

Figure 6-Continued 

SW 

0438 

ssw 

0443 

I 

southwest quadrant is marked by extensive downtuming of 

R 10 into the crater. The outermost point of initial inflection 

ranges from 720 m (2,400 ft) from ground zero near the reef 

on lines 5899 and 57 to about 870 m (2,850 ft) from ground 

zero along line 77 (fig. 18). The RIO reflector dips craterward, measured radially toward ground zero, for distances 

between 60 and 425 m (200-1 ,400 ft) from the initial zone 

of inflection, before it terminates. Termination points in the 

southwest quadrant lie between 790 m (2,590 ft) from 

ground zero on line 56 and 390m (1 ,280 ft) from ground 

zero on line 79. Near terminations, disturbed or fragmented 

reflectors are common. The large downtumed area along 

Single-Channel Seismic Survey of OAK and KOA Craters C9 
+ + + + + 

125{)()() IVY GRID (It) 

+ 

120000 + 

+ + + DATUM: llll.l W (APPII)X.) 

0 2000 FEET Z,40 L-------li....-_---J 

L 204 .ll 184/84 0240 0 500METERS 

162~'30"E 16~7'E 

Figure 7. Track lines, single-channel subbottom profiles, OAK crater area. Tics indicate 1-minute intervals; circled tics 

indicate 5-minute intervals. Labels include: SL (start of line), line no., JD Oulian day/year); and numerals (for example, 0350:00) 

are time of day. GZ=ground zero. Contour interval is 5 m. 

Figure 8. A-HH. Profiles and interpretations of Huntec subbottom profiles for OAK crater. Note that the correction for 

depth of the fish can have slight errors and that echo-sounding 

data were used for water-depth measurements. Vertical-scale 

lines on the profiles are 12.5 ms apart. Time markers (vertical 

dashed lines) are at 5-minute intervals. Date given as Julian day 

OD) and year. R1 0, R20 refer to subbottom reflectors. 

A, line 5899 

8, line 57 

C, line 56 

D, line 205 

E, line 75 

F, line 7699 

G, line 77 

H, line 78 

I, line 79 

), line 88 

K, line 89 

L, line 90 

M, line 98 

N, line 99 

0, line 82 

P, line 81 

Q, line 80 

R, line 63 

5, line 62 

T, line 61 

U, line 59 

V, line 60 

Cl 0 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 

W, line 100 

X, line 42 

Y, line 41 

Z , line 213 

AA, line 201 

88, line 212 

CC, line 203 

DO, line 210 

EE, line 202 

FF, line 211 

GG, line 209 

HH, line 204 
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SW TIME OF DAY 

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2352 2357 0002 0007 

JO 178,/984 

SW TIME OF DAY NE 

2347 2352 2357 0002 0007 o~--~----~----~----~~ 

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2320 2325 2330 2335 

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0327 0332 0337 0342 

sw JO /84 I /984 

TIME OF DAY NE 

0345 

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TIME OF DAY 

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OAK CRATER LINE 7699 

2147 2152 2157 2202 2207 

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L&.J 

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NE TIME OF DAY sw 

2147 2152 2157 2202 2207 0~----~------~----~~----~----~ 

L&.J 

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0409 0414 0419 0424 0429 

I - . ·- ... L ..... - L ___ . ··- ---- -~ -- --- ~ . .1 

Sw JO /80, /984 ur 

TIME OF DAY "r: 

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0313 0318 0323 0328 I I I 

TIME OF DAY 

0315 0320 0325 

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NE ciO 181, 1984 

0353 0358 

TIME OF DAY 

0403 0408 

sw 

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TIME OF DAY 

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TIME OF DAY 

0247 0252 0257 0302 

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0501 0506 0511 0516 0521 0526 I 

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0459 0504 0509 0514 0519 0524 o~----~------~----~------~------~~ 

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TIME OF DAY SW 

0429 0434 0439 0444 o~----~------~----~------~--~ 

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0157 0202 0207 0212 I 

sw c/0 179, /984 TIME OF DAY 

0152 0157 0202 0207 0212 

0 

0 500M I I I I I I 

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0320 0325 

TIME OF DAY sw 

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w 

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0320 0325 

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TIME OF DAY 

0330 0335 

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OAK CRATER LINE 42 

0433 0438 

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w 

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NE TIME OF DAY SW 

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IC24 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 

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0042 0047 0052 

NWJO 184,1984 

TIME OF DAY 

0035 0040 0045 0050 

SE 

0055 ::r 

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OAK CRATER LINE 210 

2303 2308 2313 2318 

I I 

NWJO 185,1984 TIME OF DAY SE 

2258 2303 2308 2313 2318 0~----~------~----~---------

LLJ 

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LLJ 

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C26 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
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2228 2233 2238 2243 

I I I I 

JO 185,1984 

E TIME OF DAY 

2226 2231 2236 2241 

Figure 8--Continued 

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0247 0252 0257 0302 

I 

S JO 184, /984 

0247 0252 

TIME OF DAY 

0257 0302 

N 

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OAK CRATER AND ENEWETAK LAGOON 

TIME OF DAY 

0042 0047 0052 0057 I I I 

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LINE 203 dl 

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Figure 9. A-1, Subbottom profiles of lagoon areas. Tracks and locations a-/ are shown on figure 4. A, line 203; B, line 204; 

C, line 205; 0, line 209; E and F, line 206; G and H, line 207; I, line 208. Date given in Julian day UD) and year. 

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TIME OF DAY 

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TIME OF DAY 

2140 2145 2150 2155 2158 I I I I I .i 

2203 2208 I I 

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Figure 9-Continued 

ENEWETAK LAGOON LINE 206 (CONTINUED) 

2308 2313 sw • . I ' ' I I ........ ~ .. ~ .. ..;< ···· ...... -.. .... ---- .- ???' __ £ ~ i~t.:.:~·r.-~ ~ ~~~ :.l s .. ~: """' _,, . .,J 

h 0 500M 0 1500FT I I I I I I I I I I 

2323 2328 2333 2338 I . 23,43 I S'W 

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· 

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C34 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 

"'0 

a 

t:: 

·-...... 

t:: 

::::;, 

~ 

I 

a'l 

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Figure 9-Continued 

ENEWETAK LAGOON AND KOA CRATER LINE 207 (CONTINUED) 

JO 185, 1984 Tl ME OF DAY NE 

0253 0258 0303 0308 0313 0318 0323 

0 500M I I I I • I 0 !500FT I I I I f 

sw 0323 0328 0338 0343 I 0358 NE 

§ 

L&J 

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~ 

0 

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ii=)-

CX) 

gzo· 

(S) 3Wil 13A\f~l A\fM-OMl 

C36 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
0'1 

Q) 

0 

0 

0 

162°1 1 '30"E 

11 °40 '30'"N + 

1 1 °40'N 

'.J 

0 

0 

0 

0 

+ 

IVY GRID (ft) 

5.0~ 

-' 0 . 0,.-------

' 

_,..._ ~ 

I 

SHALLOW SUBBOTTOM DATA 

e-1 

+ 

- LOCATION OF REFLECTION ALONG TRACKL INE 

fO DEPTH SUBBOTTOM (ms) 

~ LOCATION OF DIPPING REFLECTION TREND 

DATUM: MLLW (APPRO X.) 

'.J 

U1 

0 

0 

0 

152000 

11 °40 '30"N 

150000 

11 °40'N 

16~2°_1_1-'3-0-.. -E------~--~-L---------1~6-2-1_2_'_E _______________________ 1_62__1_2-'3_0 '" E--~ 145000 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 250 0 250 500 750 1000 

U.S. GEOLOGICAL 

SURVEY METERS 

COMPILED: June 1985 

1000 0 1000 

FEET 

2000 3000 

Figure 10. Locations and spot depths of subbottom reflectors shown by Huntec profiles in KOA crater. GZ=ground zero. 

Contour interval is 5 m. Datum is mean lower low water (approx). 

lines 77, 78, and 79 appears highly fragmented to discontinuous in its inner part (figs. 8G-/) and is absent from the 

middle of line 89, although a weak trace of R20 is present 

(line 89, figs. 8K, I8). A small segment of RIO may have 

overridden itself a few meters in small thrusts or reverse 

faults that dip toward ground zero. (See line 79, fig. 8/ at 

05I7; and line 88, fig. 81 at 0313.) 

In the southeast quadrant, RIO is less downturned 

than in the southwest quadrant, but it is disturbed at 870 m 

(2,850 ft) from ground zero along line 82 (fig. 80, 2242-

2245; fig. I8). Clear deformation and fractures of the RIO 

surface are found 640 m (2, IOO ft) from ground zero on 

line 203 (fig. 8CC, 0047-0048). Probable fragments of 

RIO lie only 580 m (1 ,900ft) from ground zero (fig. I8, and 

line 90, fig. 8L, 0434-0438). However, RIO appears to be 

absent within 730 m (2,400 ft) of ground zero in a salient to 

the east-southeast. Apparently, fragments of R20 are 

present in this quadrant at distances as close as 490 m 

(1,600 ft) from ground zero (line 89, fig. 8K). The geometry cannot be described in detail because the profiles are 

Single-Channel Seismic Survey of OAK and KOA Craters C37 
NE LINE 701 sw 

~ 

en 

-

w 

:E 

~ 

_J 

WlO 

~~ 0::: . 

I-

~ ;: 

I 

0 

3: 

I-

,,,.,,, .. ,,,,•,.,,, l, l••'••'''\••''·"' '' '• "·•·1'•'•''••.,'••11t.'•••li,,;,o o.,;, ,• 

; 1; 1 ~ I 1' It '1 I tlllll· ! tl h lll!!c tilt II fl' I t'\, 1, II' t t l 1 H'' ot 1 Jl•' 1 1'\ hill{ I '•I' ' 1 

0 500M 

0 1500FT 

Figure 11. Reflector below southwest wall and floor (line 701 ), KOA crater. Vertical scale markers are 12.5 m apart. Dashed event 

marks are 5 minutes apart. Compare figures 6E and 10. 

primarily strike lines in this area; line 203 is the only crossing line. There are crenulations and breaks in R I 0 in the 

central part of line 89 (figs. 8K, I9). A small graben in the 

bottom of the southeastward-trending valley 760 m 

(2,500 ft) from ground zero (see chap. A, fig. 9, this volume) shown in line 99 (figs. 8N, 20), appears to be about 

60 m (200ft) across, to displace the sea bottom by about I m 

(3 ft) and RIO by about 0.5 m (1.5 ft). 

The northeast quadrant shows abrupt termination of 

RIO accompanied by little downtuming (fig. I8). Fractured 

segments 60 to I 50 m (200-500 ft) in extent are found along 

lines 205, 77, and 90 (figs. 8D ,G ,L) before terminating in 

the crater. An outer limit of deformation (folding) can be 

picked at I,040 m (3,4IO ft) from ground zero on lines 7699 

and 77 (figs. 8F -G) or at about 800 m (2,620 ft) from 

ground zero on lines 89 and 90 (figs. 8K-L). Termination 

points of RIO lie at 550 m (1,800 ft) from ground zero on 

line 77 to 580 m (1 ,900 ft) on lines 79 and 89. Separated 

fragments, perhaps ofR20, lie as close as 4IO m (1,350 ft) 

to ground zero in this area (fig. I8). 

Especially in the southeast quadrant, RIO and R20 

can be traced closer to ground zero on some multichannel 

profiles than on the single-channel profiles. This is probably 

due to the greater power and depth of penetration of the 

multichannel system as well as to acquisition and processing 

techniques which enhance horizontal reflectors. (See 

chap. D, this volume.) 

C38 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
"" N "" U1 0 0 0 0 o 16205'30•E 0 162°6'E 

11 °37'N + + 

POINT PLOT REFLECTOR RIO 

NUMBER OF DIGITIZED DATA POINTS=5218 

_,...,_ t 

~ 11 °36'30•N + 

11o36'N + 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

+ 

.,. 

0 

0 

0 

162°6'30•E 0 

+ 

.,. 

1\l 

0 

0 

162°7'E 0 

130000 

+ 11 °37'N 

1 1 °36 '30·~ 

+ 

U.S. GEOLOGICAL 

SURVEY 250 0 250 500 750 1000 1250 

COMPILED: May 1985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 12. Locations of digitized points on reflector Rl 0 surface, OAK crater, used to prepare structure maps and thickness 

contours. GZ=ground zero. 

Shallow Subsurface Character of the Central Part of OAK 

Crater 

The crater is crossed by nine lines parallel to the reef, 

one line orthogonal to the reef, and two diagonal lines. 

Within the bowl of the crater, the Huntec profiles show only 

weak reflectors. Some dim subbottom laminations that may 

represent postevent deposits (figs. 21, 22) extend to at least 

13 ms (10.5 m, or 35 ft) subbottom under the crater floor, 

or to about 8 ms (6.4 m, or 21 ft) in places on the crater 

slopes. 

Single-Channel Seismic Survey of OAK and KOA Craters C39 
(N (N 

~ ~ 

~ g: 0 ~ 0 g 0 g o 162°5'30•E 162°6 'E 162°6'30•E 0 162°7 'E 130000 

11 °37'N + + + + 11 037'N 

DEPTH TO REFLECTOR R10 

CONTOUR INTERVAL: 1/2 METER _,..._ ~ DATA POINTS: 5218 l GRID CELL SIZE: 21 M 

INTERPOLATION SEARCH 

RADIUS: 210 M 

----INNERMOST EXTENT RIO 

11°36'30•N + + 11 °36'30•N 

GZ 

11°36'N + 

+ 

L1_6-2°-5-'3-0.-E----------------~--16-2°-6-'E----~~--~--------16-2-6-'-30-.-E---------L--------~1~62~~7.~E~ 119000 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

U.S. GEOLOGICAL 

SURVEY 250 0 250 500 750 1000 1250 

COMPILED: July 1985 

1000 1000 

METERS 

2000 

FEET 3000 4000 5000 

Figure 13. Structure contours (depth below sea level) for OAK crater, reflector R1 0 surface, constructed assuming velocities 

of 1,542 m/s in seawater and 1,650 m/s subbottom. 

Our mapping of arcuate faults or slide blocks and 

terraces within the crater is based on interpretations of the 

subbottom profiles integrated with bathymetric data. (See 

fig. 18.) Because of the orientation of the profiles, such 

features are delineated better on the northeast and southwest 

slopes than on the southeastern slope. One example of displacement of subsurface reflectors is illustrated on line 75 

(figs. 8£, 22) and on line 205, where a terrace-like surface 

was formed by a downslid block on the southwest slope 

(fig. 8D, about 0331). 

Surficial Debris 

We classified surficial deposits around OAK crater 

into proximal and distal debris on the basis of morphology 

and seismic signature (fig. 23). 

C40 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
(N 

N 

0 

0 

o 162°5'30"E 

REFLECTOR RlO OVERBURDEN 

CONTOUR iNTERVAL: 1 METER 

DATA POINTS: 5218 

GRID CELL SIZE: 21 M 

INTERPOLATION SEARCH 

RADIUS: 210 M 

11036'30"N + 

---- INNERMOST EXTENT R1 0 

~~ HACHURES MARK THINNING DIRECTION 

11°35'30"N + 

(N 

U1 

0 

0 

o 16206'E 

+ 

+ 

16206'30"E 

+ 

GZ 

. 

0 . 

N 0 0 0 0 0 16207 'E 0 

130000 

+ 11 °37'N 

~ -~-

j 

11 °36'N 

+ 11 °35'30"N 

~1~62~~5~'3~0="E~----------------L-1~6~2°~6~'E~~~------~-------16-2-6-'-30-.. --E--------~---------16-2-o7_'_E__J 119000 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

U.S. GEOLOGICAL 

SURVEY 

250 0 250 500 750 1000 1250 

COMPILED: July 1985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 14. Thickness of overburden (isopach) over reflector Rl 0, OAK crater, constructed assuming 1 ,650 m/s sound 

velocity. GZ =ground zero. 

The proximal accumulations of debris form mounds 

locally around the crater; RIO is deeply buried (for example, 

line 98, fig. 8M, southwest of crater). Debris mounds have 

a rough surface compared to surrounding areas. Patch reefs 

are absent or covered. Acoustically, the material is commonly amorphous or chaotic. The boundaries of the deposits 

that we identify as proximal debris coincide closely with 

areas of net accumulation determined from comparison of 

preshot and postshot bathymetry (chap. A, fig. I4, this 

volume). 

Deposits of distal debris, in contrast to the proximal 

debris, are thinner; RIO is less deeply buried, and the bottom is rough (for example, line 63, fig. 8R, 0507-05I2). In 

the subsurface, distal debris deposits are acoustically 

chaotic. They overlie a poorly definable surface and lap onto 

patch reefs. 

Single-Channel Seismic Survey of OAK and KOA Craters C41 
"" N 

0 

0 

o 162°5 '30•E 

11 °37'N + 

R10 TO R20 THICKNESS 

CONTOUR INTERVAL: 1/2 METER 

DATA POINTS: 3823 

GRID CELL SIZE: 21 M 

INTERPOLATION SEARCH 

RADIUS: 210 M 

11 °36'30•N +_--INNERMOST EXTENT R 1 O-R20 

THICKNESS 

11036'N + 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

250 

"" "' g 

0 162°6 'E 

+ 

+ 

GZ 

162°6 'E 

0 250 

... 

g 

0 

162°6'~0·E 0 

+ 

( 

162°6 '30•E 

500 750 1000 1250 

... 

N 

0 

0 

162°7'E 0 

130000 

+ 11 °37'N 

~ -~-

j 

11 °36'30•N 

1 1°36'N 

+ 

U.S. GEOLOGICAL 

SURVEY 

COMPILED: July 1985 METERS 

1000 0 1000 2000 3000 4000 5000 

FEET 

Figure 15. Thickness (isopach) of sediment between reflectors R1 0 and R20, OAK crater, constructed assuming 1 ,650 m/s 

sound velocity. GZ=ground zero. 

Surficial deposits outside the main area of debris accumulation, as shown by profiles in the lagoon, are commonly characterized by a smooth surface and even bedding 

(for example, line 209, fig. 9D ). Some are ponded sediments and local overwash deposits. Because they are thin, 

RIO is close to the_s~_a floor. 

A relatively strong and consiStent reflecting horizon 

was recorded within several mounds of otherwise amorphous or chaotic proximal debris that encircles the southeastern part of the crater (fig. 23). It crops out on both the 

inner and outer slopes of those mounds. (See lines 88, 89, 

90, 98, 99, and 203, figs. 8J -N,CC; especially line 98 in 

fig. 8M between 0226-0232 and line 203 in fig. 8CC from 

0047-0049). It is not detected near the reef in the northeast 

or southwest quadrants. The location of this intradebris reflector corresponds to areas of thick net accumulation 

C42 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
(N (N 

N Ul 0 0 0 0 o 162°5'30•E 0 162°6'E 

... ... 0 N 0 0 0 0 

162°6'30•E 0 162°7 'E 0 

130000 

11 °37'N + + + + 11 °37'N 

POINT PLOT REFLECTOR R20 

NUMBER OF DIGITIZED DATA POINTS= 3823 

+ + 11 °36'30' 

/ 

+ , , 0 35 '30" 

L1~6~2°~5='3~0.=E----------------~-,6=2~6~'E~--L-----~------~,~62~o~6.=3~0.=E---------L--------~,6=2=7~'E~ 119000 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

U.S. GEOLOGICAL 

SURVEY 250 0 250 500 750 1000 1250 

COMPILED: May 1985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 16. Locations of digitized points on reflector R20 surface, OAK crater, used to prepare structure maps and thickness 

contours. GZ=ground zero. 

derived from the comparison of preshot and postshot 

bathymetry. (See chap. A, fig. 14, this volume.) 

We suggest that this reflector may result from overthrust debris, or from an overturned flap (note especially 

line 203, 0047-0049, fig. 8CC). Further analysis of borehole samples may positively identify this important intradebris horizon. 

Patch Reefs 

Patch reefs are common features on the lagoon bottom. Our subbottom profiles show that most are young and 

have developed since the RIO surface was formed. Some 

appear to penetrate the RIO and R20 surfaces and hence are 

older. Shinn and others (chap. H, this volume) found that 

Single-Channel Seismic Survey of OAK and KOA Craters C43 
11 °37'N 

tN tN 

~ Ul 

0 0 

o 162°5'30•E g 

+ 

DEPTH TO REFLECTOR R20 

CONTOUR INTERVAL: 1 /2 METER 

DATA POINTS: 3823 

GRID CELL SIZE: 21 M 

I N"iERPOLA T I ON SEARCH 

RADIUS: 210 M 

INNERMOST EXTENT R20 

--INFERRED SUBCROP STRIKE 

SUB-R20 LAGOONWARD-DIPPI 

SEQUENCES 

11036'N + 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 

162°6'E 

+ 

+ 

... 

g 

162°6'30•E g 

+ 

~ 

GZ 

... 

N 

0 

0 

162°7 'E 0 

130000 

+ 11 °37'N 

t 

-I'llj 

125000 

11 °36'N 

+ 

U.S. GEOLOGICAL 

SURVEY 250 0 250 500 750 1000 1250 

COMPILED: July 1985 

1000 0 1000 

METERS 

2000 

FEET 3000 4000 5000 

Figure 17. Structure contours for reflector R20 surface, OAK crater, constructed assuming velocities of 1,542 m/s in 

seawater and 1,650 m/s subbottom. Dashed line shows inferred strike of lagoonward-dipping sub-R20 sequences. 

GZ=ground zero. 

one patch reef 2 krn (1.1 nmi) southwest of ground zero is 

capped with a thick section of Holocene deposits. The patch 

reefs presumably are all preevent features. 

Debris accumulations lap onto or bury patch reefs 

around the crater, whereas in the lagoon farther from the 

crater, some are surrounded by moats. Patch reefs at the 

outer parts of the crater area in the southeast quadrant may 

have served as buttresses that, in part, controlled the movement of debris. · 

Physiography of R20 Surface and Geologic Characteristics 

of the Pre-R20 Deposits 

Lagoon ward-dipping sequences below R20, which 

form its topography along the back reef, are best seen on the 

C44 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
-+= EXPLANATION 

--- INNERMOST EXTENT RIO + ---- INNERMOST EXTENT R20 

~ RIO CONTINUOUS,BUT SUBSIDED OR DEFORMED 

E3 RIO WEAK, DISCONTINUOUS 

bd R 10 PRESENT AS FRAGMENTS 

RIO INFLECTION (direction of apparent dip) 

""1.6 RIO DIP CACULATED FROM STRUCTURE CONTOURS 

X RIO FRACTURE I 

RIO THRUST (teeth on overthrust block) 

NORMAL OR Slut.f> FAU...TS IN CRATER 

(hochures on downthrown side, dashed wher 

TERRACE STEPS IN CRATER 

IVY GRID (ft) 

MAP PROJECTION: UTM 

SPHEROID: CLARK 1866 

250 0 

COMPILED : MAY 1985 

1000 0 

250 

1000 

500 

METERS 

2000 

FEET 

750 

3000 

+ 

1000 

§ 

~ 

1250 

4000 5000 

+ II 0 35'30"N 

""' 120000 

~ 

"" 

I62°71E 

U.S. GEOLOGICAL 

SURVEY 

Figure 18. In-crater faulting and structural features of reflector R10 surface, OAK crater, GZ=ground zero. Track lines are 

screened. Bathymetry contour interval is 5 m. 

seven profiles perpendicular to the reef. The R20 reflector 

appears to be an erosional surface that truncates two or three 

or more separate sequences that dip lagoonward along the 

trend of the back reef (for example, line 201 , fig. 8AA). The 

resistant members of these sequences are expressed as cuestas and hogbacks, which are associated with depressions or 

swales and probably a wave-cut terrace in the lagoonward 

part of the area. The trends of these features are reflected by 

Single-Channel Seismic Survey of OAK and KOA Craters C45 
-en 

w 

:!: 

i= 

_J 

WIO 

~~ a::· 

t-

>-

~ I 

0 

3: 

tNE 

I 

I 

I 

I 

I 

~ ·:~~~~~~:!::~~~~~~~~~~: ~:· 

'! 

I 

. I 

I 

..... ·:~ :: ;, ··· ··· .,,., ·,_ . 

. · ~..::~ . •. ·· ,,. 

0 

0 

LINE 89 

500M 

I 500FT 

Figure 19. Weak trace of reflector R20 along line 89, southwest quadrant. 

sw LINE 99 

--·: -~.ELSH-_.MQV~ENI ·· ··-··-----------

~--~-----.,Jr:--. . ..... .. . ,, ... '"""'-·-··· ·····-·' ' ~,~~1~~-v~ ....... ." >"""¥ :; 

~-L .. .:·;~-: . r~ . . ~ -~· ~'1\-: ..... ~P.-,- ~:.(;.:~·.-;;;;(21 ~.:6~ 3•;; · .. :: '' ~ ~, 

0 

0 

500M 

I 

!500FT 

·! 

Figure 20. Graben(?) forming valley floor on line 99, southwest quadrant. R10 and R20 surfaces are also shown. 

C46 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 

sw 

NE 
sw LINE 77 NE 

.. . ' 

.. :· .. :~~-.-..~~~ ··--·--.... -- . .. ' ~ ... .. 

~; I i -; ~;; I ; ; ij;;; I I ~ ;11 . ~ "~·- ~~ --<-...,_,., ~-...... ,.. ... "~·· ..... ..........,.... .. v.w~~. · / .,...,. ...... , ......... ..,._..,...,.,~~· · ... ; ........... .... · .

-

en 

-

w 

~ 

t-

....J 

WIO >N 

<tO 

a::· 

t-

~ 

~ 0 

3: 

t-

....... ... . , . 

0 

0 

. . . : ..... ' ... ~ ~:: ; ~~ · 

· ......... . , 

t 

. ~ ..... .. ··, 

. • ... ·~ • ·r . 

·- ·- \" ·' 

500M 

I 

I 500FT 

Figure 21. Postevent fill and fault in the central part of OAK crater (line 77). See also line 75, figure 22. 

the structure contours of figure 17. Also shown is the inferred subcrop strike of the lagoonward-dipping sequences. 

The pre-R20 sequences probably include beachrock, which 

is more indurated than surrounding sediments. These sequences are discontinuous, similar to present -day deposits 

of beachrock on the lagoon ward beaches of the islands of the 

atoll (fig. 24). 

Geologic Control of Patch-Reef locations 

Some of the patch reefs appear to have grown upon 

topographic highs on strata resistant to erosion, such as the 

cuestas and hogbacks of the pre-R20 sequences. Consequently, beach ridges inundated by rising sea level may 

serve as bases for patch-reef growth, explaining why some 

Single-Channel Seismic Survey of OAK and KOA Craters C47 
sw 

,..._ 

en 

-

LaJ 

~ 

t- ' ' ' _J 

WI.O 

~~ 0:: 

t-

~ 3: 

I 

0 

3: 

tLINE 75 ------------ ------- -r-: : 

:J ··: 

... ~--

-:. : V:,. 

0 

0 

500M 

I 

1500FT 

NE 

.__, .. ____ . .------~-----

Figure 22. Fault in central part of OAK crater; note displacement of subsurface reflectors and terrace at surface {line 

75). Faint reflectors from crater fill may represent postevent deposits. 

C48 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
EXPLANATION 1 

II 037'N &S:'] PROXIMAL DEBRIS (massive, thick) I + 

I 

WNN APPROXIMATE 1Nt£R ~DARY, PROXIMAL DEBRIS 

l _ .,' APPROXIMATE OUTER BOUNDAR~, PROXIMAL DEBRIS 

(includes transitional areas) 1 

btttJ LOCATION OF INTRA- DEBRIS kEFLECTOR 

I ~ DISTAL DEBRIS(chaotic,thimerlrunbe~s Cl'e representative 

approximate thicknesses in meters (ossuming 1600 m/sl 

POST -EVENT CRATER FILL: ! ~2 LOCATION ALONG TRACKLINE ANO THICKNESS 

(meters, assuming 1600 m/s) 

+ 

+ 

+ 

IVY GRID 

162°5'30"E 

MAP PRO..ECTION: UTM 

SPHEROID : CLARK 1866 

COMPILED: MAY 1985 250 0 250 

1000 0 1000 

+ 

_· I_ 

+ 

+ 

162°7'E 

U.S. GEOLOGICAL SURVEY 

500 750 1000 1250 METERS 

2000 3000 4000 5000 FEET 

Figure 23. Surface sediments of OAK crater area showing distribution and thickness of debris and postevent fill in crater. 

lntradebris reflector may be inverted surface of R 10 in an overturned flap, or it may be R 10 thrust over the southeastern edge 

of the crater. GZ=ground zero. Bathymetry contour interval is 5 m. 

Single-Channel Seismic Survey of OAK and KOA Craters C49 
Figure 24. Beachrock on lagoon side of Bokoluo (Alice) Island. 

C50 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
patch reefs appear to be in lines subparallel to the reef. (See 

chap. A, fig. 7, this volume.) 

SUMMARY 

In summary, two seismically reflecting layers, RIO 

and R20, which predate the OAK nuclear event, were 

mapped in the subsurface around the crater. These are shallower than about 60 m (200 ft) below sea level and are less 

than 20 m (65 ft) subbottom. The RIO reflector is absent 

within about 600 m (1 ,970 ft) of ground zero in most areas, 

but one inward-dipping segment extends as close as 400 m 

(1,3IO ft) southwest of ground zero. The termination of the 

deeper lying R20 reflector is commonly about 550 m 

(I,800 ft) from ground zero, but ranges between 440 and 

700 m (1 ,500-2,300 ft) from it. 

The debris deposits surrounding the crater have characteristics that can be distinguished seismically from the 

surrounding naturally deposited sediments. The debris deposits are amorphous, chaotic, or not clearly structured and 

have a rough surface at the sea bottom. Deposits proximal 

to the present crater are about IO m (33 ft) thick, but their 

base cannot be clearly distinguished seismically. They extend as far as 1 ,400 m ( 4,593 ft) from ground zero along the 

reef and about 800 m (2,625 ft) into the lagoon. Distal 

deposits make up a thinner outer apron, 1.5 to 5 m (5-I6 ft) 

thick, that can be recognized as far as 1,700 m (5,577 ft) 

from ground zero. 

REFERENCES 

Ristvet, B. L., Tremba, E, L., Couch, R. F., Fetzer, J. A., Goter, 

E. R., Walter, D. R., and Wendland, V. P., 1978, Geological 

and geophysical investigations of the Eniwetok nuclear 

craters: Air Force Weapons Laboratory Technical Report TR77-242, Kirtland Air Force Base, New Mexico 87117, 263 p. 

Tremba, E. L., Couch, R. F., and Ristvet, B. L., 1982, Enewetak 

Atoll Seismic Investigation (EASI): Phases I and II (final 

report): Air Force Weapons Laboratory Technical Report 

TR-82-20, Kirtland Air Force Base, New Mexico 87117, 

124 p. 

Single-Channel Seismic Survey of OAK and KOA Craters C51 

Chapter D 

Multichannel Seismic-Reflection 

Survey of KOA and OAK Craters 

By J. A. GROW, M. W. LEE, J. J. MILLER, W. F. AGENA, 

J. C. HAMPSON, D. S. FOSTER, and R. A. WOELLNER 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction Dl 

Purpose Dl 

Setting Dl 

Previous work Dl 

Geophysical methods Dl 

Acquisition Dl 

Multichannel processing techniques D2 

Seismic velocities and check -shot surveys D2 

Side-echo problems D9 

Seismic character of craters and adjacent lagoon D9 

Results: KOA crater Dll 

Depth of penetration of multichannel seismic profiles at KOA Dll 

Correlation of seismic profiles to KOA drill holes D13 

Deformational effects: north-south profile 306V1 through KOA ground 

zero D17 

Deformational effects: east-west profile 304V1 through KOA ground zero DIS 

Deformational effects: northwest-southeast profile 305V1 through KOA ground 

zero D19 

Deformational effects: northeast-southwest line 302V1 through KOA ground 

zero D19 

Structure-contour maps D19 

Structural summary of KOA crater based on multichannel seismic data D22 

Results: OAK crater D26 

Key seismic horizons at OAK: reference drill sites OAR-2 and OOR17 D28 

Deformational effects: northwest-southeast profile 103V2 through OAK ground 

zero D29 

Deformational effects: southwest-northeast profile 101 V 4 through OAK ground 

zero D34 

Deformational effects: southwest-northeast profile 124V1, offset 76 m from 

OAK ground zero D34 

Deformational effects: north-south line 105V2 through OAK ground zero D34 

Deformational effects: east-west line 104V2 through OAK ground zero D34 

Structure-contour maps D37 

Structural summary of OAK crater based on multichannel seismic data D41 

References D46 

FIGURES 

1-3. Maps showing: 

1. Sites of USGS refraction profiles and multichannel seismic-reflection 

profiles in Enewetak Atoll, 1984 D3 

2. Multichannel seismic-reflection track lines and drill sites, KOA 

crater D4 

3. Multichannel seismic-reflection track lines and drill sites, OAK 

crater DS 

4-5. Graphs showing relation between travel time and depth below sea level for 

seismic velocity check shots: 

4. KOA crater D6 

5. OAK crater D7 

6. Graph showing relation between velocity and depth for KOA and OAK 

craters D8 

7. Three-dimensional circular model of OAK crater DlO 

8. Deep-penetration seismic profile of KOA crater (line 306V2) Dll 

Contents Ill 
IV Contents 

9. Seismic-reflection profile 405V2 across OAK crater D12 

10. Seismic-reflection profile 101 V5 across OAK crater D13 

11. Map showing bathymetry and track lines, KOA crater, with drill sites D14 

12. Migrated depth section and interpretation for profile 306Vl at drill site KAR-l, 

KOA crater D15 

13-16. Seismic display and interpretation for: 

13. North-south profile 306Vl through KOA ground zero D16 

14. East-west profile 304V1 through KOA ground zero D18 

15. Northwest-southeast profile 305Vl through KOA ground zero D20 

16. Northeast-southwest profile 302V1 through KOA ground zero D21 

17-20. Structure-contour map of: 

17. Reflector R45, KOA crater D22 

18. Reflector R75, KOA crater D23 

19. Reflector R80, KOA crater D24 

20. Reflector R95, KOA crater D25 

21. Map showing bathymetry with multichannel seismic-reflection track lines and 

drill sites, OAK crater D26 

22-30. Seismic display and interpretation of: 

22. Northwest-southeast profile 103V2 through OAK ground zero D27 

23. Southwest-northeast profile 101V4 through OAK ground zero D28 

24. Southwest-northeast profile 124V1 offset 76 m southeast of OAK 

ground zero D29 

25. Southwest-northeast profile 108Vl at drill site OAR-2, OAK 

crater D30 

26. Southwest-northeast profile 109V1 at OOR-17, OAK crater D31 

27. Southwest-northeast profile 109V1 (northeast end), OAK 

crater D32 

28. Southeast-northwest profile 121V1, OAK crater D33 

29. North-south profile 105V2 through OAK ground zero D35 

30. East-west profile 104V2 through OAK ground zero D36 

31. Map of OAK showing location of multichannel seismic-reflection profiles, drill 

sites, figure numbers for profiles, and 1-m bathymetry D37 

32-35. Seismic display and interpretation of: 

32. Southwest-northeast profile 108Vl at drill site OQT-19, OAK 

crater D38 

33. Southwest-northeast profile 123V1 at drill site ORT-20, OAK 

crater D39 

34. Northwest-southeast profile 112V1 at drill site OQT-19, OAK 

crater D40 

35. Northwest-southeast profile 113V1 near drill site OQT-19, OAK 

crater D42 

36-39. Structure-contour maps showing: 

36. Reflector R20, OAK crater D43 

37. Reflector R40, OAK crater D44 

38. Reflector R75, OAK crater D45 

39. Reflector RIOO, OAK crater D46 
Chapter D 

Multichannel Seismic-Reflection Survey of KOA and 

OAK Craters 

By j. A. Grow1 

1 M. W. Lee1 

1 j. j. Miller1 

1 W. F. Agena 1

1 j. C. Hampson21 D. S. Foster21 

and R. A. Woellner2 

INTRODUCTION 

Purpose 

Because postevent processes obscure depth, diameter, and ejecta debris of submarine nuclear craters, highresolution multichannel seismic reflection data are essential 

to define their subsurface geometry. To optimize the field 

data, we mobilized a Phoenix Seismic Processing Computer 

System for on-site processing. The primary goal of the multichannel seismic survey was to acquire data on the distribution, continuity, and structure of subsurface reflectors to a 

depth of at least 500 m ( 1 ,640 ft) below bottom. A secondary goal was to provide data for the selection of drilling 

sites. 

Shallow (0-30 m, or 0-100 ft) subsurface reflectors 

were mapped with the single-channel seismic system 

(chap. C, this volume), and deeper reflectors were mapped 

with the multichannel seismic system. 

Setting 

During the period of the survey (August 21-

September 20, 1984), the lagoon provided us with light 

winds and low seas, an ideal meteorologic and oceanographic setting for a multichannel survey. However, the 

physiographic and geologic settings were far from ideal. 

Handling a long seismic streamer in shallow water close to 

the reef at OAK crater was difficult to impossible in some 

areas. KOA was even more confining because of the shoal 

on the lagoonward side of the crater (chap. A, fig. 16, this 

volume). The number of lines that could be run was limited 

by the Egabrag 's size and by the length of the seismic 

streamer needed to acquire the desired penetration and 

resolution. 

1

U.S. Geological Survey, Branch of Oil and Gas Resources, Denver, 

Colo. 80225. 2U.S. Geological Survey, Branch of Atlantic Marine Geology, Woods 

Hole, Mass. 02543. 

Previous Work 

Reflection seismic profiling in the lagoon has apparently been limited to studies associated with the craters. The 

first subbottom reflection profiling was part of project 

EXPOE (Exploratory Program on Enewetak) (Ristvet and 

others, 1978). Twelve reflection profiles (two-way travel 

time) collected in OAK, MIKE, and KOA craters with an 

8-kJ sparker system and 20-element single-channel analog 

streamer array (length not reported) revealed reflectors depressed toward the crater center; these reflectors become 

discontinuous and lose their identity at a distance of 0.5 to 

0. 75 apparent crater radii (chap. A, figs. 9, 17, this volume; 

table 2, Introduction to the volume). One successful profile 

was run off Medren (Elmer) Island. However, the authors 

expressed reservations about the usefulness of the data due 

to limitations in the accuracy of navigation, high noise levels aboard ship, and abundant multiples in the data. To 

resolve these problems, an extensive multichannel seismicreflection program using an 8-kJ sparker source and a 12-

channel, 75-m streamer was carried out by Fairfield Industries in late 1979 during Project EASI (Enewetak Atoll 

Seismic Investigation) (Tremba and others, 1982). 

Approximately 260 km (140 nmi) of seismic lines 

were shot during Project EASI in three grids controlled by 

a Motorola Miniranger navigation system. The grid in OAK 

and the grid in MIKE and KOA had line spacings of about 

140 m ( 450 ft) near ground zero, whereas the grid in the 

control area off Enjebi (Janet) Island had line spacings of 

about 600 m (970ft). The data acquired in the EASI survey 

show strong reflectors at about 0.3 s (two-way travel time) 

in many areas, but the 8-kJ sparker had a very complex 

source signature, and the 75-m streamer did not have 

enough offset to resolve the velocity structure accurately. 

However, EASI multichannel seismic data were useful for 

developing the design of the USGS multichannel program. 

GEOPHYSICAL METHODS 

Acquisition 

We experimented with four different streamer configurations and acoustic sources to determine what system 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 01 
would give optimum penetration and resolution of the complex atoll strata. These included: (1) a 75-m (246 ft) 

streamer cable with 12 channels in 6.25-m (21 ft) groups; 

(2) a 150-m (492ft) cable with 24 channels in 6.25-m (21 ft) 

groups; (3) a 300-m (984 ft) cable with 24 channels in 

12.5-m (41 ft) groups; and (4) a 300-m (984ft) cable with 

12 channels in 25-m (82 ft) groups. The acoustic sources 

included 15- and 80-in3 waterguns, and 1-, 20-, 40-, and 

540-in3 airguns. The digital recording system had the option 

of recording either 12 channels at a 0.5-ms sampling rate or 

24 channels at a 1.0-ms sampling rate. The data were 

recorded aboard ship using a Texas Instruments DFS-V 

seismic-data-acquisition system and ANSI (American National Standards Institute) standard nine-track magnetic tape 

at 1,600 BPI (bits per inch). 

Based on 4 days of testing and processing, one combination of sound-source and receiving systems gave best 

results in the craters, and another combination gave best 

results for regional work. Best for the detailed crater work 

was a 15-in3 watergun and a 150-m (492ft) streamer cable 

with 24 channels in 6.25-m (21 ft) groups at a sampling rate 

of 1.0 ms and a recording time of 1.0 s. Best for regional 

lines across and outside the atoll was an 80-in3 watergun and 

a 300-m (984 ft) cable with 24 channels in 12.5-m (41 ft) 

groups at a sampling rate of 1 ms and a recording time of 

2.0 s. Shot-spacing for the 15-in3 watergun was 3.1 m 

(10.2 ft) for 24-fold common-depth-point (CDP) coverage, 

while the shot-spacing for the 80-in3 watergun was 12.5 m 

(41 ft) for 12-fold CDP coverage. 

We collected 53 km (29 nmi) of data adjacent to the 

lagoonward side of KOA and southwest of Enjebi Island 

(200-series lines, fig. 1), 34 km (18 nmi) in KOA crater 

(300-series lines, fig. 2), and 69 km (37 nmi) of data in 

OAK crater (100-series lines, fig. 3). Tie lines between, 

OAK and KOA craters (lines 401-404, fig. 1) totaled 45 km 

(24 nmi). Regional lines across and outside the reef (lines 

405-415, fig. 1) totaled 181 km (98 nmi). The combined 

total of all multichannel seismic lines was 382 km 

(206 nmi). 

As many as five passes were made along parts of 

some lines. The profiles are designated, for example, OAK 

line 405V2, for the second pass on line 405. 

Multichannel Processing Techniques 

Field tapes from Enewetak were processed on a VAX 

11/780 computer in Denver, Colo., using Digicon DISCO 

software. While the 15- and 80-in3 waterguns were considerably cleaner sound sources than the 8-kJ minisparker used 

in the EASI multichannel seismic survey, source-signature 

deconvolution before stack was desirable to remove a lowfrequency precursor and small oscillations following the 

main pulse. USGS software, developed in-house for the 

signature deconvolution, was successful in collapsing the 

watergun signature into a clean spike. Watergun signature 

deconvolution was implemented by applying Gray's (1979) 

variable-norm deconvolution technique to the original seismic data. A single-source signature for each seismic line 

was extracted from a CDP gather. On the basis of the extracted wavelet, a deconvolution operator was derived and 

applied to the corresponding seismic line. When this source 

signature deconvolution was applied within the craters, a 

seismically transparent sedimentary unit could be clearly 

identified. Drilling confirmed that the base of this transparent layer marked the probable base of the transient crater 

(B. Wardlaw, written commun., July 15, 1985). 

Seismic Velocities and Check-Shot Surveys 

Multichannel velocities (interval velocities derived 

from the stacking velocity) were erratic until a careful analysis of water-bottom reflections and multiple times indicated that 6 ms needed to be subtracted from the raw data to 

correct for cumulative phase shifts within the acquisition 

and processing systems. Subsequent velocity analyses 

showed much-improved precision, and two sediment layers 

were resolved. Sediments between the sea floor and 100m 

(330 ft) had an average interval velocity of 1, 752 rnls 

(5,748 ft/s) in KOA and 1,687 m/s (5,535 ft/s) in OAK. 

Sediments between 100 and 300 m (328-984 ft) had an 

average interval velocity of 2,095 m/s (6,873 ftls) at KOA 

and 1,995 m/s (6,545 ft/s) at OAK. Scatter within the data 

suggested that the accuracy was within ±5 percent. No 

consistent velocity anomaly was detected within either 

crater. Check shots from subsequent drilling within and 

outside KOA crater did show a 7-percent velocity decrease 

between 100- and 220-m (328-722 ft) depth, but the average velocities were within the range predicted by the multichannel analyses. Check shots from subsequent drilling at 

OAK showed good agreement with the multichannel velocities outside the crater. A large velocity decrease was observed in the check shots between 100- and 300-m (328-

984 ft) depth within the crater not detected by the multichannel velocities. A complete analysis of this discrepancy is 

beyond the scope of this report, but the diffraction effects of 

fractured discontinuous reflectors and side echoes from the 

slopes around the crater are probably major factors. 

After the source-signature deconvolution and 6-ms 

time-shift corrections were applied, a conventional processing sequence was effective. Water-bottom multiples were 

not severe, but a poststack predictive deconvolution was 

applied and improved the data in many cases. All OAK and 

KOA lines were migrated and converted to depth using 

check-shot velocities. 

Depths refer to sea level at the time profiles were run. 

Resolution of the data is not sufficient to warrant correcting 

depths to the Holmes and Narver datum (mean low water 

spring minus 0.18 m (0.6 ft)). 

Both EASI and USGS reflection profiles over OAK 

crater recorded a band of discontinuous strong reflectors at 

02 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
8 

0 

0 

<t 

+ 

Enjebi (Janet) 

K 

t ,.L 

0 

0 

0 

0 

~ 

+ 

8 

0 

0 

~ 

+160000 

ENEWETAK ATOLL 

<'l.._~V N /PREVAILING WIND 

.#'<&~ '1-o"d' ~o l;o;<> ···~-~ """'" 

'li+<&o;""o/ ~u vwlojwa (Ursula) 

+ 120000 

~~ ~. ~ 

0 

0 

Bike~ ·~ ~ (Leroy) ·-~ 

-l-

<;QUTHWEST ~ 

PASSAGE 

+ 

+IVY GRID {fl) 

+ 

IOPOO 20,000 30,000 4QPOO 50POO 60,000 FEET 

5 10 NAUTICAL MILES 

+ + 

• (Oscar) 

SOUTH CHANNEL 

+ 

"~~nit <Yvonne) -r ~\ ~} 

-~ 

/fc 

+soooo 

~~ .~Japtan (David) 

.:>. EAST CHANNEL 

Medr~IQ:Ier) 

/

40000 

A:wetak (Fred) 

Figure 1. Sites of USGS refraction profiles and multichannel seismic-reflection profiles in Enewetak Atoll, 1984. Uppercase 

letters are first letters of site names. (See table 1 of the Introduction to the volume.) 

approximately a 300-ms depth outside the crater; these reflectors are delayed by up to 50 ms beneath ground zero. 

The 75-m (246ft) streamer used by Fairfield Industries was 

too short to obtain reliable velocities even outside the 

craters. We anticipated that the 150- and 300-m (492- and 

984-ft) streamers used during this survey might provide 

velocities that would yield depths accurate to approximately 

5 percent. This approach would allow us to determine what 

part of the time delay beneath OAK crater was due to velocity decreases (or increases?) beneath ground zero and what 

part was due to actual depression of the rocks. Calibration 

of the seismic velocities by check-shot surveys were run in 

the drill holes at ground zero and reference holes for both 

KOA and OAK craters (figs. 4, 5). In addition to allowing 

a more accurate time-to-depth conversion of the reflection 

profiles, the interval velocities derived from the check shots 

provide a direct measure of the depth of detonation effects 

on rock properties. 

A plot of one-way seismic travel time versus seismometer depth in the KOA drill holes shows a delay below 

ground zero of up to 8 ms in one-way travel-time at approximately a 220-m (721 ft) depth (fig. 4). Between the sea 

floor and approximately a 130-m ( 426 ft) depth, the observed values of both reference and crater holes appear to be 

similar within the scatter of the data. Between 130 and 

220m of depth (426-721 ft), the ground zero hole (KBZ-4) 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 03 
II 0

39'N + 

140000 IVY GRID (ft 

DATUM: MLLW (APPROX.) 

MAP PROJECTION: UTM 

SPHEROID: CLARK 1866 

COMPILED: MAY 1985 

+ 

162°II'30"E 

250 0 250 

1000 0 

311 

0 

0 

~ 

500 

1000 

309 308 310 

306-t--

162°12'E 

750 IOOOMETERS 

2000 3000FEET 

+ 

162°12'30"E 

U.S. GEOLOGICAL SURVEY 

Figure 2. Multichannel seismic-reflection track lines and drill sites, KOA crater. GZ=ground zero. Contour interval is 5 m. 

04 Sea-Floor Obser:vations and Subbottom Characteristics, OAK and KOA Craters 
W36'N 

II 0 35'30"N 

+ 

+ 

125000 IVY GRID (ft) 

+ 

120000 

+ 

DATUM' MLLW (APPROX) 

MAP PROJECTION · UTM 

SPHEROID CLARK 1866 

COMPILED MAY 1985 

1000 

+ 

0 

Q 

~ r<) 

162°61E 

250 250 500 METERS 

1000 2000 

FEET 

+ + 

+ 

_,.._ t 

j 

+ 

+ + + 

0 405 0 

0 

~ 

16~6'30"E 162°7'E 

U.S. GEOLOGICAL SURVEY 

750 1000 1250 

3000 4000 5000 

Figure 3. Multichannel seismic-reflection track lines and drill sites, OAK crater. GZ=ground z~ro. Contour interval is 5 m. 

value is progressively delayed with respect to the reference 

hole (KAR-l) value. Between 220 and 350 m (721-

1, 148ft) of depth, the delay decreases to about 2 ms. With 

possible errors of± 1 m (3.3 ft) in depth and ± 1 ms of time, 

interval velocities from individual pairs of readings at 10-m 

(33 ft) intervals were noisy. Therefore, interpreted curves 

were averaged over 50-m (164 ft) intervals. 

Plots of interval velocity versus depth averaged over 

50-m (164ft) depth intervals for KOA show large velocity 

decreases between 100 and 200 m (328-656 ft) beneath 

KOA crater (fig. 6). Between a depth of 200 and 250 m 

(650-820 ft), the KOA interval velocities are similar. Both 

KOA sites show small velocity inversions between 250 and 

300 m (820-984 ft) in a zone reported in preliminary drilling 

reports to be organic-rich. Between a depth of 300 and 

350m (984-1, 138 ft), the KOA ground zero velocities are 

approximately 400 rnls (1,312 ftls) faster than those of the 

reference hole. This effect is probably due to a natural facies 

transition from unconsolidated sediments in the lagoon at 

KAR-l to more massive consolidated reef facies at KBZ-4 

(ground zero at KOA). The lack of any measureable time 

delays or velocity anomalies in the upper 130 m ( 427 ft) at 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters D5 
ONE WAY TRAVEL TIME IN MILLISECONDS 

0 50 100 150 200 

0 0 

_.-- KDT -6 seafloor (17 m) 

---- KCT -5 sea floor (30 m) 

--KAR-1 sea floor (32 m) 

50 

KBZ-4 sea floor (33 m) 

?? 

·? -Seismometer depth or KOA CHECK SHOTS ? coupling problems? "? 

DEPTH versus TIME 

U) 100 

a:: tLLJ LLJ 

t- LLJ 

LLJ IJ... 

~ z 

z 150 500 _J 

_J LLJ 

LLJ (ij > _J LLJ 

_J <( 

<( 200 200 m LLJ 

LLJ U) 

U) ;= ~ NOTE: GZ Hole delayed 8 ms 0 

g (7%) with respect to _J 

LLJ 

LLJ 250 reference hole. CD 

a:l 

:X: 

:X: REFERENCE tt- a.. 

a.. HOLE (KAR-1) LLJ 

LLJ Q 

Q 300 

1000 

SYMBOLS 

• KBZ-4 KOA GROUND ZERO (GZ) 

• KCT-5 540 ft SOUTH OF KOA GZ 

350 & KDT-6 1200 ft SOUTH OF KOA GZ 

-$- KAR-1 KOA REFERENCE HOLE 

(8400 ft south of GZ) 

400~------------------------------------------------------------------~ 

Figure 4. Seismic velocity check shots, KOA crater. 

KOA ground zero is probably due to the absence of lithification at KAR-l. The significant velocity decrease between 

100 and 200m (328-658 ft) deep beneath KOA ground zero 

is not easily explained by natural geologic facies changes 

and is probably due to fracture effects of the KOA detonation. 

A plot of one-way seismic time versus depth for OAK 

shows an increasing delay between 100 and 300 m (328-

984 ft) of depth where a remarkable 20-ms delay was observed (fig. 5). Interval velocities averaged over 50-m bands 

for OAK are also shown in figure 6 for comparison with 

KOA. Check shots for the OAK reference holes (OAM-1, 

OAR-2) only reached 295m (968ft), while the ground zero 

hole (OBZ-4) reached a depth of 540 m (1,772 ft). As in 

KOA, no significant delays or velocity anomalies were observed in the upper 100m (328ft) of OAK crater. Both the 

OAK reference and ground zero holes have approximately 

the same position with respect to the reef, and the large 

velocity decreases at OAK ground zero between 100 and 

300 m (328-984 ft) of depth are probably due mostly to 

detonation effects, although some geologic facies changes 

are probably present. A sharp increase in velocity at OBZ-4 

(ground zero, KOA) between 300 and 350m (984-1,148 ft) 

suggests that the detonation effects below 300 m (984 ft) are 

probably less significant. 

For the purpose of time-to-depth conversion of the 

multichannel seismic-reflection profiles, reflection time 

versus interval-velocity tables for KOA and OAK were 

derived from the time-versus-depth plots (figs. 4, 5). Examination of downwarping reflectors around the craters suggests that the lateral effect around KOA extends out to a 

457-m (1,500 ft) radius, while around OAK it extends out 

06 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
ONE WAY TRAVEL TIME IN MILLISECONDS 

0 50 100 150 200 250 

0 0 

/OAM-1, OAR-2, Projected sea floor (34.1 

r_OCT -5, Projected sea floor (50.5 m) 

50 ? ? OBZ-4, Projected sea floor (60 n) 

·~p . . ?;?{ oor seismometer couplmg or 

depth errors in upper 100 m OAK CHECK SHOTS 

100 

DEPTH versus TIME 

en 

a:: 150 500 ~ w w ~ w 

w lJ... 

:E 

200 GROUND ZERO HOLE (OBZ-4) z 

z 

....J 

....J w 

w > 

> 250 w 

w J,y ....J 

....J 

'1)-~ <{ 

<{ w 

w • I)> en 

en 300 NOTE: GZ Hole delayed---- • • - 300 m V~ 

:= 

1000 := 

20 ms (14%) and \ \ IP OC' 0 

0 transition hole /'-. /)').-- ....J 

....J w 

w 350 delayed 13 ms (9%) '$)~ ~ • f7.s_ CD 

CD ..(\ '1,-. -350 m IP <)TO with respect to ~ 

~ ro ~ ¢..>-. IJJ /. ~ 

~ reference holes 

a.. "'1~ ~ "6 ~) a.. 

w 

w 400 ~ 1--0 ~ -400 m IP 0 

0 Jo ~ ~o 

"'1~ ~0 ('«' 

450 ' '( ~ ~ro IP 

1500 

0/-

'6' . 500 ~ 

550~------------------------------------------------------------------~~ 

Figure 5. Seismic velocity check shots, OAK crater. GZ=ground zero. 

to 610 m (2,000 ft). Therefore, the appropriate referencehole velocities were applied outside 457 m and 610 m 

(1 ,500 and 2,000 ft) at KOA and OAK, respectively, and 

linear transition to ground zero was assumed. Line 109 in 

OAK crater (fig. 3) was run parallel to the reef and approximately 427 m (1 ,400 ft) southeast of ground zero, and it 

showed no beds deeper than 50 m (subbottom) disrupted or 

significantly delayed by velocity decreases near the crater. 

Therefore, during the depth conversion of the OAK seismic 

profiles, OAK reference velocities were assumed to apply 

all along line 109. This assumption required that the transition from OAK ground zero to OAK reference velocities 

take place over 427 m (1 ,400 ft) on the southeast side of 

OAK, while the transition was assumed to take place over 

610 m (2,000 ft) on the southwest, northwest, and northeast 

sides of OAK. For OAK lines southeast of line 109 (fig. 3), 

interval velocities were gradually reduced to take into account the increasing water depth, assuming that the underlying velocity layers dipped to the southeast, parallel to the sea 

floor. 

Preliminary results from check shots at the OOR-17 

reference drill site on the southwest end of line 109 (fig. 3) 

show the same trend as OAR-2 and confirm the assumption 

of OAR-2 velocities for line 109. 

Concern over the accuracy of the OOR-17 check 

shots was expressed on the basis of shipboard analysis, and 

an 8-ms error was suspected because the check shots at the 

sea floor appeared too slow. No analog or digital magnetic 

tapes were recorded by the contractor, and the paper galvocamera records from the field do not have the resolution 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 07 
VELOCITY IN METERS/SECONDS 

1500 2000 2500 3000 

0 .---~~--T---~----,----,----~----r----r----~---T----~--~----,---~~--~ 0 

100 

• • • • • 

/·········~ 

KOA VELOCITY vs. DEPTH 

From interpolated check shots 

500 

200 Ground zero hole : ........... ._ ____ ___, 

Reference hole 

(KAR-1) 

en 3oo 

a: 

w 

1-

w 

(KBZ-4) • • • • • ••••• 

-~···················· . • 

. • 

• 

1000 1-

w 

w 

u. 

~ 

z 

I 

1-

a. 

w 

0 

3000 

rL----,------.-----,,-----.------.--------r-----,------r--------.-----~-----------.--------~-----------. 0 

1500 

0 

2000 2500 z 

I 

1-

100 

200 

• 

. • 

. 

........................ 

• 

. • 

. • 

.... • 

• • • • • •• • ••••• • • • 

OAK VELOCITY vs. DEPTH 

From interpolated check shots 

Reference hole 

/ (0AM-1,0AR-2) 500 

a. 

w 

0 

300 

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(OBZ-4) : 

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500 

NOTE: 

A 1 ms time delay error for 

a layer 50 m thick with an 

actual velocity of 2000 m/s 

results in a 77 m/s velocity error. 

Figure 6. Interval velocities, KOA and OAK craters. 

necessary to resolve this issue. However, a visual examination of the galvo-camera records from KAR-l, OCT-5, and 

OOR-17 does not confirm any obvious 8-ms error and implies that no relative change occurred between the first and 

last check-shot surveys. A systematic error might still occur 

if electrical crosstalk between the airgun firing wire and the 

• 

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shot-detector hydrophone wire took place. Alternatively, if 

the shot-detector hydrophone was too sensitive, it might 

pick up the airgun solenoid or shuttle motion in advance of 

the main pressure pulse from the airgun. However, because 

we lack magnetic tapes for a more detailed analysis, the 

galvo-camera records must be accepted as the best available 

D8 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
data source, and in the absence of more conclusive evidence, the contractor's check shots have been applied without a systematic 8-ms correction. 

Side-Echo Problems 

Conventional multichannel seismic-reflectionprofiling techniques are based on an assumption of twodimensional topography and structure; reflections of topography or buried structures from outside the plane of the 

profile result in side echoes that cannot be easily corrected 

with presently available acquisition and processing 

techniques. 

In Enewetak, side echoes off the main reef, crater 

rims, and random pinnacle reefs presented difficult interference problems, especially on profiles parallel to the main 

reef or those grazing the crater rims. While the reflection 

profiles cannot be corrected for these effects, computer 

models using simplified topography illustrate these problems. For instance, a circular model of OAK crater with an 

inner crater and an intermediate terrace (fig. 7) illustrates the 

side-echo and diffraction effects that complicate the interpretation of the multichannel seismic data from Enewetak. 

The first layer of the model simulates the sea floor of 

the apparent OAK crater. The second layer models a 

shallow-subsurface reflector which is truncated beneath the 

inner crater. The third layer simulates a deep reflector which 

is down-dropped beneath the inner crater. The impulse-like 

strong seismic response under the center of the crater (fig. 

7 B) results from the focused diffractions from the edges of 

the discontinuous subsurface layer. Numerous diffraction 

tails, some originating from the topographic relief of the 

apparent crater and others from the truncational edge of the 

second layer, can be clearly observed under the crater. This 

diffraction effect is a major side-echo problem in interpreting seismic data. 

Figure 7C shows the seismic response when the seismic line is offset 35 m ( 115 ft) from the center of the crater. 

Notice the synclinal appearance of the diffraction tails under 

the crater. When the line-offset distance is 75 m (246 ft) 

(fig. 7D), the truncated second layer appears as a continuous reflector, and the water-bottom reflection reveals a very 

complicated interference pattern. The seismic responses of 

the deep crater have a synclinal appearance. Similar sideecho problems due to the three-dimensional topographic and 

subsurface effects mentioned above can be observed in multichannel seismic data from Enewetak. 

Because of the side-echo problems, caution is required in making overly detailed interpretations of the 

records. The least contaminated lines are those directly 

through the topographic center of the crater, while lines 

even 35 m ( 115 ft) away from the crater center are partially 

contaminated. 

SEISMIC CHARACTER OF CRATERS AND 

ADJACENT LAGOON 

Profiles using the 80-in3 watergun and the 24-channel 

300-m (984 ft) streamer were recorded to 2 s on a northsouth profile through KOA ground zero, approximately perpendicular to the reef, and both parallel and perpendicular to 

the reef at OAK. Strong reflectors were observed to approximately a 1.0-s depth in both areas, the deepest of which are 

inferred to correspond with the volcanic rock drilled at 

1,405 m (4,610 ft) on Elugelab (Flora) Island prior to the 

MIKE detonation (fig. 8). Profile 306Vl, the north-south 

line through KOA ground zero, has been converted to depth 

using the check-shot velocities in the upper 350m (1, 148ft) 

and assuming that the reflector at approximately 1.0 sis the 

volcanic horizon. This approach assumes an average interval velocity of 3,200 m/s (10,490 ft/s) between 0.350 and 

1. 000 s. The unmigrated data for line 306V2 had better 

signal-to-noise ratio than the migrated data for depth conversion, and several strong reflecting zones can be seen 

between the volcanic basement at 1,400- to 1 ,500-m 

(4,593-4,921 ft) depth and the present sea floor (fig. 8). 

A band of shallow reflectors between 60 and 120 m 

(197-394 ft) deep can be seen across the entire profile 

except where the reflectors are downwarped or terminated 

near KOA ground zero. The most obvious pattern observed 

within the sedimentary section on profile 306V2 is a series 

of highly reflective zones at approximately 300-m (984 ft), 

600-m (1 ,968 ft), and 800-m (2,624 ft) depths, which appear to migrate from south to north (fig. 8). The band of 

reflectors between approximately 280- and 400-m (919-

1 ,312 ft) depth has much stronger amplitude in the north 

near KOA crater than in the south. The shingled pattern of 

the deeper reflecting zones between 600 and 1,000 m 

(1 ,968-3,280 ft) also shows large horizontal variations of 

reflectivity. The shallowing of these high-amplitude reflectors from south to north is interpreted to be evidence of a 

prograding reef system where the strong reflectors are probably well-cemented reef plate or back-reef facies and their 

weaker reflecting equivalents to the south are a deeper water 

lagoon facies. The absence of obvious forereef reflectors on 

the north side may be due to the steepness of the forereef or 

to masking by the strong reflectors shallower in the section. 

Profiles perpendicular and parallel to the reef at OAK 

ground zero are shown in profiles 405V2 and 101 V5, respectively (figs. 3, 9, 10). Line 405V2 shows a similar 

pattern of shingled reflectors prograding away from the lagoon as depths become shallower. Line 101 V5 is parallel to 

the reef and displays much better lateral continuity of reflectors than the lines perpendicular to the reef. Note that the 

resolution on line 101 V5 is not as good as on line 405V2. 

This "wormy" low-resolution pattern is common to most of 

the profiles run parallel and close to the reef. While the 

continuity of reflectors is good, probably because of similar 

lithologic facies, the resolution appears to be degraded by 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 09 
Om 360 m 

I 

INTERMEDIATE 

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1-----.-_ Bath\metry TERR~ACE-----

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Circular Model 

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Figure 7. Three-dimensional circular model of OAK crater. A, Cross-section of model. B, Modeled reflectors at 0 offset. C, 

Modeled reflectors at 35-m (115ft) offset. 0, Modeled reflectors at 75-m (246ft) offset. Horizontal distance scale in meters. Depth 

in A is in meters; in 8, C, and 0, is in seconds. 

01 0 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
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80-in3 watergun and 300-m streamer. GZ=ground zero. 

side echoes off the reef topography and subsurface structures on the reef side of the profile. 

While strong reflection bands are observed down to 

basement, the lateral continuity of individual seismic 

wavelets is relatively poor, even on lines perpendicular to 

the reef. The lack of contrasting lithologies within the homogeneous carbonate reef environment results in generally 

poorer impedance contrasts and reflection quality than in 

typical continental margin sediments. Because of the poor 

lateral continuity of individual seismic wavelets, relatively 

few horizons can be mapped around and into the craters with 

a high degree of confidence. 

In summary, KOA and OAK craters are located 

within a reef-lagoon system where seismic amplitudes 

change rapidly perpendicular to the reef and very slightly 

parallel to the reef. High-amplitude reflecting areas only 0.5 

to 1 krn (0.3-0.5 nmi) wide can be seen prograding from the 

lagoon toward the ocean. In spite of the poor continuity of 

individual seismic wavelets and side-echo problems, a few 

groups of reflectors within the sedimentary section allow the 

general character of deformation within and adjacent to the 

craters to be mapped with reasonable confidence. 

RESULTS: KOA CRATER 

Depth of Penetration of Multichannel Seismic 

Profiles at KOA 

Because of reefs and shoals in the north, east, and 

southeast side of KOA crater (figs. 1, 11), and a badly 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 011 
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GZ=ground zero. 

disturbed zone in MIKE crater to the west and southwest 

side of KOA, most of the 1984 multichannel seismic lines 

were run near a north-south heading. The production lines 

were acquired using the 15-in3 watergun source and 24-

channel, 150-m (492 ft) streamer. Shots were fired at 2-s 

intervals and the ship speed was held as close as possible to 

3.0 kn (5,556 m/hr) so that shot distances averaged 3.09 m 

(10.1 ft), a close approximation to the 3.125 m (10.25 ft) 

needed for 24-fold CDP coverage. Digital recordings at 

1-ms sampling rate were recorded to 1.0 s. Good reflections 

were observed all the way to 1.0 s. 

A sharp, single pulse from the crater floor near KOA 

ground zero demonstrates the highly successful signature 

deconvolution that was possible with the 1984 data. The 

seismically transparent sediments between the crater floor 

(34 m, or 111 ft) and 80 m (262 ft) probably represent 

transient crater fill beneath the inner bathymetric crater (fig. 

11). The overall depth of penetration and the resolution 

within the crater itself gives confidence that the discontinuous reflectors and lateral amplitude transitions are evident in 

the seismic lines in spite of the expected side-echo and 

facies-change difficulties. 

Correlation of Seismic Profiles to KOA Drill 

Holes 

Six drill holes at KOA were sited along north-south 

profile 306Vl through KOA ground zero (figs. 2, 11-13). 

Reference hole KAR-l was located 2,594 m (8,509 ft) 

south of KOA ground zero in an area assumed to be undisturbed by KOA or any of the other nuclear weapons tests in 

Enewetak (fig. 12). The five other drill sites were located at 

KOA ground zero (KBZ-4) and between KOA ground zero 

and 404 m (1,326 ft) south of ground zero (sites KCT-5, 

KDT -6, KET -7, and KFf -8). Portions of seismic line 

306V 1 at KAR-l and the five drill sites near KOA ground 

zero are displayed in figures 12 and 13, respectively. Eleven 

reflectors with considerable lateral continuity have been annotated with labels RIO, R20, and so forth, down through 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 013 
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MAP PROJECTION: UTM 

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U.S. GEOLOGICAL 

SURVEY COMPILED: SEPT 1985 METERS 

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1000 0 1000 

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Figure 11. Bathymetry and track lines, KOA crater, with drill sites. Contours are in meters. GZ=ground zero. 

RllO (fig. 12B). Some of these reflectors correlate with 

disconformities and sediment package boundaries identified 

in the preliminary drilling results (B. Wardlaw, written 

commun., July 15, 1985). Intermediate reflectors (R45, 

R75, and R95) are discussed later. 

At the KAR-l drill site (figs. 2, 12), RIO occurs at a 

depth of 47 m (154 ft) below sea level and correlates with 

the boundary between Holocene and Pleistocene sediments 

( disconformity "1," separating sediment packages 1 and 2; 

B. Wardlaw, written commun., July 15, 1985). R20 occurs 

at 55 m (180ft) and correlates with Wardlaw's disconformity "2" within the Pleistocene (sediment package 2). R30 

is a weak reflector at 80 m (262ft) near KAR-l and correlates with Wardlaw's disconformity "5" within the Pleistocene. R30 becomes a much stronger reflector near KOA 

ground zero. R40 occurs at 98 m (321ft) and correlates well 

with Wardlaw's disconformity "8" within the Pleistocene. 

R50 occurs at 115 m (377ft) and falls between Wardlaw's 

disconformities "11" and "12". R60 occurs at 124m (407ft) 

near Wardlaw's disconformity "12" which separates Pleistocene and Pliocene sediments (sediment packages 2 and 3, 

014 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
FIX#4665 4660 4655 4650 

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Figure 12. North-south profile 306V1 at drill site KAR-1, KOA crater. A, Migrated depth section; B, Interpretation. GZ=ground 

zero. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 015 
FIX#4630 4615 

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numbers from B. Wardlaw, written commun., July 15, 1985. 

respectively). R70 occurs at 168 m (551 ft), 11 m (36 ft) 

above disconformity "15" which separates sediment packages 3 and 4, both Pliocene. R80 occurs at 245m (804ft), 

9 m (29ft) above disconformity "16," which separates sediment package 4 from the underlying organic-rich sediment 

package 5. R90 occurs at 280 m (919 ft), within sediment 

package 5. RIOO occurs at 334 m (1 ,096 ft), within the 

lower part of sediment package 6. RllO occurs at 343 m 

(1,125 ft), 13 m (43 ft) above disconformity "18" which 

separates sediment packages 6 and 7 (middle-to-late 

Miocene). R90, R 100, and R 110 are part of a band of 

strong, discontinuous reflectors between 280 and 350 m 

(918-1, 148 ft) which increase in reflectivity near the reef, 

suggesting paleo-reef and back-reef facies. 

While some reflectors correspond to disconformities 

drilled at KAR-l , there are no strong reflectors that can be 

016 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
correlated easily between the six drill holes. The relatively 

homogeneous carbonate composition of the Enewetak sediments appears to result in small velocity and density contrasts related to subtle depositional and diagenetic effects 

which appear to have limited lateral continuity. 

Projection of the reflectors from KAR -1 (fig. 12) into 

KOA crater (fig. 13) was hampered also by reverberation 

beneath the shoal south of the crater, and only reflectors 

R60, R80, and R90 tie to Wardlaw's disconformities in the 

crater holes within_± 10 m. While most of the reflectors 

were obviously weak and discontinuous, R30 appeared to be 

a good reflector beneath the shoal. However, R30 projected 

into the KDT-6 well approximately 35 m (115 ft) deeper 

than disconformity "5" with which it should have tied. The 

cause of the mis-tie is uncertain, but it might be due to the 

seismic reflector following a diagenetic boundary rather 

than a primary depositional boundary. Alternatively, thrusting and uplift of disconformity "5" may have occurred during the cratering process. Since the drilling results have not 

supported any evidence of thrusting or uplift of disconformity "5," a mis-tie must be assumed, and the reflector under 

KOA crater has been relabeled from R30 to R45. R45 is a 

relatively good reflector beneath KOA and appears to downwarp into the crater on many of the seismic profiles. 

Reflectors RIO, R20, R50, R70, RlOO, and RllO 

from KAR-l could not be reliably tied into KBZ-4, and 

intermediate reflectors R75 and R95, which are mappable 

within KOA, have been identified. Structure-contour maps 

were prepared for R45, R75, R80, and R95. However, 

before discussion of the maps, the deformation effects seen 

on seismic lines 306, 304, 305, and 302 (fig. 11) will be 

interpreted in detail. 

While the absolute ties between KAR-l and KBZ-4 

for the reflectors have been difficult, the relative downwarping, fracturing, and truncation ofR45, R75, and other intermediate reflectors are still valid indications of deformational 

effects within KOA crater. 

Deformational Effects: North-South Profile 

306 Vl through KOA Ground Zero 

Within the center of KOA crater, transparent sediments are seen between the crater floor (34 m, or 111 ft, 

below sea level) and 80 m (262ft) (fig. 13A ,B). The bottom 

of this transparent zone is interpreted as the base of the 

transient crater, and the preliminary drilling reports indicate 

that drilling lithologies and age units are unstratified and 

mixed above this level (B. Wardlaw, written commun., July 

15, 1985). Below this level, a stratified mixed rubble was 

encountered. 

RIO and R20, approximately 47 and 55 m (154 and 

180 ft) deep at KAR -1 (fig. 12A ,B), could not be traced 

with any confidence within 600 m (1 ,968ft) south of KOA 

ground zero, but a few shallow reflectors (which may be 

RIO or R20) are present between 300 and 400 m (984 and 

1,312 ft) to the north of KOA ground zero. These shallow 

reflectors are difficult to discriminate from water-bottom 

multiples. Scattered reflecting points occur at 10- to 30-m 

(33-98 ft) depth beneath the terraces beginning 100 m (328 

ft) north and 200m (656ft) south of ground zero. These may 

be chaotic slump masses or coherent down-dropped fault 

blocks. 

R45 forms the most mappable shallow reflector 

around KOA crater (fig. 13A ,B). As discussed in the previous section, R45 within KOA crater and R30 at KAR-l 

appeared to be the same reflector, but this possibility did not 

agree with the stratigraphic drilling results. In spite of the 

correlation problem to KAR-l, R45 is a good mappable 

surface within KOA crater and can show relative deformational effects. Beginning about 500 m (1 ,640 ft) south of 

KOA ground zero, R45 drops from 77 m (253 ft) to a depth 

of 92 m (302ft) at a distance of 240m (787ft) from KOA 

ground zero, where it is too broken to be closely followed. 

On the north side of KOA ground zero, a shallow reflector, 

perhaps R45, slopes from 65 m (213ft) at the north end of 

the profile, 400 m (1 ,312ft) from ground zero to a depth of 

89 m (292 ft) at a point 200 m (668 ft) from ground zero. 

The effects of the KOA detonation on R45 are slightly 

asymmetric with intense fracturing and downward depression extending approximately 40 m (131 ft) further from 

ground zero on the south side (240 m, or 787 ft) than on the 

north side (200m, or 656ft) (fig. 13B). 

R75 forms an intermittent horizon toward KOA crater 

(fig. 13A ,B). South of ground zero, it occurs at a depth of 

approximately 145 m (476 ft) at a point 700 m (2,296 ft) 

south of ground zero and drops to 162 m (531 ft) at a 

distance of 380m (1,246 ft). A strong reflector at 180-m 

(590ft) depth, directly below KOA ground zero and out to 

100 m (328 ft) south of it, may correlate with R75. The 

apparent deformation on R75 is deeper than reported in the 

preliminary drilling reports (B. Wardlaw, written commun. , 

July 15, 1985). The interpreted 180-m (590ft) reflector may 

be a side echo. However, a similar strong event occurs on 

several KOA profiles. Possibly the drilling disconformities 

"12" and "15" (fig. 13B), which appear uplifted near KOA 

ground zero, are underlain by more depressed strata. This 

would require rebound at a shallow level, but complexities 

of this sort may be possible in craters. For the present 

analysis, our preferred interpretation is that the downwarped 

R75 is a valid seismic reflector. 

Below a depth of approximately 220m (722ft), R80, 

R90, and R95 appear to pass beneath KOA ground zero with 

little disruption (figs. 13A ,B). While the preliminary 

drilling report (B. Wardlaw, written commun., July 15, 

1985) indicates that some fracturing extended to 297 m (975 

ft) below KOA ground zero, the travel-time and velocity 

anomalies beneath KOA ground zero in the check shots 

(figs. 4, 6) only extended down to approximately 220 m 

(722 ft). Before discussing a more general interpretation of 

crater deformation at KOA, presentation of profiles 304Vl, 

305Vl, and 302Vl is appropriate. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 017 
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I I 

0 

4590 

KBZ-4(Cl) I 

- CRATER FLOOR\ 

---=-- ---: 

- - - ---...__ --- -R45? 

- - - - - - - -- -RSO?- - - - - -- -? - - - -- --- -- - - - - --

4594 

EAST .. 

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I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 

3000 2500 2000 1500 1000 500 0 500 

DISTANCE FROM KOA GZ (FT} 

1000 500 0 200 

DISTANCE FROM KOA GZ (M} 

Figure 14. East-west profile 304V1 through KOA ground zero (GZ). A, Seismic display. B, Interpretation. 

I 

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Deformational Effects: East-West Profile 304V1 

through KOA Ground Zero 

A portion of line 304 V 1, beginning approximately 

1,000 m (3,280 ft) west of KOA ground zero and extending 

approximately 200m (656ft) east of ground zero, is shown 

in figures 14A ,B . The shallow reflectors west of KOA 

ground zero are difficult to interpret and have probably been 

disturbed by MIKE, APACHE, and HURON in addition to 

KOA. A reflector possibly equivalent to R45 may be 

present, but it is too weak to interpret confidently. R75 is 

not obvious as a mappable horizon on line 304 V 1. 

018 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
R80 and R95 are mappable beneath KOA ground zero 

on line 304V1 at depths of 230m (754ft) and 285 m (935 

ft), respectively. They are relatively flat, having little or no 

downward depression or major fracturing. 

In summary, line 304 V 1 shows that R80 and R95 at 

depths greater than 230m (754ft) below KOA ground zero 

are relatively undisturbed and agrees with the observations 

at that depth derived from the north-south line 306V1 (figs. 

13A,B). 

Deformational Effects: Northwest -Southeast 

Profile 305V1 through KOA Ground Zero 

Line 305V 1 begins approximately 600 m (1, 968 ft) 

northwest of KOA ground zero and extends 300 m (984 ft) 

southeast of ground zero. R45 appears as a weak reflector on 

both ends of the profile (figs. 15A ,B). Beginning about 

450 m (1 ,476 ft) northwest of KOA ground zero, R45 begins to drop from 80 m (262ft) to a depth of 98 m (321 ft) 

200m (656ft) from ground zero, where it is too broken to 

be followed more closely to ground zero. On the southeast 

side of KOA ground zero, another reflector, apparently 

R45, can be identified approximately 200 m (656ft) from 

ground zero. The interpreted R45 on line 305V1 terminates 

approximately 200m (656ft) from KOA ground zero as did 

R45 on the north end of line 306V1 (figs. 13A ,B). 

A weak northwest-dipping reflector between depths 

of 150 and 180m (492-590 ft) southeast of ground zero on 

line 305V1 may be R75. R80 and R95 appear at 230- and 

290-m (754 and 951 ft) depth below KOA ground zero on 

line 305V 1 (fig. 15B) and are relatively undisturbed except 

for a gentle apparent southeast dip. 

When compared to line 306V1 (fig. 13), line 305V1 

(fig. 15) shows the best seismic resolution of R45 and confirms that R45 begins to bend down about 450 m (1 ,476ft) 

northeast of KOA ground zero and is intensely fractured 

within 200m (656ft) of ground zero. Over this 250-m (820 

ft) interval, R45 is downwarped from 80 m (267ft) to 95 m 

(312 ft). Within 200 m (656 ft) of ground zero, R45 is 

apparently too severely fractured to provide a mappable 

seismic reflector. 

Deformational Effects: Northeast-Southwest Line 

302V1 through KOA Ground Zero 

The portion of line 302V 1 shown in figures 16A and 

B starts 800 m (2,024 ft) southwest of KOA ground zero and 

extends 300 m (984 ft) northeast of ground zero. On the 

southwest end, line 302V1 passes south of MIKE ground 

zero and APACHE ground zero and over HURON ground 

zero (fig. 2). Shallow reflectors on this line are too broken 

to provide a mappable horizon. Deep reflectors, R80 and 

R95, again are relatively flat and undisturbed below KOA 

ground zero. 

The most interesting feature on line 302V 1 is a reflector, apparently R75, on the southwest side of ground zero. 

It is flat at about 145 m (476ft) in the southwest but begins 

to downwarp 300 m (984 ft) southwest of ground zero and 

drops to 180m (590ft) at ground zero. Although this downwarp of R75 is most clearly seen on line 302V1, it also 

downwarps on the southwest side of KOA ground zero on 

lines 301V1, 303V1, 306V1, and 309V1 (not shown). 

The downwarp of R75 to 180 m (590 ft) beneath 

ground zero is in conflict with the preliminary drilling data 

from KOA ground zero hole KBZ-4, where the deepest 

horizon with apparent downwarp is disconformity "12" at 

112 m (369 ft) (B. Wardlaw, written commun., July 15, 

1985). Although it is possible that the apparent downwarp 

of R75 on line 302V1 is a side echo or other interference 

effect, a reexamination of the drill log between 112 and 

180m (369-591 ft) at KOA ground zero is recommended. 

Although data conflict on this issue, the R75 downwarp 

appears to be real. Within KOA crater, at drill site KBZ-4 

(ground zero), disconformity "12" appears to be uplifted 

8 m (26 ft) above its depth at sites KFf -8 and KCT -5 

(fig. 13B) (B. Wardlaw, written commun., July 15, 1985). 

Even if disconformities "12" and "15" are level or slightly 

uplifted near KOA ground zero (fig. 13B), this uplifting 

could be a rebound phenomenon leaving deeper horizons, 

such as R75, still depressed. No disconformities were reported between 147 and 258m (481-847 ft) at KBZ-4, and 

rebound within this zone may be possible. 

Structure-Contour Maps 

Using migrated depth sections of KOA lines 301 V1, 

303V1, 308V1, 309V1, 310Vl, 311 VI, and 312Vl in addition to the four lines discussed above, structure-contour 

maps of R45, R75, R80, and R95 have been prepared that 

depict the three-dimensional warping or termination of these 

four reflectors (figs. 17-20). 

The R45 map (fig. 17) shows a relatively flat surface 

75 m (246 ft) deep at distances more than 500 m (1 ,640 ft) 

south ofKOA ground zero. The surface downwarps to more 

than 110 m (361 ft) 240 m (787 ft) south of ground zero 

where R45 becomes too fractured to map. Southwest of 

KOA ground zero and south of MIKE ground zero and 

APACHE ground zero, R45 appears to downwarp to approximately 120 m (394 ft). North of KOA ground zero, 

R45 (or a slightly shallower reflector on line 306V1, fig. 13) 

downwarps to a depth of 90 to 100 m (295-328 ft) at a 

distance approximately 200 m (656 ft) from ground zero. 

Within 200 m (656 ft) of ground zero, R45 is too fractured 

to map. 

The R75 map (fig. 18) shows mappable events primarily in the area south-southwest of KOA ground zero. 

R75 lies between 140 and 150m (459-492 ft) at distances 

more than 300 m (984 ft) from ground zero and then drops 

to 180 m (590 ft) directly below ground zero. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters D19 
FIX# 

0 

-

~ 

_J 100 w 

> w 

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FIX# 

4600 4605 

KBZ -4 (Gl) 

1500 1000 500 0 

DISTANCE FROM KOA GZ (FT} 

400 200 6 

DISTANCE FROM KOA GZ (M} 

4610 

SOUTHEAST .. 

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KBZ-4 (Gl) SOUTHEAST .. 

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~::~~~~==::::::'~:J~~~~~~='==~~~~~~~~~o BASE OF TRANSIENT CR~TER iRATER FLOOR 7LUMPS 

---R20?~------ SLU~PS- \ •• ~.? 

~ ---R45?- :-------2_............ . ·····?···· ............. ·· · · · · ·?·.. ...--? 

W

_J 100- J - - ~............ INTENSE FRACTURING ~__...._.., _J DOWNWARP OF R45 7 R45 ------- . BEGINS 450m FROM GZ. R30 ENDS 200m ...... ----?- AND DEPRESS I~-? '.R45? ENDS W > FROM GZ · - - - - 1- - -?- · 200m FROM GZ > 

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8 2000 1500 1000 500 0 500 1000 

Dl STANCE FROM KOA GZ ( FT) 

I 

600 400 200 6 

DISTANCE FROM KOA GZ (M) 

I 

200 

Figure 15. Northwest-southeast profile 305V1 through KOA ground zero (GZ). A, Seismic display. 8, Interpretation. 

Figure 16 (right). Northeast-southwest profile 302V1 through KOA ground zero (GZ). A, Seismic display. 8, Interpretation. 

020 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
-

390M SE OF 

MIKE Gl 

... FIX#4515 

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... 4520 4525 4530 

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DISTANCE FROM KOA GZ (ML HURON 

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KBZ -4 (Cl) NORTHEAST .. 

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DOWNWARPS AT 300m --::::>---- -- ,_ _J 

FROM GZ DOWNWARP OF R75 ALSO SEEN ON LINES 30IVI, _.,. 303VI, 306VI,AND 309VI ,_ --. 

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DISTANCE FROM KOA GZ (M) 

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162°12'30"E o 152000 

R45 ?? 

(SHALLOWER R.EFLECTOR ?) 

11°40'30"N + 

~ -f"!!l1 

DEPTH TO 

REFLECTOR 45 

+ 

+ 

STRUCTURE CONTOUR: 

Bo I d Ll ne , 5-M Contours 

- --InFerred Contours 

x x x x x Data Points 

Bathymetry: 

FIne LIne, 5-M Contours 

Da lum , MLLW (Approx.) 

11 °40 '30"N 

11 °40'N 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 250 0 250 500 750 1000 

U.S. GEOLOGICAL 

SURVEY 

COMPILED: June 1985 1000 0 

METERS 

1000 

FEET 2000 3000 

Figure 17. Structure-contour map, reflector R45, KOA crater. GZ=ground zero. 

The structure-contour map of R80 (fig. 19) shows 

variations between 215 and 245 m (705-804 ft) of depth 

with less than a 10-m (33 ft) downwarp beneath KOA 

ground zero and no simple trend outside KOA crater. 

The structure-contour map of R95 (fig. 20) shows 

variations from 280 to 335 m (918-1 ,099 ft) with no obvious downwarp at KOA ground zero and a generally southward dip in the southern half of the map area. 

Structural Summary of KOA Crater Based on 

Multichannel Seismic Data 

Seven multichannel seismic radial lines over KOA 

ground zero show a transparent pond of sediments in the 

center of the crater which extends down to an average depth 

of approximately 80 m (262 ft). In a water depth of about 

34m (111 ft) (33 m, or 108 ft, corrected to Holmes and 

Narver datum), this 46-m-thick (151-ft-thick) transparent 

sediment layer is interpreted as chaotically mixed debris and 

postevent sediment fill. Results from drilling this transparent sediment at KBZ-4 were in good agreement with the 

seismic data. Mapping of the lateral extent of this debris has 

been hampered by the numerous side echoes off the crater 

rim and may have been further complicated by low-angle 

sediment slumps (fig. 13B). Unfortunately, the Huntec 

near-bottom sonar (single-channel) system was also unsuccessful in KOA and, therefore, neither seismic system has 

produced a clear picture in the upper 50 m (164ft) of the 

lateral extent of the transient crater. 

A zone of intense fracturing can be inferred on reflector R45 within 200m (656ft) of KOA ground zero. R45 is 

last seen at a depth of approximately 100m (328ft), but on 

022 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
0 

~ 0 

162°11 '30•E 

11 °40'30~ 

/ 

...... 

0 

0 

0 

0 

5.0.._/'-

162°12 '30-E 

DEPTH TO 

REFLECTOR 75 

+ 

STRUCTURE CONTOUih 

Bold LIM, 5-M Contoure 

...... 

Ul 

0 

0 

0 

- - - Infrr.d Contour• X X X X X Dete Pointe 

hthiJ-try• Fin. LIM, 5-M Contoure 

Datum, MLLW (Approa.) 

152000 

11 °40'N 

16.2~~1~1~'3~0~-E~----------~---------16_2_-12-,-E----------------------16_2_-12-'-30-.-E--~ 145000 

MAP PROJECTION: UTM 

SPHERO I 0: C I arlc: 1 866 2!50 0 2!50 500 7!50 1000 

U.S. GEOLOGICAL 

SURVEY 

COMPILED: June 1985 AHA A A• 

1000 0 

METERS 

1000 

FEET 

Figure 18. Structure-contour map, reflector R75, KOA crater. 

th1;! basis of the general absence of reflectors on line 306V 1 

near KOA ground zero (fig. 13B), the intensely fractured 

zone may project to a depth of 140 m (459 ft) directly 

beneath ground zero. Because good marker reflectors shallower than R45 are lacking, the lateral extent of the intensely fractured zone above R45 beyond 200 m (656 ft) 

from ground zero is speculative and is based only on the 

multichannel seismic data. On line 306V1 (fig. 13B), intense fracturing extends laterally out to a distance of about 

500 m (1 ,640 ft) on the south side and at least 300 m (984 

ft) on the north side of KOA ground zero. 

2000 3000 

The down warping of the R45 and R75 reflectors indicates an outer zone of deformation where coherent reflectors 

are depressed downward and fracturing effects are only 

moderate. For R45, this downwarp begins at approximately 

500 m (1 ,640 ft) from ground zero, and it drops from 75 to 

100m (246-328 ft) before intense fracturing occurs within 

200m (656ft) of ground zero. For R75, the apparent downwarp begins 300m (984ft) from ground zero, and it drops 

from 150 to 180 m (492-590 ft) (fig. 18). The relatively 

undisturbed reflectors R80 and R95 suggest that moderate 

fracturing and downwarp do not occur below 230m (754ft). 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 023 
0'1 

(I) 

0 

0 

0 

162°11 '3o•E 

11 °40'.30•N + 

11 °40'N 

0 "' 0 

0 

0 162°12'30•E 

DEPTH TO 

REFLECTOR 80 

+ 

+ 

STRUCTURE CONTOUih 

...., 

Ul 

0 

0 

0 

8o I d LIne, 5-M Contour• 

---InF.,-r-ed Contour• x x x x x D•t• Point• 

8•thy-try• FIne LIne, 5-M Contour• 

Datum, MLLW (ApproxJ 

152000 

11 °40'30•N 

11 °40'N 

MAP PROJECTION: UTM 

SPHEROID: Clark 1866 250 0 250 500 750 1000 

U.S. GEOLOGICAL 

SURVEY ~ETERS 

1000 0 

COMPILED: June 1985 AHHHHI 

1000 

FEET 

Figure 19. Structure-contour map, reflector R80, KOA crater. 

The check-shot surveys from the KOA drill holes 

indicated an increasing travel-time delay down to a depth of 

220m (722ft) below KOA ground zero (fig. 4). The interval velocities, which were averaged over depth intervals of 

50 m (164ft), show a major velocity anomaly down to at 

least 200m (625ft) (fig. 6). The preliminary drilling reports 

(B. Wardlaw, written commun., July 15, 1985) indicate 

some fracturing down to 297 m (975 ft) beneath KOA 

ground zero. The interpretation of line 306Vl over KOA 

ground zero (fig. 13B) assumes that the base of the moder2000 3000 

ately fractured and depressed zone corresponds with the 

maximum time delay in the check shots, that is, 220m (722 

ft). While some fracturing may occur to a depth of 297 m 

(975ft) in the core samples, the effects appear minor in the 

check -shot and multichannel seismic data below 220 m (722 

ft). 

In spite of limited success in tying the individual 

seismic reflectors to the disconformities in the KAR-l, 

KBZ-4, and other KOA drill sites, the relative warping, 

fracturing, truncation of reflectors, and the transparent 

024 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
0'1 

(X) 

0 

0 

0 

162°11 '30'"E 

11 °40'30'"N 

11 °40'N 

162°11 '30'"E 

....... 

0 

0 

0 

0 162°12 '30'"E 

+ 

'\ 

~ -r-1-

j 

+ DEPTH TO 

REFLECTOR 95 

STRUCTURE CONTOUR• 

Bold Ll,., 5-M Contour. 

....... 

Ul 

0 

0 

0 

- - - InFrred Contour• 

X X X X X Data Pointe 

S.thyMtryl 

Fl,. Ll,., 5-M Contoura 

Datum, MLLW (Approx.l 

152000 

11 °40'30'"N 

150000 

11 °40'N 

MAP PROJECTION: UTM 

SPHEROID: C I ark 1866 250 0 250 500 750 1000 

U.S. GEOLOGICAL 

METERS SURVEY 

1000 0 

COMPILED: June 1985 HHHHHI 

1000 

FEET 

2000 3000 

Figure 20. Structure-contour map, reflector R95, KOA crater. 

crater fill within KOA crater observed in the multichannel 

seismic reflection data are valid evidence for the deformation effects within the crater. 

In terms of first-order deformational effects at KOA, 

both the multichannel seismic reflection and drill-hole results indicate a transient crater depth of 75 to 80 m (246-262 

ft). Moderate fracturing effects can be identified to a depth 

of220 m (722ft) in the reflection and check-shot data, while 

fracturing was observed to 297 m (974 ft) in the drilling 

results. Downwarp of horizons directly beneath ground zero 

varies between 112 m (369 ft) for the drilling results and 

180m (590 ft) for the seismic profiles; internal rebound 

effects may be responsible for this apparent discrepancy. 

The lateral effects of the KOA event can be seen in downwarp of the R45 reflector out to distances of 500 m (1 ,640 

ft) from ground zero, and the drilling results indicate fracturing down to a depth of 27 m (88 ft) at the southernmost 

drilling site (KET -7, fig. 13), which is 404 m (1 ,326 ft) 

south of KOA ground zero. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 025 
11 °36 '30"N 

(N 

(N 

0 

0 

0 

DATUM: MLLW(APPROX.) 

MAP PROJECTION: UTM e:----3 I 

100 0 100 200 300 400 500 

METERS 600 700 800 900 1000 U.S. GEOLOGICAL 

SURVEY COMPILED: SEPT 1985 A RAHAl 

500 0 500 I 000 I 500 2000 2500 3000 

FEET 

Figure 21. Bathymetry with multichannel seismic-reflection track lines and drill sites, OAK crater. GZ=ground zero. 

Bathymetric contour interval is 1 m. 

RESULTS: OAK CRATER 

Because one side of OAK crater lies mostly in deep 

water open to the lagoon, Egabrag II was able to maneuver 

successfully with the long streamer. A grid of 25 profiles 

was acquired with the 15-in3 watergun and 24-channel, 150-

m streamer (figs. 3, 21). Good-quality reflections were generally observed to the base of the recording window at 1. 0 s. 

The primary lines perpendicular and parallel to the reef 

through OAK ground zero are lines 103V2 and 101V4, 

respectively (figs. 22, 23). Because the geometric center of 

OAK crater lies 100 m (330 ft) southeast of ground zero, 

line 101 V4 grazes the northwest flanks of the crater and has 

more severe side echoes than line 124 V 1 , which also lies 

parallel to the reef but passes closer to the geometric center. 

Therefore, lines 103V2 and 124Vl are the reefperpendicular and reef-parallel lines which pass closest to 

the geometric and physiographic center of OAK crater. 

Clean pulses off the crater floor at approximately 60 m ( 197 

ft) are observed on both lines 103V2 and 124V1, and a 

transparent zone can be observed down to approximately 

110 m (361 ft). We interpret this zone as the base of the 

transient crater (figs. 22B, 24B). In OAK, as in KOA, we 

observed good reflections down to 1.0 km (3,280 ft), but the 

lateral continuity of individual reflectors was usually 

limited. 

026 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
CROSS LINE# 122 

FIX# 1835 

~ 

-

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~ 200 w 

_J 

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w 

Cf) 300 

3: 

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CROSS LINE# 122 

FIX# 1835 

109 102 123 108 124 101 125 

I 1845 I 

OHT-10 0/T-11 

3000 2500 2000 1500 1000 500 0 

DISTANCE FROM OAK GZ (FT) 

800 600 400 200 0 

DISTANCE FROM OAK GZ (M) 

~ 

LL 

_J 

w 

> 

w 

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500 1000 

200 

OUT-24 ~~~~~~~~~--~----~~~~--~~~~~------~o 

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~ 500 

&()~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

8 3000 2500 2000 1500 1000 500 0 

DISTANCE FROM OAK GZ (FT) 

500 1000 

1000 800 600 400 200 0 200 

DISTANCE FROM OAK GZ (M) 

~ 

LL 

500 Ll:f 

> 

w 

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Figure 22. Northwest-southeast profile 1 03V2 through OAK ground zero (GZ). A, Seismic display. B, Interpretation. Major 

disconformity numbers after B. Wardlaw, written commun., july 15, 1985. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 027 
:::?! 

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CROSS LINE# 

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DISTANCE FROM OAK GZ (FT) 

800 600 400 200 0 200 

DISTANCE FROM OAK GZ (M) 

CROSS LINE# I II 113 114 103 115 116 

FIX# 1740 1730 I I rr25 

118 

400 

118 

NORTHEAST .. 

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600 800 

NORTHEAST• 

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~ 500 

2000 

t 

001-19 

1000 0 1000 2000 

I 

1500 5: 

3000 

w 

a 

DISTANCE FROM OAK GZ (FT) 

800 600 400 200 0 200 400 600 800 

DISTANCE FROM OAK GZ (M) 

Figure 23. Southwest-northeast profile 101V4 through OAK ground zero (GZ). A, Seismic display. B, Interpretation. Major 

disconformity numbers after B. Wardlaw, written commun., July 15, 1985. 

Key Seismic Horizons at OAK: Reference Drill 

Sites OAR-2 and OOR-17 

Reference drill sites were located near the reef, both 

northeast and southwest of OAK crater, at sites OAR-2 and 

OOR-17, respectively (fig. 3). OAR-2 was located near the 

northeast end of seismic line 108V1 (fig. 25) and OOR-17 

on the southwest end of line 109V1 (fig. 26). The most 

continuous reflectors observed outside OAK crater are 

shown on the northeast end of line 1 09V 1 , which is parallel 

to the reef and 412 m (1,350 ft) southeast of ground zero 

(fig. 27). Reflectors R10 and R20 correlate with shallow 

horizons mapped by the Huntec sonar (chap. C, this volume), and preliminary drilling results indicate that R 10 is 

the Holocene-Pleistocene boundary. Reflectors R40, R60, 

R75, R80, and R100 are interpreted as the most correlatable 

seismic horizons. Line 121 V 1 illustrates the character of a 

profile perpendicular to the reef well northeast of OAK 

ground zero and OAR-2 (fig. 28). 

028 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Ill 112 113 114 103 104 115 116 118 119 NORTHEAST. 

w 

~ CD 

I 

~ 

a_ 

w 

Cl 

r----

800 600 400 200 0 200 400 600 800 

p 

u... 

G:1 

> 

w 

....J 

<( 

~ 

3: 

0 

....J 

w 

CD 

DISTANCE FROM LINE 103 (M) 

NOTE: LINE 124VI IS PARALLEL TO REEF AND OFFSET 76m (250ft.) FROM GZ TOWARD LAGOON 

CROSS LINE# Ill 112 113 105 114 103 104 115 116 

FIX# 2340 2350 

0 0 

-100 p 

~ ~ 

....J ? - 500 G:1 

~200 > 

w _J ....J 

<( <( 

w w 

(/) 300 (/) 

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w w CD40Q CD 

I I ~ 

a_ 1500h= 

~500 w 

Cl 

600 

3000 0 1000 2000 3000 

8 DISTANCE FROM LINE 103 (FT) r----

800 600 400 200 0 200 400 600 800 

DISTANCE FROM LINE 103 (M) 

NOTE: LINE 124VI IS PARALLEl TO REEF AND OFFSET 76m (250ft.) FROM GZ TOWARD LAGOON 

Figure 24. Southwest-northeast profile 124V1 offset 76 m (250 ft) southeast of OAK ground zero (GZ). A, Seismic display. 

8, Interpretation. Line 124 is parallel to the reef and offset 76 m (250ft) from ground zero toward the lagoon. 

The major disconformities identified in the preliminary drilling reports at sites OAR-2 and OOR-17 (B. 

Wardlaw, written commun., July 15, 1985) are shown in 

figures 25, 26, and 28. While there are some mappable 

reflection horizons, their lateral continuity is only fair, at 

best. This appears to be an inherent characteristic of the 

relatively homogeneous carbonates throughout the lagoon at 

Enewetak.. Mapping the reflectors into OAK crater is difficult but not impossible. While any single reflector may die 

out laterally, others above or below it can be followed which 

allows the general deformational character within the crater 

to be mapped with a reasonable degree of assurance. 

Deformational Effects: Northwest -Southeast 

Profile 1 03V2 through OAK Ground Zero 

The most revealing single profile over OAK crater is 

the northwest-southeast line perpendicular to the reef 

through ground zero (line 103V2, fig. 22). The key reflectors, as well as numerous intervening beds, are well represented. The greatest downwarp of subsurface reflectors lies 

about 120m (394ft) southeast of ground zero, close to the 

geometric center of the crater based on bathymetry. The 

transient crater extends to a depth of 110m (361 ft) beneath 

the geometric center and appears to extend laterally for a 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 029 
CROSS LINE# 

FIX#I965 

-

~ 

_J 

w 

> w 

_J 

~ 

0 

_J 

w 

CD 

:I: 

~ 

a.. 

w 

0 

2200 

A 

800 

CROSS LINE# 

FIX #1965 

0 I 

-

~ .... - -

120 

1955 

3000 4000 5000 

DISTANCE NORTHEAST OF LINE 103 (FT.) 

1000 1200 1400 

DISTANCE NORTHEAST OF Llr£ 103 (M) 

120 I 

'I~ 1955 

OAR-2 

I 

SEA FLOOR"'"\ 

~~·;2 

R20::7 tRIO 5 

NORTHEAST .. 121 

1950 

5800 

1600 

NORTHEAST .. 121 

1950 

I 

- - -

d 100 ----=-=~ -R40- 8 - > w 

_J 

~200 1-

(/) 

~ 

3300 1-

II 

R60?- 12 

-- - - - - - 15 

- -R75?- - - R85 -16 

--- - - 17 - MAJOR DISCONFORMITY NUMBERS 

.---, ----~ -RIOO _..-~ -

-

-

- - ~ ~ ----- - -- - -- --:.... 

:I: ~ 

1- t400 

w 

0 

500 I 

2200 

8 

I 

- _::::::::-:- -- - - -~ 

V.E.= 1.1 X I I I I I I I I I ' 3000 4000 5000 5800 

DISTANCE NORTHEAST OF LINE 103 (FT.) 

800 1000 1200 1400 1600 

DISTANCE NORTHEAST OF LINE 103 (M) 

NOTE: LINE IOSVI IS PARALLEL TO REEF AND 152 m(500ft} SOUTHEAST OF OAK GZ 

~ 

~ 

!-

-

... 

-

-

-

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~ 

CROSS LINE# 

FIX# 2055 

OOR-17 OSR- 21 

0 

6000 

A 

CROSS LINE# 

5000 

1500 

2050 

106 Ill 105 112 

4000 3000 2000 1000 0 

DISTANCE SOUTHWEST OF LINE 103 (FT.) 

1000 500 0 

DISTANCE SOUTHWEST OF LINE 103 (M) 

SOUTHWEST END NORTHEAST. 

106 Ill 105 112 113 114 103 

p 

~ 

......J 

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> 

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~ 

2050 I 2045 204b I I 2035 I I 12030 

O~~~~~c~RM~-;~~~SE~JA~F~W~OR~~~~~~;;~~;;~_:_~R~IO~~~~~~T~R~ro~~~~~~~~~;;:;;;::~==~=-rO ~ FIX# 2055 

OOR-17 OSR-21 

d 100 

4 

[!! 4 RIO AND R20 DISTURBED AT -500L/ ~ e -R40 ~ - --__ ---- _ ---R40 (164011.) FROM GZ -- ......J 

> w 

......J 

12 12 NOTE: NO DEFORMATION EFFECT AT R40 AND DEEPER L&..J 

15 

~200 (/) 

3: 16 

g 300 17 

NOTE: R60,R75,R85 AND 

RIOO ALL IRREGULAR NEAR OOR-17 ? 

? --R60 -- - -- r-500 Gj 

......J 

R75 --- <( 

------ ~ 

-- R85?- 3: 

~ - r- ----::: - :r -::::..::~_.7--------...;....::---- RIOO? ;..__ - ------

--..:::::-==-- _ _ _ ~HOOO ~ RIOO? --- -.:::::: ::::::::._ ---- m 

b: 400 MAJOR N~:ro:ORIIITY 

w 

a 

:I: 

1-

Cl. 

w 

V.E.=1.2X -1500 a 

500 1 I I I I I I 

6000 5000 4000 3000 2000 1000 0 

8 DISTANCE SOUTHWEST OF LINE 103 (FT.) 

1500 1000 500 0 

DISTANCE SOUTHWEST OF LINE 103 (M) 

NOTE: LINE 109 IS PARALLEL TO REEF AND 412m(f350ft) SOUTHEAST OF OAK GZ 

Figure 26. Southwest-northeast profile 1 09V1 at OOR-17, OAK crater. A, Seismic display. 8, Interpretation. Line 109 is parallel 

to the reef and 412 m (1,350 ft) southeast of OAK ground zero (GZ). Major disconformity numbers after B. Wardlaw, written 

commun., July 15, 1985. 

distance of 480 m (1 ,575 ft) southeast of ground zero. The 

profile extends only 300 m (984 ft) northwest of ground zero 

toward the reef where possible shallow slumping makes 

interpretation and correlation more uncertain. 

From the lagoon (southeast) side of OAK crater, the 

R40 horizon begins to downwarp at a distance of 500 m 

(1 ,640 ft) and then appears to become severely fractured at 

360 m (1, 181 ft) from ground zero. On the reef (northwest) 

side of the crater, R40 may come to within 120 m (394 ft) 

of ground zero before becoming severely fractured. Beneath 

the geometric center of the crater, the absence of coherent 

reflectors between approximately 110 and 180m (361-590 

Figure 25 (left). Southwest-northeast profile 1 08V1 at drill site OAR-2, OAK crater. A, Seismic display. 8, Interpretation. Line 108 

is parallel to the reef and 152m (500ft) southeast of OAK ground zero (GZ). Major disconformity numbers after B. Wardlaw, written 

commun., July 15, 1985. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 031 
~ 

_J 

w 

> 

w 

_J 

<X: 

w 

(/) 

S; 

0 

_J 

w 

m 

A 

115 116 118 104 119 

2000 3000 4000 

DISTANCE NORTHEAST OF LINE 103 ( FT) 

6 56o 1obo 

DISTANCE NORTHEAST OF LINE 103 (M) 

NOTE: LINE 10915 PARALLEL TO REEF AND 412m (1350ft) SE OF OAK GZ 

I 

1500 

NORTHEAST .. 121 

NORTHEAST .. CROSS LINE# 103 115 116 118 104 119 120 121 

F~lx_#~ .. ~-~~3o~l~~~--~~2~~~-? ____ ~l ___ 2~~2_o ________ l_2~oi_5 ____ ~ ___ 20~Io _____ ~l--ro 0 RIO 8 R20 DISTURBED DUE TO OAK ? /R 10 /SEA FLOOR 

. ' - R2o-

- 100-- -'----------"==-- R40-=====---=-·-...;.- --=---- ~ NOTE: R40 8 DEEPER HORIZONS UNDISTURBED ------ ,..._: 

l..i_ 

ijj ---- __ ------RGO ... - - - -500 :; 

Gj 200 _J -

<X: 

w (/)300r-

------------ -R75- - --------....... ------------

- - -- -.......-- --- - ---Ras-------

w 

> 

~ 

<X: 

w 

HOOO (/) S; 

g _ ----- --RIOO- --- ----------- - !': ----- ----- g w m400 

:t: 

tCL 

~500 

w 

m 

:t: ~1500 5:: 

w 

0 

600~--..-~~~~-~.~~--.-~~.-~~~~--~~~~~-~~,--~-.-~~-~.,-~~-~ 

0 1000 2000 3000 4000 5000 

V.E. = 1.2X 

8 DISTANCE NORTHEAST OF LINE 103 ( FT) 

6 56o 1obo 

DISTANCE NORTHEAST OF LINE 103 ( M) 

NOTE: LINE 109 IS PARALLEL TO REEF AND 412 m (1350 ft) SE OF OAK GZ 

<X: 

w 

(/) 

S; 

0 

_J 

w 

m 

Figure 27. Southwest-northeast profile 109V1 (northeast end), OAK crater. A, Seismic display. 8, Interpretation. Line 109 is 

parallel to the reef and 412 m (1,350 ft) southeast of OAK ground zero (GZ). Major disconformity numbers after B. Wardlaw, written 

commun., July 15, 1985. 

ft) suggests that rocks are intensely fractured as deep as 

180m (590ft). Horizons R60, R70, and underlying reflectors have been downwarped by 10 to 40 m (33-131 ft), 

down to a depth of approximately 280m (918ft). Because 

reflectors R85 and RlOO dip slightly toward the lagoon even 

outside the crater area (see profile 121Vl, fig. 28), downwarps of 10 m (33 ft) or less are difficult to resolve. However, the deep penetration seismic line parallel to the reef 

Figure 28 (right). Southeast-northwest profile 121 V1, OAK crater. A, Seismic display. 8, Interpretation. Line 121 is perpendicular 

to the reef and 1,615 m (5,300 ft) northeast of OAK ground zero (GZ). Major disconformity numbers after B. Wardlaw, written 

commun., July 15, 1985. 

D32 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
CROSS LINE# 

FIX# 3055 

I 

1--

Q.. 

w 

0 

A 

1000 

3000 

110 

3050 

109 102 123 

3045 

2000 1000 

124 101 

3b4o 

OAR-21 

244M 

0 

DISTANCE SOUTHWEST OF LINE 101 (FT} 

500 0 

DISTANCE SOUTHWEST a= LINE 101 (M} 

NOTE: LINE 121 IS PERPENDICULAR TO REEF AND 1615m (5300ft.} NE OF OAK GZ 

125 

CROSS LINE# 110 109 102 123 124 101 125 

OAR-21 

t-= 

LL. 

500__J -

w 

> w 

__J 

<X: 

w 

1000 en 

~ 

g 

w 

CD 

I 

1500 h: 

w 

0 

FIX# 30553 r0

1

50 3045 3b4o ~ 

• 244M s~. NORi HwEsT• 

SEA LOOR~ RIO REEF+~ 0 0 1 

+LAGOON 

- '- - - -==- R20 - ---~ ~ RIO ,......_ 

~-

- 100 -~---- - ::::E --- R40------ - 8 I~ -

12 

- -- -~ .- __J ~=-=----- ----------Rso- - -- 15_.. _____ -

~ 200- w 

__J 

-------- R75 ----- -1------

8 ___ 16 --

<X: ~--- - - - - ----- R 5---- 17 '-MAJOR DISCONFORMITY NUMBERS 

~ 300- ----

q 

3: ~ - - - ---------=--- - -......-RIOO--:::----- - - ~ ==--= ~ - I __, -----_,.. / 

MIGRATION 

t-= 

LL. 

- -500 __J 

w 

> 

w 

__J 

<X: 

w 

-1000 en 

~ 

g 

w 

CD 

EDGE EFFECTS I 

""' / -1500 h: 

~ 400-

I 

_,_, w 

0 

h: 

~ 500-

- V.E. = I.IX - 600~ .• --~.-~,~~.--.~.~-,-~,-.-~.---.~,-~.~.~~.-r-,~,~------

8 3000 2000 1000 

DISTANCE SOUTHWEST OF LINE 101 (FT) 

0 

1000 500 0 

DISTANCE SOUTHWEST CF LINE 101 (M) 

NOTE: LINE 121 IS PERPENDICULAR TO REEF AND 1615m (5300ft.) NE OF OAK GZ 
(line IOI V5, fig. IO) suggests that reflectors 300m (984ft) 

deep are down warped about I 0 m (33 ft) beneath OAK 

ground zero. Note also that the check-shot velocities between 250 and 300 m (820-984 ft) at OAK ground zero are 

approximately 30 percent slower than at reference site 

OAR-2 (fig. 6). Since the OAK reference hole only went to 

305 m (1 ,000 ft) below sea level, reduced velocities at 

ground zero may extend deeper. Therefore, while the seismic reflection data (line I03V2, fig. 22) suggests minor 

fracturing or depression effects below 280 m (9I8 ft), it 

should be noted that the check -shot velocities suggest that 

reduced velocities, presumably due to significant fracturing, 

may extend to depths of 300 m (984 ft) or greater. Finally, 

the refraction data (chap. E, fig. I9, this volume) suggest 

that the velocities beneath OAK may be lowered at depths 

of more than 400 m (1,3I2 ft). 

Deformational Effects: Southwest-Northeast 

Profile 101 V4 through OAK Ground Zero 

Line I 0 IV 4 is parallel to the reef and passes almost 

directly over OAK ground zero (fig. 23). Because of the 

offset of the geometric center of the crater to the southeast 

of ground zero, side echoes off the reef and crater rim create 

more severe noise problems on this profile than the side 

echoes present on line I03V2 (fig. 22), which is perpendicular to the trend of the reef. Line I 0 IV 4 grazes the crater 

slope on the northwest (reef) side of the crater (fig. 2I), 

where slumps appear to have transported debris into the 

crater. (See chap. A, fig. I5B, this volume.) Therefore, the 

acoustically transparent sediment pond seen on line I03V2, 

which marks the base of the transient crater, is not obvious 

on line I 0 IV 4. 

The shallow RIO and R20 reflectors are clearly apparent on this line and begin to downwarp at distances of 550 

to 600 m (1 ,804-I ,968 ft) from ground zero. RIO appears 

to drop from 40 m ( I3I ft) down to approximately 80 m (262 

ft) at distances of 220m (722ft) from ground zero, where 

the reflectors appear to be truncated by the excavation. The 

R40, R60, and R75 reflectors are not as clear on line IOI V4 

as on the perpendicular line, but R40 appears to begin downwarping 400 m (1 ,3I2 ft) from ground zero. Reflectors R85, 

R95, and RIOO lie between 260 and 340m (853-I,115 ft) 

deep. Although all reflectors show considerable depositional variability along the line, RIOO appears to be downwarped approximately IO to 20m (33-66 ft) beneath ground 

zero due to the OAK detonation. 

Deformational Effects: Southwest-Northeast 

Profile 124V1, Offset 76 m (250 ft) from OAK 

Ground Zero 

Line I24VI is parallel to the reef and passes nearly 

through the geometric center of the crater (fig. 24) approximately 76 m (250ft) southeast of OAK ground zero. As on 

line I03V2 perpendicular to the reef, the transparent crater 

fill can be observed down to IOO m (328ft). Reflectors RIO 

and R20 are relatively clear on the line and appear to have 

some downwarp (3-IO m, or I0-33 ft) at distances beyond 

600 m (1,968 ft) from ground zero. They are downwarped 

as much as 70 m (230 ft) at a distance of 300 m (984 ft) from 

ground zero, and possibly fragments of RIO and R20 can be 

traced down to depths of90 m (236ft) at a distance of IOO m 

(328 ft) from ground zero. 

Downwarps of reflectors R40, R60, and R75 begin 

between 400 and 500 m (1 , 3I2-I, 640 ft) from ground zero 

with 20 to 40 m (66-13I ft) downward displacement suggested beneath ground zero. Reflectors R85 and RIOO appear to have downwarps of less than IO m (33 ft) beneath 

ground zero. 

Deformational Effects: North-South Line 1 05V2 

through OAK Ground Zero 

Line I05V2 illustrates complexities within the excavated zone and shallow reflectors not observed on the previously discussed lines (fig. 29). While the base of the transient crater appears to be at II 0 m (36I ft) just south of 

ground zero, line I05V2 reveals a buried block that rises to 

a depth of 85 m (279ft) directly beneath OAK ground zero. 

The R20 reflector on the south side of ground zero also has 

remarkable coherence and can be traced to within 300 m 

(984ft) of ground zero. RIO and R20 appear to begin their 

downwarp about 600 m (1 ,968ft) south of ground zero. R40 

begins to downwarp approximately 500 m (1 ,640 ft) south 

of ground zero. The downwarps on reflectors R60 and R75 

are not as clear as on line I03V2 but appear to be on the 

order of 20m (66ft) beneath ground zero. Reflectors R85 

and R I 00 are again a discontinuous band of reflectivity 

which appears to have been downwarped IO m (33 ft) or 

less. 

Line I05V2 displays the same dramatic offset of 

deepest deformation to the southeast of ground zero, as is 

observed on line I03V2. 

Deformational Effects: East-West Line 1 04V2 

through OAK Ground Zero 

Line I 04 V2 recorded a broad transparent zone down 

to a depth of 110 m (36I ft) which marks the base of the 

transient crater and extends to over 400 m (I , 3I2 ft) east of 

ground zero (fig. 30). Reflectors R40 and R60 are also 

relatively undisturbed at a distance of 400 m (1 , 3I2 ft) or 

more from ground zero. Downwarps of R60 and R75 are 

difficult to measure accurately, but they appear to be in the 

range of IO to 40 m (33-13I ft) beneath ground zero. R85 

and RIOO are relatively flat, and they lie at depths of 280m 

(9I8 ft) and deeper. The line confmns the offset of deepest 

deformation southeast of ground zero, seen on line I03V2 

(fig. 22) and line I05V2 (fig. 29). 

034 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
~ 

_.J 

w 

> 

w 

_.J 

<X: 

~ 300 

~ 

g 

~ 400 

I 

..._ 

a_ 

w 

0 

A 

_.J 

~ 200 

w 

_.J 

<X: 

~ 300 

~ 

g 

~ 400 

I 

..._ 

a_ 

~ 500 

CROSS LINE# 102 

FIX# 2175 

2000 1500 1000 

DISTANCE FROM OAK GZ (FT) 

1000 800 600 400 

DISTANCE FROM OAK GZ (M) 

NOTE: LINE 105V2 PASSES 37m (120ft.) WEST OF OAK GZ 

CROSS LINE# 102 

FIX# 2175 

__ ----- -R85 ------ ---==.._ ----===--__ ---- ---- ---- ;::...__________________ -~ =--=---==---=-....===. 

• ------~-- -RIOO-

...... ....--. -----.... -===------ NOTE : ASYMETRIC DAMAGE 

EFFECTS; WORSE ON 

SOUTH a SOUTHEAST 

3500 3000 2500 2000 1500 1000 

DISTANCE FROM OAK GZ (FT) 

1000 800 600 400 

DISTANCE FROM OAK GZ (M) 

NOTE: LINE 105V2 PASSES 37m (120ft.) WEST OF OAK GZ 

108 124 

200 0 

NORTH. 

108 124 101 8103 125 

I 

0Bl-4(Gl) 

MINOR DOWNWARP (SIOm) a FRACTURING EFFECTS 

BELOW 280m (918ft.) 

500 

200 

0 

0 

t-= 

LL 

_.J 

w 

> 

w 

_.J 

<X: 

w 

(/) 

500 cd 

> 

w 

_.J 

<X: 

w 

(/) 

1000~ _.J 

w 

0) 

I 

..._ 

1500 EJ 

0 

Figure 29. North-south profile 1 05V2 through OAK ground zero (GZ). A, Seismic display. 8, Interpretation. Line 1 05V2 passes 

37 m (120 ft) west of OAK ground zero. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 035 
_j 

w 

~ 200 

_j 

<( 

w 

(/) 

3: 

0 

_j 

~ 400 

CROSS LINE# 109 102 123 

FIX# 2135 2140 

125 WEST .. 

2150 

0 

-!--= 

LL 

........... 

500 Ljj 

> 

w 

_j 

<( 

w 

(/) 

1000 3: 

g 

w 

CD 

I 

1500 h= 

w 

0 

3000 2500 2000 1500 1000 500 0 500 1000 

_j 

w 

A 

0 

~ 200 

_j 

<( 

w 

(/)300 

3: 

0 

_j 

~ 400 

I 

._ 

a_ 

~ 500 

1000 800 

CROSS LINE# 

FIX# 2135 

DISTANCE FROM OAK GZ (FT) 

0 

SLUMPS? 

200 

125 WEST • 

2150 

SLUMPS 

MINOR DOWNWARP(~IOm) 

AND FRACTURING EFFECTS 

BELOW 280m (918ft.) 

V.E.= 1.2X 600~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

0 

-!--= 

LL 

........... 

500 Li:1 

> 

w 

_j 

<( 

w 

(/) 

1000 3: 

g 

w 

CD 

I 

1500 h= 

w 

0 

3500 3000 2500 2000 1500 1000 500 0 500 1000 

8 DISTANCE FROM OAK GZ (FT) 

1000 800 600 400 200 0 200 

DISTANCE FROM OAK GZ (M) 

Figure 30. East-west profile 104V2 through OAK ground zero (GZ). A, Seismic display. B, Interpretation. 

D36 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
22 

122000 E 

DATUM: r.t..LW (APPROX.l 

MAP PROJECTION: UTM 100 100 200 300 U.S.GEOLOGICAL 

SURVEY METERS 

COMPILED: SEPT 1985 !100 1000 

FEET 1!100 2000 

Figure 31. Map of OAK showing location of multichannel seismic-reflection profiles, drill sites, figure numbers for profiles, 

and 1-m bathymetry. 

Structure··Contour Maps 

Reflectors R20, R40, R75, and RIOO were coherent 

enough to map. The single-channel data were used to prepare structure-contour maps for both RIO and R20 (chap. C, 

figs. 13, I7, this volume). Both single-channel and multichannel data were generally in close agreement for these two 

reflectors, except for the area south of ground zero where 

they could be carried much closer toward ground zero on the 

multichannel data. Prior to discussing the R20 structure 

contour map, we will interpret four more key profiles south 

of ground zero (fig. 31). 

The area southwest of OAK ground zero had extremely coherent and continuous RIO and R20 reflectors. 

Line 108Vl (fig. 32), which is parallel to the reef and offset 

I52 m (500ft) toward the lagoon from line IOIV4 (fig. 23), 

shows very clear RIO and R20 reflectors at about 38 and 

45 m (125 and I48 ft), respectively, at the southwest end of 

the line. Beginning at about 750 m (2,460 ft) southwest of 

ground zero, RIO and R20 start downwarping toward 

ground zero to depths of 55 and 62 m (180 and 203 ft), 

respectively, near the OQT -19 drill site at approximately 

400 m (1 ,328 ft) southwest of ground zero. Fragments of 

R 10 and R20 continue at a depth of approximately 80 m 

(262ft) at a distance of 250m (820ft) south of ground zero. 

Very similar R I 0 and R20 reflectors can be observed on the 

next line toward the lagoon, line I23VI (fig. 33). RIO and 

R20 correlate with disconformities "I" and "2" at the ORT20 and OQT-19 wells. Lines 112V1 and 113VI are perpendicular to the reef, crossing the previously discussed line 

108V1 and 123Vl. Line 112 is 415 m (1 ,360ft) southwest 

of ground zero and shows that R 10 and R20 dip toward the 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 037 
~ 

_J 

w 

> w 

_J 

3: 

g 

w 

CD 

I 

....... 

Q_ 

w 

0 

CROSS LINE# 

FIX#I990 

Ill 113 

1985 

1000 500 

CROSS LINE# 

FIX#I990 

DISTANCE SOUTHWEST OF LINE 103 (M) 

Ill 112 113 

1985 

NORTHEAST .. 

114 103 I 

1975 

OP. 

0 

0 

-

~ 

I.J... 

_J 

w 

500 ~ _J 

<X: 

w 

(f) 

3: 

IOOOg 

~ 

I 

....... 

a... 

w 

1500° 

NORTHEAST .. 

114 103 I 

1975 

I 

1980 

i I ORT-20 OOT-19 0,-~----------------~--~~~~-----~~----~--------~--~~ ==~==-=~L===_.S~IEA~~FI.J~OO~R:l1~-- SLUMPS CRATER FLOOR 

OPZ-18 

0 

~ 

_J 100 - --- w 

> w 

_J 

<( 200 - - w 

(f) 

- -RIO - ~-"'- \ R20 - I 2--1 2 -~- ~- t 5 5 --~ --~RI~0~8~R~20~--+-....... 

-.--- \-- R40----- ~~- ~ _? ........__FRAGMENTS??· ••• : ••• 

NOTE: RIO AND R20 12 12 /----- I . START DOWNWARPING MULTIPLE ? BASE OF 

750m (2460 ft) 15 TRANSIENT 

SOUTHWEST OF GZ CRATER 

r 

. ... r 

,_. II 

12 r 

-

R75- ~----~ 16 -R75- -..L 

500 

~ 

I.J... 

_J 

w 

> 

w 

_J 

<( 

w 

(f) 

~ HIGH AMPLITUDE BUT MAJOR ' DISCONFORMITY ~ 3: 

g 300_ VERY IRREGULAR REFLECTORS NUMBERS - ...._ r-IOOO 0-...J 

m w ---- \~ -- - - - ---- - -- - ---==-....---- -- .,..._:::-::::::: w ,..._ - - . --"'-- .,....:-,..--- - m 

I 

5= 400-

- -:::::::...---~-...........: =-----....-----RIOO? --.r ----_....-~ _.,. __ r I 

....... w Q_ 

0 

500 V. E.= 1.2X ~~--.-~--.-.-.-1-r--~-~,-r--r-1-r--r--r--r--r-1~~~~~--~.~ 

8 3000 2000 1000 

DISTANCE SOUTHWEST OF LINE 103 (FT) 

0 

1000 500 0 

DISTANCE SOUTHWEST OF LINE 103 (M) 

NOTE: LINE 108 IS PARALLEL TO REEF AND 152m (500ft) SOUTHEAST OF OAK GZ 

w 

-1500° 

Figure 32. Southwest-northeast profile 1 08V1 at drill site OQT -19, OAK crater. A, Seismic display. B, Interpretation. Line 108 

is parallel to the reef and 152m (500ft) southeast of OAK ground zero (GZ). Major disconformity numbers after B. Wardlaw, written 

commun., July 15, 1985. 

D38 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
CROSS LINE# 

FIX#2455 

~ 

-

~ 100 

> w 

_J 

<l: 200 w 

(/) 

3: 

S3oo 

w 

en 

I 

........ 

a_ 

w 

0 

A 

800 

Ill 105 113 

2000 1000 

DISTANCE SOUTHWEST OF LINE 103 (FT) 

600 400 200 

DISTANCE SOUTHWEST OF LINE 103 (M) 

Ill 112 

I 

OOT-19 

105 113 

2465 

NORTHEAST .. 

114 103 115 

247d 

0 

0 

NORTHEAST .. 

114 103 115 

247d 

~ 

-

_J 100 

w 

--R40 1-

RIO 8 R20 BEGIN 

DOWNWARP 700m 

(2296 ft) FROM GZ 

?... . ... ··~······· 

> w 

_J 

<l: 200 

w 

(/) 

?- 16 

MAJOR DISCONFORMITY • 

NUMBERS 

BASE OF 

TRANSIENT 

CRATER 

500~----~~--~~~~~~--~~~~~~--~-r----

~)000 

8 

2000 1000 

DISTANCE SOUTHWEST OF LINE 103 (FT) 

0 

800 600 400 200 0 

DISTANCE SOUTHWEST OF LINE 103 (M) 

NOTE: LINE 123VI PARALLEL TO REEF AND 213m(700ft) SOUTHEAST OF OAK GZ 

<l: 

w 

(/) 

3: 

g 

w 

CD 

~ lJ_ 

_J 

w 

500 ~ _J 

<l: 

w 

(/) 

3: 

IOOOg w 

CD 

I 

........ 

a_ 

w 

1500° 

Figure 33. Southwest-northeast profile 123V1 at drill site ORT -20, OAK crater. A, Seismic display. 8, Interpretation. Line 123 is 

parallel to the reef and 213m (700ft) southeast of OAK ground zero (GZ). Major disconformity numbers after B. Wardlaw, written 

commun., July 15, 1985. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 039 
CROSS LINE#IIO 

NORTHWEST .. 109 105 ICY2 123 108 124 101 125 I 

FIX# 2275 2280 

-

~ 

-_J 

L&.J 

> L&.J 

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0 

A 

CROSS LINE#IIO 

FIX# 

2000 1000 0 

DISTANCE FROM LINE 101 (FTI 

I I I 

600 400 200 

I 

0 

2275 

DISTANCE FROM LINE IQI(M) 

NORTHWEST .. 

109 105 102 123 108 124 101 125 

~ IOQ-~----=R20 - --R40 RIO.~ t/~ ~ ~ NOTE: RIO AND R20 DOWNWARPED--

2 

r-5QQ W.....J> 

W AT CLOSEST POINT TO GZ 15 

.....J 415m (1360ft) 

~ 200- R75-- - (f) 

~ 

16 

t 

MAJOR DISCONFORMITY 

-----.-

g 300- NUMBERS 

-----

1-

<( 

w 

(f) 

~ 

r-1000 wg w 

CD - _-__ RIOQ__ -=-- _ _________.:-- CD 

I I 

b: 400- ra_ 

w 

0 

w 

""""1500° V.E.=I.2X soo~-----~~~~----~-~~~.--~~.--r-~~--~ 

8 2000 1000 0 

DISTANCE FROM LINE 101 (FT) 

600 400 200 0 

DISTANCE FROM LINE IOI(M) 

-..._ 

lL.. 

-

<X: 

w 

(f) 
reef and reach their greatest depths, 53 and 60 m (174 and 

197ft), respectively, near the crossing of lines 101V4 and 

112 (fig. 34). Line 113Vl is 262 m (860 ft) southwest of 

ground zero and also shows RIO and R20 dipping toward the 

reef (fig. 35). On line 113Vl, two small normal faults can 

be resolved that downfault both RIO and R20 to the northwest to a depth of 78 m (256 ft) before the reflectors terminate in an intensely fractured zone near ground zero. Lines 

108Vl, 123Vl, 112V1, and 113Vl (figs. 31-35) show that 

R 10 and R20 on the southwest side of OAK ground zero 

begin to downwarp at a distance of at least 700 m (2,296 ft) 

from ground zero; this observation is in excellent agreement 

with the single-channel map of R20 (chap. C, fig. 17, this 

volume). While the single-channel data can follow RIO and 

R20 to within 400 m (1 ,328 ft) of ground zero, the multichannel data follow down warped fragments of R 10 and R20 

to within 250 m (820 ft) of ground zero. Closer to ground 

zero, RIO and R20 are intensely fractured and probably 

were partly excavated during the formation of the transient 

crater. A structure-contour map of R20 on the southwest 

side of ground zero summarizes the downwarping and truncation of R20 (fig. 36). 

Reflector R40 correlates with disconformity "8" at a 

depth of approximately 92 m (302 ft) in the OAR-2 and 

OOR-17 OAK reference sites. Mapping of R40 into OAK 

crater was successful to within 300 to 400 m (984-1 ,328 ft) 

of ground zero (fig. 37) where it drops down to a depth of 

approximately 110m (361 ft) before becoming too fractured 

to follow. While the correlation of R40 with disconformity 

"8" was good at OIT -11 (southeast of ground zero on line 

103V2, fig. 22) and at ORT-20 and OQT-19 (southwest of 

ground zero on line 108V1, fig. 32), R40 is 10 to 25 m 

(33-82 ft) too deep on the northeast side of the crater near 

drill sites OFf-8 and OET-7 (line 101V4, fig. 23). Although the discrepancy may be due to the weak, discontinuous character of R 40 along the northeast end of line 10 1 V 4 

(fig. 23), it also may be due to thrusting. The structurecontour map for R40 notes other possible areas where thrust-

. ing may have occurred. Subsequent analysis of the drill-hole 

samples and logs can probably clarify this discrepancy. 

However, for this report, we have noted only where the 

seismic data do or do not agree with the drilling data. 

In summary, R40 comes in to about 300 m (984 ft) 

before becoming too fractured to map. Surprisingly, the 

overlying RIO and R20 group was mapped into 250m (820 

ft) before breaking up. The difference is probably due to 

RIO and R20 being extremely strong formations relative to 

R40 and other reflectors around OAK. The outer limit of 

downwarp for R40 is not as clear as for R20 but varies from 

400 m (1,312 ft) on line 104V2 (fig. 30) to 500 m (1,640 ft) 

on line 103V2 (fig. 22) to as much as 550 m (1,804 ft) on 

the southwest end of line 101V4 (fig. 23). The net vertical 

downwarp for R40 is only from 90 m (295 ft) down to a 

maximum of 115 m (377 ft) within distances of 600 and 

300m from ground zero (1 ,968 and 984ft). Drilling results 

from sites OBZ-4, OPZ-18, OKT-13, and OCT -5 (figs. 

22, 23) have not reported disconformity "8," but have reported "mixed rubble" down to 127 m (415 ft) and 

"stratified, unmixed rubble" down to 229 m (752 ft). The 

shallowest disconformity reported at OBZ-4 is "12" at 

166 m (546 ft), within the "stratified, unmixed rubble." 

Disconformity "8" was 29 m (95 ft) shallower than disconformity "12" in OAR-2, so disconformity "8" and R40 

might be expected at 136 m (446 ft) at OBZ-4, that is, 

within the "stratified, unmixed rubble." The failure to identify disconformity "8" at OBZ-4 may be due to the intense 

fracturing within the "stratified, unmixed rubble." 

The structure-contour map for reflector R75 (fig. 38) 

shows an elliptical depression with R75 downwarped from 

about 210m (689ft) outside the crater down to 230m (754 

ft) beneath ground zero. The elliptical shape with the long 

axis parallel to the reef and the offset of the ellipse 150 m 

( 492 ft) south of ground zero is well defined but not really 

understood. Downwarping of R75 begins at least 400 m 

(1 ,328ft) southwest of ground zero, maybe as far as 700 m 

(2,296 ft). The downwarping of R75 to 230 m (754 ft) in 

OAK is considerably deeper than the downwarping of R75 

in KOA to a depth of 180 m (590 ft). 

The structure-contour map prepared for R 100 shows 

an irregular surface varying between 325 and 350m (1 ,066-

1,148 ft) deep (fig. 39). An irregular, elongate depression 

occurs parallel to the reef with a 10- to 20-m (33-66 ft) 

downwarp. While R75 had a clear 20-m (66 ft) downwarp 

associated with OAK ground zero, the elongate depression 

of R 100 is irregular enough to suggest a combination of 

pre-OAK relief with a 10-m (33ft) or less additional downwarp due to the OAK detonation. 

Structural Summary of OAK Crater Based on 

Multichannel Seismic Data 

The seismic lines over the center of OAK crater show 

a transparent pond of sediments that extends down to 110 m 

(361 ft). In a water depth of 60 m (197 ft, corrected to 

Holmes and Narver datum), this 50-m-thick (164-ft-thick) 

transparent layer is interpreted as chaotically mixed debris 

and postevent sediment fill. Results from drilling this transparent sediment at OBZ-4 indicate that the boundary between "stratified, mixed rubble" and "stratified, unmixed 

rubble" occurred at 110 m (362 ft) (B2 and B3 layers of 

Figure 34 (left). Northwest-southeast profile 112V1 at drill site OQT -19, OAK crater. A, Seismic display. 8, Interpretation. Line 

112 is perpendicular to the reef and 415 m (1,360 ft) southwest of OAK ground zero (GZ). Major disconformity numbers after 

B. Wardlaw, written commun., July 15, 1985. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 041 
CROSS LINE#IIO 

FIX# 

~ 

-_.J 

w 

> w _.J 

<( 

~ 

A 

CROSS LINE#IIO 

FIX# 

~ 

-_.J 100 

w 

> 

w 

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<l: 

~ 

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w 

CD 

I 

1-400 Cl... 

w 

a 

500 

1800 

8 

105 NORTHWEST .. 109 102 123 108 J24 101 125 I 

1000 

DISTANCE FROM LINE 101 (FT) 

400 200 0 

DISTANCE FROM LINE IOI(M) 

109 

-- - ----~ 

-------;:R~IO::o~-:::::;::.- - - - ~ 

----- -------

V.E.=I.OX 

1000 0 

DISTANCE FROM LINE 101 (FT) 

400 200 0 

DISTANCE FROM LINE 101 (M) 

_.J 

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1500a 

NOTE: UNE 113VI IS PERPENDICULAR TO REEF AND 262m(860ft) SW OF OAK GZ 

Figure 35. Northwest-southeast profile 113V1 near drill site OQT-19, OAK crater. A, Seismic display. B, 

Interpretation. Line 113 is perpendicular to the reef and 262 m (860 ft) southwest of OAK ground zero (GZ). 

042 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
<N 

01 

0 

0 

0 

<N 

cr. 

0 

0 

0 

<N 

'-1 

0 

0 

0 

<N 

(X) 

0 

0 

0 

W36'N 

123000~~~-------+--~~~~~-+----~~~~--~~------~--~~~~----~ 

STRUCTURE CONTOUR \ 

--5 M Contours l - - - Inferred Conto~rs 

x x x x x Tracklines 'b 

....&-. Thrust Fault 

(Teeth on overthrust) 

-'- Normal Fault ( Hachu re on downthrown side) 

lfW.MN Reflector Terminates 

- : , Bathymetry ) -- 5M Contours 

1 Oatum,I\IW.W (Approx.) 

) i_) 122000 ~~~~~5~'4~5'~'E--~~----~--L-----LI6-2°-6~'E-L ____ _L ____________ I_6l2°-6~'15L"-E------~----~ 

MAP PROJECTION: UTM 100 

HH~ 

100 200 300 -400 500 600 U.S. GEOLOGICAL 

SURVEY METERS 

COMPILED: SEPT 1985 500 500 1000 1500 2000 

FEET 

Figure 36. Structure-contour map, reflector R20, OAK crater. Thrust faults are dashed where inferred. GZ=ground zero. 

B. Wardlaw, written commun., July 15, 1985). The lateral 

extent of the transparent sediment layer is obscured by side 

echoes and slumps near the rims of the crater but appears to 

be between 400 and 500 m (1,328-1,640 ft) from ground 

zero on lines 103V2 and 101V4 (figs. 22, 23). The base of 

this transparent sediment layer at 110m (361 ft) is inferred 

to be the base of the transient crater. 

A zone of intense fracturing extends from the base of 

the transparent sediments at 110m (361 ft) down to approximately 180m (590ft) in the center of OAK crater (figs. 22, 

24). Within this zone, the downwarped edges of reflectors 

R10, R20, and R40 become too intensely fractured to map 

within 250 to 400 m (820-1 ,328 ft) of OAK ground zero. 

Beyond the zone of intense fracturing, downwarping 

of coherent sedimentary reflectors can be observed at varying distances from and depths below OAK ground zero. 

Downwarping of the R10 and R20 reflectors was demonstrated with the single-channel seismic data which showed 

that some effects of cratering could be observed out to 

750 m (2,460 ft) to the southwest and 900 m (2,952 ft) to 

the northeast (chap. C, fig. 13, this volume). The singlechannel data showed downwarping of R10 and R20 continuing to within 400 to 450 m ( 1 ,312-1 ,4 76 ft) of ground zero 

where the reflectors were either truncated or too deep to be 

detected. The multichannel seismic system confirmed these 

data, but especially southwest of ground zero, the system 

was able to trace R20 to within 250 m (820 ft) of ground 

zero. At distances of more than 750 m (2,460 ft) southwest 

of ground zero, R20 occurs at approximately a 40-m (133 ft) 

depth (fig. 36). The multichannel seismic lines parallel to 

the reef show that downwarping of R10 and R20 begins 

between 550 and 750 m (1 ,804-2,460 ft) from ground zero 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 043 
(1'1 

(1'1 

0 

0 

0 

,. 

0 

0 

162°6'30•E g r-lt-----~-~-H-T-0-----.------------------------------------------~ 128000 

REFLECTOR 40 

~THRUST FAULT (TEETH ON OVERTHRUST SlOE) 

11 °36'30•N 

-JIIII- '\N LACK OF RESOLUTION 

- - - Inferred Cantours j 

STRUCTURE CONTOUR• 

- 511 Contours 

X X X X X X TrackJJnel 

Bathymetry: 

-- 5 M Contours 

Datum, MLLW (Approx.) 

IVY GRID (ft) 

162°6'E 

xx 0 

X X 

Q ~-

() 

11°36 '30•N 

125000 

)~ 0 

~ 

~ 11 °36'N 

·0 /< 

tfJ 

0 

!,) 

-~ () F 

o· ~ 

121000 

162°6'30•E 

MAP PROJECTION: UTM 1 00 0 1 00 200 300 400 1500 600 700 800 !NlO 1000 U.S. GEOLOGICAL 

SURVEY METERS 

COMPILED: SEPT 1985 0 1500 1 000 1 1500 2000 21500 3000 

FEET 

Figure 37. Structure-contour map, reflector R40, OAK crater. Thrust faults are dashed where inferred. GZ=ground zero. 

(figs. 23, 24, 32, 33). A gradual downwarp occurs from 

40 m (131 ft) in the southwest to a depth of 80 m (262 ft) at 

250m (820ft) from ground zero where R20 is either truncated or obscured by fracturing. It is important to note that 

only a few small faults (1-2 m, or 3-6 ft) were observed to 

disrupt RIO and R20 over this broad zone of downwarp. 

Downwarping at the next deeper level can be observed with reflector R40 which shows downwarp effects 

beginning between 400 and 500 m {1,312-1,640 ft) from 

ground zero (figs. 29, 30). R40 drops from a level of approximately 95 m (312 ft) outside the crater to a depth of 

115m (377ft) at a distance of 300m (984ft) from ground 

zero before becoming too fractured to map (fig. 37). 

The next deeper level of downwarp can be mapped on 

reflector R75 which occurs at a depth of approximately 

210m (689ft) outside the crater (fig. 38). R75 downwarps 

to 230m (754ft) near OAK ground zero. R75 begins downwarping at distances up to 350 m (1,148 ft) southwest of 

ground zero (fig. 38). 

The deepest level of possible downwarped reflectors 

can be inferred from reflector R 100 which occurs at a depth 

of 325 to 350m (1 ,066-1,148 ft) outside the crater (fig. 39). 

Approximately 10m (33ft) of downwarp are suggested on 

R95 in the 80-in3 watergun line parallel to the reef over 

OAK ground zero (line 101 V5, fig. 10) and on RlOO in the 

15-in3 watergun line over the same location (fig. 23). Inspection of line 103V2 perpendicular to the reef (fig. 22) 

shows clear downwarp of reflectors to approximately 280 m 

D44 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
(1'1 

(1'1 

0 

0 

0 

,.. 

0 

0 

0 

0 

--~------------~-------------------------------------------. 128000 t DEPTH TO 

11 °36'3Q•N 

11 °36'N 

-JIIIII- REFLEClOR 75 

I 

STRUCTURE C<W1'lXR --5M Contours 

- - - Inferred Contours 

x x x x x Tracklines 

Bathymetry: 

-- 5 M Contours 

Datum, MLLW ( Approx.l 

11 °36 '3Q•N 

11 °36'N 

MAP PROJECTION: UTM 1 oo o ·1 oo zoo 100 400 eoo AI 100 too !NIO 1 ooo U.S. GEOLOGICAL 

SURVEY ~TERS 

COMPILED: SEPT 1985 eoo o 1100 1000 11100 2000 2t100 JOOO 

FEET 

Figure 38. Structure-contour map, reflector R75, OAK crater. GZ=ground zero. 

(918 ft), and this has been interpreted as the base of 

"moderate fracturing and depression." The possible 10-m 

(33 ft) downwarp of RlOO is getting down within the random natural variations in depth to R 100 outside the crater 

area. Therefore, the maximum depth of moderate downwarp 

or depression effects, that is, those downwarps greater than 

10 m (33 ft), is 280 m (918 ft). 

The check-shot velocities from the OAK ground zero 

site, OBZ-4, also show that the interval velocities at ground 

zero are 10 to 30 percent slower than the reference sites 

down to a depth of 300m (984ft). Because the check-shot 

interval velocities were averaged over the 250- to 300-m 

(820-984 ft) interval, the 280-m (918ft) depth of"moderate 

fracture and depression" observed on line 103V2 is consistent with the check-shot data. A depth of 280m (918ft) is 

considered the best overall estimate of damage effects due 

to the OAK event, on the basis of a combination of multichannel and check-shot velocity data. 

The preliminary drilling results indicate a slightly 

deeper level of downward displacement at 325 m (1 ,065 ft) 

with "undisplaced, fractured" rock extending down to 

340m (1,115 ft) (B. Wardlaw, written commun., July 15, 

1985). The overall consistency between the seismic and 

drilling results from OAK crater is excellent. While the 

drill-hole data are generally the most accurate absolute measure of depth, the seismic data still provide the best relative 

measure of trends between the drill holes. 

Multichannel Seismic-Reflection Survey of KOA and OAK Craters 045 
~ 

~ 

0 

0 

0 

~ ~ 

~ 0 0 0 

8 162°6 'E 162°6 '30"E 8 r-~--------------~----~~------------------------------------~ 128000 t DEPTH TO 

11°36 '30"N 

11 °36'N 

-r-1- REFLEClOR 100 

I 

STRUCTURE CONTOUR 

--5M Contours - - - Inferred Cant urs x x x x x TrackliMs 

Bathymetry: 

---- 5 M Contours 

'Datum, MLLW ( Approx.l 

11 °36 '30"N 

11 °36'N 

o· ~--~~~--~--~~----~--~~~--~~~~~--~~----------~~ 121000 

MAP PROJECTION: UTM 100 0 100 200 300 400 500 600 700 800 900 1000 

METERS 

COMPILED: SEPT 1985 500 0 500 1000 1500 2000 2500 3000 

FEET 

Figure 39. Structure-contour map, reflector Rl 00, OAK crater. GZ=ground zero. 

REFERENCES 

U.S. GEOLOGICAL 

SURVEY 

Gray, W. C., 1979, Variable norm deconvolution: Palo Alto, Calif., Stanford University, Ph.D. 

thesis. 

Ristvet, B. L., Tremba, E. L., Couch, R. F., Fetzer, J. A., Goter, E. R., Walter, D. R., and 

Wendland, V. P., Geological and geophysical investigations of the Eniwetok nuclear craters: 

Air Force Weapons Laboratory Technical Report TR-77-242, Kirtland Air Force Base, New 

Mexico 87117, 263 p. 

Tremba, E. L., Couch, R. F., Ristvet, B. L., 1982, Enewetak Atoll Seismic Investigation (EASI): 

Phases I and II (final report): Air Force Weapons Laboratory Technical Report TR-82-20, 

Kirtland Air Force Base, New Mexico 87117, 124 p. 

046 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Chapter E 

Seismic-Refraction Survey of OAK 

Crater 

By H. D. ACKERMANN, J. A. GROW, and J. M. WILLIAMS 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction El 

Purpose El 

Setting El 

Previous refraction work El 

Methods E2 

Results E3 

Refraction sideswipe E4 

Shallow crater structure-horizons 1, 2, and 3 ES 

Deep crater structure-horizons 4 and 5 E9 

A generalized crater model El3 

References El8 

FIGURES 

1. Maps showing locations of refraction lines R1 (reef line) and R2 (lagoon line) and 

hydrophone positions at OAK crater E2 

2. Drawing showing sonobuoy mooring configuration E3 

3-5. Seismograms for: 

3. Recording station A3 from 80-in3 watergun E4 

4. Recording station A3 from 540-in3 airgun ES 

5. Recording station B23 showing worst case of crossfeed between 

channels E6 

6. Velocity section for horizons 1, 2, and 3 at OAK crater interpreted from reef-line 

data E6 

7. Diagram showing two hypothethical ray paths, one of which transects crater-fill 

material and another that passes beneath it E7 

8. Graph showing extrapolated travel-time curves for horizon 3 from all the receiver 

stations on the reef line E7 

9. Interpretation of figure 6 E8 

10. Velocity section for horizons 1 and 3 at OAK crater interpreted from the lagoonline data E9 

11-18. Graphs showing: 

11. Travel-time curves for horizon 4 ElO 

12. Profile of horizon 4 southeast of OAK crater Ell 

13. Profile of horizon 4 assuming its velocity to be 2.8 km/s El2 

14. Profile of horizon 4 assuming that its velocity changes abruptly from 

2.8 to 2.25 km/s 800 m from ground zero El3 

15. Profile of horizon 4 assuming that its velocity changes gradually 

from 2.8 km/s 850 m from ground zero to 2.4 km/s at ground 

zero El4 

16. Profile of horizon 4 modified from that of figure 15 by introducing a 

southeast dip toward the lagoon El5 

17. Travel-time curves for horizon 5 El6 

18. Profile of horizon 5 computed from the travel-time curve of figure 17 El7 

19. Final interpreted model of OAK crater showing four zones in which velocities 

have been altered by the OAK event El7 

TABLE 

1. Interval velocities measured in boreholes at the OAK ground zero site and the 

reference site Ell 

Contents Ill 

Chapter E 

Seismic-Refraction Survey of OAK Crater 

By H. D. Ackermann1, j. A. Grow2, and j. M. Williams1 

INTRODUCTION 

Purpose 

Two refraction lines were included in the data collected at OAK crater. These refraction measurements were 

augmented by stratigraphic data acquired by multichannel 

seismic-reflection techniques aboard Egabrag II and then by 

down-hole velocity data acquired aboard the drilling vessel. 

We anticipated that refraction measurements would 

provide critical data concerning the velocity structure in the 

OAK crater area in comparison to the velocity structure of 

the undisturbed atoll. We hoped, thereby, to improve on the 

refraction data collected as part of project EASI (Enewetak 

Atoll Seismic Investigation) (Tremba and others, 1982) by 

using a variety of sound sources such as 15-in3 and 80-in3 

water guns, a 540-in3 airgun, and numerous recording channels. 

Setting 

Although a low sea state is ideal for profiling, interpretation of seismic data collected in an atoll is difficult 

because of the discontinuous and porous nature of the sedimentary strata. The level of detail desired to decipher crater 

structure requires closely spaced source and receiver points. 

OAK crater was formed on the edge of the reef plate, half 

in consolidated reef plate sediments and half in unconsolidated to poorly consolidated lagoonal sediments. This geometry resulted in refraction sideswipe from the reef front 

and possibly from the steep crater walls. 

Previous Refraction Work 

Raitt (1957) carried out the first refraction measurements at Enewetak Atoll. The measurements comprised four 

lines in the atoll and five outside. One unique characteristic 

of these data was that receiving geophones for several lines 

were set 76 m (250 ft) below the surface in the hole drilled 

at Elugelab Island (Ladd and others, 1953), which was later 

destroyed by the MIKE detonation. Six layers were identified with the following velocities: 2.44, 3.06, 4.15, 5.59, 

IU.S. Geological Survey, Branch of Regional Geophysics, Denver, 

Colo. 70225. 

2U.S. Geological Survey, Branch of Oil and Gas, Denver, Colo. 70225. 

6.90, and 8.09 km/s (8,000; 10,000; 13,600; 18,400; 

22,600; 26,500 ft/s). The first two layers correlated with 

carbonate rock overlying a volcanic third layer which had 

been drilled both on Elugelab and Medren (Elmer) Islands 

(chap. A, fig. 1, this volume) (Ladd and others, 1953; Ladd 

and Schlanger, 1960). Raitt's shallow-depth results will be 

discussed in more detail near the end of this section. 

No further refraction data were collected until 1971-

72 when approximately 15 km (8 nmi) of lines were run 

mostly on Aranit (Aomon, or Sally) and Runit (Yvonne) 

Islands (Henny and others, 1974) as part of Project PACE 

(Pacific Atoll Cratering Experiment). Subsequently, approximately 76 km (41 nmi) of refraction lines were run on 

12 islands in the atoll, and 30 km (16 nmi) of marine data 

were collected in various craters plus one 1 ,000-m (3,280 ft) 

line off Enjebi (Janet) Island as part of project EXPOE 

(Exploratory Program on Enewetak) (Ristvet and others, 

1978). Two of these lines were shot in KOA-MIKE craters, 

but they defined only the top surface of one layer below the 

sea floor. None were run at OAK crater, but several lines 

were shot on the islands nearest the crater, Biken (Leroy) 

and Bokoluo (Alice) (Ristvet and others, 1978). 

The last refraction data collected prior to this study are 

five lines in OAK and KOA craters and off Enjebi (Janet) 

Island as part of Project EASI (Tremba and others, 1982). 

Three zones were defined below OAK crater by combining 

refraction and reflection data. The first, from the crater floor 

to 87 m (285ft) below it, has a velocity of 1,530 m/s (5,000 

ft/s); the second, extending to a depth of 141m (462ft), has 

a velocity of 1,770 m/s (5,800 ft/s); and the third, and 

deepest, has three zones ranging in velocity from 2,010 to 

2,440 m/s (6,600-8,000 ft/s). (See fig. 30, Tremba and 

others, 1982.) A similar model was developed for KOA 

(Tremba and others, 1982). 

In the shallow sediments, Raitt (1957) determined a 

lagoonal velocity function which increases from 1. 9 kmls 

(6,200 ft/s) in reef detrital material to about 2.4 kmls (7 ,900 

ft/s) in partly consolidated calcareous sediments at a depth 

of about 106 m (348 ft); another increase to 3.0 kmls 

(10,000 ft/s) is observed at a depth of about 600 m (1 ,969 

ft), also in partly consolidated calcareous sediments. In 

marked contrast to these results, Henny and others (1974) 

showed that the onshore velocities of the reef plate surrounding the lagoon may exceed 3.0 kmls (10,000 ft/s) at 

Seismic-Refraction Survey of OAK Crater E1 
only 50 m (160ft) of depth compared to 700 m (2,300 ft) 

in the lagoon as determined by Raitt. 

Reynolds (see appendix B, Tremba and others, 1982) 

did a refraction study both at OAK crater and at a nearby 

reference area using a sparker sound source and two hydrophone receivers. He concluded that within 60 m (200ft) of 

the sea surface, the reference area contains several stringers, 

or solution unconformities, each only a few meters thick, 

which are cut off within OAK crater. In addition, the velocity within the crater to a depth of about 150 m (490 ft) is 

slightly less than that at the reference site, whereas below 

150m, the velocities at the crater are, if anything, slightly 

greater than those at the reference site. 

MAP PROJECTION ' UTM SPHEROID' CLARK 1866 

COMPILED' MAY 1985 

+ 

+ 

+ 

+ 

1000 

+ 

+ 

0 

0 

,.., 

~ 

162°6'E 

250 250 

1000 

METHODS 

Sites for deployment of hydrophones were selected on 

the basis of bathymetric and seismic data collected during 

Project EASI (Tremba and others, 1982). These sites lay 

along two refraction lines passing close to OAK ground 

zero. The longer line, 3. 8 km (or 2.1 nmi), of 23 sonobuoy 

sites lay roughly parallel to the reef (reef line R1), and the 

shorter line, 1.5 km (or 0.8 nmi), of 7 sonobuoy sites was 

perpendicular to it (lagoon line R2) (fig. 1). Source-point 

positions extended several kilometers beyond the ends of the 

reef line and also beyond the southeast end of the lagoon 

line. 

500 METERS 

2000 

FEET 

+ 823~Refraction line Rl + 

+ + DATUM: MLLW (APPRO)(.) 

IVY GRID (II 

+ + 

+ 

+ + + 

REFRACTION STATIONS 

• Moored sbnobuoy locations 

0 

0 

~ 

" 16~6'30"E 162°7'E 

U.S. GEOLOGICAL SURVEY 

750 1000 1250 

3000 4000 5000 

Figure 1. Locations of refraction lines R1 (the reef line) and R2 (the lagoon line) and hydrophone positions at OAK crater, 

Enewetak Atoll. GZ=ground zero. 

E2 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Sonobuoys were attached to Norwegian floats 0.6 m 

(2ft) in diameter that were taut-line moored with polypropylene line 1 em (3/8 in.) in diameter to anchors weighing 

about 35 kg (77 lb). Buoys not within about 40 m (130ft) 

of the desired site were repositioned and resurveyed. The 

configuration of a mooring is depicted in figure 2. 

The recording systems comprised several components. Refraction Technology (Ref Tek) 18L lowfrequency, analog, reusable radio sonobuoys were used for 

transmission of seismic data to the research vessel. Companion HS-1 FET (Field Effect Transistor) suspension hydrophones coupled the acoustic pressure wave to the radio 

telemetry system. Each buoy had an associated whip antenna cut to length for the transmission frequency of each 

channel. The adjustable gain of each buoy was set at 0 dB. 

In areas of shallow water, cables from sonobuoys to hydrophones were shortened to keep hydrophones off the bottom. 

Maximum length of the cables was 18.3 m (60ft). 

The sonobuoy receiving system consisted of a dual 

high-gain directional antenna that enabled reception of radio 

signals while approaching or departing each sonobuoy along 

the line. The signal was amplified by 20 dB prior to a Ref 

Tek-64 antenna distributor which split the radio signal to 

three Ref Tek-20 receiver mainframes, each of which incorporated 10 RefTek-19 receiver modules. The receiver modules were configured to minimize any adjacent receiver 

crosstalk. The seismic analog data were monitored using a 

galvo-recorder and recorded digitally by a 24-channel Texas 

Instrument DFS-V seismic data acquisition system in SEGB nine-track magnetic tape format. 

The survey was conducted over a period of 3 days, 

during which three passes were made along the two lines 

(fig. 1) with 15-in3 and 80-in3 water guns and a 540-in3 

airgun towed by Egabrag II at approximately 3 kn (150 

cm/s). Shot-point spacings were approximately 6 and 12m 

(20 and 40ft) for the 15-in3 and 80-in3 water guns and 37m 

(120 ft) for the 540-in3 airgun. 

Ship position was fixed every 20 seconds with the 

Motorola Falcon IV miniranger system. Because there was 

no provision for recording fix positions on the data tapes, 

shot locations were obtained at regular intervals by manually correlating fix positions with field file numbers 

recorded on tape headers. Final shot positions were obtained 

by interpolation from a 1 :6,000-scale map of fix positions. 

Seismic-refraction record sections for each receiving 

station were produced at the U.S. Geological Survey 

(USGS) processing center in Denver, Colo. A 100-ms automatic gain control operator, which effectively suppressed 

background noise prior to the onset of initial refraction arrivals, was applied to each seismic trace. Both filtered and 

unfiltered versions of the data were generated, and both 

were used to pick arrivals. 

The three different energy sources (15- and 80-in3 

waterguns, 540-in3 airgun) were used to provide a broad 

range of choice between frequency spectrum, signal ampliFigure 2. Sonobuoy mooring configuration. 

tude, and data density. In the final analysis, the data from 

the 540-in3 airgun were superior despite the gun's lower 

repetition rate which resulted in fewer traces per recording 

station and a lower data density. The rapid signal attenuation 

due to the porous nature of the coral sediments necessitated 

a large amplitude signal. The signal from the water guns 

contained a 30-ms-long precursor to the main pulse, causing 

difficulty in delineating initial onsets. The thin solution unconformities (Tremba and others, 1982) were transparent to 

all three energy sources. 

Data quality was generally good. Typical examples of 

seismograms, in compressed form, are shown in figures 3 

and 4. The most severe problem encountered was crossfeed 

between channels on the northeast end of the reef line resulting in some loss of data. The most severe case is shown in 

figure 5. Independent measurements of sonobuoy receiver 

locations showed a drift of roughly 30m (100ft), but relative locations with respect to source positions could be determined within a few meters. Another complication was 

that the track of the recording vessel did not lie precisely 

along the line of sonobuoys, resulting in a few mis-ties on 

reciprocal times along the reef line by as much as 15 to 

20 ms. The distance between the source and individual 

sonobuoys was most often less than 50 m (160ft). Stations 

A2, A8, and B15 malfunctioned. Such problems with individual channels were readily overcome by the large quantity 

and redundancy of collected data. 

Large numbers of recording stations and closely 

spaced shot points are essential in areas of varying subsurface conditions such as those at OAK crater. Methods of 

interpreting such closely packed and overlapping data sets 

fall into the realm of engineering seismology (Ackermann 

and others, 1982). 

RESULTS 

The computer-processed results are presented as 

depth-distance diagrams, velocity sections which show layers of increasing velocity that may vary laterally (fig. 6). 

The upper boundary of each layer represents the path of a 

Seismic-Refraction Survey of OAK Crater E3 
2.00 SE 

(f) 

~ 1.00 ~ 

1--

0.00 .00 

Figure 3. Seismogram for recording station A3 (fig. 1) from 80-in3 watergun. 

seismic ray that propagated from the source point to a receiver along a subsurface velocity discontinuity where the 

velocity is assumed to increase abruptly with depth. This 

upper boundary is called a horizon. Horizons represent ray 

paths which can cut across or transect geological or physical 

layers, as in OAK crater where ray paths cut across the 

crater fill (fig. 7), because it has approximately the same 

velocity as the fractured and porous rocks on the flanks. 

Velocity sections show the paths of rays; therefore, the 

shape of the crater could not be resolved from the refraction 

results. Information about the nature of the rocks or material 

along the ray paths could be deduced from the value and 

variations in velocity along them. 

Refraction Sideswipe 

At an apparent depth of about 600 m (2,000 ft), the 

ray velocity for the reef line is about 4.3 km/s (14,000 ft/s) 

compared to only 2.9 km/s (9,500 ft/s) beneath the lagoon 

line, suggesting an unrealistic anisotropy ratio of 32 percent. A velocity of 4.3 km/s implies dense rocks, perhaps 

limestone or fractured intrusive or lava flows, but not porous 

coral reef sediments found beneath back-reef or lagoon. 

However, the work of Henny and others (1974) proves that 

velocities within the reef are substantially higher than those 

in the lagoon (Raitt, 1957) and at depths of several hundred 

meters onshore may attain values of 4.3 km/s (14,000 ft/s). 

Because the reef line was offset only a short distance from 

the reef edge, the apparent discrepancy is readily resolved; 

it appears that the 4.3-km/s (14,000 ft/s) velocity simply 

represents sideswipe from a wave refracted along the reef 

edge. The reef-edge sideswipe originated from shallow 

depths, perhaps less than 100 m (330 ft). On land, experience has been that refraction sideswipe from steep discontinuities is seldom recorded because the direction of the returning wave is essentially perpendicular to the geophone 

orientation. Hydrophones, however, respond equally well to 

horizontal and vertical pulses. 

This result severely limited analysis of the reef-line 

data. Reef-edge refractions arrived at the recording stations 

ahead of the slower arrivals from beneath the crater, resulting in the hidden-layer problem. Efforts at solving this problem were virtually fruitless. Therefore, the major portion of 

the interpretations for horizons deeper than the approximate 

E4 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
3.00 3.00 

2.00 2.00 

1.00 1.00 

0.00 

'....-----3700 M------. 

12,139 Ft. 

Figure 4. Seismogram for recording station A3 (fig. 1) from 540-in3 airgun. 

transient crater floor were obtained from the lagoon line 

using the six operational hydrophones. Data from the reef 

line, with its greater concentration of receiver stations and 

hence greater resolving power, were used to determine the 

shallower horizons. 

Shallow Crater Structure-Horizons 1, 2, and 3 

Signal returns were obtained from five horizons; that 

is, five critically refracted ray paths were observed. Computed ray paths and velocities for horizons 1, 2 and 3 of the 

lagoon line are shown in figure 6. Although horizons are 

bowed toward the crater interior, the true crater structure is 

not evident because of the small contrasts in velocity between undisturbed materials outside the crater area and 

those materials within the crater. 

No returns were observed from the sea floor, indicating that the velocities of the porous shallow reef deposits 

outside the crater area and the shallow debris beneath the 

crater floor are indistinguishable from the velocity of sea 

water (about 1.54 km/s, or 5,050 ft/s). The initial critically 

refracted ray (horizon 1) outside the crater area was recorded 

from subsea-floor depths of between 5 and 30m (16-100 ft) 

with propagation velocities between 1. 7 and 1. 8 krnls 

(5,600-5,900 ft/s). Beneath the crater floor, it was recorded 

from 10 to 20m (30-70 ft) with velocities between 1.6 and 

1.7 km/s (5,200-5,600 ft/s). The difference in velocity is 

small and almost indistinguishable. Data from horizon 2 

were observed only on the northeast side of ground zero, 

with velocities between 1.8 and 1.9 km/s (5,900-6,200 

ft/s). The third recorded critical ray again has remarkably 

similar propagation velocities both outside and inside the 

crater-2.0 to 2.3 km/s (6,500-7 ,500 ft/s) outside and 1.8 

to 2.1 km/s (5,900-6,900 ft/s) inside. At ground zero, the 

path of this ray agrees well with the estimated base of the 

floor of the transient crater. (See chap. D, this volume.) 

Because of the lack of contrast in velocities, the minimumtime ray path for horizon 3 does not follow the crater floor 

but grazes it and travels through deeper coral-reef deposits. 

The reversed travel-time curves, including extrapolations (Ackermann and others, 1982) from which horizon 3 

Seismic-Refraction Survey of OAK Crater E5 
823 Nf sw 

w 

~ 

1.00 

..__ 0.50 

0.00 

Figure 5. Seismogram for recording station B23 (fig. 1) showing the worst case of crossfeed between channels. 

sw 

85 89 811 813 819 

81 82 83 84 86 87 88 810 812=GZ 814 816 817 818 820 821 822 

.. ' ''' .... ',, . ' .... ' 

SEA FUDOR SEA LEVEL 

823 

• 

::E 

z 

0 

~ZON I "'1.54 • 1.54 .~----...::::::===---

1.8 ~~ ~ 1.7 ......__ 

~ > 

UJ 

.....J 

UJ 

HORIZON3~ 1.6 ·~· 1.6 1.~ 

2.0 ~ 1.7 HORIZON 2 

1.9 ----. 1.8 / -~·~- . 2.2 2.3 

1.8 '· _ _____, 1.9 ...._...- 20 2.1 . 

V.E.= 8.6X 

VELOCITIES IN KM/5 

-1600 -1200 -800 -400 

Figure 6. Velocity section for horizons 1, 2, and 3 at OAK crater interpreted from the reef-line data (line R1, fig. 1 ). Solid 

circles indicate boundary between velocity values. GZ=ground zero. 

NE 
SOURCE PO I NT 

1P·--------------------------------------------------------SEALEVEL 

.-----SEA FLOOR 

Figure 7. Two hypothetical ray paths, one of which transects crater-fill material and another that passes beneath it. 

SW NE 

81 82 

o~~~~--.. --~~~~~~~~~~-L~~~~~--~~-L--~~ 

O=GZ 1000 2000 

DISTANCE (M) 

Figure 8. Extrapolated travel-time curves for horizon 3 from all of the receiver stations (B 1-B23) on the reef line (fig. 1 ). Note the 

increase in delay time toward ground zero (GZ) (sta. B12). 

was computed, are shown in figure 8. Note the steady increase in delay time approaching station B 12 at ground zero 

where this ray path is approximately tangent to the transient 

crater floor. 

Figure 9 shows a diagrammatic interpretation of the 

physical significance of the ray paths of figure 6. Line AB 

outlines the approximate excavated zone based on a depth of 

120 m (390ft) at ground zero. (See chap. D, this volume) 

Seismic-Refraction Survey of OAK Crater E7 
sw NE 

85 89 811 813 819 

81 82 83 84 86 87 88 810 812=GZ 814 816 817 818 820 821 

'1400 

822 823 

0 ' ' ' ''' ' .. ''' '' ' ' ' ' ' ' ' ~ \ 

- 1.5 \ 

z -.............. 0 1.8 

t-

~ ~ 

_J 

~ 

-120 

-160 

V.E = 9.6X 

VELOCITIES IN KM/5 

A SEA LEVEL 

F SEA FLOOR 

/HORIZON I / 

HORIZON 2 

~RIZON3 2.2 2.3 

-1600 0 

OISTANCE(M) 

Figure 9. Interpretation of figure 6. Line AB encloses the initial excavation. Line CD estimates preevent horizon 3, and the x's 

below CD indicate highly fractured rock, reduced in velocity from preevent values of 2.2 to 2.3 km/s (7,200-7,550 ft/s) down to 

1.7 to 1.9 km/s (5,600-6,250 ft/s). Lines CE and DF extrapolate this fractured zone to the sea floor. Solid circles indicate boundary 

between velocity values. GZ=ground zero. 

and a maximum crater radius of about 600 m (2,000 ft). (See 

chap. A, this volume.) The ray representing horizon 1 warps 

downward beneath the crater with little change in velocity, 

conforming more to depth of the sea floor than to any other 

crater characteristic. Horizon 3, on the other hand, begins to 

warp downward well outside the probable transient crater 

and is tangent to it below ground zero. Dashed line CD was 

drawn to estimate the subhorizontal preevent horizon 3. A 

hypothetical preevent ray path would have approximately 

followed path CD, whereas the present minimum-time path 

bends beneath it, due not only to debris deposited in the 

bowl, but also to ruptures and fractures in wedges flanking 

both sides of the crater. Marked by x' s in figure 9, these 

wedges, composing a ring or halo when viewed in three 

dimensions, constitute a large area, or volume, in which the 

blast phenomena lowered velocities from over 2 krn!s 

(6,600 ft/s) to 1. 7 or 1.8 krn!s (5,600-5,900 ft/s). These 

wedges are projected toward the surface by dashed lines CE 

and DF and intersect the sea floor above the zone in which 

the velocity of horizon 3 reaches 2.2 to 2.3 krn!s (7 ,200-

7,500 ft!s); this velocity suggests the possibility that nearsurface fractures may extend as far as 1,400 m (4,600 ft) 

from ground zero. Lines CE and DF are permitted to transect the layers below the sea floor on the assumption that 

additional fracturing only negligibly affected their initial inplace velocity, which was not much greater than that of sea 

water. Fracturing extending to a considerable distance from 

ground zero is also suggested by a reference boring which 

encountered fractured rock at distance of 1,325 m (4,347 ft) 

from ground zero (A. Schenker, oral commun., May 1985). 

If blast-induced fractures exist at distances of over 

1 ,000 m (3 ,300 ft) from ground zero, then the possible 

significance of the lateral increase in velocity of horizon 3 

from 1.9 to 2.1 krn!s (6,200-6,900 ft/s) beneath ground 

zero to 2.2 to 2.3 krn!s (7 ,200-7,500 ft/s) under station 

locations B21 and B22 1,400 m (4,600 ft) northeast of 

ground zero must be evaluated. Is the lower velocity around 

the crater the result of blast-induced fractures, or is it due to 

the laterally varying nature of the coral reef deposits? Data 

from several sources suggest that 2.2 to 2.3 krn!s (7 ,200-

7,500 ft/s) is the in-place velocity of coral deposits in this 

area, approaching the velocity (2.4 krn!s or 7,900 ft/s) measured by Raitt (1957) and Tremba and others (1982); and the 

E8 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
lower velocities of I.8 to 2.I km/s (5,900-6,900 ft/s) are the 

result of blast-induced fractures. 

On the northeast side of the crater, the shallow-depth 

data from the reef line clearly indicate a slight increase in 

velocity with depth between horizons I and 3. (See horizon 

2 in figs. 6 or 9.) The data are inadequate for attaching any 

other physical significance to them. 

The shallow horizons beneath the lagoon line, computed from the 540-in3 airgun data, are shown in figure IO. 

Although depth and velocity values on this line approximately agree with those on the reef line at ground zero, these 

data could not be interpreted in the same detail as were the 

reef-line data (figs. 6, 8) because only six operational hydrophones at larger intervals were emplaced there. 

Deep Crater Structure-Horizons 4 and 5 

Crater analyses for depths below I20 m (390ft) were 

relegated to the lagoon-line data due to the interference by 

refraction sideswipe along the reef line. However, as shown 

NW SE 

1.54 

10 

< 

• 

SEA LEVEL 

~ SEA FLOOR 

~.5~RIZON I 1.8 • 1.7 

0 400 800 1200 

DISTANCE(M) 

Figure 10. Velocity section for horizons 1 and 3 at OAK Crater 

interpreted from the lagoon-line data (line R2, fig. 1 ). Solid 

circles indicate boundary between velocity values. 

GZ=ground zero. 

in figure I, the lagoon line could not be extended northwest 

of the crater, resulting in only single-ended (unreversed) 

seismic coverage from the deep structures beneath the 

crater. Therefore, an infinite number of solutions satisfying 

the data from greater depths are possible. The range of 

solutions was constrained to a large degree by the drilling 

results, which are described in some detail through the remainder of this section. 

The travel-time curves for horizon 4 are shown in 

figure II, where curves in parentheses and labeled A I 

through A 7 are from receiver positions AI through A 7. The 

travel-time curve in parentheses and labelled X represents 

the arrival times at all the six recording stations obtained 

with the source point at I, 790 m (5,870 ft) on the distance 

axis. The posted inverse slopes of curves A I through A 7 and 

the slope for curve X southeast of receiver position A6 

suggest a subsurface ray velocity of approximately 2.8 km/s 

(9,200 ft/s)-or a horizon of negligible dip southeast of the 

crater with a velocity of about 2.8 km/s. An increased slope 

of curve X is observed northwest of receiver position A6 

from the posted values of lower apparent velocity (fig. I1), 

suggesting that OAK crater and the fractured rocks around 

it have disrupted this travel-time curve. 

Subsurface velocity distributions must satisfy curve X 

and curves AI through A 7. Through an iterative process, we 

obtained velocity distributions that also satisfy the velocity 

survey of the boring at ground zero. The process is simplified by first extrapolating both curve A 7 and curve X to their 

origin points at 860 and 1, 790 m (2,820-5,870 ft), respectively, with the inverse slope of2.8 km/s (9,200 ft/s). These 

curves are labeled in figure II as Y and Z, respectively. Any 

solution that simultaneously satisfies curves X, Y, Z, and 

A 7 will also satisfy curves AI through A6. 

Curves Y, Z, and A 7, now representing reversed 

data, may be solved uniquely using the overlying velocity 

distribution of figure IO. The computer-generated output is 

shown by the horizontal row of open circles in the bottom 

right side of figure I2, representing a horizon or ray path 

approximately 300 m (1 ,000 ft) below sea level with a 

propagation velocity of2.8 km/s (9,200 ft/s). Because curve 

X is unreversed in the subsurface, any set of velocity values 

within limits may be used to generate a subsurface profile of 

a ray path which satisfies it and the overlying velocity distribution. An initial value of 2.8 km/s generated the profile 

shown by the open circles in figure 13; namely, a horizontal 

bed southeast of station A6 which then drops precipitously 

an untenable 200m (660ft) within a distance of about 450 m 

(1 ,500ft) below the crater floor. To raise horizon 4 beneath 

the crater, it is necessary to decrease its velocity, thereby 

implying that the rocks about 300 m (1 ,000 ft) beneath the 

crater floor are fractured. Successive iterations, gradually 

decreasing the velocity of horizon 4 to a limiting value of 

approximately 2.25 km/s (7 ,400 ft/s), resulted in figure I4. 

This solution is also untenable because the velocity survey 

Seismic-Refraction Survey of OAK Crater E9 
NW SE 

1200 AI A3 A4 A5 A6 A7 X I I I I I I 

-~ o/' ~ 2.4 y v' 2.7~ i\ 

1000 

~ ' 

~~ v;y'h 

"" ~2.6 "~ l A;-~Y.V 800 

"' ~8 

' ~ 

\/ ~ 

" K / ~ ' 

~y 

2~ 400 

\ / "' ~ ~ 200 

0 I I I I I I 

O=GZ 1000 

DISTANCE (M) 

2000 

Figure 11. Travel-time curves for horizon 4. The line segments 

labelled A 1 through A7 represent the data from receiver stations A 1 through A7 (fig. 1 ), and the segment labeled X represents the arrival times recorded at the receiver stations with the 

source situated at point X on the distance axis. Curves Y and Z 

in the ground-zero boring (table 1) showed that at approximately 300 m (1 ,000 ft) below sea level, the velocity increased abruptly from about 2.0 km/s (6,600 ft/s) to over 

2.5 km/s (8,200 fUs), not to 2.25 km/s (7 ,400 fUs). The only 

way to attain a value as high as 2.5 km/s (8,200 fUs) for 

horizon 4 beneath the center of the crater and yet satisfy the 

travel-time curves of figure 11 is to increase significantly the 

average velocity of the overlying materials outward from 

ground zero. Although in apparent contradiction to the ray 

velocities in figure 14, justification for tapering the velocities comes from three other sources: First are the results 

from the ground zero and reference borings. The latter were 

drilled on the reef line 1,325 m (4,347 ft) northeast of 

ground zero. A comparison of the velocity survey results 

(table 1) shows a significant velocity difference in the interare extrapolations of curves A7 and X, respectively. Apparent 

velocities are in kilometers/second. Curves between parameters are measured data; curves between brackets are extrapolated data. GZ=ground zero. 

vals between 100 and 300m (330-980 ft) below sea level. 

The velocities below ground zero are lower. Second, refraction results from the reef line (fig. 6) suggest that velocities 

of refracting horizon 3 increase outward from the center of 

the crater. Third, the results of Raitt (1957) and Tremba and 

others (1982) in undisturbed sediments indicate that velocities below 100 m (330 ft) depth are 2.4 km/s (7 ,900 fUs) 

and not the 2.0-km/s (6,600 fUs) average determined from 

the ground-zero hole. 

For the final set of computations in this iterative process, we estimated the average velocities for the material in 

layer 3 as 2.0 km/s (6,600 fUs) in the center of the crater 

(consistent with the ground-zero velocity survey), increasing to 2.2 km/s (7 ,200 fUs) between stations A4 and A5, and 

to 2.3 km/s (7 ,500 fUs) beyond station A6, approximately 

El 0 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
NW 

100 I 

r0 

-

-100 -

I ' I I 

AI A3=GZ A4 

' • • 

~ 1.54 

r.~- "-~1.54 1.7 

SE 

' I ~ I I ' ' ' I 

- A5 A6 A7 

• • • 

- HORIZON I 

HORIZON 3 - 2.0 ----.~· 2.1 - 2.2 . - :E 

-

z 

0 

t= -200 

~ LLJ 

_J 

LLJ 

-300 

-400 

-500 

rrrr-

-

-

I 

V.E.=2.9X 

VELOCITIES ·IN KM/S 

I I I I I 

-400 0 400 

-

- HORIZON 4 

····················- 2.8 

-

-

- I I I I I I I I 

800 1200 1600 2000 

DISTANCE (M) 

Figure 12. Profile of horizon 4 southeast of OAK crater. Profile was computed using the overlying velocity distribution obtained 

from figure 10 and the travel-time curves Y, Z, and A7 of figure 11. Solid circles indicate boundary between velocity values. Open 

circles are computed depth points connected by a best-fit line. GZ=ground zero. 

Table 1. Interval velocities measured in boreholes at the OAK ground zero site and the reference site 

[See fig. 19; SL, sea level; - - -, below depth of penetration] 

Depth interval Reference hole 

m ft m/s ft/s 

SL-50 SL-164 1,543 5,062 

50-100 164-328 1,634 5,360 

100-150 328-492 2,058 6,751 

150-200 492-656 2,513 8,244 

200-250 656-820 2,294 7,526 

250-300 820-984 2,778 9,114 

300-350 984-1,148 

350-400 1,148-1,312 

400-450 1,312-1,476 

450-500 1,476-1,640 

500-550 1,640-1,804 

650 m (2100 ft) from ground zero (fig. 15). We note that 

these are average velocities in a layer, which may exceed the 

ray velocity recorded from the upper surface. The sea-floor 

topography and the crater bowl have also been removed 

from figure 15 because of their negligible effect on vertical 

travel times. 

OAK 

ground-zero hole Difference 

m/s ft/s m/s ft/s 

1,543 5,062 0 0 

1,634 5,360 0 0 

1,650 5,413 408 1,338 

2,016 6,614 497 1,630 

2,165 7,103 129 423 

1,961 6,433 817 2,680 

2,538 8,326 

2,632 8,635 

2,273 7,457 

2,674 8,772 

2,513 8,244 

Trial velocity values from horizon 4 were then successively iterated until a nearly horizontal profile satisfying the 

travel-time curves of figure 11 was obtained. This model 

(fig. 15) shows horizon 4 to be about 350m (1, 150ft) deep, 

with a velocity of 2.4 km/s (7 ,900 fils) below ground zero 

that increases outward to 2.7 km/s (8,900 ft/s) 640 m (2,100 

Seismic-Refraction Survey of OAK Crater E11 
A3=GZ 

• 

A4 

• 

1.7 

A5 

• 

A6 

• 

A7 

• 

SE 

HORIZON I 

~ ·--- 2.2 • --------· 2.0 2.1 HORIZON 3 

-

z 

0 

...... 

~ UJ 

.....J 

UJ 

-300 

2.8 

V.E.= 2.9X 

VELOCITIES 

-500 

-400 0 400 800 1200 1600 2000 

DISTANCE (M) 

Figure 13. Profile of horizon 4 assuming its velocity to be 2.8 km/s. Profile was computed from the travel-time curves offigure 11 

and the overlying velocity distribution obtained from figure 10. Solid circles indicate boundaries between velocity values. Open 

circles are computed depth points connected by a best-fit line. GZ=ground zero. 

ft) from ground zero and 2.8 km/s {9,200 ftls) beyond 900 m 

(2,950 ft) from ground zero. 

This profile mis-ties the drilling results for horizon 4 

at ground zero by 60 m (200ft) of depth and 0.15 km/s (490 

ftls) of velocity. However, the computer analysis showed 

that by using a velocity of 2.5 km/s (8,200 ftls) for horizon 

4 below ground zero, a slightly greater downwarp towards 

the center of the crater is introduced, increasing the mis-tie 

there by an additional 30m (100ft). One way of raising the 

entire profile the required 60 m would be to decrease all 

velocities below horizon 3 by roughly 0.35 km/s {1,100 

ftls). However, this approach would severely compromise 

the well-velocity survey results. A more realistic approach 

is to force horizon 4 to dip downward toward the center of 

the lagoon by lowering the southeast end of the profile and 

raising the northwest end. We did this by modifying curves 

Y and Z in figure 11. A trial run produced the profile shown 

in figure 16, raising horizon 4 on the crater side by the 

desired amount and downwarping it toward the crater interior but with a velocity of only 2.4 km/s (7,900 ftls). 

The tie with drilling data at ground zero can undoubtedly be further improved by readjusting the average velocities of layer 3 and performing additional iterative computations on the data from horizon 4. However, we believe that 

further attempts at fine-tuning are unwarranted due to the 

limited data from the lagoon line. The data for greater 

depths are not reversed; there were only six working receiver stations, and coverage into the lagoon was insufficient to obtain an unambiguous precrater reference profile. 

Refraction arrivals from horizon 5 were analyzed in 

the same manner as those for horizon 4. The travel-time 

curves are shown in figure 17. The overlying velocity distribution used is shown in figure 15, with the exception that a 

depth of 500 m (1 ,640 ft) below sea level was chosen as a 

cutoff beneath which lateral changes in velocity and thus 

differential fracturing were not permitted. A uniform velocity of 2.5 km/s (8,200 ftls) was assumed for the velocity 

below a 500-m (1 ,640 ft) depth. 

Analysis of the travel-time curves for horizon 5 resulted in a horizon at approximately a 700-m {2,300 ft) 

depth (fig. 18) with a corresponding ray velocity of a little 

over 2.9 km/s (9,500 ftls), which does not appear to change 

laterally to the southeast of OAK ground zero. 

The remainder of this section presents conclusions 

inferred from the analysis of horizons 4 and 5 of the lagoon 

line. 

The rocks beneath ground zero, down to and including a hard layer (horizon 4) at approximately a 300-m ( 1 ,000 

E12 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
NW 

100 I 

~ 

0 

-

-100 -

I 

AI 

• 

1.8 

I I I 

A3=GZ A4 

• • 

~1.547 1.54 

, ... 

SE I I I I I I I I I 

- A5 A6 A7 

• • • 

-

1.7 HORIZON I 

-

2.0 ~ ~· 2.0 2.1 HORIZON 3 - -

- -

- -

HORIZON 4 

- 0 o liMP c o c 8 8 ,_ CIIQa ~ • • • ~ • a 8 • e e e 8 • e • e -

.7; 2.8 - ... 0 aZ8 e - -

-300 

2.25 

-400 ~ -

~ V.E.=2.9X - VELOCITIES IN KM/S 

-500 _l I I I I I I I I I I I I I 

-400 0 400 800 1200 1600 2000 

DISTANCE (M) 

Figure 14. Profile of horizon 4 assuming that its velocity changes abruptly from 2.8 to 2.25 km/s (9,200-7,400 ft/s) 800 m (2,625 

ft) from ground zero (GZ). Profile was computed from the travel-time curves of figure 11 and the overlying velocity distribution 

obtained from figure 10. Solid circles indicate boundaries between velocity values. Open circles are computed depth points 

connected by a best-fit line. 

ft) depth, have a lower velocity than those in the surrounding undisturbed area and hence are fractured. 

Velocities and, thus, fractures taper radially from the 

crater center. Between ground zero and 650 m (2,100 ft) 

radially to the southeast, the average velocity of the 200-mthick (660-ft-thick) layer 3 increases from 2.0 km/s (6,600 

ft/s) to about 2.3 km/s (7 ,500 ft/s), thus approaching the 

value of 2.4 km/s (7 ,900 ft/s) assumed for undisturbed rock. 

However, the critically refracted ray velocity for horizon 3 

(fig. 10) tends to be less than that of the average layer 

velocity, suggesting that fractures near the surface of this 

layer extend further from ground zero than they do at depth. 

For example, on the reef line, critically refracted ray velocities do not reach 2.3 km/s (7 ,500 ft/s) until roughly 

1 ,400 m ( 4,600 ft) from ground zero. 

The velocity of layer 4, approximately 300m (1,000 

ft) deep, also increases radially from 2.4 to 2.5 krnls 

(7 ,900-8,200 ftls) at ground zero to 2. 7 km/s (8,900 ft/s) 

between 640 m (2, 100ft) and 800 m (2,600 ft) from ground 

zero. Assuming a value of 2.8 km/s (9,200 ft/s) for undisturbed rocks in layer 4 suggests that fracturing in this layer 

may extend further from ground zero than in the overlying 

softer materials. 

The depth of fracturing in layer 4 cannot be determined. Layer 5 with a 2.9-km/s (9,500 ft/s) velocity at 

approximately a 700-m (2,300 ft) depth does not appear to 

be fractured, and the crater delay times for this horizon can 

essentially be accounted for by velocity decreases above the 

level of horizon 4. Our interpretations suggest a maximum 

depth of 500 m (1,600 ft) beneath which no fracturing occurs. It is likely that layer 4 is a thin bed less than 100 m 

(330 ft) thick, which acted as a wave guide or channel for 

seismic energy, explaining why fractures in this layer may 

radiate further from ground zero than in the overlying layer. 

Finally, figures 15 and 16 suggest a possible 10- to 

20-m (30-60 ft) downwarp of horizon 4 toward the center 

of the crater. However, the analysis of the fme structural 

details of this layer may have been carried out beyond the 

limits of data accuracy. 

A GENERALIZED CRATER MODEL 

Figure 19 depicts our final generalized crater model. 

The shallow features are inferred from the analysis of the 

reef line and the deep ones from the lagoon line. Therefore, 

Seismic-Refraction Survey of OAK Crater E13 
NW 

100 I 

-

0 

-100 ~ 

I 

AI 

' 

I I I 

A3=GZ A4 

' ' 

1.54 

1.6 

SE I I I I I I I I 

- A5 A6 A7 

' ' ' 

-

:!: -· 

z 

0 

~ -200 

~ LU 

_J 

LU 

-300 

~ 2.0 

~ 

~ 

f-

- 2.2 2.3 HORIZON 3 (MODIFIED) -

-

-

- HORIZON 4 ~ ...-·v ea? a Po%% a en88 @) %nJ8 6 a I? • • • oA> • e e eeeeeeaaaaa2.4 ~ 2.5 $ 2.7 $ 2.8 -400 f- -

f- V.E.= 2.9X - VELOCITIES IN KM/S 

-500 I I I I _l_ I I _l_ I J I I I I 

-400 0 400 800 1200 1600 2000 

DISTANCE (M) 

Figure 15. Profile of horizon 4 assuming that its velocity 

changes gradually from 2.8 km/s (9,200 ft/s) 850 m from 

ground zero (GZ) to 2.4 km/s (7,900 ft/s) at ground zero. Profile 

was computed from the travel-time curves of figure 11 and with 

the additional constraint that the velocity of the overlying rnathe problem of possible crater asymmetry, due to the highdensity rocks composing the reef, has not been addressed. 

Figure 19 shows four zones, A through D, outlined 

with heavy lines. Each zone includes an area, or volume in 

three dimensions, in which velocities are less than those of 

the surrounding country rock. The velocity reduction is due 

to fracturing produced by the OAK event. Individual zones 

are labeled with a range of velocities rather than a single 

value because velocities increase with distance from ground 

zero. In zone C, for example, the velocity below ground 

zero is 2.0 km/s (6,600 ft/s); nearer the crater margin where 

fractures diminish, it increases to 2.2 km/s (7 ,200 ft/s). 

Zone A probably represents the infill of the transient 

crater which was excavated below the level of preevent 

horizon 3. It could not be distinguished by the refraction 

travel-time data from the surrounding ruptured and fractured 

coral (zone B) because of the small velocity differential 

terial grades from 2.0 km/s (6,600 ftls) at ground zero to 2.3 

km/s (7,500 ft/s) 700 m from ground zero. Solid circles indicate 

boundaries between velocity values. Open circles are computed depth points connected by a best-fit line. 

(1.6-1.7 krnls vs. 1.7-1.8 km/s, or 5,200-5,600 vs. 

5,600-5,900 ft/s) between them. Hence the boundaries of 

zone A (120-m, or 390-ft, depth; and 600-m, or 2,000-ft, 

maximum radius) had to be obtained from seismic reflection 

data (chap. C and D, this volume). 

The base of zone B, defined by the reef-line data, 

surrounds the transient crater. It is characterized by a wedge 

(in cross section) shown by x's in figure 19; the wedge is 

adjacent to and surrounds the crater floor. This wedge represents highly fractured rock because the velocity in it has 

been lowered by about 33 percent, from 2.3 to 2.4 km/s 

(7 ,600-7,900 ft/s) preeventto 1. 7 to 1.8 km/s (5,600-5,900 

ft/s) postevent. Rocks in zone B that are shallower than 

those in preevent horizon 3 have undergone little change in 

velocity (from 1.8-1.9 km/s to 1.7-1.8 km/s, or 5,900-

6,200 to 5,600-5,900 ft/s). This similarity does not mean 

E14 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
IOONW 

I 

~ 

-

z 

0 

0 

-100 

t= -200 

<t 

> 

~ 

....J 

~ 

~ 

~ 

~ 

~ 

~ 

fI 

AI 

~ 

I I I 

A3=GZ A4 ~ f 

1.54 

1.6 

- 2.0 

SE I I I I I I I I I 

- A5 A6 A7 ~ ~ ~ 

~ 

2.2 -· 2.3 HORIZON 3 (MODIFIED) -

-

-

-

~Q-·~·· 0 - HORIZON 4 

2.4 ~ 2.6 m~v&e ... ~..,sa ............... 

-300 

f- - 2.7 ~ 2.8 -·· e e o e c o 0"1 

f- ~ 2.9 -400 -

~ V.E.=2.9X - VELOCITIES IN KM/S 

_l_ I I I _l_ I 1 I t I I I I I -500 -400 0 400 800 1200 1600 2000 

DISTANCE (M) 

Figure 16. Profile of horizon 4 modified from that of figure 15 by introducing a southeast dip toward the lagoon. Solid circles 

indicate boundaries between velocity values. Open circles are computed depth points connected by best-fit line. GZ=ground zero. 

that the rocks above preevent horizon 3 have not been altered but rather that their initial porosity was not significantly changed by fracturing. For example, the velocity of 

crater debris, completely rearranged from its initial state, is 

only slightly less than the velocity of the undisturbed materials at depths of less than 70 m (230ft). Zone B has been 

extrapolated above horizon 3 to intersect the sea floor, suggesting that near-surface fractures may extend radially out 

about 1 ,400 m ( 4,600 ft) from ground zero. 

Zones C and D were defined from the lagoon-line data 

with additional control from the velocity surveys in the 

ground zero and reference holes and the results of the refraction surveys of Raitt (1957), Ristvet and others (1978), and 

Tremba and others (1982). In zone C, which extends from 

below zone B to a depth of about 300 m (1 ,000 ft), the 

velocities of the country rock have been lowered from 2.3 

to 2.4 km/s (7 ,500-7,900 ft/s) to 2.0 to 2.2 km/s (6,600-

7 ,200 ft/s). It is floored by a high-velocity bed of unknown 

thickness represented by horizon 4, which had a preevent 

velocity of approximately 2.7 to 2.8 km/s (8,900-9,200 

ft/s) that has been reduced to values of 2.4 to 2.6 kmls 

(7 ,900-8,500 ft/s) below the crater floor. These reduced 

velocities are shown in figure 19 as zone D. Reduced velocities in zoneD extend radially 640 to 800 m (2,100-2,600 

ft/s) from ground zero. The reference boring, 1,325 m 

(4,347 ft) from ground zero, also penetrated assumed eventinduced fractures at approximately a depth of 300 m (984ft) 

(A. Schenker, oral commun., May 1985). 

Refraction travel-time delays from horizon 5, approximately 700 m (2,300 ft) deep, may be attributed entirely to 

lowered velocities in zones A through D, suggesting that 

fracturing extends at most 100 to 150 m (330-500 ft) beneath horizon 4 or to a maximum depth of about 425 m 

(1,400 ft). The denser rocks of layer 4 may have acted as a 

barrier to fracturing at greater depths. 

Seismic-Refraction Survey of OAK Crater E15 
en 

~ 

-

2000 I 

1600 

1200 

800 

400 

0 l 

AI A3 A4 A5 A6A7 X ..... I I I I I I I I I 

I~ A!/' '/" ~" ..... ~" 

~ ~ 

~yW " 2.6 AV 

X ' \#: ~.9 

"' 9 

7 

" 

~ ;t';x , 

~ ;/ 

v 

/- ~ 

/ ~ ' , ' v 

I' ~ 

I l I I l I I I I I I 

O=GZ 1000 2000 3000 4000 5000 

DISTANCE (M) 

Figure 17. Travel-time curves for horizon 5. The line segments labeled A 1 through A7 represent the data for receiver stations A 1 

through A7 (fig. 1 ), and the segment labeled X represents the arrival times recorded at the receiver stations with the source situated 

at point X on the distance axis. Curves Y and Z are extrapolations of curves A7 and X, respectively. Apparent velocities are in 

kilometers/second. Curves between parentheses are measured data; curves between brackets are extrapolated data. 

Figure 18 (top right). Profile of horizon 5 computed from the travel-time curve of figure 17, using a velocity of 2.9 km/s (9,500 ft/s). 

The assumed overlying velocity distribution is that of figure 15, except for the velocity reversal at 500-m (1,640 ft) depth. Solid 

circles indicate boundaries between velocity values. Open circles are computed depth points connected by a best-fit line. 

GZ=ground zero. 

Figure 19 (bottom right). Final interpreted model of OAK crater showing four zones (labeled A through D) in which velocities have 

been altered by the OAK event. Small x's indicate zone of highly fractured rock. Reference is the position of drill hole OAR-2 (See 

fig. 21, chap. D, this volume.) GZ=ground zero. 

E16 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
NW 

200 

~ 

0 

-

-200 .... 

~ 

I I 

AI 

• 

2.0 

I T I I I I 

A3=GZ A4 A5 A6 A7 

• • • • • 

1.54 

1.6 

--... - -·--· 2.2 2.3 

__.__. 

:E 

z 

Q 

~ 

~ -400 UJ 

_J 

~ 2.4 2.5 2.7 2.8 

UJ 

2.5 

-600 - ooe--.&,o ~ 0 oo 

-800 ~ V.E. = 3.0X 

VELOCITIES IN KM/S 

I I I I I I I I 

0 800 

2.3 to 2.4 

2.7 to 2.8 

V.E. = 2.2X 

VELOCITIES IN KM/S 

-1600 -800 

I 

I 

I T T T r I I I T T 1 I I I I 

MODIFIED HORIZON 3 

HORIZON 4 

HORIZON 5 

e !II; scos .~ •• ""'" o o o u o o o o e o 0 e 8 8 8 • 8 

2.9 

I I I I I I I 

1600 2400 

DISTANCE (M) 

c 2.0 to 2.2 

0 2.4 to 2.6 

2.9 

0 

DISTANCE (M) 

I I I I l I I J 

3200 4000 

2.3 to 2.4 

HORIZON 4 

HORIZON 5 

800 1600 

SE 

T T 

-

-

-

-

-

-

-

-

I J 

NE 

Seismic-Refraction Survey of OAK Crater E17 
REFERENCES 

Ackermann, H. D., Panbratz, L. W., and Dansereau, D. A., 1982, A comprehensive system for 

interpreting seismic-refraction arrival-time data using interactive computer methods: U.S. 

Geological Survey Open-File Report 82-1065, 265 p. 

Henny, R. W. , Mercer, J. W. , and Zbur, R. T., 197 4, Near surface geologic investigations at 

Eniwetok Atoll: Proceedings of the Second International Coral Reef Symposium 2, Great 

Barrier Reef Committee, Brisbane, p. 615-626. 

Ladd, H. S., Ingerson, E., Townsend, R. C., Russell, M.,and Stephenson, H. K., 1953, Drilling 

on Enewetak Atoll, Marshall Islands: American Association of Petroleum Geologists, v. 37, 

p. 2257-2280. 

Ladd, H. S., and Schlanger, S. 0., 1960, Drilling operations on Enewetak Atoll: U.S. Geological 

Survey Professional Paper 260-Y, p. 863-903. 

Raitt, R. W., 1957, Seismic refraction studies on Eniwetok Atoll: U.S. Geological Survey Professional Paper 260-S, p. 685-698. 

Ristvet, B. L., Tremba, E. L., Couch, R. F., Fetzer, J. A., Goter, E. R., Walter, D. R., Wendland, 

V. P., 1978, Geologic and geophysical investigations of the Enewetak nuclear craters: Air 

Force Weapons Laboratory Technical Report TR-77-242, Kirtland Air Force Base, New 

Mexico 87117, 263 p. 

Tremba, E. L., Couch, R. F., Ristvet, B.L., 1982, Enewetak Atoll Seismic Investigation (EASI): 

Phases I and II (final report): Air Force Weapons Laboratory Technical Report TR-82-20, 

Kirtland Air Force Base, New Mexico 87117, 124 p. 

E18 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Chapter F 

Observations of OAK and KOA 

Craters from the Submersible 

By R. B. HALLEY, R. A. SLATER, E. A. SHINN, D. W. FOLGER, 

J. H. HUDSON, J. L. KINDINGER, and D. J. RODDY 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction Fl 

Purpose Fl 

Setting Fl 

Previous work Fl 

Methods Fl 

Results F5 

OAK crater F5 

Crater floor F5 

Inner slope F5 

Terrace F6 

Fluted slope F8 

Hummocky sea floor FlO 

Benches Fll 

Talus slope and reef plate Fl2 

KOA crater Fl4 

Crater floor Fl5 

Slope and terrace area Fl6 

Fluted slope Fl6 

MIKE crater Fl7 

Forereef dives Fl7 

References Fl7 

Appendix Fl9 

FIGURES 

1-3. Maps showing: 

1. Location of 1984 submersible dive tracks in OAK crater F2 

2. Location of 1984 submersible dive tracks in KOA and MIKE 

craters F3 

3. Location of 1985 submersible dive tracks in OAK crater F4 

4. Areas with similar bottom sediment and physiographic characteristics in OAK 

crater: 

A. Superimposed on 1-m bathymetry F6 

B. Superimposed, with 5-m bathymetry, on vertical aerial photograph F7 

5. Map showing generalized sea-floor characteristics along submersible dive tracks in 

OAK crater F8 

6-10. Photographs showing: 

6. Features of the OAK crater floor F9 

7. Features of the OAK inner slope FlO 

8. Features of the OAK terrace Fll 

9. Features of the lagoon ward edge of OAK crater Fl2 

10. Features of OAK benches and the talus slope Fl3 

11. Areas with similar bottom sediment and physiographic characteristics of KOA 

crater: 

A. Superimposed on 1-m bathymetry Fl4 

B. Superimposed, with 5-m bathymetry, on vertical aerial photograph Fl5 

12. Map showing generalized sea-floor characteristics along submersible dive tracks in 

KOA and MIKE craters Fl6 

13. Photographs (including one aerial photograph) of features in the rockfall area 

seaward of MIKE crater Fl8 

Contents Ill 

Chapter F 

Observations of OAK and KOA Craters from the 

Submersible 

By R. B. Halley1, R. A. Slater2, E. A. Shinn3, D. W. Folger4 , J. H. Hudson3, and j. L. 

Kindinger4, and D. j. Roddy5 

INTRODUCTION 

Purpose 

The purpose of this part of the study was to observe 

and document characteristics of the crater bottom, to collect 

rock samples, and to make detailed observations of such 

important features as scarps, slumps, and debris. We used 

a research submersible, which was essential for conducting 

comprehensive reconnaissance surveys at all depths andespecially in waters more than 30m (100ft) deep where scuba 

dive time was limited and where sharks were common. 

Setting 

Most submersible dives were conducted within 

Enewetak lagoon in the crater areas of OAK, KOA, and 

MIKE in waters between 5 and 60 m (16-197 ft) deep (figs. 

1-3). Current speeds were less than 50 cm/s (1.0 kn), and 

waves were less than 1 m (3.3 ft) high, except during rare 

local squalls. Visibility varied from less than 1 m (3.3 ft) to 

more than 30 m ( 100 ft) but was commonly about 5-10 m 

(16-33 ft). Water temperature was 28± 1 °C. On several 

dives, we examined control areas as far as 2 km (1.1 nmi) 

into the lagoon from the craters. 

In 1984, we made 2 dives outside the atoll in a rockfall area off MIKE crater to a maximum depth of 198 m ( 650 

ft) and, in 1985, 13 dives to a maximum depth of 359 m 

(1,180 ft). There, gentle swells did not interfere with diving, 

but abundant sharks precluded supporting scuba 

observations. 

Previous Work 

During July and September 1981, eight dives were 

made with the submersible Makalii as part of Project EASI 

1U.S. Geological Survey, Branch of Oil and Gas Resources Denver, 

Colo. 80225. 2U.S. Geological Survey, Consultant, Oxnard, Calif. 93030. 3U.S. Geological Survey, Branch of Oil and Gas Resources, Fisher 

Island Station, Miami, Fla. 33139. 4U.S. Geological Survey, Branch of Atlantic Marine Geology, Woods 

Hole, Mass. 02543. 5U.S. Geological Survey, Branch of Astrogeophysical Studies, 

Flagstaff, Ariz. 86001. 

(Enewetak Atoll Seismic Investigation) (Tremba and others, 

1982). Six dives took place in OAK crater, one in KOA 

crater, and one outside the cratered areas. The observations 

made during these dives are incorporated into the following 

discussion of our results. 

METHODS 

During July and August of 1984, we conducted 144 

dives aboard the research submersible Delta , which was 

operated by MARFAB, Torrence, Calif. (figs. 1, 2; appendix). The two-person submersible dove on 29 consecutive days, averaging 5 dives per day, with a maximum of 10 

dives in 1 day. Egabrag II, support vessel for both submersible and scuba operations, monitored the submersible's 

location and depth continually during all reconnaissance, 

sampling, and demonstration dives, except when Halimeda 

or a whaler was used in shallow water or when equipment 

aboard Egabrag II malfunctioned. 

The submersible was tracked from Egabrag II with a 

Honeywell Hydrostar Navigation System that was interfaced with the Motorola FALCON IV Miniranger System 

used to position Egabrag II. Inaccuracies in positioning the 

submersible developed in three situatipns: (1) when 

Egabrag was directly above the submersible; (2) when the 

submersible was rapidly ascending or descending; and 

(3) when the submersible was more than four times its depth 

away from Egabrag. The first situation occurred rarely; the 

second situation occurred when the submersible was crossing steep slopes inside the craters and patch reefs outside of 

OAK crater; the third situation was initially a problem in 

shallow water but subsequently was corrected by placing the 

Miniranger System aboard Halimeda or a whaler and tracking a float attached to the submersible. Some of the submersible locations may be inaccurately plotted in shallow 

water or in areas where slopes were steep. 

During reconnaissance dives, video recordings were 

made of all sea-floor features along each track, and separate 

sound recordings were made of observers' comments. A 

1.3-cm (0.5 in.) VHS video system was used for recording 

all observations and the observer's narrative from within the 

submersible. The recorder was powered by a 12-volt motorcycle battery in the submersible to allow as much as 2 hours 

Observations of OAK and KOA Craters from the Submersible Fl 
II 037'Nr-------------r----------------------r-----------, 

# 

·#"> 

_,.._ t 1)-&P 

I 11>f;, 

II 0 36'30"N + 

# 

SLI37 

125000 IVY GRID (fl) 

SL 77 

+ 

EL76 

EL74 

~ 0 

0 

§ f'() 

0 500 SL 75 1000 METERS + + 0 1000 2000 3000 4000 5000 FEET 

162°5'30"E 162°6'E 162°6'30"E 162°7'E 

Figure 1. Location of 1984 submersible dive tracks in OAK crater. SL 75=start line 75; EL 76=end line 76. Navigational fixes 

shown by tics on dive tracks, annotated with fix number at 5-fix or greater intervals. Tracks dashed where location uncertain. 

GZ=ground zero. 

of continuous recording on a TS I20 video tape. The camera 

displayed date, title, and elapsed time in the field of view 

throughout a dive. During dives, the elapsed time on the 

screen was repeated verbally, so that it would be recorded 

on the audio track along with verbal descriptions of the 

bottom, Greenwich time, depth, and navigation fix numbers. A second recording (audio only) of the observers' 

comments was also made during most dives. The 43 observation dives averaged I hour and 20 minutes in duration; 

some were as long as 2 hours and 30 minutes, and others as 

short as 20 minutes. The dives included six transects 

through ground zero and along several contours in each 

crater; the observations on these dives served as a framework for subsequent dives to investigate specific features. 

During rock sampling dives, we attached a nylon 

mesh bag to the starboard side of the submersible's hull. The 

observer manipulated the submersible's mechanical arm and 

claw to pick up rock samples and deposit them in the sample 

bag. Up to 22 kg (48 lbs) of samples were collected at 59 

sites (chap. G, fig. I, table I, this volume). Usually one 

dive was devoted to each site. 

F2 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
0 

r 

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1'-

_,.._ ~ 

+ ~ IVY GRID (ft) 

+ 

0 500 1000 

METERS 

0 1000 2000 3000 4000 

FEET 

162°12'E 162°12'30"E 

Figure 2. Location of 1984 submersible dive tracks in KOA and MIKE craters. SL 87=start line 87; EL 87=end line 87. Navigational fixes shown by tics on dive tracks, 

annotated with fix number at 5-fix or greater intervals. Tracks dashed where location uncertain. GZ=ground zero. 
~ i 127000~------~---~~----.;-----------r-----------.------=c.::....::, 

53 FIX NUMBER 

('!) SEDIMENT HILL 

DATUM: MLLW (APPROX.) 

CONTOURS IN METERS 

MAP PROJECTION: UTM 

SPHERO lD: C I ark 1 866 

COMPILED: Ju I y 1985 

0 

0 

100 200 300 

500 1000 

400 500METERS 

1500 2000 2500 FEET 

_,.._ t 

j 

U.S. GEOLOGICAL 

SURVEY 

Figure 3. Location of 1985 submersible dives in OAK crater. Shown are positions of sediment hills (H) in the crater terrace 

area. (See fig. 4A.) SL 356=start line 356; EL 356=end line 356. Navigational fixes shown by tics on dive tracks, annotated 

with fix number at 5-fix or greater intervals. GZ=ground zero. 

Forty-five field demonstration dives, averaging about 

30 minutes each, were conducted for visiting project scientists and administrators, other project personnel, and for 

visiting officials from the Marshall Islands. 

In 1985, 57 dives were conducted. Of these, 29 were 

in OAK, 13 in KOA, 13 outside the atoll off MIKE crater, 

and 2 near Medren Island. Most dives were made so that 

members of the Defense Nuclear Agency could observe and 

study important characteristics of each crater. No navigation 

was available except for seven dives in OAK that were 

specifically designed to document and sample critical features observed the previous year. 

During those seven dives, the submersible towed a 

float that was tracked by a whaler equipped with the Falcon 

IV Miniranger System. Because the submersible traversed 

the crater slope over considerable depth changes, inaccuracies in this method of positioning were inevitable. These 

errors were later corrected by comparing submersible observations with features on a 1:1 ,500-scale bathymetric map 

contoured at a 0.5-m (1.5 ft) interval. The corrected tracks 

are shown in figure 3. 

F4 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
RESULTS 

Observations from the submersible have been 

grouped into areas with similar bottom sediment and physiographic characteristics. Some of these areas correlate with 

the provinces defined on the basis of bathymetry in chapter 

A of this volume, for example, the crater floor, inner slope, 

and terrace in OAK crater. Other areas show less correlation. It should be emphasized that the boundaries of these 

areas are estimated from the submersible. The swath of 

visibility generally averaged about 20m (67 ft) wide along 

the track of the submersible and area boundaries, particularly lagoonward of ground zero, and the boundaries should 

be taken as approximate. 

OAK Crater 

OAK crater lies on the leeward side of Enewetak 

Atoll, 12 km (6.5 nmi) from MIKE crater, the nearest other 

blast site. Apparently, little sediment has washed from the 

adjacent reef flat into the crater during the 27 years since it 

was formed (Tremba and others, 1982). Aerial photographs 

clearly show ejecta rays tens of meters wide and hundreds 

of meters long on the adjacent reef flat until the 1970's, but 

these have now mostly eroded away. We have superimposed the map of bottom characteristics and the 5-m bathymetric contours on an aerial photograph of the crater to show 

their relations to the reef plate (fig. 4B). 

In OAK crater, the provinces defined from the submersible are: (1) crater floor, (2) inner slope, (3) terrace, 

(4) fluted slope, (5) benches, (6) talus slope, (7) hummocky 

sea floor, and (8) reef plate (fig. 4A ,B). The dive results are 

discussed by province. 

Crater Floor 

Within the 59-m (194ft) isobath (fig. 4A), the deepest 

part of OAK crater, the flat floor is covered by very finegrained sediment (figs. 5, 6A ). Although the sediment contains a mix of sand (2 mm-62 ~J.m), silt (62 mm-4 ~J.m), and 

clay (less than 4 ~J.m), a predominance of silt and clay causes 

the sediment to be cohesive; therefore a clump of this material picked up with the claw of the submersible stays intact. 

The crater floor is gently undulating. Its surface is 

smooth or is pockmarked with 2- to 5-cm-diameter (0. 8- to 

2-in.-diameter) funnel-shaped holes constructed by burrowing organisms, perhaps crabs, anemones, or worms (figs. 

6D ,E). Shrimp mounds, volcano-shaped piles of sediment, 

generally 15 to 20 em (6-8 in.) high, are widely scattered 

throughout this area (figs. 6A-C). About three mounds are 

present in every square meter of the crater floor. 

The sediment surface of the crater floor is widely 

covered by a thin brown microbial mat, presumably constructed of algae and bacteria. It is disrupted by the slightest 

disturbance. Along the outer edge of the crater floor and in 

depths greater than 50 m ( 164 ft) are hundreds of echinoids 

(sea urchins) (fig. 6F). The echinoid groups appear to be 

grazing on the brown microbial mat, which was removed to 

reveal white sediment where they had grazed. Individual 

echinoids are about 5 em (2 in.) in length and are covered 

by fragile spines. Groups contain as many as 50 individuals 

per square meter ( 11 ft2

) and may cover tens of square 

meters of the sea floor. Scattered among and apparently 

moving with the echinoid groups are helmet shells (gastropods) averaging 10 to 30 em (8-12 in.) in diameter. They 

evidently are feeding on the echinoids. 

The flat crater floor represents the present level of 

infilling of the central crater area. The primary mechanism 

of infilling today appears to be deposition of fine-grained 

sediment that is transported in suspension to this deep and 

restricted portion of the crater, supplemented perhaps by 

sediment moving down the common gully thalwegs 

(chap. A, fig. 9, this volume). These crater floor sediments 

are the most fine-grained, most porous, and least dense of 

any encountered in the crater. 

Movement of sediment as a traction load may also 

account for transport of some sediment to the crater floor. 

During June 1985, currents of about 50 cm/s (1 kn) were 

observed moving sediment along the bottom. Such currents 

appear to be short-lived, possibly associated with extreme 

tidal ranges, and none lasted more than a few hours. 

Inner Slope 

The base of the inner slope lies 100m (330ft) from 

the geometric center of the crater at about the 59-m (194ft) 

isobath (fig. 4A). Although this slope averages only about 

6-8°, it is locally as steep as 17°. The slope area is 75 to 

100m (246-328 ft) wide along the radius of the crater and 

extends 200 to 260 m (650-853 ft) beyond the geometric 

center of the crater. The top of the inner slope falls between 

the 54- and 49-m (177 and 161 ft) contours. 

The microbial mat-echinoid-helmet community 

seems to be restricted to the low energy and poor circulation 

area of the lower inner slope and the crater floor (figs. 

7 A-C). Groups of grazing echinoids were not seen above 

about 53 m (175 ft) on the crater wall. 

The surface sediments change slightly from the base 

of the inner slope, where they are extremely muddy and 

similar to the sediment on the crater floor, to the upper edge, 

where they contain more silt and sand (fig. 7D). The cohesiveness of the sediment reflects this textural change. No 

material coarser than sand was observed on the inner slope. 

The inner slope forms an inner bowl, within the central crater area, that is partially filled with sediment to produce the flat floor of the crater. Probably because the inner 

slope has been modified by physical and biological sediment 

movement, no fault scarps are evident. Single-channel highresolution seismic data do not show any coherent reflectors 

that extend as close to ground zero as this area (see chap. C, 

Observations of OAK and KOA Craters from the Submersible FS 
W36'N + 

u•35'30"N + 

DATUM: MLLW (APPROX) 

CONTOURS IN METERS 

MAP PROJECTION: UTM 

_,..,_ t 

~ + 

+ 

+ 

U.S. GEOLOGICAL 

SURVEY 250 0 250 500 750 1000 1250 

COMPILED: SEPT. 1985 METERS 

1000 0 1000 2000 

FEET 3000 4000 5000 

Figure 4A. Areas with similar bottom sediment and physiographic characteristics in OAK crater superimposed on 1-m 

bathymetry. 

this volume); however, they do show complex downfaulting that may, at least in part, be responsible for the 

terrace and slope. 

Terrace 

From the top of the inner slope to a distance of 310 to 

427 m (1 ,017-1,400 ft) from the geometric center of the 

crater is the terrace area, rising gently (2-3°) to the outer 

slope of the crater wall (fig. 4A). The terrace ranges from 

100 to 200m (330-660 ft) wide and is widest on the reef and 

lagoon sides of the crater. 

The sediment on the terrace varies in character, depending on depth, proximity to the reef, and local topography. In most places, sediments are more sandy and burrowed than are sediments that cover the inner slope or crater 

floor. The most characteristic features in this depth zone are 

large shrimp burrows and mounds, up to 0.3 m (1 ft) high 

and 1 m (3 ft) in diameter, often so close together that they 

overlap. The mounds are most common at the outer edge of 

the terrace and decrease in frequency toward the inner slope. 

In addition, the benthic flora and fauna are more diverse 

here than those deeper in the crater. Common features on the 

F6 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
0 

0 

162°630"E 

500 METERS 

1500 FEET 

162°613011[ 

W36'30"N 

Figure 48. Areas with similar bottom sediment and physiographic characteristics and 5-m bathymetry superimposed on 

vertical aerial photograph of OAK crater to show their relation to the reef plate. (Photograph courtesy of B. L. Ristvet.) 

terrace surface are Callianassa sp. mounds, large burrowing 

anemones (up to 25 em, or 10 in. , in diameter, figs. 8A -D), 

small burrowing anemones (about 5 em, or 2 in., in diameter), and a vmiety of calcareous and noncalcareous green 

and brown algae. 

Smaller terraces or steps, less than 1 m (3ft) high and 

having slopes less than 20°, lie within the terrace. Commonly, between two and five steps were observed on any 

transect across the terrace area. All are oriented with steep 

slopes facing ground zero. Some steps rise up to level portions of the terrace, and others slope slightly (a few degrees) 

away from ground zero, thus reversing the inward slope of 

the crater wall for short distances. Steps are 30 to 100 m 

(100-330 ft) apart and are oriented circumferentially with 

respect to the geometric center. Most occur on the reef side 

of the crater. The origin of these ridges is unknown, but they 

may be related to faults, slumps, and dewatering features 

(see chap. C, this volume). 

Observations of OAK and KOA Craters from the Submersible F7 
,/ 

/"" 74 

F lMUD,SILT,SAND ~~ MUD,SILT,SAND \~ /:HGRAVEL,COBBLES, C<§:•J} RUBBLE >lm, ~PROMONTORY I •*•!PATCH REEFS, RUBBLE 

W/ECHINOIDS WITHOUT BOULDERS <lm LEDGES (< lm) ,SCATTERED CORALS 

ECHINOIDS c:=:::J RIDGES ~ CLIFFS 

0 500 M 

0 !500FT 

Figure 5. Generalized sea-floor characteristics along submersible dive tracks in OAK crater. The width of the tracks approximates 

the swath of visibility (20m average). Track numbers shown with small arrow. GZ=ground zero. 

Within the area of the terrace, we observed many 

small sediment hills apparently made up in part of darkbrown, coarse detritus. They contrasted with the surrounding finer textured terrace sediments. In 1985, we located 

eight of the hills with the Miniranger System (fig. 3) but 

observed others during dives when the navigation system 

was not being used. Most hills that we observed lie in the 

terrace area, but they may be more widespread. They are 

less than 3 m ( 10 ft) high and are round or elongate. Some 

reach dimensions of 5 to 10 m (16-33 ft) across and up to 

30 m (100 ft) long, usually elongated in the direction of a 

crater circumference. Most hills contain granules (2-4 mrn) 

and pebbles (4-64 mm) and even, in one area, a few cobbles 

(64-256 mm) (fig. 8£). Coarse material is concentrated 

near the tops of hills, possibly due to winnowing. The 

coarse material often consists of dark-brown coral fragments, typical of sediment that was cored 230 m (755 ft) 

below bottom (T. W. Henry and J. I. Tracey, oral commun. , April 1985). Thus, the sediment hills may be composed in part of material transported upward by waters escaping from fractures to the sea floor from strata that were 

dewatering due to the compressional effects of the blast. 

Similar features have been noted in KOA crater (B. L. 

Ristvet, written comrnun., July 1984). 

Also in the terrace area are large, gentle hills and 

swales, a hundred meters in width, that radiate from the 

crater center; they are obvious on the bathymetric map (fig. 

4A) but are too large to be obvious with limited visibility 

from the submersible. 

No rocks crop out and there is no evidence of debris 

blocks on the terrace; the rocks may have been buried by 

finer textured ejecta or by a veneer of crater sediments. 

Fluted Slope 

The fluted slope lies at the outer edge of the terrace, 

where the angle of the slope increases to as much as 20°. 

F8 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
IOcm 

E 2cm 

Figure 6. Features of the OAK crater floor: A, Isolated Callianassa burrow mound on pockmarked sea floor. 8, Multiple 

Callianassa burrow mounds with groove produced by sediment slide down side of mounds in foreground. C, Callianassa 

mound (arrow) in foreground with depression of unknown 

(See figs. 3, 4A .) Gentle hills and swales extend from the 

terrace through the fluted slope (fig. 4A ). But the fluted 

slope surface is mainly characterized by smaller promontoorigin in background. 0, Small burrow (arrow) with radiating 

track pattern around opening. E, Tentacles of a worm that 

produces small burrows in craterfloor. F, Echinoids commonly 

found at edges of crater floor. 

ries and reentrants (fig. 5); it is steepened by these promontories and is gentle where they are absent. The fluted slope 

surface merges with the back-reef talus slope on the northObservations of OAK and KOA Craters from the Submersible F9 
Figure 7. Features of the OAK inner slope: A, Isolated Callianassa mound and echinoids. B, Submersible arm and 

claw sampling echinoids on current-smoothed inner slope. C, Field of echinoids browsing on inner slope. 

0, Callianassa burrow mounds composed of slightly coarser sediment than those on the crater floor (compare with 

fig. 68). 

west (reef) side of the crater and with hummocky sea floor 

on the southeast (lagoon) side. 

The promontories are a few meters high and several 

tens of meters wide and long. They are flat-topped, project 

into the crater, and merge smoothly upslope. The sides of 

the promontories are typically covered with fleshy macroalgae and scattered corals. Promontories are about as wide 

as the areas that separate them, producing a fluting effect on 

the scale of tens of meters. We did not see any promontories 

elongated parallel to radials (rays). Some promontories are 

covered by sand, silt, and clay, and others by gravel and 

cobbles, particularly on the side facing ground zero (figs. 

9A -E). It is these rocky terrains that we commonly sampled 

for debris blocks. (See chap. G, this volume.) 

The promontories may be mounds of debris that lie 

outside the transient crater, or they may be ridges that have 

been incised by postblast erosional processes within the 

transient crater, similar to but larger than the ridges in 

LACROSSE crater illustrated by Ristvet and others ( 1978, 

fig. 5.17). 

Hummocky Sea Floor 

Two areas of hummocky sea floor, one east of and 

one south of ground zero, are separated by a channel leading 

from the central crater area to the lagoon floor (fig. 4A ). The 

areas of hummocky sea floor contain mounds or hills, some 

of which are up to 10 m (33 ft) high. Some of these hills 

appear to be mounds of debris; others are patch reefs that 

have been eroded or partially buried by debris. Areas between them look like the lagoon floor (fig. 9B) as does the 

channel leading from the lagoon to the crater. The southern 

area is less rough and more like lagoon floor than the eastern 

area. 

Within this and the fluted slope areas, debris mounds 

were identified as close as 305 m (1 ,000 ft) to ground zero 

and as far away as 640 m (2,100 ft). The area covered by 

debris is at least 335 m (1, 100 ft) wide in some areas. The 

mounded portion of the debris blanket is easily recognized 

from the submersible, but flatter portions of the blanket may 

extend beyond the mounds where it is hard to recognize. 

F1 0 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
,E lm 

Beyond the mounded debris (outward from ground 

zero) are small sediment ridges (figs. 4A, 5) parallel to the 

circumference of the crater, several tens of meters long, a 

meter to several meters high, and up to 15 m (50 ft) wide. 

They are generally larger than those in the crater terrace. 

IIJenches 

Prominent benches up to 5 m ( 16 ft) high and as much 

as 200 m (660 ft) long, occur roughly parallel to the crater 

I 

.. 

IOcm 

Figure 8. Features of the OAK terrace: A, Area of coarse 

sediment with Callianassa mound and dark clumps of 

sea anemones. B, Log on terrace with two Callianassa 

burrow mounds in foreground, one of which is spewing 

sediment. C, A large burrowing anemone. 0, the 

anemone withdrawing into burrow after being disturbed 

by submersible arm.£, Coarse sediment and cobbles on 

flanks of sediment hills characterize reef side of the 

terrace. 

circumference north and west of ground zero (fig. 4A ). 

Much of their surface is covered by fleshy and calcareous 

algae (figs. lOA ,B). They generally lie in water between 

30m (100ft) and 40 m (130ft) deep, but a particularly large 

feature on the western bench may be traced upward to water 

as shallow as 20m (66ft) (fig. 4A ). It may be a break in the 

talus slope as interpreted in figure 10 of chapter A (this 

volume). The benches appear to pinch out against the talus 

slope northwest of ground zero. Perhaps the western and 

Observations of OAK and KOA Craters from the Submersible Fll 
northern benches are actually connected, forming a continuous feature along the reef side of the crater but with the 

central part buried beneath talus. 

As with some other features observed from the submersible, the origin of the benches cannot be determined 

from their surface characteristics and must await additional 

subsurface exploration. Their position below the talus slope 

and the absence of similar features on the lagoon side of the 

crater suggests that they may have slumped from the talus 

slope to their present position. If so, the reef plate must have 

B 20cm 

Figure 9. Features of the lagoonward edge of OAK crater: 

A, Muddy and 8, sandy portions of the hummocky sea floor 

with scattered Halimeda tufts and algal mats on sediment 

surface. C and 0, Rubble-covered promontory surfaces with 

various types of fleshy algae growing on gravel and cobbles. 

E, Coral-covered margin of patch reef at southern edge of 

crater. 

been stripped away prior to slumping because we found no 

evidence of it on the benches. 

T a Ius Slope and Reef Plate 

Along the reef on the northwest side of OAK crater, 

the benches and fluted slope merge with a talus slope (fig. 

4A ). The talus slope, composed mainly of sand and reef 

plate rubble, covers much of the steepest parts of the crater 

wall. Some terraces in the talus slope may have been formed 

F12 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
r 

by reef plate that is partly covered by sand. Other terraces 

on the slope probably comprise slumps that have moved or 

are moving material into the crater. Numerous small (1-5m, 

or 3-16ft) sand slumps were noted; however, some other areas 

have been stabilized by scattered algal plants and mats. 

The top of the talus slope is rimmed by the lagoonward edge of the reef plate, rock that is formed of cemented 

reef rubble and occurs in a band several hundred meters 

wide lagoonward of the living reef (fig. 4A ). The reef plate 

Figure 10. Features of OAK benches and the talus slope: 

A and B, Views of algae-covered slope on one of several 

hills that lie on the northern bench. C, Outcrop of reef 

plate at 20-m (67ft) depth on talus slope. 0, Outcrop of 

reef plate at top of talus slope at 8-m (26 ft) depth with 

coral that has grown since the OAK event. E, Cobbles 

and boulders of reef plate on the talus slope at 25-m 

(83ft) depth. 

is partly exposed above the talus slope where it forms prominent escarpments as much as 5 m (16ft) high (figs. 10C,D). 

Where the slope has a thicker veneer of sand, it barely 

protrudes. In other areas, there seems to be no reef plate at 

all; it may have been stripped away during the blast. (See 

chap. H, fig. 23, this volume). 

The reef plate commonly underlies water less than 

1 m (3 ft) deep, but it has been depressed along the edge of 

the crater to about 20m (65ft) at its,closest point to ground 

Observations of OAK and KOA Craters from the Submersible F13 
0'1 

CD 

0 

0 

0 

162°11 '30'"E 

11°40'30'"N 

11 °40'N 

+ 

+ 

DATUM: MLLW (APPROX.) 

CONTOURS IN METERS 

'I 

0 

0 

0 

0 

REEF PLATE FLUTED SLOPE 

. . ~(j ~*'" ' (J~ 

(y~ ~ 

162°12 '30'"E 

'I 

(J1 

0 

0 

0 

+ <~·~:·:~:: .,, .. :: 

IVY GRID (ft) 

' ff7 

_,..,._ t 

j 

+ 

152000 

11°40 '30'"N 

150000 

11 °40'N 

162L_1_1_'-30-,.-E------------~---------1-6_2_-12_'_E _______________________ 1_6_2_-12_'_3_0_'"E--~ 145000 

MAP PROJECTION: UTM 

250 0 250 500 750 1000 

U.S. GEOLOGICAL 

SURVEY COMPILED: SEPT 1985 METERS HHHHHI 

1000 0 1000 

FEET 

2000 3000 

Figure 11A. Areas with similar bottom sediment and physiographic characteristics of KOA crater superimposed on 1-m bathymetry. GZ=ground zero. 

zero (figs. 10C,D). The escarpment at the lagoonward edge 

of the reef plate is obviously eroding away from the crater. 

The original crater rim may have been several tens to as 

much as 100m (330ft) closer to ground zero. Evidence of 

this retreat are large blocks, boulders, and cobbles of reef 

plate that have slid downslope from the escarpment and 

cover the talus slope in many areas (fig. 10£). Where the 

slope is covered by sand, it presumably overlies much 

coarser reef plate detritus. 

KOA Crater 

KOA crater lies in a more windward position than 

OAK on the Enewetak Atoll in an area of considerable 

carbonate-sand production; as a result, sand islands have 

developed along this portion of the reef and beachrock has 

formed in the surficial sediments. Beachrock is lithified, 

well bedded, and contains moderately to steeply dipping 

layers of lithified beach sand. It forms around and beneath 

F14 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
162 o 11'30" E 

0 

I 

0 

II 0 40'N 

500 METERS 

I 

1500 FEET 

'""'- II 0 40'30"N 

Figure 118. Areas with similar bottom sediments and physiographic characteristics and 5-m bathymetry superimposed on 

vertical aerial photograph of KOA crater to show their relation to the surrounding reef plate and beachrock. 

islands as the result of the precipitation of naturally occurring carbonate minerals between sand grains. At the KOA 

site, reef plate lies along the seaward (reefward) side of the 

crater, and both reef plate and beachrock lie along the other 

sides of the crater (fig. liB). Major features and sediment 

texture of KOA crater are shown in figures llA and 12. 

Figure 11 shows the provinces in KOA defined from the 

submersible. They include: (1) crater floor, (2) slope and 

terrace, (3) fluted slope, and (4) reef plate. 

Crater Floor 

The crater floor at KOA (fig. 11A) dips gently toward 

ground zero from a water depth of about 30 m (100 ft) to 

33 m (108ft); it is covered by sandy mud that appears to be 

coarser than the sediment on the floor of OAK crater, and 

it is covered by mounds 15 to 30 em (0.5-1 ft) high that 

surround the burrows of Callianassa sp. shrimp. Large burrowing anemones, 15 to 20 em (0.5-0.7 ft) across, lie 

widely scattered between the burrow mounds. 

We observed no rock or gravel on the crater floor. 

Though visibility was poor (often <1 m, or 3ft), we could 

see bottom sediments from the lower ports of Delta . Because the visibility was limited, large features might have 

been missed; however, no large features were encountered 

during several crater transects. 

Observations of OAK and KOA Craters from the Submersible F15 
~'i=m MUD,SILT,SAND fi~·i}i"iiiti:':\l GRAVEL, COBBLES, BOULDERS< I m 

0 

r:~=~/~ RUBBLE > lm, LEDGES 

500 M 

[2J PROMONTORY 

....._____.__.....____.__~ 

0 ~OOFT 

Figure 12. Generalized sea-floor characteristics along submersible dive tracks in KOA and MIKE craters. The width of the tracks 

approximates the swath of visibility (20m average). Track numbers shown with small arrows. GZ=ground zero. 

The crater floor of KOA, in contrast to that of OAK, 

appears to be receiving considerably more and coarser sediment that apparently is swept from the reef flat on the north 

and northeast sides of the crater. Sediment deposited on the 

crater floor is quickly reworked by burrowing organisms 

and is blended with older sediments. 

Slope and Terrace Area 

The crater floor is surrounded by a distinct but gentle 

slope area (fig. 11A ). Slight variations of slope suggest that 

terraces are present in this area, but they are difficult to 

delineate from submersible observations. The sediments appear identical to those of the crater floor. The slope area lies 

at distances between 150 and 400 m (492-1,312 ft) from 

ground zero and extends from depths of 30 m (100 ft) to 

depths as shallow as 15 to 20m (49-66 ft). The area of low 

relief around the crater floor (fig. llA) is equivalent to the 

inner slope and terrace of OAK crater. No hills or steps were 

observed in this area. Although an inner slope and terrace 

may be differentiated on the basis of bathymetry (chap. A, 

fig. 18, this volume), no such distinctions were possible 

from the submersible. The entire sea floor in this area appears to be gently sloping, silty, and muddy sand burrowed 

by Callianassa sp. 

Fluted Slope 

At a radius of about 300 m (1 ,000 ft) from ground 

zero, the wall of KOA is scalloped or fluted by a series of 

reentrants and promontories similar to those in OAK (figs. 

4A , 11A). Promontories are a few tens of meters wide, 

several meters high, are steep-sided toward ground zero, 

and merge gradually with the slope away from the crater 

F16 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
center. These reentrants caused the submersible to follow a 

sinuous course during dives along contours in this area. The 

promontories of KOA are covered with sand, gravel, and 

coarse rubble like those observed at OAK. 

At the top of the fluted slope, the rim of the crater 

takes on different aspects. Southwest of ground zero, KOA 

is connected to MIKE through a 25-m-deep (82-ft-deep) 

channel. To the north, the crater wall is rimmed by reef 

plate; to the southeast and south, the crater wall is rimmed 

by beachrock. The sediment surface of the crater wall is 

mostly covered by sand which appears to be thicker on the 

northeast and east sides where the influx of detritus from the 

reef is greatest. On the northwest and southeast sides, there 

is a significant contribution of coarser material, composed 

of beachrock, reef plate, and living coral. 

Where the crater is shallower than 25 m (82 ft), 

mostly fluted slope, the bottom is covered by patches of 

fleshy and calcareous macroalgae, commonly Halimeda 

sp., Dictyota sp., Caulerpa sp., and Padina sp. In some 

areas, these plants completely cover the bottom; elsewhere, 

they are widely scattered. There is only the most general 

correlation of the abundance of the plants with depth. The 

major controls of their distribution are not known. 

The complex bathymetry of the area between the 

craters, made up of mounds and channels, probably resulted 

from several of the cratering events which were sited near 

this area (chap. A, fig. 17, this volume). 

Slabs and outcrops of beachrock that occur on the 

south and southeast margins of the crater represent remnants 

of rocks that were originally beneath islands and along the 

lagoonward margins of islands in the KOA area. The bedding in undisturbed beachrock is typically steep and may be 

mistaken for rotated horizontal bedding. 

MIKE Crater 

MIKE crater has a large, flat, burrowed crater floor, 

probably indicating substantial sediment filling. The crater 

is rimmed with material that is more coarse than that found 

on the crater floor. Several promontories occur around the 

crater margin, some capped by coarse rubble and boulders. 

The promontories lie at a distance of about 550 m (1 ,800 ft) 

from MIKE ground zero (fig. 12). 

Forereef Dives 

In 1984 and 1985, we made 15 submersible dives 

along the seaward edge of the atoll near MIKE and KOA 

craters. These were reconnaissance dives to examine a natural portion of the forereef slope and to compare it with a 

slump scarp that was formed by a submarine rockfall (fig. 

13A) created in the area during or shortly after the KOA 

event. Photographs (Ristvet and others, 1978) show that the 

escarpment was not present on November 8, 1952, after the 

MIKE event; but it is present on June 3, 1958, after the 

KOA event. 

The natural slope is characterized in this area by three 

zones: (1) the reef plate, crest, and near forereef, which 

extend from sea level to 16m (52ft), with slopes ofless than 

10°, (2) the bypass slope, from 16 to 275 m (52-900 ft), 

with slopes of 55° that decrease to 35° near the base; and 

(3) a talus slope below 275 m (900 ft) with slopes that are 

less than 35°. Vertical walls, grooves and chutes, common 

on Caribbean forereef slopes (Enos and Moore, 1983) are 

sparse on the northwestern slope of Enewetak. 

The slump scarp comprises three units that were differentiated initially by surficial appearance: ( 1) a rough 

near-vertical wall from the reef crest to 76 m (250 ft) composed mainly of coral, with common rubble-covered ledges 

(fig. 13B); (2) a vertical to overhanging wall from 76 m 

(250ft) to 220m (720ft) that is massive and fractured, with 

smooth, blocky surfaces (figs. 13C,D ); and (3) inclined 

strata from 220m (720ft) to more than 400 m (1,310 ft), 

composed of hard, dense white limestone and dolomite (fig. 

13£). Caves occur in all three units. Open, cement-lined 

fractures and voids layered with cements are most common 

in the middle unit. Exposed on the southeastern side of the 

scarp are forereef boulder beds dipping at 30° toward the 

open sea. The steeper (55°) dipping natural surface truncates 

these beds, revealing the erosional nature of the bypass 

slope. 

REFERENCES 

Couch, R. F., Jr., Fetzer, J. A., Goter, E. R., Ristvet, B. L., 

Walter, D. R., and Wendland, V. P., 1975, Drilling operations on Eniwetok Atoll during Project EXPOE: Air Force 

Weapons Laboratory Technical Report TR-7 5-216, Kirtland 

Air Force Base, New Mexico 87117, 270 p. 

Enos, Paul, and Moore, C. H., 1983, Fore-reef slope environment, 

in Scholle, P. A., Bebout, D. G., and Moore, C. H., Carbonate depositional environments: American Association of 

Petroleum Geologists, Memoir 33, p. 507-538. 

Ristvet, B. L., Tremba, E. L., Couch, R. F., Jr., Fetzer, J. A., 

Goter, E. R., Walter, D. R., and Wendland, V. P., 1978, 

Geologic and geophysical investigations of the Eniwetok nuclear craters: Air Force Weapons Laboratory Technical Report TR-77-242, Kirtland Air Force Base, New Mexico 

87117, 298 p. 

Observations of OAK and KOA Craters from the Submersible F17 
Figure 13. Features of the rockfall area seaward of MIKE: A, Vertical aerial photograph of MIKE and KOA craters, 

showing rockfall area on outer reef. (Photograph courtesy of B. L. Ristvet). B, Rough, near-vertical wall at 76-m (250 

ft) water depth. C, View looking down a vertical to overhanging massive and fractured limestone wall at 168-m (550 

ft) water depth. 0, Close-up photograph of C, showing detailed features including a vertical fracture. E, Slope of 

dense white limestone and dolomite at 332 m (1 ,090 ft). 
Appendix 
DIVE no. JULIAN DAYLocal Date/ 

Greenwich 

Date 

73 197-15JUL84 

198-16JUL84 

74 

75 

76 198-16JUL84 

198-16JUL84 

77 198-16JUL84 

199-17JUL84 

78 

79 199-17JUL84 

80 199-17JUL84 

199-200-

17-18JUL84 

Chapter F - Appendix 

SUBMERSIBLE DELTA LOG 

OAK and KOA Crater Survey 

Enewetak Atoll 

15 July-10 August 1984 

TIME 

Local/ 

G.m.t.(Z) 

1230-1245 

0030-0045Z 

1330-1530 

0130-1330Z 

1610-1720 

0410-0520Z 

0858-1104 

2058-2304Z 

1215-1400 

0015-0200Z 

1520-1710 

0320-0510Z 

0940-1110 

2140-2310Z 

1155-1250 

2355-0050Z 

F20 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 

LOCATION DESCRIPTION 

Pilot 

Observer 

Mission 

GZ OAK SLATER 

SHINN 

TEST 

OAK SLATER 

SHINN 

TRANSECT 

OAK SLATER 

HALLEY 

TRANSECT 

OAK SLATER 

FOLGER 

TRANSECT 

OAK SLATER 

SHINN 

TRANSECT 

OAK PRIVITT 

HALLEY 

CRATER FLOOR 

SURVEY 

OAK SLATER 

FOLGER 

RECCO REEF 

OAK SLATER 

FOLGER 

RECCO REEF 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

81 199-17JUL84 1325-1500 OAK SLATER 

200-18JUL84 0135-0300Z FOLGER 

TV REEF 

MARGIN 7-M 

CONTOUR 

82 1542-1626 OAK PRIVITT 

0342-0426Z RISTVET 

ORIENT. 

83 1634-1716 OAK PRIVITT 

0434-0516Z BRAMLETTE 

ORIENT. 

84 200-18JUL84 0935-1035 OAK SLATER 

2135-2235Z HALLEY 

TV REEF RECON 

18-M CONTOUR 

85 200-18JUL84 1122-1250 OAK SLATER 

200-201 2322-0050Z HALLEY 

18-19JUL84 TV REEF RECON 

30-M CONTOUR 

86 200-18JUL84 1410-1545 OAK SLATER 

201-19JUL84 021--0342Z HALLEY 

TV RECON 

LAGOON EJECTA 

BLANKET 

87 201-19JUL84 0850-0953 KOA SLATER 

2050-21532 FOLGER 

TRANSECT 

88 201-19JUL84 1029-1109 KOA PRIVITT 

2229-23092 FOLGER 

TRANSECT 

89 201-19JUL84 1130-1205 KOA PRIVITT 

201-202 2330-00152 HALLEY 

19-20JUL84 TRANSECT 

Observations of OAK and KOA Craters from the Submersible F21 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

90 201-19JUL84 1231-1321 KOA SLATER 

202-20JUL84 0031-0121Z FOLGER 

TRANSECT 

91 1525-1555 OAK SLATER 

0327-0355Z SEVIN 

SURVEY 

92 1607-1637 OAK SLATER 

0405-0437Z KREYENHAGEN 

SURVEY 

93 202-20JUL84 0931-1106 OAK SLATER 

2131-2306Z KR.EYENHAGEN 

SURVEY 

CRATER FLOOR 

94 202-20JUL84 1220-1245 OAK SLATER 

203-21JUL84 0020-0045Z JONES 

ORIENT. 

95 1300-1320 OAK SLATER 

0100-0120Z DAI 

ORIENT. 

96 1325-1405 OAK SLATER 

0125-0205Z SCHENKER 

ORIENT. 

97 1417-1440 OAK SLATER 

0217-0240Z BROWN 

ORIENT. 

98 1449-1509 OAK SLATER 

0249-0309Z SEVIN 

ORIENT. 

99 1524-1545 OAK SLATER 

0324-0345Z MATTHEWMAN 

ORIENT. 

100 1553-1608 OAK SLATER 

0353-0408Z HOLLAR 

ORIENT. 

F22 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

101 203-21JUL84 0928-0950 KOA SLATER 

2128-2150Z SHINN 

BOW PLANE 

SEARCH 

102 1019-1149 KOA SLATER 

2219-2349Z SHINN 

TV 

103 203-21JUL84 1300-1329 KOA PRIVITT 

204-22JUL84 0100-0129Z SEVIN 

ORIENT. 

104 1345-1430 KOA PRIVITT 

0145-1230Z JONES 

ORIENT. 

105 1452-1522 KOA PRIVITT 

0252-0322Z DAI 

ORIENT. 

106 1539-1605 KOA SLATER 

0339-0405Z KREYENHAGEN 

ORIENT. 

107 204-22JUL84 0852-1052 KOA SLATER 

2052-2252Z HALLEY 

TV 

108 1111-1157 KOA SLATER 

2311-2357Z HALLEY 

TV 

109 204-22JUL84 1307-1452 KOA SLATER 

205-23JUL84 0107-0252Z RODDY 

TV 

110 1523-1623 KOA PRIVITT 

0323-0423Z JOHNSON 

BOW PLANE 

SEARCH 

Observations of OAK and KOA Craters from the Submersible F23 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

111 205-23JUL84 0915-1115 OUTSIDE SLATER 

205-23JUL84 2115-2315Z ATOLL HALLEY 

OFF MIKE TV 

112 205-23JUL84 1140-1310 AA SLATER 

205-206 2340-0110Z SHINN 

23-24JUL84 TV 

113 206-24JUL84 1435-1645 KOA SLATER 

207-25JUL84 0235-0445Z RODDY 

TV 

114 207-25JUL84 0924-1017 OAK SLATER 

2124-2217Z RODDY 

TV 

115 1040-1111 OAK SLATER 

2240-2311Z RODDY 

TV 

116 207-25JUL84 1340-1415 KOA PRIVITT 

208-26JUL84 0140-0215Z RISTVET 

RAIL SURVEY 

117 1420-1445 KOA SLATER 

0220-0245Z RODDY 

TV 

118 1456-1519 KOA SLATER 

0256-0319Z RODDY 

TV 

119 1536-1612 KOA PRIVITT 

0336-0412Z NUNES 

ORIENT. 

120 1433-1655 KOA PRIVITT 

0233-0455Z TRUMBORE 

ORIENT. 

121 208-26JUL84 1015-1153 KOA SLATER 

2215-2353Z HALLEY 

TV 

F24 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

122 208-26JUL84 1315-1500 MIKE SLATER 

209-27JUL84 0115-0300Z HALLEY 

TV 

123 1543-1618 MIKE SLATER 

0343-0418Z HALLEY 

TV 

124 209-27JUL84 0944-1110 OAK SLATER 

2144-2310 PRIVITT 

TV 

125 209-27JUL84 1140-1222 OAK SLATER 

209-210 2340-0022Z PRIVITT 

27-28JUL84 SAMPLING 

126 209-27JUL84 1255-1320 OAK SLATER 

210-28JUL84 0055-0120Z PRIVITT 

SAMPLING 

127 1340-1440 OAK SLATER 

0140-0240Z PRIVITT 

SAMPLING 

128 1505-1625 OAK SLATER 

0305-0425Z PRIVITT 

SAMPLING 

129 210-28JUL84 1004-1059 MIKE SLATER 

2204-2259Z RODDY 

TV 

130 210-28JUL84 1134-1247 MIKE SLATER 

210-211 2334-0047Z RODDY 

28-29JUL84 TV 

131 210-28JUL84 1331-1435 MIKE SLATER 

211-29JUL84 0131-0235Z RODDY 

TV 

132 1534-1614 ENGEBI SLATER 

0334-0414Z RODDY 

TV 

Observations of OAK and KOA Craters from the Submersible F25 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

133 1640-1820 ENGEBI PRIVITT 

0440-0620Z JOHNSON 

ORIENT. 

134 211-29JUL84 1020-1145 MIKE PRIVITT 

2220-2345Z RODDY 

TV 

135 211-29JUL84 1350-1445 KOA PRIVITT 

212-30JUL84 0150-0245Z RODDY 

SAMPLING 

136 212-30JUL84 1019-1124 SW OF OAK SLATER 

2219-2324Z HALLEY 

TV 

137 212-30JUL84 1235-1345 NW OF OAK SLATER 

212-213 0035-0145Z RODDY 

29-30JUL84 TV 

138 212-30JUL84 1437-1532 OAK SLATER 

213-31JUL84 0237-0332Z RODDY 

TV 

139 213-31JUL84 1006-1036 OAK SLATER 

2206-2236Z KIND INGER 

SAMPLING 

140 1056-1130 OAK SLATER 

2256-2330Z KIND INGER 

SAMPLING 

141 213-31JUL84 1237-1252 OAK SLATER 

214-1AUG84 0037-0052Z KIND INGER 

SAMPLING 

142 1313-1345 OAK SLATER 

0113-0145Z KIND INGER 

SAMPLING 

F26 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

143 1413-1443 OAK SLATER 

0213-0243Z KIND INGER 

SAMPLING 

144 1500-1524 OAK SLATER 

0300-0324Z KIND INGER 

SAMPLING 

145 214-1AUG84 1015-1020 OAK SLATER 

2215-2220Z SHINN 

ABORT 

146 214-1AUG84 1030-1210 OAK SLATER 

214-215 2230-00lOZ SHINN 

1-2AUG84 TV 

147 214-1AUG84 1322-1426 OAK SLATER 

215-2AUG84 0122-0226Z SHINN 

TV 

148 1505-1613 OAK PRIVITT 

0305-0413Z HUDSON 

SAMPLING 

149 215-2AUG84 1012-1045 OAK SLATER 

2212-2245Z PRIVITT 

SAMPLING 

150 215-2AUG84 1110-1204 OAK SLATER 

215-216 2310-0004Z HUDSON 

2-3AUG84 SAMPLING 

151 215-2AUG84 1300-1315 LAGOON PRIVITT 

216-3AUG84 0100-01152 JOHNSON 

INSPECT REEF 

152 1345-1410 OAK PRIVITT 

0145-0210Z MEYERS 

RAD MEAS. 

153 1426-1520 OAK SLATER 

0226-0320Z HUDSON 

SAMPLING 

Observations of OAK and KOA Craters from the Submersible F27 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

154 1553-1638 OAK SLATER 

0353-0438Z HUDSON 

SAMPLING 

155 216-3AUG84 1016-1059 OAK SLATER 

2216-22592 KIND INGER 

SAMPLING 

156 1109-1149 OAK SLATER 

2309-23492 KIND INGER 

SAMPLING 

157 216-3AUG84 1236-1314 OAK SLATER 

217-4AUG84 0036-01142 KIND INGER 

SAMPLING 

158 1336-1400 OAK SLATER 

0136-02002 KIND INGER 

SAMPLING 

159 1415-1445 OAK PRIVITT 

0215-0245Z JAMES 

ORIENT. 

160 1506-1534 OAK SLATER 

0306-0334Z KIND INGER 

SAMPLING 

161 1554-1610 OAK SLATER 

0354-0410Z KIND INGER 

SAMPLING 

162 217-4AUG84 1114-1328 OAK SLATER 

217-218 2324-01282 SHINN 

4-SAUG84 TV 

163 217-4AUG84 1451-1542 OAK SLATER 

218-SAUG84 0251-0342Z HUDSON 

SAMPLING 

164 1553-1649 OAK SLATER 

0353-0449Z HUDSON 

SAMPLING 

F28 Sea-Floor Observations and SubboHom Characteristics, OAK and KOA Craters 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

165 1925-2025 ENGEBI PRIVITT 

0725-08252 WILSON 

RECCO 

166 218-5AUG84 0945-1053 OAK SLATER 

2145-2253Z SHINN 

SAMPLING 

167 1118-1154 OAK SLATER 

2318-2354Z KIND INGER 

SAMPLING 

168 218-5AUG84 1237-1337 OAK SLATER 

218-219 0037-0137Z HUDSON 

5-6AUG84 SAMPLING 

169 218-5AUG84 2015-2150 ENGEBI PRIVITT 

219-6AUG84 0815-0950Z JOHNSON 

RECCO 

170 219-6AUG84 1000-1230 OAK SLATER 

219-220 2200-00302 HALLEY 

6-7AUG84 TV 

171 219-6AUG84 1309-1344 OAK SLATER 

220-7AUG84 0109-0144Z HALLEY 

SAMPLING 

172 1405-1440 OAK PRIVITT 

0205-02402 JOHNSON 

SEARCH FOR 

SAMPLE BAG 

173 1455-1535 OAK SLATER 

0255-03352 HALLEY 

SAMPLING 

174 1551-1636 OAK SLATER 

0351-04362 HALLEY 

SAMPLING 

Observations of OAK and KOA Craters from the Submersible F29 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

175 220-7AUG84 0950-1035 OAK SLATER 

2150-2235Z KIND INGER 

SAMPLING 

176 1058-1123 OAK SLATER 

2258-2323Z KIND INGER 

SAMPLING 

177 220-7AUG84 1141-1230 OAK SLATER 

221-222 2341-0030Z KIND INGER 

7-8AUG84 SAMPLING 

178 220-7AUG84 1307-1343 OAK SLATER 

221-8AUG84 0107-0143Z KIND INGER 

SAMPLING 

179 1356-1420 OAK SLATER 

0156-0220Z KIND INGER 

SAMPLING 

180 1445-1520 OAK SLATER 

0245-0320Z KIND INGER 

SAMPLING 

181 1506-1555 OAK SLATER 

0306-0355Z KIND INGER 

SAMPLING 

182 1603-1628 OAK SLATER 

0403-0428Z KIND INGER 

SAMPLING 

183 221-8AUG84 0951-1016 OAK SLATER 

2151-2216Z HUDSON 

SAMPLING 

184 1031-1056 OAK SLATER 

2231-2256Z HUDSON 

SAMPLING 

185 1107-1142 OAK SLATER 

2307-2342Z HUDSON 

SAMPLING 

F30 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

186 221-8AUG84 1153-1222 OAK SLATER 

221-222 2353-0022Z HUDSON 

8-9AUG84 SAMPLING 

187 221-8AUG84 1334-1411 OAK SLATER 

222-9AUG84 0134-0211Z HUDSON 

SAMPLING 

188 1421-1446 OAK SLATER 

0221-0246Z HUDSON 

SAMPLING 

189 1454-1527 OAK SLATER 

0254-0327Z HUDSON 

SAMPLING 

190 1546-1620 OAK SLATER 

0346-0420Z HUDSON 

SAMPLING 

191 1636-1701 OAK SLATER 

0436-0501Z HUDSON 

SAMPLING 

192 222-9AUG84 1004-1034 OAK SLATER 

2204-2234Z KIND INGER 

SAMPLING 

193 1054-1124 OAK SLATER 

2254-2324Z KIND INGER 

SAMPLING 

194 222-9AUG84 1141-1211 OAK SLATER 

222-223 2341-0011 KIND INGER 

9-10AUG84 SAMPLING 

195 222-9AUG84 1235-1300 OAK SLATER 

223-10AUG84 0035-0100Z KIND INGER 

SAMPLING 

196 1319-1342 OAK SLATER 

0119-0142Z KIND INGER 

SAMPLING 

Observations of OAK and KOA Craters from the Submersible F31 
DIVE no. JULIAN DAY- TIME LOCATION DESCRIPTION 

Local Date/ Local/ Pilot 

Greenwich G.m.t.(Z) Observer 

Date Mission 

197 1359-1415 OAK SLATER 

0159-0215Z KIND INGER 

SAMPLING 

198 1429-1448 OAK SLATER 

0229-0248Z KIND INGER 

SAMPLING 

199 1504-1524 OAK SLATER 

0304-0324Z KIND INGER 

SAMPLING 

200 1535-1615 OAK SLATER 

0335-0415Z KIND INGER 

SAMPLING 

201 1630-1700 OAK PRIVITT 

0430-0500Z PAROLSKI 

RECCO 

202 223-10AUG84 1000-1046 OAK SLATER 

2200-2246Z HALLEY 

TV 

203 1104-1124 OAK SLATER 

2304-2324Z HALLEY 

TV 

204 223-10AUG84 1220-1300 OAK PRIVITT 

224-11AUG84 0020-0100Z HALLEY 

F32 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Chapter G 

Preliminary Analyses of OAK Debris 

Samples 

By R. B. HALLEY, R. P. MAJOR, K. R. LUDWIG, 

Z. L. PETERMAN, and R. K. MATTHEWS 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction G 1 

Purpose G1 

Methods G1 

Lithology G 1 

Trace elements and stable isotopes of carbon and oxygen G5 

Strontium isotopy and paleontological studies G5 

Conclusion G 10 

References G 11 

FIGURES 

1. Map showing debris sample locations and bathymetry in OAK crater, and photographs showing lithology of Pleistocene or older limestones G2 

2. Photomicrographs showing petrographically distinct limestones from OAK debris 

samples G6 

3. Strontium and magnesium profiles for core XEN-3 G9 

4. Oxygen and carbon isotope profiles for core XEN-3 G9 

5-7. Graphs showing: 

TABLES 

5. Rate in change of strontium isotope ratio in seawater during the last 8 

million years G10 

6. Change in seawater strontium during the last 8 million years G10 

7. Depth estimates for OAK debris samples using KAR-l for calibration G10 

1. Preliminary data on bottom samples from OAK crater G3 

2. Geochemical characteristics of samples from core XEN-3 G7 

3. Chemical and isotopic analyses of selected OAK debris samples G10 

4. Strontium analyses from OAK crater debris samples G11 

Contents Ill 

Chapter G 

Preliminary Analyses of OAK Debris Samples 

By R. B. Halley1, R. P. Major1, K. R. Ludwig2, Z. L. Peterman2, and R. K. Matthews3 

INTRODUCTION 

Purpose 

The OAK event produced debris that lies on the reef 

plate and on the lagoon floor around the crater. In this pilot 

effort, we are attempting to ascertain, within narrow limits, 

which subsurface horizons were the sources of the debris 

samples. To do this, we have correlated physiCal and chemical properties of debris samples recovered in the lagoon 

(fig. 1) with those of samples recovered from EXPOE (Exploratory Program on Enewetak) hole XEN-3, drilled on 

the south shore of Enjebi (Janet) Island (see fig. 9d, Couch 

and others, 1975), and with samples recovered from hole 

KAR-l (see chap. D, fig. 2, this volume) drilled during 

1985 south of KOA crater as part of the second phase of this 

program. 

METHODS 

We take several approaches to correlating the debris 

samples with samples from the core borings. These approaches include assessment of lithology and petrography 

and analyses of cathodoluminescene characteristics, trace 

elements, carbon and oxygen isotopes, and strontium isotopes. Most of these methods require samples having distinc:tive characteristics that can be correlated with specific 

strata found in the drill holes. The last two techniques rely 

on known temporal changes and can supply independent 

measures of age. 

The locations of bottom samples collected in OAK 

crater are shown in figure 1. Preliminary data concerning 

the samples are presented in table 1. Pleistocene or older 

rocks were found at 21 of 59 sample sites. Pleistocene and 

older sediments as used here indicate strata that have been 

mineralogically altered in freshwater prior to and during a 

lowering of sea level about 18,000 years ago. Pre senti y, 

except for crater areas, Pleistocene and older rocks are not 

exposed at the surface of the atoll and are generally covered 

by 10 to 20 m (33-66 ft) of younger sediments. Recovery 

1

U.S. Geological Survey, Branch of Oil and Gas Resources, Denver, 

Colo. 80225. 2U.S. Geological Survey, Branch of Isotope Geology, Denver, Colo. 

80225. 3Brown University, Geology Department, Providence, R.I. 02912. 

of such rocks at the sea floor indicates that they have been 

transported from the subsurface during cratering and are 

probably debris or ejecta. Some of this material may have 

been airborne (ejecta), but we shall use the more inclusive 

term, debris. 

In the lagoon, debris occurs as close as 300 m (984 ft) 

and as far as 640 m (2, 100 ft) from ground zero at OAK; 

most was found at distances greater than 350 m (1, 150 ft). 

It is not known to what degree this material has been redistributed by currents or waves. 

About 80 percent of the samples consist of coral fragments that are not distinct from each other or from coral 

recovered from numerous horizons in core borings. The 

remaining 20 percent of the samples consist of a variety of 

limestone clasts that are more distinctive in their lithologic 

and geochemical character and therefore may be correlatable with sediments cored near the blast area. No sampling 

was conducted in KOA because debris in the area could 

have come from other nearby events. 

LITHOLOGY 

On the basis of the limestone classification of Dunham (1962), most of the limestone debris are packstones and 

wackestones. Three samples were grainstones. Typical debris limestones are shown in figure 1. They are commonly 

welllithified, and some show features, such as brown discoloration and vuggy porosity, associated with prolonged 

subaerial exposure during times of lowered sea level. 

Debris was initially compared to cores from hole 

XEN-3 acquired during the EXPOE project (Couch and 

others, 1975; Ristvet and others, 1978) and from core 

KAR-l acquired during the second phase of this study near 

KOA crater. Better correlations obviously can be made with 

samples from cores nearer OAK crater; such cores presently 

are being analyzed but are not yet available. 

Most of the debris samples are well-lithified limestones. In the cores examined thus far, similar limestones 

occur primarily at solution unconformities (Couch and 

others, 1975). Many of these unconformities developed 

when the rocks were at the surface during periods of lowered 

sea level. Several of them are correlative, some as shallow 

as 7 m (23 ft) below the sediment surface in the OAK area. 

At KAR-l, which is about 12 km (6.5 nmi) from OAK 

Preliminary Analyses of OAK Debris Samples G1 
• BOTTOM SAMPLE LOCATONS 

IVY GRIO(ft) 

0 

DATUM: MLLW (APPROX } § 

t0 

MAP PROJECTION UTM 

COMPILED SEPT 1985 250 

1000 0 

0 250 500 750 

1000 2000 3000 

0· 

0 

0 

r· 

D "' l\ r §:'\\_ 

~c V 

' _/\ "vr 

0 

0 

8 

<;j" 

t 

-Nl 

U.S GEOLOGICAL SURVEY 

1000 1250 METERS 

4000 5000 FEET 

Figure 1. Debris sample locations and bathymetry in OAK crater with photographs showing lithology of Pleistocene 

or older limestones. Sample from locality 158 is a grainstone with most grains dissolved during alteration in fresh 

water. All other samples are packstones and wackestones in the terminology of Dunham (1962). Examples from sites 

78, 168A, and 168C illustrate vuggy porosity and brown discoloration suggesting alteration at a solution unconformity. 

GZ=ground zero; GC=geometric center. Contour interval is 5 m. 

ground zero, the uppermost horizons are not as welllithified 

as the debris samples. Presumably, much of the debris sampled around OAK comes from deeper horizons which, at 

KAR-l, lie about 20 to 30m (66-100 ft) below bottom. 

Some of the petrographic vanatwns in debris are 

shown in figure 2. One sample exhibits bright luminescent 

bands of calcite cement (fig. 2F) which should be easy to 

recognize if it is present in core samples. 

G2 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Table 1. Preliminary data on bottom samples from OAK crater 

[Sample sites are shown in figure 1; samples combined at sites are noted with letter (A,B,C). X=Pleistocene age or older; N=age not determined. ---, 

no thin-section prepared and no coral or rock fragments recovered] 

Sample Navigational No. of No. of No. of No. of Pleistocene thin coral rock site fix samples sections fragments fragments age 

78 --------- 1622 7 6 6 X 

124 --------- 1912 N 

125 --------- 1915 6 8 4 2 N 

126 --------- 1918 17 9 15 2 N 

127 --------- 1921 11 5 4 7 X 

128 --------- 1926 17 10 5 11 N 

139 --------- 2329 3 3 X 

140 --------- 2339 N 

141 --------- 2343 N 

142 --------- 2348 N 

143 --------- 2351 7 2 7 N 

144 --------- 2355 2 N 

148 --------- 2481 4 4 3 X 

149 --------- 2488 5 4 3 2 Probable X 

150 --------- 2504 3 2 3 X 

153 --------- 2518 4 3 4 N 

154A -------- 2521 } 154B -------- 2525 4 4 4 X 

155 --------- 2527 6 5 6 X 

156 --------- 2536 4 5 2 2 X 

157 --------- 2539 5 3 5 N 

158 --------- 2546 2 X 

160 --------- 2549 4 4 N 

163 --------- 2551 5 5 N 

164 --------- 2558 5 5 Probable X 

166A -------- 2565 } 166B -------- 2561 6 3 5 X 

166C -------- 2560 (gravel) 

Preliminary Analyses of OAK Debris Samples G3 
Table 1. Preliminary data on bottom samples from OAK crater-Continued 

[Sample sites are shown in figure 1; samples combined at sites are noted with letter (A,B,C). X=Pleistocene age or older; N=age not determined. ---. 

no thin-section prepared and no coral or rock fragments recovered] 

Sample Navigational No. of No. of No. of No. of Pleistocene 

thin coral rock site rix samples age sections fragments fragments 

167 --------- 2569 3 3 3 X 

168A -------- 2571 } 168B -------- 2575 7 5 4 3 X 

168C - - - -- - -- 2573 

170 --------- 2630 2 2 N 

171A -------- 2694 } 5 4 5 X 171B -------- 2691 

173 --------- 2698 6 2 6 X 

174 --------- 2701 4 3 3 X 

175A -------- 2703 } 175B -------- 2707 3 3 N 

176 --------- 2714 3 3 N 

177 --------- 2721 X 

178 --------- 2725 4 2 4 N 

179 --------- 2729 5 2 5 N 

180 --------- 2736 9 3 9 N 

181 --------- 2742 5 2 5 N 

182 --------- 2745 2 N 

183 --------- 2749 2 2 N 

184 --------- 2751 N 

185 --------- 2756 N 

186 --------- 2764 3 3 N 

187 --------- 2869 5 5 N 

188 --------- 2772 3 2 2 Probable X 

189 ----------2774 2 2 N 

190 --------- 2780 2 2 N 

191 --------- 2784 2 2 N 

G4 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Table 1. Preliminary data on bottom samples from OAK crater-Continued 

[Sample sites are shown in figure 1; samples combined at sites are noted with letter (A,B,C). X=Pleistocene age or older; N=age not determined.---, 

no thin-section prepared and no coral or rock fragments recovered] 

Sample Navigational No. of No. of No. of No. of Pleistocene thin coral rock site fix samples sections fragments fragments age 

192 --------- 2787 

193 --------- 2791 8 

194 --------- 2797 4 

195 --------- 2803 3 

196 --------- 2809 2 

197 --------- 2812 

198 --------- 2814 5 

199 --------- 2821 3 

200 --------- 2824 3 

201 --------- 2827 

125 --------- 1915 6 

Total ------------ - - - --------- 246 

TRACE ELEMENTS AND STABLE ISOTOPES OF 

CARBON AND OXYGEN 

In core XEN-3, 175 samples were analyzed for magnesium, strontium, iron, and manganese as well as for carbon and oxygen-isotope ratios. These data are presented in 

table 2 and in figures 3 and 4. Analyses for the same elements in 10 debris samples are presented in table 3. 

Some of these measurements can be used to estimate 

the preevent depth of debris samples. First, magnesium 

values of 1 percent or greater are limited to depths of less 

than 16 m (52 ft) in hole XEN-3 (fig. 3). Two selected 

debris samples (table 3, sites 125B, 147) have magnesium 

values that suggest that they are from this interval. Second, 

samples with 8180 of less than -3.75 occur deeper than 

14 m (46 ft) in XEN-3 (fig. 4), and seven debris samples 

have 8180 less than -3.75 (table 3). Third, samples with 

813C less than -5.0 are not found above 67 m (220 ft) 

(fig. 4), and two debris samples have 813C less than -5.0 

(table 3). These data suggest that a geochemical profile of 

a core near OAK may provide the basis for estimating the 

original depth of debris samples. Further statistical analyses 

of these data combined with mineralogical analyses will 

establish the reliability of such correlations. 

4 

3 

3 

2 

5 

3 

2 

8 

160 

N 

7 N 

4 N 

3 N 

2 N 

N 

4 N 

3 N 

3 N 

X 

4 2 X 

203 42 

Coral: rock= About 5/1 

Several of the debris samples have uncommonly negative values for both oxygen and carbon, as may be expected 

from rock samples that originate near subaerial exposure 

horizons (Allen and Matthews, 1982). These isotopic analyses support lithologic inferences that debris samples may be 

derived from well-lithified limestones at or near solution 

unconformities. 

STRONTIUM ISOTOPY AND 

PALEONTOLOGICAL STUDIES 

A continuously changing, unidirectional marker during the Tertiary that left a record in OAK debris samples can 

be used to establish age and original depth of debris much 

more precisely than the comparative methods described 

above. Two such markers exist -one provided by the ratio 

of strontium isotopes 87 and 86 in marine carbonates, a ratio 

that has increased steadily through the time interval represented by OAK debris, and the other provided by evolutionary changes of organisms. 

Paleontologic study of Enewetak material presently is 

focused on establishing a zonation in reference cores and 

documenting the disruption of this zonation in crater and 

Preliminary Analyses of OAK Debris Samples G5 
Figure 2. Photomicrographs showing petrographically distinct limestones from OAK debris samples. A, Algal (Halimeda) 

packstone. B, Mollusk (gastropod and bivalve) wackestone. C, Coral and algal grainstone. 0, Echinoderm packstone with 

calcite-filled vugs. E, calcite cement in plain light. F, same as E during cathodoluminescence showing bright luminescent 

zones. 

G6 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Table 2. Geochemical characteristics of samples from core XEN-3 

[ ---, no data; ppm, parts per million] 

Depth of Depth of 

sample sample ~~ (ft) (m) 

Sr Fe 

(ppm) (%) 

Mn 

(ppm) 

10 

11.3 

18 

20 

22.5 

24.5 

27.5 

28.4 

33 

40 

47.7 

48 

51.7 

53.8 

54.7 

57.1 

57.5 

58.5 

59.5 

60.8 

63 

63.7 

64 

64.5 

64.7 

66 

67.7 

68.7 

70.4 

72.1 

73.3 

75 

76.2 

76.5 

77.8 

79.7 

80.8 

82 

82.5 

82.8 

84.8 

86 

87 

88 

89 

89.5 

89.8 

91.4 

92.8 

93.3 

94.5 

95.8 

3.04 

3.44 

5.48 

6.09 

6.85 

7.46 

8.38 

8.65 

10.0 

12.1 

14.5 

14.6 

15.7 

16.3 

16.6 

17.4 

17.5 

17.8 

18.1 

18.5 

19.2 

19.4 

19.5 

19.6 

19.7 

20.1 

20.6 

20.9 

21.4 

21.9 

22.3 

22.8 

23.2 

23.3 

23.7 

24.2 

24.6 

24.9 

25.1 

25.2 

25.8 

26.2 

26.5 

26.8 

27.1 

27.2 

27.3 

27.8 

28.2 

28.4 

28.8 

29.1 

1.8 5,600 0.02 

1.9 5,100 .04 

2.3 4,100 .10 

2.2 4,700 .09 

.33 7,500 .02 

.51 7,300 .01 

6.0 -1.94 

7.0 -2.81 

13.0 -2.67 

10.0 -2.53 

<4.0 -2.84 

5.0 -3.28 

2.13 

1.50 

.98 

1.27 

.21 

.39 

-2.12 1.43 

.73 7,200 <.005 <4.0 -3.57 .50 

1.0 6,700 .05 6.0 

.66 7,200 .03 <4.0 -3.38 .34 

.36 7,600 .05 8.0 -3.00 -.08 

2.0 4,500 .27 44.0 -2.88 .99 

.48 5,100 .01 5.0 -4.41 -.54 

.36 5,100 .04 9.0 -4.72 -.64 

.57 5,200 .07 12.0 -3.86 -.19 

.51 4,800 <.005 <4.0 -4.94 .11 

.42 5,200 .01 <4.0 3.67 .68 

.59 3,700 <.005 <4.0 -4.89 -.15 

.67 3,100 .007 <4.0 -5.29 -.61 

.62 3,200 .03 7.0 -4.53 -1.18 

.51 4,200 .02 6.0 -4.46 .01 

.43 4,500 .02 6.0 -4.17 -.41 

.49 4,000 .02 6.0 -4.62 -.27 

.54 4,400 .05 11.0 -4.33 -.05 

.23 5,900 .005 <4.0 -3.90 -1.75 

.46 3,900 .02 5.0 -4.37 -1.03 

.47 4,500 .06 10.0 -4.36 -.26 

.71 2,700 .05 12.0 -5.96 -.99 

.22 7,400 .01 <4.0 -2.54 1.84 

.41 5,000 .02 5.0 -3.97 .26 

.38 5,600 .03 7.0 -4.00 .52 

.34 4,900 .005 <4.0 -3.77 .26 

.57 4,200 .4 10.0 -4.82 -.53 

.54 2,100 .01 5.0 -5.63 -1.70 

.33 5,500 .01 <4.0 -4.14 -1.38 

.43 3,700 .01 5.0 -4.74 -1.59 

.57 2,900 .02 6.0 -4.70 -1.81 

.50 3,800 .02 6.0 -4.68 -1.76 

.63 2,500 .02 

.33 6,200 .02 

.57 4,200 .03 

.55 3,900 <.005 

.36 5,900 .007 

.23 6,900 .01 

.76 3,100 .03 

.68 4,000 .07 

.72 3,600 .02 

.59 3,600 .01 

.71 3,500 .02 

.54 4,200 .01 

-4.62 -1.90 

7.0 -4.68 -2.35 

4.0 -4.58 -.71 

8.0 -5.19 -.86 

<4.0 -4.56 - .29 

<4.0 -3.87 -.07 

<4.0 -4.30 -1.15 

8.0 -4.88 -1.41 

12.0 -3.96 .13 

5.0 -4.41 -.17 

<4.0 -4.44 -.52 

5.0 -4.17 -.41 

<4.0 -4.16 -.27 

-3.81 .09 

Depth of Depth of 

sample sample Mg 

(ft) (m) (%) 

Sr Fe 

(ppm) (%) 

Mn 

(ppm) 

96.8 

98.3 

100 

100.2 

103.5 

104.5 

107.2 

108.1 

110.2 

111.2 

113 

113.7 

113.9 

114.1 

114.2 

115.6 

116.1 

118.1 

120 

122.9 

124 

126.6 

128.3 

130.2 

131.2 

132.0 

132.6 

133.4 

134.5 

135.8 

137.2 

140 

141.9 

143.7 

145.4 

147.6 

149.3 

151.2 

152.2 

153.5 

155.3 

157.3 

158.9 

162.0 

163.9 

165.8 

167.0 

169.8 

170.6 

172.1 

174.7 

176.0 

178.8 

29.5 

29.9 

30.4 

30.5 

31.5 

31.8 

32.6 

32.9 

33.5 

33.8 

34.4 

34.6 

34.7 

34.7 

34.8 

35.2 

35.3 

35.9 

36.5 

37.4 

37.7 

38.5 

39.1 

39.6 

39.9 

40.2 

40.4 

40.6 

40.9 

41.3 

41.8 

42.6 

43.2 

43.7 

44.3 

44.9 

45.5 

46.0 

46.3 

46.7 

47.3 

47.9 

48.4 

49.3 

49.9 

50.5 

50.9 

51.7 

51.9 

52.4 

53.2 

53.6 

54.4 

0.31 6,500 0.02 <4.0 -4.49 -.79 

.60 4,300 .007 <4.0 -3.82 .25 

.62 4,100 .04 6.0 -4.25 -.26 

-3.68 .28 

.50 5,000 <.005 <4.0 -4.61 -.58 

.62 3,900 .007 <4.0 -4.72 -.28 

.48 6,400 .06 10.0 -3.61 .16 

.62 4,300 .03 5.0 -4.09 -.17 

.55 4,900 .02 <4.0 -4.44 .24 

.57 4,500 .007 <4.0 -4.39 .15 

.52 5,000 .007 <4.0 -4.20 .69 

.54 4,700 .009 <4.0 -4.21 .25 

.58 4,400 .04 6.0 -4.23 -.01 

.28 6,800 <.005 <4.0 -3.71 -.06 

.40 6,100 .02 4.0 -3.55 .31 

.45 4,100 .007 <4.0 -3.56 1.24 

.84 2,800 .01 <4.0 -4.67 .45 

.63 4,400 .08 12.0 -3.80 .53 

.62 4,900 .07 10.0 -3.59 .41 

.66 4,000 .007 <4.0 -4.62 .20 

.54 4,200 .007 <4.0 -4.46 .02 

.59 4,600 .02 5.0 -4.09 .23 

.21 7,200 <.005 <4.0 -3.10 1.30 

.57 4,200 .02 <4.0 -4.50 .14 

.59 4,700 .03 4.0 -3.94 .48 

.55 4,900 .007 <4.0 -4.22 .21 

-3.09 .22 

.37 2,400 .008 4.0 -5.15 -2.01 

.18 7,700 .02 <4.0 -3.06 -.25 

.53 4,600 .008 <4.0 -3.84 .46 

.55 4,200 <.005 <4.0 -4.18 .59 

.51 4,700 .008 <4.0 -3.93 .64 

.44 4,400 .02 <4.0 -3.53 -.84 

.45 3,500 .02 <4.0 -4.64 -1.65 

.60 3,500 .02 <4.0 -5.06 -1.07 

.50 4,200 .01 <4.0 -4.56 -.81 

.37 5,500 .01 <4.0 -3.72 .12 

.33 6,100 .02 <4.0 -3.88 .64 

.56 4,700 .07 13.0 -4.40 -.46 

.68 5,200 .16 26.0 -3.78 .22 

.42 4,900 .03 5.0 -4.54 -.11 

.28 6,500 .02 <4.0 -3.20 -.12 

.38 5,200 .01 4.40 -3.49 .16 

.39 5,800 .008 <4.0 -3.16 1.07 

.42 4,900 .008 4.0 -4.34 -1.13 

.25 5,300 <.005 <4.0 

.28 4,400 .01 <4.0 -4.51 -1.16 

.22 4,900 .008 <4.0 -4.49 -1.74 

.32 5,800 .01 <4.0 -3.03 .02 

.41 2,400 .02 4.0 -5.45 -1.83 

.32 4,900 .008 <4.0 -4.25 -2.43 

.44 4,900 .03 6.0 -3.72 -.86 

.29 5,200 .02 6.0 -4.30 -2.17 

Preliminary Analyses of OAK Debris Samples G7 
Table 2. Geochemical characteristics of samples from core XEN-3-Continued 

[---,no data; ppm, parts per million] 

Depth of Depth of 

sample sample Mg 

(%) 

Sr Fe 

(ppm) (%) 

Mn 

(ppm) (ft) (m) 

180.7 

181.6 

55.0 0.34 5,800 0.01 <4.0 -3.44 -0.09 

55.3 0.38 5,100 0.02 5.0 -3.56 -0.35 

82.3 

184.9 

186.9 

189.8 

191.3 

192 

193 

195 

195.8 

196.5 

197 

197R 

200 

205.1 

208 

209 

210 

55.5 0.32 4,300 <0.005 <4.0 -4.32 -2.40 

56.3 0.37 5,400 <0.005 <4.0 -4.16 -1.05 

56.9 0.19 5,500 0.01 <4.0 -4.08 -2.21 

57.8 0.25 5,600 0.01 <4.0 

58.3 0.30 3,800 0.01 <4.40 -4.75 -2.19 

58.5 -4.80 -0.59 

58.8 

59.4 

59.6 

59.8 

60.0 

0 

60.9 

62.5 

63.3 

63.7 

64.0 

211 64.3 

212 64.6 

213 64.9 

213.2 64.9 

214.0 65.2 

217.2 66.2 

218.3 66.5 

220.6 67.2 

222 67.6 

223 67.9 

223.2 68.0 

223.5 68.1 

224 68.2 

76.5 23.3 

225 68.5 

228.3 69.5 

229.3 69.8 

230.4 70.2 

232 70.7 

234 71.3 

235.3 71.7 

237.3 72.3 

239.6 73.0 

241.6 73.6 

243.2 74.1 

245.2 74.7 

247.0 75.2 

249.0 75.8 

249.9 76.1 

251 76.5 

251.8 76.7 

252.7 77.0 

261.3 79.6 

263.3 80.2 

-4.21 -2.07 

-4.91 -3.25 

-4.81 -4.22 

-3.37 -0.78 

0.24 4,400 0.02 <4.0 -5.25 -3.11 

-4.47 -1.97 

0.41 3,800 0.07 10.0 -4.53 -2.50 

0.47 4,100 0.008 <4.0 -4.79 -0.62 

-4.79 -0.67 

-4.62 -1.76 

-5.08 -1.54 

-5.69 -0.25 

-5.57 -0.38 

0.47 3,900 0.04 8.0 

--- --- -4.90 -0.89 

0.38 4,100 <0.005 <4.0 -4.66 -0.49 

0.45 4,100 0.008 <4.0 -5.24 0.05 

0.35 4,400 0.02 <4.0 -4.81 0.03 

-4.86 -0.18 

-4.46 -1.04 

-5.64 -5.67 

-6.45 -7.11 

-5.50 -7.71 

-3.47 -1.15 

.54 2,100 .01 5.0 -5.63 -1.70 

.53 3,100 

.59 3,800 

.54 4,300 

.54 4,200 

.50 4,300 

.37 5,000 

.44 4,700 

.50 3,800 

.25 4,400 

.27 4,200 

.24 5,000 

-4.46 -2.36 

-3.97 1.54 

.04 5.0 -4.31 -2.86 

.02 4.0 -4.62 -1.32 

.01 <4.0 -5.02 -1.32 

<.005 <4.0 -4.32 -1.06 

.02 <4.0 -4.55 -2.20 

.02 <4.0 -4.11 -2.20 

.007 <4.0 -4.43 -1.17 

.008 <4.0 -4.56 -1.63 

.005 <4.0 -4.49 -2.21 

.04 4.0 -4.48 -2.07 

.04 <4.0 -4.97 -2.04 

-4.35 -1.76 

-5.71 -8.12 

-4.01 -2.48 

.31 1,400 <.005 <4.0 -5.61 -5.12 

.29 570 .04 <4.0 -5.97 -6.69 

.19 6,500 .02 <4.0 -5.12 -3.34 

.49 3,300 .008 <4.0 -5.29 -2.49 

Depth of Depth of 

sample sample Mg Sr Fe Mn 

(ppm) (ft) (m) (%) (ppm) (%) 

264.7 

268.6 

271.1 

273.1 

274.9 

275 

276.3 

276.6 

278.1 

280.8 

283 

284.1 

286.3 

288.1 

293.2 

296.5 

80.6 0.53 3,400 0.01 <4.0 -4.92 -1.83 

81.8 .36 3,900 .007 <4.0 -4.18 -2.25 

82.6 .21 3,400 <.005 <4.0 -5.19 -2.98 

83.2 .22 3,600 .03 <4.0 -4.45 -2.24 

83.7 .46 4,100 .02 <4.0 -4.55 -3.00 

83.8 .12 7,700 .02 <4.0 -4.10 -.97 

84.2 .51 2,800 .008 <4.0 -5.32 -4.15 

84.3 

84.7 

85.5 

86.2 

86.5 

87.2 

87.8 

89.3 

90.3 

.22 2,700 .02 

.69 3,100 .02 

.62 3,200 .02 

.66 2,900 .02 

.45 3,400 <.005 

.39 4,800 .02 

.92 3,800 .30 

.30 6,200 .04 

.54 3,600 .01 

(suspect) 

<4.0 

<4.0 -4.70 -2.98 

<4.0 -4.40 -2.02 

<4.0 

<4.0 

<4.0 

43 

<4.0 

<4.0 

-4.93 

-4.58 

-3.93 

-3.65 

-4.01 

-5.89 

-2.86 

-3.29 

-1.68 

-1.23 

-1.38 

-1.93 

transition cores. A few of the debris samples contain enough 

characteristic microfossils to place them within this biostratigraphic zonation. 

Table 4 presents data from analyses of 21 debris samples from OAK crater. Sr isotope analyses are given as 

epsilon-Sr, which is defined as 

. ( ( 87Sr/86Sr) sample ) Epsdon-Sr= 1 x 104 

(

87Sr/86Sr) modem seawater 

The 87Sr/86Sr of seawater is taken from a modem clam shell 

from Enewetak. Similarly, calculated epsilon units from the 

data of DePaolo and Ingram ( 1985) indicate a rate of change 

of -0.524 epsilon units per million years (fig. 5). Using this 

relation, an age may be estimated for each debris sample 

(table 4). 

Analyses of samples from KAR-l have allowed us to 

construct a profile of epsilon-Sr for the core (fig. 6). The 

original depth of debris samples may be estimated by comparing epsilon-Sr of debris with the KAR-l profile. A similar profile should be constructed for a core near OAK crater 

as soon as analyses are available. 

Figure 3 (above right). Strontium and magnesium profiles for 

core XEN-3. (See table 2.) 

Figure 4 (below right). Oxygen and carbon isotope profiles for 

core XEN- 3. (See table 2.) 

G8 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
~ 

::I: 

~ 

a... 

w 

a 

~ 

:::c 

1-

CL 

w 

0 

40 

60 

0 

40 

60 

4000 

Sr (ppm) 

6000 

::!: 

40 

::I: 

~ 

a... 

w 

a 

60 

8000 0 2 3 

%Mg 

~ 40 :::c 

ICL 

w 

0 

60 

-10 

Preliminary Analyses of OAK Debris Samples G9 
Table 3. Chemical and isotopic analyses of selected OAK 

debris samples 

[Sample sites are shown in figure 1; ppm, parts per million] 

Sample Mg Sr Fe Mn 

site (%) (ppm) (%) (ppm) 01a0 o13C 

78 0.35 520 0.008 <4.0 -6.16 -5.83 

125B 4.9 1,600 .02 6.0 -0.86 2.55 

125A .38 1,500 .008 <4.0 -5.94 -6.28 

127 .15 7,900 .01 <4.0 -1.48 +1.95 

147 1.0 5,500 .005 <4.0 -3.40 +1.08 

156 .36 4,800 .008 <4.0 -3.94 .58 

158 .47 1,600 .008 <4.0 -4.87 -1.92 

166B .44 990 <.005 <4.0 -5.05 -3.42 

168A .40 3,000 <.005 <4.0 -5.08 -3.59 

201 .33 2,300 .008 <4.0 -4.49 -0.74 

The depth distribution of 21 OAK debris samples is 

shown in figure 7. The distribution suggests that the majority of samples fall in the depth range of 30 to 60 m ( 100-200 

ft). With proper calibration, debris samples can probably be 

located to within ±6 m (20 ft) of their original depth. 

8 

X6 

(f) 

a:: 

<t 

LLJ 

>-

LLJ 

~4 

2 

SLOPE =-:524 ± 0.009 

\ 

\. 

~ 

c:::::::J"' ~ \. 

o~~--~--~~--~--~--~-"~~ "" 

-4 -3 -2 -1 0 

EPSILON Sr 

Figure 5. Rate in change of strontium isotope ratio in seawater 

during the last 8 million years, based on eight analyses of 

DePaolo and Ingram (1985), for core KAR-1. Columns indicate analytical error. 

CONCLUSION 

Of the various techniques used to analyze OAK debris, recent advances in Sr isotope analyses offer most 

promise for establishing original depths of samples. Analyses of error indicate that epsilon-Sr may be measured to 

within ±0.2 and perhaps to within ±0.1 units, resulting in 

an age estimate to within± 100,000 to ±200,000 years and 

a depth estimate within roughly ±6 m (20ft). 

300 

-

~200 

I: 

to.. 

w 

0 

100 

0--~~--~~~--~~~--~=-~ 

-10 -8 -6 -4 -2 0 

EPSILON- Sr 

Figure 6. Change in seawater strontium during the last 8 million years, from data published by DePaolo and Ingram (1985). 

Horizontal bars show analytical error. 

200 

~ 0 

:c 

fa.. 

~100 0 

o~~--~--~--~~--~--~~~--~~ 

-4 -3 -2 -1 0 +I 

EPSILON -Sr 

figure 7. Depth estimates for OAK debris samples using 

KAR-1 for calibration. Each open circle represents one data 

point except where more (2, 3) are indicated. 

G10 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Table 4. Strontium analyses from OAK crater debris samples 

Sample sites are shown in figure 1. 

Epsilon-Sr is defined as the difference, in parts per 10,000, between the 

87Srf86Sr ratio of the sample to the 87Sr/86Sr ratio of a modem clamshell 

standard. 

The calculated age is based on DePaolo's data (DePaolo and Ingram, 1985) 

for late Tertiary carbonates, which indicate a rate of change of 0.524 

epsilon units per million years. The calculated depth is based on the relationship we observed for preliminary data on samples from undisturbed 

core KAR-l, and has an uncertainty of roughly 15 m (50ft). 

Estimated preevent 

Sample site Epsilon Age depth below sea floor 

-Sr (Ma) (m) (ft) 

125A -------- -1.18±.15 2.25 ±.28 58 190 

125C -------- -.4 ±.13 .76 ±.24 18 58 

150 ---------- -.39±.15 .74 ±.28 17 57 

158 ---------- -1.32±.14 2.51 ±.26 66 216 

166B -------- -2.78±.18 5.3 ±.34 167 547 

166C -------- -.82±.18 1.56 ±.34 38 126 

167A -------- -.95±.2 1.81 ±.38 45 148 

168A -------- -.87±.18 1.66 ±.34 41 134 

168C -------- -1.8 ±.24 3.43 ±.45 96 314 

171 A -------- -1.13±.23 2.15 ±.43 55 180 

171BD ------ -1.04±.16 1.98 ±.3 50 164 

171(5) ------ -.88±.16 1.67 ±.3 41 136 

173B -------- -.78±.15 1.48 ±.28 36 119 

174 ---------- -1.18±.15 2.25 ±.28 58 190 

174(1) ------ -1.36±.14 2.59 ±.26 68 224 

178 ---------- .15±.16 -.29 ±.3 0 0 

179 ---------- .16±.14 -.31 ±.26 0 0 

180D -------- -.22±.15 .41 ±.28 10 32 

188 ---------- -.18±.17 .335±.32 8 26 

189 ---------- -.01±.12 .45 ±.22 1 3 

191 ---------- -.23±.14 .43 ±.26 10 33 

REFERENCES 

Allen, J. R., and Matthews, R. K., 1982, Isotope signatures associated with early meteoric diagenesis: Sedimentology, v. 29, p. 797-817. 

Couch, R. F., Jr., Fetzer, J. A., Goter, E. R., Ristvet, B. L., Walter, D. R., and Wendland, 

V. P., 1975, Drilling operations on Eniwetok Atoll during Project EXPOE: Air Force Weapons 

Laboratory Technical Report TR-75-216, Kirtland Air Force Base, New Mexico, 270 p. 

DePaolo, D. J., and Ingram, B. L., 1985, High-resolution stratigraphy with strontium isotopes: 

Science, v. 227, p. 938-941. 

Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in Ham, 

W. E., ed., Classification of carbonate rocks: American Association of Petroleum Geologists 

Memoir 1, p. 108-121. 

Ristvet, B. L., Tremba, E. L., Couch, R. F., Jr., Fetzer J. A., Goter, E. R., Walter, D. R., and 

Wendland, V. P., 1978, Geologic and geophysical investigations of the Eniwetok nuclear 

craters: Air Force Weapons Laboratory Technical Report TR-77-242, Kirtland Air Force 

Base, New Mexico 87117, 298 p. 

Preliminary Analyses of OAK Debris Samples Gll 

Chapter H 

Scuba Observations of OAK and KOA 

Craters 

By E.A. SHINN, J. L. KINDINGER, R. B. HALLEY, and 

J. H. HUDSON 

U.S. GEOLOGICAL SURVEY BULLETIN 1678 

SEA-FLOOR OBSERVATIONS AND SUBBOTTOM SEISMIC CHARACTERISTICS OF OAK AND 

KOA CRATERS, ENEWETAK ATOLL, MARSHALL ISLANDS 

CONTENTS 

Introduction Hl 

Purpose Hl 

Setting Hl 

Previous work Hl 

Methods Hl 

Results H6 

Baseline data H6 

Diagenesis on the reef flat H6 

Observations of OAK crater area H12 

Scour marks on the reef flat H13 

Rock ledge on the west rim H13 

Upturned flap H14 

Oak crater transect H17 

Fractures H21 

Secondary crater H21 

Patch reef H23 

Discussion H23 

Conclusions-OAK crater H29 

Observations of KOA crater area H29 

Diver observations H29 

Concrete test structure H29 

Shallow railroad rails H31 

Railroad rails within KOA crater H34 

References H39 

FIGURES 

1. Map of Enewetak Atoll showing locations of OAK and KOA craters and approximate locations of core holes drilled with diver-operated drill H2 

2. Oblique aerial photographs showing: 

A . The leeward side of Enewetak Atoll and estimated ground zero before 

detonation of OAK device H3 

B. Similar view to A showing OAK crater in 1984, approximate locations 

of diver-drilled cores, and location of secondary crater H3 

3-5. Photographs showing: 

3. Coring rig on reef flat at site of Enjebi core no. E1 H4 

4. Hydraulic coring machine at site of OAK core no. 2 H5 

5. Digging a pit in carbonate sand with the airlift H6 

6. Aerial photograph showing approximate location of Enjebi core nos. E1 and E2 

on reef flat H7 

7-9. Columnar sections showing: 

7. Lithology of Enjebi core nos. E1 and E2 H8 

8. Lithology of OAK core no. 1 drilled south of OAK crater H9 

9. Lithology of three cores drilled in a transect near OAK crater HlO 

10. Photograph showing selected slabs of core from Enjebi transect Hll 

11. Schematic diagram showing drilling set-up and generalized lithology at location 

of OAK core no. 1 H12 

12-13. Aerial photographs showing: 

12. Seaward margin of windward reef flat and well-developed erosional 

spurs and grooves H13 

13. Reef flat on leeward side of atoll in the vicinity of OAK crater H14 

14-16. Photographs showing: 

14. Smooth nature of spurs eroded by round coral boulders H15 

15. View down the precipitous slope seaward side of atoll H16 

Contents Ill 
16. Large boulder on reef flat southwest of OAK crater H17 

17. Generalized map showing probe data parallel to reef flat opposite OAK 

crater H17 

18. Photograph taken from atop boulder in figure 16 showing eroded ripple-like 

features in reef plate HIS 

19. Aerial photograph of OAK crater taken in 1972 showing location of secondary 

crater and three core holes H19 

20-22. Photographs showing: 

20. Subsided and upturned reef plate near location of OAK core 

no. 2 H20 

21. Upturned flap in reef plate at LACROSSE crater H21 

22. The precipitous dropoff formed by subsided reef plate at location of 

OAK core no. 2 H22 

23. Cross section of pits, including OAK core no. 2, showing that subsided reef plate 

is continuous beneath sand H23 

24. Photographs showing: 

A . Surface of reef plate rock exposed in pit in figure 23 H24 

B. Pit dug next to slab of rock showing that slab is actually a part of reef 

plate not stripped away H24 

25. Photographs showing: 

A . Upturned rim of secondary crater as exposed at low tide H25 

B. Underwater view of interior of secondary crater and part of upturned 

rim H25 

26. Scorpion Conch Reef: 

A . Sketched cross section showing drilling location and thickness of section drilled H26 

B . Photograph showing surface at depth of 9 m H27 

27. Oblique aerial photograph looking westward showing KOA and MIKE 

craters H28 

28. Photograph showing slightly upturned Holocene reef plate in 9 m of water in a 

small area near north edge of KOA crater H28 

29_ Vertical aerial view of the east margin of KOA crater showing location of a 

43-m-long concrete test structure H30 

30-32. Photographs showing: 

30. Concrete test structure in figures 29 and 31 H31 

31. North part of concrete test station 360.01 as it appeared before KOA 

event H32 

32. The northern part of station 360.01 subsided 2m about 8 days after 

KOA event _H32 

33. Diagram of the preevent KOA area taken from a photocopy of a Holmes and 

Narver drawing showing various structures located underwater close to or in 

the crater H33 

34-35. Photographs showing: 

34. Line of rails with corals attached leading toward collimator station H34 

35. Detail of southernmost rail, which has a colony of Pocillopora elegans 

attached H35 

36. Pre-KOA event aerial photograph showing line-of-sight tube crossing to collimator station, concrete test station 360.01, and curve in bulkheaded shoreline H36 

37. Oblique aerial photograph of KOA device in water tank and line-of-sight tube 

leading from bunker H36 

38. Underwater photographs of deep railroad rails in KOA crater H38 

TABLE 

1. Spacing of rails in KOA crater beginning in 18m (60ft) of water closest to ground 

zero and ending in 15 m (50ft) of water farthest from ground zero H37 

IV Contents 
Chapter H 

Scuba Observations of OAK and KOA Craters 

By E. A. Shinn 1, j. L. Kindinger3, R. B. Halley2, and J. H. Hudson 1 

INTRODUCTION 

Purpose 

For complete characterization of the craters, direct 

observations were essential, especially around the critical 

rim areas. Such direct observations were made by a team of 

scuba divers, all of whom were geologists with extensive 

experience in reef carbonates. The goals of the scuba team 

were: ( 1) to describe and sample the areas of the crater in 

waters less than 30m (100ft) deep; (2) to document with 

photographs, 16-mm movies, and color videotapes those 

features pertinent to the understanding of crater evolution in 

the marine environment; (3) to search for upturned or overturned strata which might be useful in differentiating the 

transient crater margin from the apparent crater margin; 

( 4) to obtain rock cores from the reef flat and other areas not 

accessible by a drill ship. 

Setting 

Scuba diving was limited to certain areas due to such 

factors as water depth, water clarity, and sharks. Almost all 

of KOA crater could be reached within the practical 30-m 

(100-ft) working depth limit for scuba diving, although the 

lagoonward rim of OAK crater was inaccessible (chap. A, 

figs. 1, 7, 16, this volume). Water clarity .. especially in 

KOA crater, was occasionally the most restrictive factor. 

Sharks were abundant enough to discourage s,;;uba diving on 

the seaward side of the reef opposite MIKE and KOA 

craters, but they were not a major problem in the lagoon. 

The environment for diving in the lagoon was usually ideal; 

winds were light and sea state low. 

The OAK device was fired from a barge anchored in 

4 m (14ft) of water near the reef edge (figs. L 2). Following 

the explosion, the 4-m (14 ft) contour lay about 500 m 

(1 ,640 ft) northwest of ground zero where waters had been 

deepened to 60 m (197 ft). 

IU.S. Geological Survey, Branch of Oil and Gas, Fisher Island Station, 

Miami Beach, Fla. 33129. 

zu.S. Geological Survey, Branch of Oil and Gas, Federal Center, Denver, Colo. 80225. 3U. S. Geological Survey, Branch of Atlantic Marine Geology, Fisher 

Island Station, Miami Beach, Fla. 33129. 

Previous Work 

The frrst intensive geologic work at Enewetak began 

in the late 1940's prior to nuclear testing, during Operation 

Crossroads. The geologic work was limited to bottom sampling with grabs and to rock coring with rotary rigs on 

islands and exposed reef flats. The results of that research 

are contained in many papers collected in U.S. Geological 

Survey (USGS) Professional Paper 260. Scuba diving was 

not yet in general use for research purposes during Operation Crossroads. Subsequently, extensive shallow coring, 

accomplished with a truck-mounted rotary rig, and seismic 

profiling were carried out on various islands as part of project PACE (Pacific Atoll Cratering Experiment) and project 

EXPOE (Exploratory Program on Enewetak). The results of 

these studies, which provided most of the background data 

for this chapter, are summarized by Couch and others (1975) 

and by Ristvet and others (1978). These studies were followed by project EASI (Enewetak Atoll Seismic Investigation) which mainly dealt with seismic profiling but which 

also employed submersibles and scuba diving in both craters 

(Tremba and others, 1982). During each of these previous 

studies, most scuba diving was apparently carried out by 

U.S. Navy divers rather than by geologists. 

METHODS 

During the second leg of the current marine field 

study, the team of scuba divers operated from Halimeda 

(fig. 2 of the Introduction to the volume), while other scientists manned the submersible Delta operated from Egabrag 

II. Because all divers, including the submersible pilot, were 

geologists with extensive experience in coral reef areas, the 

continuity of observations was assured by interchanging 

personnel on a day-to-day basis. During the third leg, scuba 

divers operated from Halimeda while seismic operations 

were being carried out aboard Egabrag II. 

The submersible's range and bearing were monitored 

continuously from Egabrag II, but locating features and 

objects identified by scuba divers was more complex. Some 

sites of interest were buoyed and their position later established with the Miniranger navigation system mounted 

aboard Halimeda or on one of the whalers. In shallow 

Scuba Observations of OAK and KOA Craters Hl 
§ 

v 

+ 

Bokaidr ik (Helen) § 

co 

+ 

+ 

+ IVY GRID(ft) + 

0 

0 

8 

~ 

+ 

8 

0 

0 

~ 

+160000 

ENEWETAK ATOLL 

/PREVAILING WIND 

u Lojwa (Ursula) 

+120000 

LACROSSE~X~unit (Yvonne) 

• (Oscar) ~z 

~\\ + •A +80000 

0 10,000 20POO 30,000 FEET ,,,,, .. , I I 

0 5,o<)O IO,OOOMETERS I I I I I I I I I I I 

Figure 1. Map of Enewetak Atoll showing locations of OAK and KOA craters and approximate locations of core holes (OAK 

nos. 1-4, Enjebi nos. E1,E2; SC no. 1) drilled with diver-operated drill. Letters, such as A, B, and C, are the first initials of site names. 

(See table 1 of the Introduction to the volume.) 

waters on the reef, positions of cores and sample stations 

were estimated. 

For coring, Halimeda carried both a hydraulic pump 

and a water pump to operate the hydraulic drill. The hydraulic pump circulates oil at approximately 1,000 psi 

(pounds per square inch) at a rate of 8-9 gal/min (or 30-34 

L/min) to drive the drill, and the low-pressure water pump 

circulates seawater used for drilling fluid. The hydraulic 

drill rig (figs. 3, 4), which can be used both above and 

below water, includes 1.5-m (5 ft) core barrel and 1.5-m 

(5 ft) lengths of standard drilling rod. The rig is capable of 

drilling to depths of 15 m (50 ft) in most limestone. It is 

generally used in water less than 15 m (50ft) deep because 

of diver decompression constraints. 

Tungsten-carbide face-flush bits, which cut 4.5-cm 

(1. 75 in.) diameter cores, were used for most drilling, and 

a similar size diamond bit was used in hard limestone near 

the reef edge. Short, large-diameter cores of reef limestone 

and living coral heads were obtained using a 10-cm (4 in.) 

diamond masonry bit. Taken in sections, several 10-cm 

cores totaling as much as 3 m (10ft) long, were removed 

from coral heads. When used in this mode, the drill was 

hand-held by one or two divers. A smaller hydraulic drill 

was also used to sample rock and coral. The small drill, 

about the size of a household electric drill, was used to take 

short, 0.6-m (2 ft) cores that were 6.5 em (2.5 in.) in 

diameter. All the hydraulic drills were driven by the hydraulic pump in the boat via a 61-m-long (200-ft-long), 

1.3-cm-diameter (0.5-in.-diameter) plastic high-pressure 

(2,000 psi maximum working pressure) hydraulic hose 

equipped with stainless steel quick-disconnect fittings. 

Figure 2 (right). A, Oblique aerial photograph, looking northwest, of the leeward (west) side of Enewetak Atoll, showing the 

estimated ground zero (GZ) before detonation of OAK device. 

The band of coral to right of ground zero is the lagoonward 

termination of cemented reef flat, or reef plate. Ground zero is 

located over uncemented carbonate sand. Water depth in 

lower left-hand corner of photograph is greater than 30 m (1 00 

ft). (Photograph courtesy of B. L. Ristvet.) B, Similar view to 

figure A, showing OAK crater in 1984, approximate locations 

of three diver-drilled cores, and location of 30-m-diameter 

(100-ft-diameter) secondary crater. Position of ground zero 

(GZ) is estimated. 

H2 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Scuba Observations of OAK and KOA Craters H3 
Figure 3. Coring rig set up on reef flat at site of Enjebi core no. E1. (See fig. 1.) Hydraulic power source and water pump are aboard 

Halimeda. Photograph taken at high tide. At low tide, reef flat was exposed and boat was aground. 

Still photographs were taken underwater with 

Nikonos cameras using subsea 150-watt strobes; 16-mm 

movies were taken both above and below the water with a 

Bolex movie camera in an underwater housing. Videotape 

records were made with a miniature JVC 1.3-cm (0.5 in.) 

VHS camera and recorder in a plastic underwater housing 

fitted with two battery-powered 100-watt movie lights. 

A device called an airlift was constructed aboard 

Egabrag II to dig holes through sand cover. It consists of a 

4.6-m (15 ft) length of 15-cm-diameter (6-in.-diameter) 

thin-walled aluminum tubing with a garden hose fitting near 

one end (fig. 5). A length of hose connects the airlift to an 

air compressor aboard Halimeda. Underwater, the end of 

the tube is pointed down into the bottom sediment. Air, 

pumped into the tube through the garden hose, rushes up the 

tube, creating suction that draws sediment into the tube at 

the bottom end and ejects it from the upper end. The tube is 

usually slanted so that the material coming out of the upper 

end does not fall back into the hole being excavated. The 

slightest current usually is sufficient to carry the suspended 

sediment from the working area. The airlift was used to dig 

six shallow holes along a transect at OAK and one deep 

(1.5 m, or 5 ft) hole in 18 m (60 ft) of water at KOA. 

Figure 4 (right). Hydraulic coring machine at site of OAK core 

no. 2 in 9 m (30 ft) of water near edge of OAK crater. (See 

fig. 1.) Submersible Delta is in background. 

H4 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Scuba Observations of OAK and KOA Craters H5 
Figure 5. Digging a pit in carbonate sand with the airlift. Location is pit closest to OAK crater. 

RESULTS 

Baseline Data 

To establish baseline data, we made underwater observations of reef flats and lagoonal patch reefs out of the 

area affected by the blast. We also drilled two cores on the 

windward-facing reef flat near Enjebi (see figs. 6, 7) and 

one core on the leeward-facing reef flat (lagoonal side) south 

of OAK (figs. 8, 9). The OAK core no. 1 was useful for 

comparison with three cores (OAK nos. 2, 3, 4) drilled in 

a transect extending from the edge of OAK crater to a 

position near the seaward margin of the reef flat. Pleistocene 

limestone was not reached in OAK core nos. 2, 3, or 4 

(fig. 9). 

The sediments, which were penecontemporaneously 

cemented, consist mainly of coral fragments, coralline 

algae, fragments of various calcified green algae, and whole 

in-place corals. Near the high-energy, wave-battered reef 

margin, thin flat-lying corals dominate. In cross section, 

these corals appear as a series of flat to undulose plates 

interlayered with sediment (fig. 10). Farther from the reef 

margin, larger massive corals as well as finger corals increase in number. The massive and branching corals, which 

have high relief, cannot survive the battering by waves on 

the reef margin. Lagoonal sediments consist mainly of sandsize algal and coral fragments and, in some areas, almost 

pure foraminiferal sands. 

Diagenesis on the Reef Flat 

Previous coring during Operation Crossroads, PACE, 

and EXPOE convincingly demonstrated that the degree of 

cementation and hardness of the reef-flat rock increases 

toward its seaward edge. Conversely, cementation decreases gradually toward the lagoon; a point is reached 

where cementation abruptly ceases and the sediment is unconsolidated. The cementation gradient was very pronounced in the Enjebi cores (figs. 6, 7). Core no. E-1 was 

successfully drilled on the reef flat 230m (755 ft) from the 

reef edge using a carbide bit. The same carbide teeth were 

immediately stripped at the location of core no. E-2, 34m 

(110ft) from the reef edge, and a diamond bit had to be used 

instead. Figures 6 and 7 show that both cores were in the 

Holocene layer of reef-flat rock, or reef plate. In the lower 

portion of both cores, we encountered uncemented 

Holocene carbonate sand. The increase in cementation near 

the reef margin is probably related to increased wave action 

which pumps supersaturated seawater (the cement source) 

through the sediment and rock pores. The cement is mainly 

aragonite and high-Mg calcite. This reduction in cementation from reef edge to backreef has been documented elsewhere in modem reef sediments (James and Ginsburg, 

1979; Shinn and others, 1982) and on Enewetak in deeper 

units by Ristvet and others (1978). In the older units, especially those of Pleistocene age, the relationship is complicated by an overprint of freshwater diagenesis. Each time 

relative sea level dropped, the rocks and sediments were 

subaerially exposed and penetrated by meteoric water, 

which caused cementation and leaching within the first 

meter or two of the sediment or rock surface. Because of the 

subaerial diagenesis overprint, submarine-cemented unconformities can be traced lagoonward to a point where the 

horizon had been lithified entirely with freshwater cements 

consisting of low-Mg calcite. 

OAK core no. 1 was drilled about 3.5 km (1.9 nmi) 

southwest of OAK crater on the reef flat (fig. 1) a few 

meters from the abrupt change from rock to sediment. The 

environmental situation there is considerably different from 

that on the northeast reef flat where the Enjebi cores were 

drilled. On the west side of the atoll, the lagoonal edge of 

the reef flat receives considerable wave action because the 

prevailing northeasterly winds pass over 15 to 20 km (8-11 

nmi) of open lagoon (fig. 1). The deep water and long fetch 

allow waves as high as 1.3 m (4ft) to develop during 25-kn 

(12.5 rnls) winds. Swells as high as 1.5 m (5 ft) are not 

uncommon during winter. Because of this constant wave 

action, the lagoonal side of the reef flat has some of the 

characteristics of the windward (seaward) side of the reef 

where corals grow in abundance and reef flat sediments are 

moderately well cemented. Particularly large coral heads 

(Porites lutea) grow along a narrow depth zone (3-5 m, or 

10-15 ft) of water just lagoonward of the reef flat. The reef 

flat is barely awash at spring low tide. Safe drilling sites on 

the lagoonal side of the reef flat were difficult to find, 

H6 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Figure 6. Aerial photograph showing approximate location of Enjebi core nos. E1 and E2 on reef flat. (See fig. 1.) 

(Photograph taken in 1944; courtesy of B. L. Ristvet.) Scale is approximate. 

because Halimeda could be easily driven onto the reef by 

breakers. To drill OAK core no. 1, Halimeda was anchored 

with three moorings in a small, 3-m-deep (10-ft-deep), 

cove-like reentrant within the reef flat, and the drill, drill 

rods, and tripod were set up on the reef flat about 10 m 

(33ft) away. Water at OAK core no. 1 was less than 30 em 

(1 ft) deep at low tide. The approximate location of OAK 

core no. 1 is shown in figure 1 , and a cross-section sketch 

(fig. 11) shows the depth of cementation. From 1.5 to 1.8 m 

(5-6 ft) downhole, Holocene sediment comprised uncemented carbonate sand; underlying Pleistocene sediment 

was cemented. 

Figures 12 and 13 are aerial views of the windward 

reef flat on the east side of Enewetak Atoll and the leeward 

reef flat on the west side of the atoll. The windward or 

seaward side (fig. 12) is ornamented with spectacular spurs 

and grooves of erosional origin (fig. 14). They are similar 

to those described by Cloud (1959). On the west (leeward) 

Scuba Observations of OAK and KOA Craters H7 
• c:: 

• C.) 

0 

-

0 

:a: 

• c:: 

IJ 

E - 1 

0 

2 

3 

E- 2 

0 - 5 ft Interval • 90% rec 0 - 2 5 ft Interval = 99% rec 0 0 

Bdst - skel & shell trag, well 0 Bdst - lam corln alg w/Bor & 

intlll, skel & corln alg 

trag, Homotrema sp., 

well cmt 

cmt, Homotrema sp., Bor 

& intill, Halimeda, Brhg 

cor 

mass cor encrust w I corln alg 

Grst - shell trag & Halimeda 

sd, Homotrema sp. 

5 - 10 ft Interval • 80% rec 

mass cor, Bor, corln alg, 

in till 

lam corln alg crust 

lse pieces mass cor 

10 - 15 tt Interval = 60% rec 

10 cor w/corln alg, hvy Bor 

w/some intlll, 

Homotrema sp. 

Grst - Halimeda, skel trag, 

Brhg cor 

lge Moll shell trag 

15 - 20 tt Interval • n. rec 

15 

20 - 25 tt Interval = 1% rec 

20 

mass cor 

lge Moll trag 

25 - 33 tt Interval "' n. rec 

25 

2 

3 

5 

Brhg cor 

Bdst - Brhg cor w/lam corln 

alg, well cmt 

lam corln alg 

well cmt w/Homotrema sp. 

lam corln alg crust 

Brhg cor w/rdsh oxide, 

corln alg, skel & echino 

frag, Homotrema sp. 

Bdst(rbl) - well cmt, sdy 

skel trag 

Bdst(rbl) - dk grysh skel trag, 

10 a.a. 

lge Moll shell trag 

It? Bdst - grvly, well cmt, Bor & v-~ 4 o 0 intlll, corln alg 

~ 1a Bdst(rsb:!~-t~;::~·o::~;:~:":t;. 

6 

7 

OJ mass cor· /jf:xK Brhg cor w/rdsh oxide 

mass cor 

Bdst - poorly cmt, Bor & intlll, 

sdy, Halimeda 

Grst - poorly cmt, Halimeda 

20 

Bdst(rbl) - poorly to no cmt, 

cor ,corln alg, sdy & 

lse skel trag, some 

w /rdsh oxide 

Grst - cor & skel trag, 

...... , .... ,.. .. ~ 2 5 Homotrema sp. 

Meters Feet 

30 

33 - 35 ft Interval "' 30% rec 

lge Moll trag 

poorly cmt cor trag, corln alg, 

caliche-like Mat 

~";./\...?'('.~ 3 5 Grst - Halimeda 

0::: Meters Feet 
Figure 7 (left). Lithology of Enjebi 

core nos. E1 and E2. (See fig. 1.) Pleistocene was reached in E1 at approximately 10 m (33 ft). 

The following abbreviations in figures 

7-9 are mostly after Swanson ( 1 981): 

a.a. as above 

alg algae 

Bdst boundstone 

Bor bored 

brhg branching 

chlky chalky 

cmt cemented 

cor coral 

corln coralline 

dk dark 

echino echinoid 

encrust encrusted 

frag fragment 

grsh grayish 

or grysh 

Grst grainstone 

grvly gravelly 

infill infilled 

hvy heavy (-ily) 

lam lamina (-ated) 

lge large 

lse loose 

mass massive 

Mat material 

Moll mollusc 

n. no 

Pkst packstone 

rbl rubble 

rdsh reddish 

rec recovery 

sd sand 

sdy sandy 

skel skeletal 

sol solution 

sp. species 

(singular) 

spp. species (plural) 

tn tan 

w/ with 

wh white 

whsh whitish 

Figure 8 (right). Lithology of OAK 

core no. 1 drilled south of OAK 

crater. (See fig. 1.) Note the significant 

section of uncemented carbonate 

sand below cemented reef plate. 

Rock presumed to be Pleistocene in 

age was encountered at approximately 7.5 m (25 ft). 

OAK - 1 

Q) 

c 

Q) 

(J 

0 

0 

J: 

Q) 

c 

Q) 

(J 

0 

.... 

(/) 

Q) 

a. 

0 

Meters 

0 - 5 ft Interval "' 60% rec 0 Bdst - well cmt skel frag, Halimeda, Homotrema sp., 

Moll frag, lam corln alg 

Bdst - hvy Bor corln alg & Brhg cor, Homotrema sp. 

rbl - lam corln alg 

rbl -mass cor w/corln alg, Homotrema sp. 

5 5 - 2 5 ft Interval "' n. rec 

10 

15 

20 

25 - 30 ft Interval "' 15% rec 

rbl - 2 spp. mass cor 

30 30 - 38 ft Interval = 20% rec 

mass cor 

lam corln alg 

Bdst - corln alg & skel frag w/ Homotrema sp. 

35 

lam corln alg 

mass cor 

Grst - corln alg, Brhg cor-mass cor-skei-Moll-echino 

frag, n. Homotrema sp. 

Feet 

Scuba Observations of OAK and KOA Craters H9 
OAK- 2 OAK- 3 OAK- 4 

0 

2 

3 

Meters 

0 0 - 5 ft Interval = 60\ rec 0 Bdst - grsh, lam corln alg, 

skei-Moll trag, 

Homotrema sp., Halimeda, 

Brhg cor, Bor, mass cor 

Bdst - mass cor, well cmt sd, I 

5 

10 

a. a. 

5 - 10 ft Interval = 30% rec 

Bdst - corln alg, Homotrema 

sp., skel trag, Halimeda, 

Brhg cor 

a. a. 

mass cor 

Feet 

3 

4 

5 

6 

8 

9 

0 - 5 ft Interval • 30% rec 0 0 0 

~ 

0 - 5 ft Interval • 40% rec 

Brhg cor w/eorln alg, cor 

w/rdsh oxide 

5 

Bdst(rbl) - Brhg a mass cor, 

corln alg, Moll-skelechlno trag 

rbl - chlky wh corln alg 

mass cor w/corln alg 

5 - 10 ft Interval • 40% rec 

Bdst(rbl) - lam corln alg, 

Homotrema sp. 

mass cor w/corln alg 

2 

10 

10 - 15 ft Interval • 40% rec 3 

Bdst(rbl) - corln alg, a. a. 

mass cor 

mass cor hvy Bor 

Brhg cor w/lam corln alg 

Bdst - lam corln alg w/skel 

frag,a. a., Homotrema sp. 

4 

15 - 20 ft Interval • 30% rec 

15 Bdst - lam corln alg, a. a. 

Bdst - a. a. 

mass cor w/crust of Halimeda 

20 - 25 ft Interval • n. rec 

5 

20 6 

25 - 30 ft Interval • 40% rec 

5 

Pkst - tn ad, skei-Moll trag, 

Homotrema sp., lam corln alg 

Brhg cor w/lam corln alg whsh 

mass cor 

Grst - Halimeda ad 

5 - 10 ft Interval = n. rec 

10- 15ft Interval • 30% rec 

10 cor w /lam corln alg, 

Homotrema sp., hvy Bor 

15 - 20 ft Interval • 10% rec 

15 corln alg 

mass cor 

lam corln alg 

mass cor 

20 - 25 ft Interval. n. rec 

20 

25 25 

mass cor hvy Bor Meters Feet 

a. a. 

a. a., Homotrema sp. 

30 

Meters Feet 

Figure 9. Lithology of three cores drilled in a transect near 

OAK crater. (See fig. 1.) OAK core no. 2 could not be drilled 

deeper because of fractured rock. Recovery of core was poor 

in OAK core no. 3 also due to fracturing, but thickness of 

cemented reef plate was approximately 6 m (20ft). Pleistocene 

limestone was not reached in OAK core no. 3. Rock in OAK 

core no. 4 was not fractured, and only the upper meter was 

cemented. The remainder of the core was mainly uncemented 

carbonate sand and coral debris. (See definitions of abbreviations, previous page.) 

reef flat, spurs and grooves are mostly lacking and the 

dropoff is precipitous (fig. 15). On the west (seaward) side 

of the atoll, the water depth increases from only 30 em (1 ft) 

to over 100 m (330 ft) over a distance of less than 20 m 

(66ft). Spurs are poorly defined there and appear to be more 

constructional than erosional. (See Shinn, 1963; Shinn and 

others, 1981 , for a discussion of constructional versus erosional spurs and grooves.) On the east (seaward) margin, 

Figure 10 (right). Selected slabs of core from Enjebi transect. 

Two slabs below the scale are from 4ft (1 .3 m) downhole in the 

highly cemented E2 corehole drilled 33m (108ft) from the reef 

edge. (See fig. 6.) Voids in E2 sample are lined with amber 

coating of aragonite, and rock is extremely hard. Material is 

mainly cemented coral. Two slabs above the scale are from 

same interval in core E1 drilled 210m (690ft) from reef edge. 

E1 sample consists mostly of poorly cemented carbonate sand. 

Hl 0 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Scuba Observations of OAK and KOA Craters Hll 
Prevailing wind and waves .-. ._. 

oro 

ft m 

5 1.5 

Figure 11. Schematic diagram showing drilling set-up and generalized lithology at location of OAK core no. 1. (See fig. 1.) 

however, erosional spurs and grooves extend 30 to 100 m 

(100-330 ft) or more seaward from the exposed reef flat into 

water that is as much as 10 m (33 ft) deep in places. The 

major dropoff, where depths in excess of 100 m (330 ft) 

begin, lies another few hundred meters seaward along most 

of the east margin. 

Small patch reefs, consisting mainly of large massive 

head corals, occur in 3 to 6 m (10-20 ft) of water lagoonward of both the east and west reef flat, as they do near the 

island of Biken (Leroy). This zone is more than a kilometer 

wide. It is only a few hundred meters wide between the 

islands of Bokoluo (Alice) and Enjebi (Janet). (See fig. 1 in 

the Introduction to the volume.) Within the lagoon, numerous patch reefs rise up to or near the surface from depths 

greater than 30 m ( 1 00 ft). 

The width of the reef flat is variable, 200 to 300 m 

(656-984 ft) wide along the east side of the atoll and as 

much as 1 km (0.54 nmi) or more on the southwest side of 

the atoll and in the transitional zone to the north. In the 

vicinity of KOA and MIKE craters, the reef flat was originally more than 1 km (0.54 nmi) wide (fig. 1). Nuclear 

devices shot here were, for the most part, on the reef flat, 

although the reef plate apparently was not thick or well 

cemented, according to Ristvet and others ( 1978) and our 

observations. 

Observations of OAK Crater Area 

Two large, upside-down, 3-m-diameter (10-ftdiameter) Porites lutea head corals lie high on the reef flat 

both northeast and southwest of OAK crater (fig. 16). These 

heads served as useful landmarks throughout the study, and 

both are located within 50 m (164ft) of the lagoonal margin 

of the reef flat. Heads reaching this diameter grow only in 

a narrow zone on the lagoonal side of the reef flat, and these 

heads apparently were emplaced upon the reef flat by the 

OAK event. Large storm waves capable of moving coral 

heads most often come from the seaward side, where no 

H12 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Figure 12. Aerial view of seaward margin of windward (east) reef flat showing well-developed erosional spurs and grooves. 

Grooves have overhanging edges near surf zone and in places form tunnels perpendicular to reef front. Scale is approximate. 

similar heads were observed. Other debris is scattered over 

the flats which, at low tide, protrude above water both 

northeast and southwest of the crater. Few rocks like those 

in figure 16 are visible above water on the reef flat north west 

of ground zero. Following a survey of overburden thickness, which was conducted by probing through unconsolidated sediment to the reef plate with a metal rod (fig. 17), 

we concluded that the area has subsided from 1 to 2 m 

(3.3-6.6 ft) . 

Scour Marks on the Reef Flat 

Pronounced ripple-like grooves oriented normal to the 

trend of the reef occur intermittently along the lagoonal 

margin of the cemented reef flat. They measure roughly 5 to 

10 em (4-6 in.) high, are 10 to 15 em (4-6 in.) wide, and 

are separated by grooves 10 to 15 em ( 4-6 in.) wide 

(fig. 18). Although these features have the appearance of 

ripple marks often preserved on bedding surfaces of ancient 

limestone, they apparently were scoured by small pebbles 

and sand that wash back and forth as waves break on the reef 

flat eroding the reef plate. We used these features in addition 

to lithology to determine the origin of limestones found in 

8 to 21 m (25-70 ft) of water along the west margin of OAK 

crater. 

Rock Ledge on the West Rim 

We made numerous scuba and submersible dives 

along the precipitous west rim of OAK crater to investigate 

the origin and age of the limestone and to determine if the 

rim was part of the transient crater or was created by 

postevent subsidence. Although much of the surface within 

about 200 m (656 ft) of the crater rim is covered with 

carbonate sand (fig. 19), a prominent rock ledge 1.5 to 

2.5 m (5-8ft) thick, crops out to form a precipitous escarpment. The ledge has been broken to form vertical and sometimes slightly overhanging scarps. Rock fragments and 

sandy sediment cover the lower part of the escarpment. The 

rubble slope, with some large boulders several meters in 

Scuba Observations of OAK and KOA Craters H13 
Figure 13. Aerial view of reef flat on leeward side of atoll in the vicinity of OAK crater. Prevailing wind produces waves which 

break on lagoon side of reef flat. View is to southwest, and approximate location of OAK core no. 1 is shown by arrow. 

diameter, is sometimes as steep as 30° degree from the base 

of the escarpment to water at least as deep as 33m (110ft). 

The surface of the rock ledge lies in water as shallow 

as 6 m (20 ft) to as deep as 22 m (70 ft). It can be traced 

laterally for 30 to 40 m (100-130 ft) in some places but in 

many places is covered by sand. The ledge is faulted, causing its depth to change dramatically. In one area in the 

center of the arc formed by the west rim of the crater, the 

ledge has been completely covered by sand and rubble for 

a distance of more than 100 m (330 ft). 

Upturned Flap 

Close to the area where OAK core no. 2 was drilled 

(see fig. 19), in 7 to 8 m (22-25 ft) of water, the ledge dips 

away from ground zero at an angle of about 10° (figs. 

20A,B). Part of it is ornamented with ripple-like scour 

marks having the same spacing and orientation as those 

observed along the lagoonal margin of the nearby reef flat 

(fig. 20A). The presence of these scour marks, as well as the 

presence of red H omotrema sp. and the similarity of the rock 

matrix to that drilled at OAK core no. 1, are considered 

sufficient evidence that the rock forming the west margin of 

OAK crater is reef plate. In most places, it has subsided to 

a depth of about 12 m (40 ft), but at one locality it was 

observed as deep as 22 m (70 ft). Whether the upturned 

Figure 14 (right). Underwater photograph showing smooth nature of spurs eroded by round coral boulders. Groove is about 

1 m (3 ft) wide. 

H14 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Scuba Observations of OAK and KOA Craters H15 
:I: 

.... 

0'1 

V'l 

~ 

~ .,., 

0 

~ 

0 

0" 

"' ~ 

~ 

~ 

s· 

al 

Slol 

= Q. 

V'l 

1: 

0" 

[ 

8' 

3 

~ 

:::r 

~ Slol 

I") 

1i 

~­

;::;· 

~ 

0 

> 

~ 

Slol 

= Q. 

~ 

0 

> 

Q 

Slol 

1i 

"' 

Figure 15. Underwater view looking down the precipitous slope on seaward (west) side of atoll. Propeller to right is only a few feet above bottom. Slope 

is approximately 80°, and spurs and grooves like those shown in figures 12 and 14 are absent. Propeller is about 0.25 m (0.8 ft) in diameter. 
ledge, identical in appearance to portions of the upturned 

ledges surrounding LACROSSE crater (fig. 21), is an upturned flap or a rotated slump block was impossible to 

distinguish. A thick layer of sand covers much of it. 

Oak Crater Transect 

The observation of subsided reef plate with a possible 

upturned flap led to investigation of the reef flat west of the 

crater. Three core holes were drilled in a line. The first was 

located a few meters from the crater rim in 9 m (30 ft) of 

water (fig. 22); a second hole was placed in approximately 

1.5 m (4 ft) of water and 244 m (800 ft) from the crater 

Figure 16 (left). Large boulder (a single head of the coral Porites lutea) on reef flat southwest of OAK crater. Boulder was 

undoubtedly thrown there by the OAK event. Location of boulder is shown on figures 18 and 19. Man standing in water is in 

same position as in figure 18. 

Figure 17 (below). Generalized map showing probe data parallel to reef flat opposite OAK crater. Data show the surface of 

reef plate is about 1 m (3ft) deeper opposite the crater, indicating localized subsidence caused by the OAK event. Crater 

debris present adjacent to the crater on the reef flat does not 

protrude above water at low tide as it does northwest and 

southeast of the crater. Porites lutea boulder at left is the one 

shown in figure 16. See figure 19 for aerial photograph of same 

area. GZ=ground zero. 

-- ------ ------- --------- ----------~~~~~!~--~~--~-~J--~~-~!-~l~~-----------------------------------------

2• Boulder Rid&e 

----- ------------------~;------------------------------ --------------------- ------------------ -- -------

•• •• 

6• 

5• 

Inter-Reef Flat Channel 

------------------ -------------------4----------------- - -------------------------------------------------------

4• • 

3 3 3 4 3 43 445455656676765656 5454543434 3 4 • • • • • • • 5•• • • • • • • • • • • • • • • • • • • • • • • • .3 

4• 

3• 6 7 • 12 9 7 •

4 

. . . . . . . . . . -------- •11 , .. 

3• 17 ........ ' Porites lutea • ............ 

... ,,',,.¥Approximate Crater Ed1e 

' ' boulder • ;: ,/ 

/ / 

~\\e Ed1e of Reef P a 

0 2000 ft 

~~~~~--~~ 

0 610 m 

distances and locations approximate 

GZ 

+ 

\ 

' \ 

\ 

\ 

\ Porites lutea 

boulder • 

t p\at.e t 1lee £d~e o 
Figure 18. Photograph taken from atop boulder in figure 16 showing eroded ripple-like features in reef plate. Note cobbles in 

grooves and wave action which causes them. 

margin; and a third was drilled near the seaward side of the 

reef flat about 823 m (2, 700 ft) from the crater margin. 

Fracturing of the rock was evident in the two core sites 

closest to the crater, which were within the area affected by 

subsidence. The transect location is shown in figure 19. See 

figure 9 for lithologies. To obtain additional information, 

we dug six pits with the airlift through the sand cover along 

the transect line. The transect cross section is shown in 

figure 23. The pits allowed important observations of the 

underlying rock. First, there are no ripple-like grooves 

there, such as those we observed southwest of the crater area 

(fig. 18) and at the crater rim (fig. 20A ) ; second, we observed no faunal borings in the rock (fig. 24A ) , whereas 

they are common southwest of the crater area; third, two 

large rock slabs were found to be exposures of reef plate and 

not slabs of debris. (See fig. 23 and detail in fig. 24B.) The 

slabs stand 30 to 100 em (1-3 ft) above the surrounding 

rock, which is covered by 30 to 60 em (1-2 ft) of sand. 

Apparently, the sand has filled in areas where about an equal 

thickness of reef plate has been stripped away by the blast. 

Why selective stripping occurred at this distance from the 

crater but not adjacent to the crater, where the scoured 

ripple-like features and bored surface are still intact, is not 

clear. 

Unfortunately, no rock was encountered in the 1.5-mdeep (5-ft-deep) pit that we dug closest to the crater margin. 

The digging indicates that the reef plate does extend continuously beneath the sand from the crater rim to the point 

where it is exposed near OAK core no. 2. This observation 

suggests that the conclusions of Tremba and others ( 1982, 

H18 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
til 

n 

c 

cAl 

0 

c-

(11 

tD 

~ 

e:. 

s· 

::I 

(II 

s. 

0 

> 

~ 

AI 

::I 

c. 

~ 

0 

> 

n 

~ tD 

~ 

Edge of 

Reef Pia 

• 

Figure 19. Aerial photograph of OAK crater taken in 1972 shows location of 30-m-diameter (100-ft) secondary crater (outlined by dots) and three core holes. Data from 

; cores are shown in figure 9. Dashed line shows location of transect in figure 23. Most of the debris rays have been removed by subsequent storm waves. Locations and 

..o scale are approximate. (Photograph courtesy of B. L. Ristvet.) 

Figure 21. Upturned flap in reef plate at LACROSSE crater (fig. 1) on east side of atoll near Run it Island. Compare with figures 

20A and B. 

p. 70--71)-that the reef plate at the crater rim slid toward 

the crater a distance of about 90 m (300 ft)-may be incorrect. This transect is south of the location of reef plate that 

they suggested had moved, and their interpretation cannot 

be ruled out. To remove the thick layer of overburden in the 

area of the central escarpment required a larger airlift than 

we had available. 

Fractures 

The exposed reef plate contains numerous fractures 

and joints, especially near the site of OAK core no. 2. In 

addition, we observed several faults with down-to-the-crater 

displacement of 8.5 to 15 em (3-6 in.). The rock contains 

pervasive microfractures (fig. 24A ). These microfractures 

made drilling difficult because the angular chips continually 

jammed the core barrel. Fracturing was not apparent at the 

site of OAK core no. 4 farthest from the crater. At that site, 

only the upper 1.5 m (5 ft) of section was cemented. The 

sediment in the remainder of the hole down to 7.6 m (25 ft) 

was mostly uncemented carbonate sand with some coral 

debris. 

Secondary Crater 

We located a secondary crater along our transect, 

approximately 244 m (800 ft) from the crater rim and about 

61 m (200ft) from the site of OAK core no. 3 (fig. 19). The 

crater is 30m (100ft) in diameter and, for the most part, is 

surrounded by upturned reef plate. At extreme low tide, 

most of the upturned rocks protrude a few centimeters above 

the water surface (figs. 25A,B). Water in most of the sandand-rubble-filled crater is less than 1.5 m (5 ft) deep at low 

tide, but at the western edge it is as deep as 4 m (13 ft), 

possibly due to wave and current scour of the bottom. The 

object that created this crater was not evident. If the crater 

Figure 20A (top left). Subsided and upturned reef plate near location of OAK core no. 2. (See fig. 1.) Note ripple-like features on 

upturned rock below diver (arrows on crests). Features have same orientation and spacing as those shown in figure 18. Water depth 

8 m (25 ft). 

Figure 208 (bottom left). Expanded view of upturned reef plate shown in figure 20A. 

Scuba Observations of OAK and KOA Craters H21 
Figure 22. The precipitous dropoff formed by subsided reef plate at location of OAK core no. 2. (See fig. 1 .) 

Note fractures and postevent corals (Porites lutea) in foreground. Carbonate sand covers the rock except 

at edge. Water depth is 9 m (30 ft). 

H22 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
OAK#2 

core hole 

sea level 

200 ft to OAK#3 ~ 

,..,400ft to secondary crater~ 

original reef plate surface? 

sediment 

ft m 

2 0.6 T -- ~~~! '2 __ 

white non-bored irregular 

surface with numerous 

fractures 

ft 0 

m 0 

4 1.2 

6 1.8 

8 2.4 

20 40 

6.1 12.2 

Figure 23. Cross section of pits, including OAK core no. 2, showing that subsided reef plate is continuous beneath sand. Much 

~f the reef plate surface has been stripped by the OAK event and is shown in inset. See figures 1 and 19 for location; see also 

f1gures 24A and B. Water depth at each pit was measured within a period of 15 minutes to avoid errors caused by changing tide 

level. 

is a secondary one, it is five times larger than those previously described by Ristvet and others (1978). 

Patch Reef 

A large patch reef is located about 2 km ( 1.1 nmi) 

southwest of OAK ground zero, just beyond the area covered by our bathymetric grid. (See SC no. 1, fig. 1). Its 

steep sides rise from a depth of 34m (110 ft)to 9 m (30ft). 

A sketch of the reef, which was named "Scorpion Conch 

Reef," is shown in figure 26A . Large fractures in it were 

probably caused by the OAK event (fig. 26B). 

High-resolution seismic-reflection profiles indicate 

that many of the lagoonal patch reefs near OAK crater have 

accumulated over small Pleistocene topographic highs 

(chap. C, this volume). Many of the mounds sampled with 

the submersible in the eastern part of the crater are composed of Pleistocene or pre-Holocene reefal material 

(chap. F, this volume). Farther from ground zero, similar 

mounds are composed of living and dead Holocene corals. 

These observations suggested that many of the features closest to ground zero may be pre-Holocene coral mounds from 

which the Holocene cover has been stripped away by the 

blast. 

Cores of the upper 8 m (25 ft) of the reef consist of 

slightly cemented coral and coralgal sand (recovery was 

poor) overlying uncemented sand between 8 and 12m (25-

35 ft); the latter was not recovered. No pre-Holocene rock 

was encountered. If Scorpion Conch Reef is like others in 

the lagoon, its Holocene cap may be in excess of 15 m 

(50 ft) thick. 

Discussion 

Like Ristvet and others ( 1978), we consider that the 

deepening of the reef flat opposite OAK crater (fig. 17) and 

the clearly evident down-to-the-crater subsidence and faulting (fig. 22) are the result of post-event subsidence, probably due to shock-induced liquefaction, grain reorganization, 

and dewatering. That such subsidence would have occurred 

instantaneously several hundred meters from ground zero is 

unlikely. Outside the vaporized area, expulsion of water 

controls localized subsidence, and the large numbers of 

tortuous interparticle pathways that escaping water would 

have to follow make it impossible for the water to escape 

quickly. Subsidence was probably greatest at ground zero 

and decreased with distance from it. How fast the reef plate 

subsided is not known. At KOA crater, Ristvet and others 

(1978) reported 2 m (6.5 ft) of subsidence just outside the 

crater rim associated with sand blows and water boils caused 

by escaping water. A similar situation undoubtedly took 

place at OAK. 

The precipitous rock wall at the west margin of OAK 

may be the maximum extent of cratering at the time of the 

event, that is, the wall may be the transient crater rim. The 

upturned attitude of the reef plate, similar to the rim around 

LACROSSE crater, supports this hypothesis. The possibility that it is a rotated slab of slumped reef plate cannot be 

ruled out. We observed much of the crater rim wall dipping 

into the crater toward the area of maximum subsidence, 

which corroborates the geophysical results (chap. C and D, 

this volume). 

Scuba Observations of OAK and KOA Craters H23 
Figure 24. A, Surface of reef plate rock exposed in pit in figure 23. Note lack of pseudoripples and boring. Surface is 

irregular and fractured. Small fracture is visible at right-hand side of photograph (see arrow). Tape designations are in 

centimeters. 8, Pit dug next to slab of rock showing that slab is actually a part of reef plate not stripped away. See inset 

in figure 23 for detail. Diameter of tape in spool is about 20 em (8 in.). 

H24 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
' 

Figure 25. A, Upturned rim of secondary crater as exposed at low tide (see arrows). Man is facing northwest and standing 

on upturned rim. Crater is 30 m (100ft) in diameter. 8, Underwater view of interior of secondary crater showing part of 

upturned rim. Rim is exposed at low tide. 

On the reef flat, post-Pleistocene material consists of 

well-cemented limestone, 1.5 to 2.5 m (5-8 ft) thick, that 

is underlain by 4.6 to 6 m (15-20 ft) of uncemented or 

poorly cemented sand; the sand, in tum, lies on cemented 

Pleistocene limestone. (See 0 AK core no. 1 , figs. 8, 11; 

OAK core nos. 2, 3, fig. 9). The unconsolidated sand probably facilitated the removal and peeling of the cemented reef 

plate from the underlying Pleistocene limestone. Peeling 

back of the reef plate may have created an upturned flap 

found along much of the west wall of OAK crater. As the 

Scuba Observations of OAK and KOA Craters H25 
• OAK GZ 

10 

m 

' 

20 

? 

• 

TD 11.6 m 

l38 ft I 

A 

Scorpion Conch Reef 

Figure 26A. Sketched cross section of Scorpion Conch Reef showing drilling location and thickness of section drilled. Brick pattern 

indicates slightly cemented coral and coralgal sand. Dots indicate uncemented coralgal sand. TD=total depth. 

wall subsided, most of the upturned ledge apparently tilted 

back toward ground zero. Only in one area (figs. 20A,B) did 

the flap retain a dip away from ground zero. If this is 

correct, then the transient crater diameter may be approximated by the distance from this flap to ground zero, about 

550 m (1 ,800ft). The reef buttressed the blast, resulting in 

a wider distribution of debris in the lagoon (chap. A, B, and 

C, this volume). 

The inrush of water to fill the crater excavated by the 

blast must have had significant effects on material in and 

around the crater. The sudden influx of water from the 

lagoon ward side of the crater, where waters were about 

45 m (150 ft) deep, undoubtedly transported much excavated material back into the crater and probably swept completely over the reef plate; however, its effect on the reef 

plate was probably less than elsewhere. The softer sediments in the lagoon were probably more affected by shock, 

by subsidence due to dewatering, and by erosion and deposition associated with the postblast rush of water back into 

the crater. 

H26 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Figure 268. Surface of Scorpion Conch patch reef at a depth of 9 m (30ft). Arrow points to fracture in 

foreground. Coral head near arrow is approximately 1 m (3 ft) in diameter. 

Scuba Observations of OAK and KOA Craters H27 
H28 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Conclusions-OAK Crater 

1. The west rocky escarpment of OAK crater is composed 

of subsided reef flat (upturned in one area) and may 

approximate the radius of the transient crater. On the 

northwest, reef-flat side of ground zero, therefore, we 

estimate that the transient crater radius is between 457 and 

550 m (1 ,500-1,800 ft). 

2. The transient crater wall northwest of ground zero subsided as much as 21 m (70 ft). 

3. Cratering was greatly influenced by the geology; energy 

from the event followed paths of least resistance, that is, 

along changes in bedding and lithology. 

4. Reef plate, 0.3 to 0.6 m (1-2 ft) thick, was stripped 

away as far as 122m (400ft) from the west crater rim. 

Observations of KOA Crater Area 

KOA was detonated in a geologically different setting 

from the one at OAK. KOA crater lies almost completely 

within a broad (greater than 1 km wide) reef flat, termed the 

"transitional area" by Ristvet and others (1978), at the northwest comer of the atoll, halfway between the windward east 

and northeast part of the atoll and leeward west rim of the 

atoll. The crater margins are clearly visible from the air, 

except on the southwest side where they merge with MIKE 

crater (fig. 27). Water surrounding the crater is less than 7 m 

(23 ft) deep in most places. To the northwest, the reef flat 

is only 30 to 60 em (1-2ft) deep at extreme low tides. 

Because of KOA' s location downwind from a reef flat 

thriving with sediment-producing organisms, the sedimentation rate, especially during storms, is high. Highresolution seismic-profile data (chapter C, this volume) indicate that about 9 m (30 ft) of postevent fill has 

accumulated. Because of this influx of sediment and the 

sluggish circulation of water in the crater, underwater visibility was seldom more than 6 m (20ft). During one 3- to 

4-day period in September, when winds blew from the west 

instead of the more common easterly direction, clear water 

moved into the crater, and underwater visibility improved to 

greater than 18 m (60 ft). 

Diver Observations 

The entire submerged margin of the crater, except for 

the part overlapping MIKE crater, was observed by two 

divers as they were towed behind Halimeda. The margin 

was even more thoroughly viewed from the submersible, 

which made dives along the 6-, 9-, and 18-m (10, 30, and 

60ft) isobaths. A ledge of rock is exposed near the north rim 

of the crater in 9 m (30ft) of water (fig. 28); this ledge was 

documented thoroughly on video and with a 10-cm-diameter 

(4-in.-diameter) core, 60 em (24 in.) long, to determine its 

age and composition. The rock is reef plate of Holocene 

age, similar to the reef plate that we observed at OAK 

crater. It is also identical to the rock we recovered in three 

cores collected from the reef flat north of KOA's rim, in 30 

to 60 em (1-2 ft) of water. 

The ledge of rock (fig. 28) is slightly upturned; however, because of its limited exposure (the ledge lies parallel 

to the crater margin for about 12m, or 40ft), we could not 

determine if it was a rotated slab partly buried in sand or part 

of a more continuous layer that had been upturned. It is most 

likely a small dislocated slab that was not overturned but has 

been down-dropped about 9 m (30 ft) from its original 

position on the reef flat. 

Most of the remainder of the the KOA crater margin 

consists of sediment. Along the north margin it is composed 

of sand-, pebble-, and cobble-size material that extends 

from 1.8 to 18 m (6-60 ft) of water depth. It lacks algal 

encrusters and appears to be freshly deposited. This sediment is probably frequently modified and supplemented by 

detritus transported into the crater from the adjacent reef 

flat. 

Along the east margin, the slope is less steep than 

along the north margin, and the sediment is finer grained. 

Staghom corals grow prolifically wherever rocks and manmade debris form a suitable substrate. We determined that 

massive head corals (Porites lute a) in the area began to 

grow on manmade objects within a year of the event. Other 

than the rock shown in figure 28, observations by scuba 

divers and from the submersible revealed few geological 

features useful to establishing the location of the transient 

crater margin. Manmade objects were discovered on the 

crater bottom; these objects provided data easily interpreted 

to indicate subsidence and a maximum crater diameter. The 

objects included a concrete test station, some railroad rails 

outside the crater in shallow water, and more significantly, 

a row of railroad rails in the crater in waters as much as 18 m 

(60ft) deep. These objects are described below, beginning 

with those located in shallow water. 

Concrete Test Structure 

The remains of a large 43-m-long (142-ft-long) test 

structure (sta. 360.01), originally emplaced to test blast 

effects, were found in approximately 2.4 m (8 ft) of water 

61 m (200ft) from the east edge of the crater (figs. 29, 30). 

Preevent photographs (fig. 31) show that the structure was 

originally located on the island just above sea level. Ristvet 

and others (1978) determined that it subsided about 2 m 

(6ft) within 8 days after the event (fig. 32). The station 

Figure 27 (top left). Oblique aerial photograph looking westward, showing KOA and MIKE craters. 

Figure 28 (bottom left). Slightly upturned Holocene reef plate in 9 m (30ft) of water in a small area near north edge of KOA crater. 

Sand covers all but the edge of rock layer. Fish are approximately 0.3 m (1 ft) long. 

Scuba Observations of OAK and KOA Craters H29 
H30 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Figure 30. Concrete test structure shown in figures 29 and 31. Test structure had three separate parts. Upper surface of structure 

was 3.5 m (11 .5 ft) above mean low water before KOA event. 

originally was 558 m (1 ,830ft) from ground zero (fig. 33), 

and its remains may have moved about 3 to 4 m (10-13 ft) 

closer to ground zero. 

Shallow Railroad Rails 

Near the concrete test structure is a line of railroad 

rails that were driven vertically into the bottom, spaced 

about 1.2 m (4ft) apart for a distance of about 30m (100ft). 

The rails form a line tangential to the crater margin (fig. 29). 

In places, timbers connect the base of the rails (fig. 34). 

Head and branching corals, including one colony of Pocillopora elegans that is 2.2 m (7ft) in diameter (fig. 35), grow 

on many of the rails. Many of the rails are bent away from 

ground zero. 

More widely spaced rails along the same line extend 

for a distance of 30m (100ft) toward the crater margin. At 

the crater margin, they lead to a considerable amount of 

debris, consisting mainly of riprap and other rails that define 

a partly rectangular shape. The rails lie in water approximately 4.6 m (15ft) deep outside the crater; some debris and 

rails extend down the slope into water as deep as 12 m 

(40ft). Examination of old photographs, especially those in 

figures 36 and 37, shows that this is the location of the 

collimator station over which the line-of-sight (LOS) tube 

led from the housing of the device at ground zero to a bunker 

on Bokaidrik (Helen) Island. The line of rails leading toward 

the collimator station was part of the 7-m-wide (24-ft-wide) 

causeway shown in figures 31 and 36. Both the causeway 

and the collimator station were constructed of rails lined 

with timbers, driven into the reef flat and infilled with riprap 

and sand. 

We verified the identity of the structure by locating 

the LOS supports (figs. 31, 36, 37). The remains of the 

23-cm-wide (9-in.-wide) I-beams protrude 0.3 to 0.6 m 

(1-2ft) from the bottom. They form a straight line leading 

from the bunker toward ground zero. We buoyed three of 

Figure 29 (left). Vertical aerial view of the east margin of KOA crater showing location of a 43-m-long (142-ft-long) concrete test 

structure (Holmes and Narver sta. 360.01 ). Railroad rails northwest of the concrete structure are the remains of a causeway leading 

to the collimator station. Southwest of the structure, a set of eight rails extends into the crater to a depth of 8 m (25ft). The east 

end of 28 more rails is marked by the circled cross at the lower left corner of the photograph. (Photograph courtesy of B. L. Ristvet.) 

Scuba Observations of OAK and KOA Craters H31 
H32 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
the beams with taut lines to establish their positions to aid 

the search for additional beams in the crater; none was 

found. 

The rails that remained standing were all on the east 

side of both the collimator station and the causeway-the 

side away from ground zero. Preevent photography indicates that water depth at the time of construction was less 

+ + 

than 1 m (3 ft). Maximum water depth was probably no 

more than 1. 8 m (3 ft). Clearly, the tops of the rails visible 

in figure 34 were about 1 m (3 ft) above water when the 

photograph in figure 31 was taken. 

The tops of the rails are now about 1.5 m (5 ft) below 

sea level. Subsidence took place as far as 100 m (330 ft) 

from the present edge of the sharp dropoff into the crater. 

+ 

COLLIMATOR STA. 

0 

0 

2 

"" 

Figure 33. A diagram of the preevent KOA area taken from a 

photocopy of a Holmes and Narver drawing (courtesy B. L. 

Ristvet), showing the locations of various structures that we 

located underwater close to or in the crater. The buoy over the 

line-of-sight (LOS) 1-beam support (triangle) and over station 

Figure 31 (top left). North part of concrete test station 360.01 

as it appeared before KOA event. Top of structure is 2.4 m (8 

ft) wide and 3.5 m (11.5 ft) above mean low water. Note 

line-of-sight (LOS) tube on top of 1-beams at top of photograph. 

LOS tube leads across collimator station at left and to KOA 

0 

I 

0 

0 

0 

200M ~ I 

360.01 (square) were located by H. V. Woodworth (of the 

Defense Mapping Agency) from shore with a theodolite. Most 

other buoys were located by the Miniranger System aboard a 

small boat. The Holmes and Narver location for station 360.01 

(shown by a cross) is 4 m (14ft) east of the position we buoyed. 

device shown in figure 37. Arrow shows vertical railroad rails 

used to make bulkhead of causeway leading to collimator station. Rails under arrow are same rails shown in figures 34 and 

35. (Photograph courtesy of B. L. Ristvet.) 

Figure 32 (bottom left). The northern part of station 360.01 subsided 2 m (6ft) about 8 days after KOA event as it appears here. 

Note water level shown by arrow. (Photograph courtesy B. L. Ristvet.) 

Scuba Observations of OAK and KOA Craters H33 
Figure 34. Line of rails with corals attached leading toward collimator station. View is to the north. Note timbers remaining at base 

of rails. These rails are the same as those shown by arrow in figure 31 . Rails on side of causeway facing KOA ground zero were 

removed by the event. 

Railroad Rails within KOA Crater 

The most significant railroad rails were discovered in 

KOA crater during submersible reconnaissance dive no. 90. 

The rails were later buoyed and examined by scuba divers. 

The depth and spacing of the 28 rails was carefully determined, and the entire line was documented photographically 

and on videotape transects. The easternmost rail lies in 15 m 

(50ft) of water; the line extends westward (toward ground 

zero) for a distance of 46 m (152ft) into water that is 18 m 

(60ft) deep (table 1). Most rails protrude above the bottom 

more than 1.8 m (6 ft), many are at least 3 m (10 ft) high, 

and one is 4.4 m (14.5 ft) long. Most are bent away from 

ground zero, and all have their flat bases oriented to the 

south (fig. 38). This set of rails is aligned with another set 

of eight rails that are located at the crater margin. The 

eastern end of this shallower line lies in 6 m (20 ft) of water 

(fig. 29) and extends westward into about 7.6 m (25 ft) of 

water. The bases of this line face south. No more rails were 

found in the area between the two sets of rails. Several 

cables, including one that was 10 em (4 in.) in diameter and 

aluminum-shielded, extend into the crater from the site of 

H34 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
Ill 

I") 

c 

0"' 

1:.1 

0 

0"' 

~ 

~ 

~ 

6' 

:I 

"' a 

0 

)I> 

;:111:: 

1:.1 

:I 

c.. 

s 

)I> 

n 

~ ;1 

::t 

w 

1.11 

Figure 35. Detail of southernmost rail, which has a 2.2-m-diameter (7-ft-diameter) colony of Pocillopora elegans attached. Note timbers at base-of line. Tape 

designations are in centimeters. 
H36 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 
the shallow rails just south of the concrete structure shown 

in figure 29. The cables pass down into the sediment in 

about 6 m (20ft) of water. 

Using the hydraulic drill suspended from an airbag, a 

3. 7-m (12ft) hole was cored 1 m (3ft) north of the last rail 

in 18 m (60ft) of water. The first 1.5 m (5 ft) of sediment 

was carbonate sand, which was not recovered; the remainder was rubble (probably riprap), and recovery was poor. 

Another site was cored 15 m (50ft) closer to ground zero, 

and the same type of material was recovered. We dug a 

1.5-m (5 ft) pit at the south side of the base of the last rail 

in 18m (60ft) of water. Most of the material there was sand, 

but the pit also contained riprap, tree limbs, and pieces of 

timber. The material appeared to be fill, possibly used to 

construct a causeway like the one that led to the collimator 

station. 

All the rails were driven in with their flat bases oriented to the south, normal for rails used to hold back an 

earthen bulkhead or causeway on their south side. Timbers 

still attached to the collimator causeway (fig. 34) attest to 

this style of construction. Engineering drawings (dated December 27, 1957, Holmes and Narver) indicate that the rails 

shown in figure 34 were driven at least 1.2 m (4 ft) into 

"coral" and 1.8 m (6 ft) into sand. The timbers were attached to the base of the rails with 1.27-cm-diameter (112-

in.-diameter) bolts. 

The line of rails has a distinct bend in it. Most rails lie 

in an east-west line perpendicular to the crater margin, but 

between rails 16 and 17, the line bends 5-10° to the northwest toward ground zero. 

The photographs and the Holmes and Narver diagrams of the preevent KOA area show a causeway located 

south of the deep rails (fig. 37). A bend in the causeway, 

shown by the arrow in figure 36, lies about 70 m (230 ft) 

west and 60 m (200 ft) south of the similar bend in the 

railroad rails, suggesting that the bend in the railroad rails is 

the same bend as in the causeway. If this is correct, the rails 

have moved at least 90 m (300 ft) to the northeast. Slumping, probably common in this area (chap. A, this volume), 

appears to have moved surface material to the northwest into 

the crater. It is improbable that a large portion of the bottom, 

Figure 36 (top left). Pre-KOA event aerial photograph showing 

line-of-sight tube crossing to collimator station (solid white 

arrow), concrete test station 360.01 (open arrow), and curve in 

bulkheaded shoreline (wide white arrow). (Photograph courtesy of B. L. Ristvet.) 

Figure 37 (bottom left). Aerial oblique photograph, looking 

east, of KOA device in water tank at lower right and line-ofsight tube leading from bunker in distance. Wide arrow indicates curve in bulkhead shown in figure 36. Riprap is visible 

below arrow. Cylindrical tank is 10 m (30 ft) in diameter. 

(Photograph courtesy of B. L. Ristvet.) 

Table 1. Spacing of rails in KOA crater beginning in 18 m 

(60ft) of water closest to ground zero and ending in 15m (50ft) 

of water farthest from ground zero 

[ GZ = ground zero] 

Rail Distance 

between rails Comments no. (meters/feet) 

1----0 

2----1.6 

3--- -1.5 

4----1.3 

5----1.2 

6----0.9 

7----1.4 

8----1.4 

9----1.8 

10----2.0 

11----2.4 

12----2.0 

13----1.5 

14----1.6 

15----1.3 

16----1.4 

17----4.8 

18----4.8 

19----1.8 

20----1.6 

21----1.3 

22----1.4 

23----1.3 

24----1.3 

25-- - -1.5 

26----1.2 

27----1.5 

28----1.6 

0 

5.3 

4.8 

4.4 

4.1 

2.9 

4.7 

4.7 

5.8 

6.4 

7.8 

6.7 

5.0 

5.2 

4.2 

4.7 

Long, bent toN. large clump of staghom 

coral at base. 

Long, bent toN. 

Clump of coral on top. 

Short, 1 m (3 ft) high, staghom between 

it and next rail. 

Tall but broken. 

Tall, 4.4 m (14.5 ft) and bent toward 

GZ, horizontal at top with coral, almost touches rail 5. 

Short, less than 1 ft long, broken. 

Long, bent toward GZ and S. 

Tall, bent away from GZ. 

Tall with clump of branching coral at 

base, top bent away from GZ. 

Tall with clump of coral midway up . 

Tall, bent away from GZ. 

Tall, bent away from GZ . 

Tall, bent away from GZ, clump of coral 

at top. 

Tall, bent away from GZ. 

Tall, bent away from GZ, coral at base, 

long space to next rail. 

Significant bend occurs between rails 16 

and 17. 

15.7 Tall, bent toward GZ?, long space to 

next rail, staghom coral to N. 

13.1 Tall. 

5. 8 Tall with parasol-shaped staghom coral 

at base. 

5.2 Medium size, staghom coral to S. 

4.4 Short with Porites on top, broken rails 

on bottom. 

Broken rail between rails 21 and 22, lots 

of debris, 4 rails on bottom. 

4.5 Long, bent away from GZ . 

4.2 Medium length, bent away from GZ, rail 

on bottom between rail 23 and 24. 

4.2 Long, bent away from GZ and touching 

rail 25. 

4.8 Long, bent away from GZ and touching 

rail 24, rail 25 is 3.5 m (11.6 ft) 

long. 

4.0 0.9 to 1.2 m (3- 4ft) high, rail straight. 

4.8 1 m (3 ft) high, straight. 

5.4 0.2 m (0.8 ft) of rail bent away from GZ. 

Scuba Observations of OAK and KOA Craters H37 
--10 15---.. 

16 

Figure 38. Underwater views of deep 

railroad rails in KOA crater. Numbers 

correspond to numbers in table 1. A, 

View looking away (eastward) from 

ground zero. 8 and C, views looking 

toward ground zero. Base of rails is to 

south, as they would be if used to 

form the bulkhead shown in figures 

36 and 37 . 

H38 Sea-Floor Observations and Subbottom Characteristics, OAK and KOA Craters 

---
at least 46.3 m or 152 ft long, could move 300 ft (90 m) 

away from ground zero while maintaining the linear integrity of 28 railroad rails. 

After reexamining our techniques and records, the 

navigation fixes appear to be correct, and the rails are accurately located. Dives 90 and 109 both locate the rails in 

roughly the position shown. We moored buoys with taut 

lines over the deep rails, and the buoys were located with the 

Miniranger system aboard a small boat. These locations 

coincide closely with those established by the submersible 

(chap. F, this volume). 

It is possible that the rails are at or close to their 

original positions and that they were not part of a structure 

included in the preevent documentation (fig. 33). If that is 

the case, the most craterward rail lies at 393 m (1 ,290 ft) 

from ground zero. Because we found materials in our core 

15 m (50ft) beyond the last rail that are similar to material 

at the last rail, we can subtract this value from the distance 

to ground zero; therefore, the resulting figure of 378 m 

(1,240 ft) may represent the maximum transient crater radius in this area. 

If no rails were set at the locations they presently 

occupy, those in deeper water must have moved from their 

original locations at either the shoreline or the causeway. If 

we assume that the bend in the rails was acquired due to 

relative movement of the substrate, the need for a correlation with the bend in the causeway is eliminated. The line 

of rails can then at least be partially reconstructed by joining 

the shallow group with the deep group (fig. 33); this means 

that the deep rails must have moved about 56 m (184 ft) 

toward ground zero in a postblast slump. Thus the riprap 

that was drilled 15 m (50 ft) craterward of the deepest rail 

would have been about 430 m (1 ,411 ft) from ground zero 

before slumping took place. This distance probably represents a better estimate for the maximum transient crater 

radius in the southeast quadrant of the crater. 

*U.S. GOVERNMENT PRINTING OFFICE: 1986-491-405:20129 

REFERENCES 

Cloud, P. E., Jr., 1959, Geology of Saipan, Marianas Islands, Part 

4-Submarine topography and shoal-water ecology: U.S. 

Geological Survey Professional Paper 280-K, p. 361-445. 

Couch, R. F., Jr., Fetzer, J. A., Goter E. R., Ristvet, B. L., 

Tremba, E. L., Walter, D. R., Wendland, V. P., 1975, 

Drilling operations on Enewetak Atoll during Project EXPOE: Air Force Weapons Laboratory Technical Report TR75-216, Kirtland Air Force Base, New Mexico 87117, 

270p. 

James, N. P., and Ginsburg, R.N., 1979, The seaward margin of 

Belize barrier and atoll reefs: International Association of 

Sedimentologists Special Publication No. 3: Blackwell Sci-

, entific Publications, Oxford, England, 191 p. 

Ristvet, B. L., Tremba, E. L., Couch, R. F., Jr., Fetzer, J. A., 

Goter, E. R., Walter, D. R., and Wendland, V. P., 1978, 

Geologic and geophysical investigations of the Eniwetok nuclear craters: Air Force Weapons Laboratory Technical Report TR-77-242, Kirtland Air Force Base, New Mexico 

87117' 298 p. 

Shinn, E. A., 1963, Spur and groove formation on the Florida reef 

tract: Journal of Sedimentary Petrology, v. 33, no. 2, p. 291-

303. 

Shinn, E. A., Hudson, J. H., Halley, R. B., Lidz, B., Robbin, D. 

M., and Macintyre, I. G., 1982, Geology and sediment accumulation rates at Carrie Bow Cay, Belize, in Rutzler, K., and 

Macintyre, I. G., eds., The Atlantic barrier reef ecosystem at 

Carrie Bow Cay, Belize, 1: Structure and communities: 

Smithsonian Contributions to the Marine Sciences, v. 12, 

p. 63-75. 

Shinn, E. A., Hudson, J. H., Robbin, D. M., and Lidz, Barbara, 

1981, Spurs and grooves revisited: Construction versus erosion, Looe Key reef, Florida: Proceedings of the 4th International Coral Reef Symposium, Manila, Philippines, v. 1, 

p. 475-484. 

Swanson, R. G., 1981, Sample examination manual: Methods in 

exploration series: American Association of Petroleum Geologists, 35 p., appendices 83 p. 

Tremba, E. L., Couch, R. F., Jr., and Ristvet, B. L., 1982, 

Enewetak Atoll Seismic Investigation (EASI): Phases I and II 

(final report): Air Force Weapons Laboratory Technical Report TR-82-20, Kirtland Air Force Base, New Mexico 

87117, 124 p. 

Scuba Observations of OAK and KOA Craters H39