Seismic-Refraction 

Studies of Eniwetok 

Atoll

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-S
Seismic-Refraction 

Studies of Eniwetok 

Atoll

By RUSSELL W. RAITT

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-S

A study of seismic-wave velocities 

down to the Mohorovfcic discontinuity

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 20 cents (paper cover)
CONTENTS

Page Page

Abstract_---_-_-----_-----------___-______-________ 685 Travel-time plots____--__-----_-___--_______- _______ 690

Introduction. ______________________________________ 685 Accuracy of travel times__-___________________.._____ 692

Acknowledgments_________________________________ 685 Velocities..-______-__- _____________________________ 694

Operating procedure_-_--____---____________________ 686 Subsurface structure_---__--_-__--___-_-___________- 695

Shore stations__________________________________ 687 Reef structure________________________________ 695

Reflection studies__-_-___-______________________ 687 First and second layers__________________________ 695

Lagoon survey.______________________________ 687 Third layer._________________________________ 696

Deep-sea survey______-___-___-_________________ 687 Fourth layer_-_---_-----__------_--_------____- 696

Firing procedure___________________________ 688 Fifth layer__________________________ 697

Receiving equipment____._______________________ 688 Sixth layer_-___---__----_-_---___-__- _________ 697

Theory of interpretation of seismic-refraction observa- Bibliography _____________________________ __________ 698

tions------------_---_-__---__-_____--_---___-_ 688

ILLUSTRATIONS

	Page 

FIGTJKE 219. Location of shot lines and receiving stations-_________________________________________________________ 686

220. Model of subsurface structure assumed for interpretation of travel-time plots_____________________________ 689

221. Travel-time plots for section through stations A. B, C, D, E, K, and K'___ ______________________________ 690

222. Travel-time plots for section northwest from station A_____-__________________________-___--____-_-_-__ 690

223. Travel-time plots for section through stations F, D, G, and L____-____________----_-_-____----_---_-__-_ 691

224. Travel-time plots for section through station J___________________________________,____________________ 691

225. Travel-time plot of short profile across reef northwest of island of Elugelab____________-__________-_______ 692

226. Travel-time plots for section through stations F, B, and A______________________________________________ 692

227. Travel-time plots for section through stations E and G_____________________________-___-_---_------___- 693

228. Travel-time plots for section through stations M and I________________-_______-______-____-_-_________- 693

229. Travel-time plots for shots received at station H_ __ ___________________________________________________ ' 694

230. Oscillograms of J>£-pound shots at short range and 80-pound shots at long range___________________________ 694

231. Hypothetical subsurface section through Eniwetok Atoll from north to soiith________________-_____________ 696

232. Hypothetical subsurface section through Eniwetok Atoll from west to east________________________________ 697

in

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

SEISMIC-REFRACTION STUDIES OF ENIWETOK ATOLL

By RUSSELL W. EAITT *

ABSTRACT

Observed seismic velocities at Eniwetok Atoll indicate the 

presence of 6 layers of rock, in which the average velocities are 

2.44, 3.06, 4.15, 5.59, 6.90, and 8.09 kilometers per second, respectively. Previous drilling permits the identification of the first 

2 layers as calcareous deposits and the third layer (4.15 

knips) as volcanic. The calculated depth of the third layer 

beneath the drilling sites was about 0.3 km greater than the 

depths at which volcanic rock was penetrated by the drill. This 

may be due to the fact that, in calculations, the vertical seismic 

velocities in the first 2 layers were assumed to be equal to the 

horizontal velocities measured in those layers, whereas the vertical velocities are probably less because of the presence of unconsolidated material between the cemented layers in the first 

2 zones.

The fourth layer (5.59 krnps) is thought to be a hard crystalline rock, probably basalt. The fifth layer (6.90 kmps), identified with the crustal layer extensively present in the Pacific 

Ocean basin, is found at a depth of 9-10 km, and the sixth layer 

(8.09 kmps), characteristic of the layer just below the 

Mohoroviclc discontinuity, is reached at a depth of 16-17 km.

INTRODUCTION

Seismic-refraction observations at Eniwetok Atoll in 

October and November 1952 had as their objective the 

delineation of the volcanic-rock layer immediately beneath the lagoon and the determination of the seismicwave velocity as a function of depth down to the 

Mohorovicic discontinuity. Primary emphasis was 

placed on determining the seismic velocity in the layer 

of calcareous sediments forming the upper surface of 

the atoll and the configuration of the volcanic rock immediately beneath.

In planning the survey, the previous seismic-refraction studies at Bikini and Kwajalein (Dobrin, Perkins, 

and Snavely, 1949; Raitt, 1954) and the drilling at Eniwetok (Ladd, Ingerson, Townsend, and others, 1953) 

were very helpful. The Bikini studies indicated the 

form of the travel-time data to be expected. The drilling at Eniwetok reached a hard formation at a depth 

of 4,610 ft (1.41 km) beneath the island of Elugelab 

and 4,170 ft (1.27 km) beneath Parry Island. At

1 Scripps Institution of Oceanography, University of California. 

NOTE.—Contribution from Scripps Institution of Oceanography, New 

Series 929.

Parry Island the hard formation was determined to be 

olivine basalt. This gave additional support to the 

identification of the 4-kmps (kilometers per second) 

layer at Bikini, found at an average depth of 4,300 

ft (1.3 km), as volcanic rock and indicated that the 

structure at Eniwetok and Bikini is similar.

ACKNOWLEDGMENTS

This work was supported by the Office of Naval Research under Contract Nonr-233(05) and the Bureau 

of Ships under Contract NObsr-43356. Support from 

the Office of Naval Research was administered by 

J. W. Smith, who arranged for the delivery of the 

large quantities of explosives and other supplies needed 

in the investigation.

A seismic-refraction operation of the magnitude of 

the Eniwetok studies requires the cooperation of many 

people, whose help is gratefully acknowledged.

H. Kirk Stephenson of the Los Alamos Scientific 

Laboratory directed the project for that organization, 

and his active participation in the survey was an essential feature of its success. J. D. Isaacs was scientific 

leader, and Comdr. C. N. G. Hendrix was liaison officer 

for the Scripps Institution scientific group. Comdr. 

John Lofland, USN, and Comdr. Theodore Eunson, 

USN, arranged for the necessary shore installations, 

transportation, and ship facilities.

The officers and crews of U. S. S. Lipan (ATF 85), 

R/V Horizon, and R/V Spencer F. Baird were of great 

help. Their commanding officers were Lt. R. M. Hughson, Noel L. Ferris, and Laurence E. Davis, respectively.

Members of the firing and recording groups were 

Joel H. Swartz, U. S. Geological Survey; Edwin M. 

Hamilton and Maxwell Silverman, U. S. Navy Electronics Laboratory: Chief Gunners Mate Daniel Howlett, USN; and Willard C. North, Daniel K. Gibson, 

Arthur D. Raff, Richard J. Smith, Alan C. Jones, 

Richard P. VonHerzen, and Winter D. Horton, Jr., 

Scripps Institution of Oceanography.

685
686 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

FIGURE 219.—Location of shot lines and receiving stations. Depth contours are in fathoms.

The author is indebted to Beauregard Perkins for 

suggestions made during his technical review of the 

paper.

OPERATING PROCEDURE

The plan of operation is shown in figure 219. The 

topography of the bottom surrounding the atoll was 

previously determined by K. O. Emery (Emery, Tracey, 

and Ladd, 1954). The letters represent receiving stations, and the heavy solid lines represent the lines along 

which shots were fired. The spacing of the stations 

and the directions of the lines in the lagoon were 

determined by the desired objective of optimum reversed control of the volcanic layer and by the necessity that the firing ship stay well clear of the coral

shoals scattered throughout the lagoon. In extending 

the lines into deep water, advantage was taken of the 

two navigable passages on the southern and eastern 

sides.

Taken together, the lines of shots provide 2 sections 

across the atoll, 1 roughly north and south, and the 

other west and east. The north-south line, with eight 

stations, was laid out not only to obtain adequate data 

concerning the upper layers but also to extend the 

lines to maximum feasible range in the hope of achieving penetration to the Mohorovicic discontinuity beneath the atoll.

Three types of stations were occupied: shore stations, A and H, where portable equipment was set up
SEISMIC-REFRACTION STUDIES, ENIWETOK ATOLL 687

in tents on Elugelab and on Parry Island; lagoon 

stations, B-G, at which the U. S. S. Lipan (ATF 85), 

at anchor, served as receiving ship, using the same 

type of portable equipment used ashore; deep-sea stations, I-M, at which the Scripps Institution of Oceanography's R/V /Spencer F. Baird served as receiving 

ship. The Baird was also used at lagoon station G 

for the line G-L. During this operation, and also 

while Baird was at K receiving shots fired on line 

D-E, the portable equipment from station A and the 

U. S. S. Lipan was used at station H.

SHORE STATIONS

In order to tie in the survey to the points at which 

drilling had determined the depth to volcanic rock 

(Ladd, Ingerson, Townsend, and others, 1953), receiving stations were occupied at the drill holes on the island 

of Elugelab and Parry Island. At Elugelab (station 

A) tests using conventional geophones at the surface 

were unsatisfactory owing to a high noise level. The 

signal-to-noise ratio of the refracted waves was much 

better for a hydrophone lowered into the drill hole. 

Consequently all shots used at station A were received 

on a hydrophone lowered to a depth of 250 ft in the 

test hole drilled on Elugelab. Shots received at this 

station were fired in the lagoon along the lines from F 

to B, and from B through C and D to E. Shot distance 

for these profiles was obtained with the help of a radio 

sonobuoy placed in the lagoon 800 yd from station A 

in about 10 fathoms depth. Shots outside the atoll 

were fired along a line bearing approximately 330° 

until well out from the base of the slope, thence bearing 0°. Shot distances for this line were obtained with 

the aid of a radio sonobuoy placed about 100 yd 

beyond the reef opposite station A.

A profile from station A to the edge of the outer reef 

was recorded to determine the differences, if any, between reef and lagoon structure, and to complete the 

short-range part of the profile extending northwestward to deep water. Shots were fired from a DUKW 

in water of about 4-foot depth normally covering most 

of the outer reef opposite Elugelab. The shot instant 

was transmitted by radio sonobuoy and the shot position 

was surveyed by taking sextant angles between known 

points near the Elugelab drill hole.

At Parry Island the noise level in the drill hole was 

higher than at Elugelab; hence the seismic waves were 

received on a hydrophone planted on the lagoon bottom 

(station H) about 500 yards from shore opposite the 

Parry Island drill hole. In this location, the noise 

level was much lower than in the drill hole. The depth 

was about 10 fathoms. At this station shots fired alongo

lines D-E and G-L were recorded.

REFLECTION STUDIES

While setting up and testing the Elugelab shore station, about 2 days were spent attempting to obtain reflections from the volcanic rock, known from drilling 

(Ladd, Ingerson, Townsend, and others, 1953) to be at 

a depth of 4,610 feet at this point. Geophones and 

drill-hole hydrophones were used. Shots were fired at 

varying distances of as much as one-half mile from 

station A. Records of all shots showed irregular disturbances for several seconds following the first arrival, with no outstanding reflections identifiable with 

the volcanic rock or other subsurface interface.

Although these tests do not preclude the possibility 

that good reflections might be obtainable elsewhere, it 

was concluded that it would be inadvisable to plan a 

reflection survey when reflections were unobtainable at 

one of the principal control points.

LAGOON SURVEY

In the initial plans for the survey, the Baird was to 

serve as a receiving ship for all stations except the shore 

stations, and it was fitted out for this purpose. However, owing to a delay of about 2 weeks in the arrival of 

the Baird, the U. S. S. Lipan (ATF 85) was used as a 

substitute. The receiving equipment, which consisted 

of the portable instruments from the Elugelab shore station, was more primitive than the multichannel installation on the Baird, but this was not a serious handicap 

to the lagoon survey as noise levels were generally low, 

and, except for the long profiles extending outside the 

lagoon to the south, the ranges were short.

At each receiving station, the receiving ship was carefully anchored with reference to gyrocompass bearings 

on the nearest island points. The receiving hydrophones were suspended at a depth of 75 ft and floated 

about 100 yd from the ship. Owing to the swinging 

of the ship at anchor, the hydrophone moved through a 

considerable arc, but the combined departure from the 

normal station positions shown in figure 219 due to this 

effect and to errors of position determination was probably no more than 200 yd. The firing ship similarly 

maintained position along the designated lines by gyrocompass bearing^, and with the same probable departure. Distances between shot and hydrophone were 

obtained much more accurately than this by measurement of the travel time of the waterborne sounds; therefore the error in the ship's position would not affect the 

value of the depth calculation but would cause a maximum error of 200 yd in the location of the depth 

value.

DEEP-SEA SURVEY

Fortunately, the Baird arrived as the lagoon phase 

of the survey was completed. The deep-water survey
688 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

could not have been carried out with the use of the 

Lipan as it had no deep-water echo sounder, which is 

essential for calculation of the water delay in deep-sea 

seismic-refraction work. The final deep-sea phase of 

the survey was completed with the Baird, and the 

portable station on the Lipan was moved to the Parry 

Island drill hole.

At deep-water stations I, M, J, K, K', and L, anchoring was not practicable and the Baird either lay to or 

was underway slowly, attempting to maintain position. 

Throughout much of this phase the wind was high, 

frequently having a velocity of 17-21 knots, and the 

combined wind and current drift while lying to was 

between 2 and 3 knots. Attempts to maintain position by turning the screw slowly were only partially 

successful. Hence the positions shown are only rough 

estimates of the average position during the shot recordings. In most cases the Baird was probably within 1 nautical mile of the indicated position. An exception was the large drift during the shooting of the 

first part of profile J, indicated by a dashed line. 

During the shooting along the southwest end of this 

line, the Baird got underway and held position reasonably well. As stated above, these changes in position of the receiving ship affect only the location of 

the depth value. Seismic-refraction operations under 

these conditions, from the standpoint of noise level, 

firing conditions, and general operational difficulties, 

-are marginal.

FIRING PROCEDURE

With the exception of a few experimental reflection 

shots and the short profile across the Elugelab reef 

(see Shore Stations), all shots were fired from the 

Scripps Institution of Oceanography's R/V Horizon, 

which proceeded along the courses indicated in figure 

219 at speeds ranging from 6 to 11 knots. Shots were 

made up from tetrytol demolition blocks. The charges 

varied from one-half pound at distances less than 5 

km from the receiving station to as much as 80 

Ib at the maximum range of 50-100 km. They were 

fired with slow-burning fuse ignited just before being 

dropped overside. The time of firing was recorded 

by picking up the direct blast on the Horizon's echo 

sounder and transmitting it by radio to the receiving 

ship, where it was recorded by the oscillograph used 

to record the seismic waves.

RECEIVING EQUIPMENT

The Baird's receiving equipment was very similar to 

that used in the Bikini survey (Raitt, 1954). Three 

hydrophones were suspended by their electrical cable 

from surface buoys to depths of 75 ft in the lagoon 

survey and 200 ft in the deep-sea survey. In the lagoon 

ihe very slow currents permitted the suspending cable

to hang vertically and the hydrophone depth was very 

close to 75 ft. In the deep sea, however, the wind and 

current drift caused the suspension to assume a substantial angle with the vertical, and the average depth 

was therefore probably close to 120 ft. A multichannel 

oscillograph on the Bcdrd recorded the outputs of the 3 

hydrophones on 3 low-frequency (3-20 cycles per second) channels, on 3 intermediate-frequency channels 

(20-200 cps), and on 3 high-frequency channels (500- 

3,000 cps). All channels recorded at 2 levels of sensitivity of 20-decibel-level difference. Except at very 

short range the seismic waves through the earth were 

received only on the low-frequency channels. The 

higher frequencies were used to identify the waterborne waves, whose travel times were used to determine 

the shot distance.

Timing on the oscillograph was obtained by timing 

lines flashed every 1/100 second, with 1/10 seconds accented. The timing disk was driven by a forkstabilized synchronous motor. The accuracy of the 

timing was controlled by frequent comparison with a 

break-circuit chronometer which was calibrated daily 

with time signals from radio station WWV.

A two-channel pen oscillograph was used at the portable station ashore and on the U. S. S. Lipan. One 

channel recorded the low-frequency seismic waves, the 

other performed the double function of recording the 

radio-transmitted firing signal and the high-frequency 

waterborne sound. Some records were made with 2 

hydrophones, both recording the low-frequency waves 

but only 1 the high-frequency waves. The low noise 

levels and good signals obtained in the lagoon gave 

little additional advantage to the use of more than one 

hydrophone. However, in the deep-sea work the noise 

levels from rough sea conditions gave a real advantage 

to using three hydrophones, as was done on the Baird.

THEORY OF INTERPRETATION OF SEISMICREFRACTION OBSERVATIONS

Knowledge of the earth's interior is yielded both by 

travel times of sound waves and by the intensity and 

spectra of the waves. However, travel tunes are more 

tractable in analysis and more easily measured. In the 

present study, as in most seismic interpretations, the 

travel times form the sole basis for the interpretation 

of the subsurface structure.

The specific methods used in the interpretation were 

the same as those used by the author in the Bikini survey (see Raitt, 1954) ; the formulas and diagrams are 

repeated here for reference.

Figure 220 illustrates a subsurface section consisting 

of three layers in which the seismic velocities are 00, 

<?i, and c2. In seismic work at sea the upper layer is 

water. For the example illustrated, the velocities G:
SEISMIC-REFRACTION STUDIES, ENIWETOK ATOLL 689

FIGURE 220.—Model of subsurface structure assumed for interpretation of travel-time plots.

and cz are 2 and 4: times as great, respectively, as <?<>, 

the velocity in water. Points A and B represent receiving stations and m and n represent shot points. 

The sound rays shown are critically refracted through 

the third layer. The symbols for the various angles 

and the thicknesses of the layers, used in the calculations, are illustrated in the figure.

The travel time, TAm, for a sound pulse to travel from 

m to a is

XAm . ZAO cos

= —— ——— \ ————— - —

c2

cos

COS QAI . 2ml COS 9ml

Ci Ci

and the travel time, TBn, for the pulse to travel from n 

to B is

T __XBn , zBO cos 0BO , zn0 cos 9n0 , 

J-Bn— ~r ~T T~ 

c2 c0 c0

——————I——————> ZB1 COS Sm , g ral COS dnl (Z)

where XAm and XBn are the horizontal distances between 

shots and hydrophones.

/ X \

In equation 1 the term ( TAm——— ) is called the inV c% /

tercept time (Gardner, 1939). The four remaining 

terms of equation 1 are called delay times and are 

designated TAO, rm0, TAI , and rml , respectively.

If the delay times for the water layer are subtracted 

from the total travel times, the corrected travel times 

are

V

T

t _-**-Am | | Am— ——— ~t~ TAI ~T Tm\ 

C2

and

rni _-^-Bn 1 Bn— ——— 

C2

(3a) 

(3b)

For gently sloping structures the seismic velocity in 

the third zone is, to a sufficient degree of approximation,

(4)

1

dXAm dXBn

If the contact between the second and third layers is 

nearly horizontal and if shots m and n are fired at the 

same point m, rm and rml may be considered equal to a 

first approximation; and if a third shot is fired at point 

A and the resulting pulse is critically refracted through 

zone G2 and recorded as a first arrival at point B, then 

the delay times at A, m, and B are determined by the 

three measured intercept times,

V

-^-A

and

C2

V

rr\t **-Bn ___ 1 Bn ————— — 

C2

rrit AB i AB —————— =TAi-f-TBi.

(5a) 

(5b) 

(5c)

The values of TAI, Tmi , and rm may be derived by 

solving equations 5a-c simultaneously. The thicknesses of the second layer beneath points A, m, and B 

to a first approximation are

(6a).

(6b)

VKU

and

(6c)

419660—57———2
690 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

r-j. r J *»• **v^» ^9 *^.^- >l! 80 J-^* -^ • <-*-• •-•"

>•-*. ^'" 5.69.,,.-'^ •2*^ .."' .

10 15 20 25 30 35 40 45 50 55

DISTANCE, IN KILOMETERS

60 65 70 75 80 85 90 95

FIGURE 221.—Travel-time plots for section through stations A, B, C, D, B, K, and K'. Times axe corrected for water delay; apparent velocities

are in kilometers per second.

The process here outlined can, in principle, be repeated indefinitely for deeper horizons of successively 

higher speeds of transmission. The practical difficulties and cumulative errors become severe if the number 

of layers with small velocity contrasts is large.

TRAVEL-TIME PLOTS

The observed travel times of the bottom-refracted 

waves, corrected for water delay, are plotted in figures 

221 to 229. In order to facilitate comparison of profiles, they are plotted together in related groups. For 

example, figure 221 contains all shots received at stations 

A, B, C, D, E, K, and K', fired along the lines that pass 

through each of these stations. For each time-distance 

graph the distance of each shot point from the corresponding receiving station is accurately plotted. Since 

the receiving station was fixed and the shot point varied 

for each profile, the zero point for each graph represents the true distance of the receiving station from the 

traverse reference station shown at zero distance (for

i r i i i r i l I r

80 75 70 65 60 55 50 45 40 35 30 25 20 15 IO 5 0 5 10

DISTANCE, IN KILOMETERS

FIGURE 222.—Travel-time plots for section northwest from station A. Times are corrected for water delay; apparent velocities are

in kilometers per second.
SEISMIC-REFRACTION STUDIES, ENIWETOK ATOLL

1—————I—————I—————I—————I—————I—————I—————I———

691

5.49

4.33^ ft'

•'* 

ff

I I I |

20 25 30 35 40 45 

DISTANCE, IN KILOMETERS

50 55 60 65 75

FIGURE 223.—Travel-time plots for section through stations F, D, G, and L. Times are corrected for water delay; apparent velocities

are In kilometers per second.

example, from station A in figure 221). Because the 

receiving stations are generally not on a straight line, 

the distances from the shot points to stations other than 

the receiving station or between stations other than the 

reference station are only approximate in the graph.

It can be seen that most of the travel-time plots can be 

represented roughly by segments of straight lines. The 

lines drawn on the plots have been fitted by inspection, 

and the corresponding apparent velocity for each segment is indicated in kilometers per second (kmps). Just 

as in the Bikini survey, the data require at least six 

velocity groups or layers to represent the range of 

values. These are, roughly, 2.5, 3,4, 5.5,7, and 8 kmps.

One significant difference from the Bikini lagoon 

profiles is the greater prominence at Eniwetok of the 

3-kmps layer, which was observed on so few profiles at Bikini that its existence throughout the lagoon was hypothetical. At Eniwetok, however, it is 

observed on every lagoon profile. This yields information about the structure of the calcareous deposits, 

but unfortunately it masks part of the important 

4-kmps segment and increases the distance between 

the shot and the receiver at which this velocity is first 

observed. Hence, the segments representing the layer 

believed to be the upper part of the volcanic rock are 

short and the degree of reversed control is considerably 

less than expected. In order to obtain the maximum 

information about this layer, and to increase the accuracy of the determination of velocities from the short 

segments in the first 15 km of profile, the number of 

shots at short range was increased significantly above 

that used in the Bikini survey.

Gaps in the profiles, indicated by dashed lines, resulted from two causes: in sections E-K in figure 221

14

I 0

Uf

1 6

7.84

6.98

s.69

4.04

4.27-, .>

80 75 70 65 60 55 5O 45 40 35 30 25 20 15 10

DISTANCE, IN KILOMETERS

10 15

FIGURE 224.—Travel-time plots for section through station J. Times are corrected for water delay ; apparent velocities are in kilometers per second.
692 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

0.6

0.5 -

0.5 I.O 1,5

DISTANCE, IN KILOMETERS

FIGURE 225. — Travel-time plot of short profile across reef northwest of 

Blugelab. The plotted distance is the direct distance V 

where A ls the horizontal distance and the hydrophone depth is 0.076 

km.

and G-L in figure 223 the lines crossed areas where 

underwater shots were prohibited because of the risk 

of damage to or interference with undersea installations, and the short-range first arrivals at the deepwater stations were masked by bottom echoes and 

direct water waves. The gaps due to the first cause 

were not serious, but the loss of the short-range information in the deep-water stations made it impossible 

to determine the velocity in sediments outside the 

lagoon.

ACCURACY OF TRAVEL TIMES

At short ranges the first-arriving elastic pulse is 

sharply defined and its arrival time can be measured 

with little difficulty to 0.01 second (see fig. 230).

At greater ranges the pulse decreases in amplitude, 

and the initial pressure pulse is frequently masked by 

the background noise. This phenomenon is illustrated 

by figure 230A, which shows the initial part of the 

pulses produced by the first four %-pound shots south 

of station B. In the uppermost record, at the shortest range, the beginning is sharp and unquestionable. 

In the lowest record the beginning is barely detectable 

and was estimated by comparison with the preceding

2.29

2.5 5.0 7.5 10.0 12.5 15.0 

DISTANCE, IN KILOMETERS

17.5 20.0 22.5

FIGURE 226. Travel-time plots for section through stations F, B, and A. Times are corrected for water delay ; apparent 

velocities are in kilometers per second.
SEISMIC-EEFRACTION STUDIES, ENIWETOK ATOLL 693

records and with other comparable records to be 0.08 

second ahead of the first negative swing.

This procedure of comparing records has been followed throughout the survey. The beginnings have 

been chosen by lining up groups of adjacent records 

on significant similar features and picking the beginnings with reference to the records which show 

the most conspicuous onsets. An example of this procedure for distant shots is shown by figure 2305, the 

record of three 80-pound shots at the end of the profile 

that extends south from station C for a distance of 

about 80 kilometers. Where similarity of form of 

the first wave is maintained for a considerable distance, 

this method of picking the beginning of the pulse yields 

an accurate slope of the travel-time curve, even though 

the arrival time may be questionable.

An ever-present potential source of error is the 

possibility that the true beginning wave group is too 

weak to observe and that the observed wave has taken 

a longer path but arrived with much greater intensity. This is particularly likely to occur in as complicated a structure as Eniwetok, where there are many 

possible paths for the waves to travel, and there is a 

good possibility that the initial ones frequently are 

much weaker than late arrivals. This possibility is 

greatest at greatest range, where even the strongest 

signals are weak. Hence, all of the sources of error

tend to increase with range. The error in picking 

the beginning of the first observed wave is roughly 

0.02Ai second, where A is the range in kilometers. Failure to observe a weak first arrival can be reduced by 

multiple profiles like section A-K' at Eniwetok, but 

there is no satisfactory way of estimating the expected 

error from this cause.

0 2.5 5.0

DISTANCE, IN KILOMETERS

FIGURE 227.—Travel-time plots for section through stations E 

Times are corrected for water delay; apparent velocities 

kilometers per second.

and G. 

are in

CO

Q

z 

o 

o

UJ 

CO

-.3.94

4.33

-4.04

7.5 5.0 2.5 0 2.5 5.0 

DISTANCE, IN KILOMETERS

7.5 10.0 12.5

FIGURE 228.—Travel-time plots for section through stations M and I. Times are corrected for water delay; apparent

velocities are in kilometers per second.
694 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

co 4

LINE G-L EAST OF H

LINE D-E

LINE G-L WEST OF H

4.15

3.99

3.01

2.37-

10 0 5 10 15 

DISTANCE, IN KILOMETERS

20 25 30

FIGURE 229.—Travel-time plots for shots received at station H. Times are corrected for water delay; apparent velocities

are in kilometers per second.

Consequently no attempt was made to determine true 

velocities for specific areas as was done at Bikini, where 

the reversed control of the third layer was better than 

at Eniwetok. Instead, velocities for each layer were 

averaged for the entire survey and a single velocity 

value, used for the entire area of the survey, was determined for each layer.

In making the averages it was assumed that the variations in apparent velocity were due primarily to ii* 

regularities in the thickness of the overburden rather 

than to changes in true velocity. Hence, following the 

procedure of equation 4, the harmonic mean of the apparent velocities is taken as the best measure of velocity 

in the formation. Not all of the data are equally suitable for this average. Some of the first arrivals are of 

poor quality, water delays may be of doubtful accuracy 

because of rough bottom topography, and drift of the 

receiving ship over a sloping bottom may give a spurious 

slope to the time-distance curve. The velocity segments 

that have been used in the averages are listed in the 

following table.

I I I I I I LL

One second 

A

FIGURE 230.—Oscillograms of (A) the first four %-pound shots south of 

point B and (B) three 80-pound shots at the outer end of the profile 

extending 80 km south of point C.

VELOCITIES

Even though many parts of the shooting lines were 

traversed by the firing ship as much as 5 and 6 times, it 

is apparent that the distance over any one layer in which 

true reversed control was established is very small.

Station

Profile

direction

Apparent

velocity

(kmps)

Station

Profile

direction

Apparent

velocity

(kmps)

First Layer

A

B

c

D----.-- — — —

NW

sw

s

sw

s

N

S 

N

SE

w

E

2.37

2.29 

2 24

2.33

2.33

2.52

2.65 

2.67

2.61 

2.56

2.38

E. ..................

G. ..................

F-_. — — -.. — — .

Harmonic mean

NW

NE 

SW

W

E

NE

E

2.40

2.65 

2.48

2.40

2. 5& 

2.2»

2.61

2.44
SEISMIC-KEFKACTION STUDIES, ENIWETOK ATOLL 695

Station

Profile

direction

Apparent

velocity

(kmps)

Station

Profile

direction

Apparent

velocity

(kmps)

Second layer

A

B

C

T-)

SW

S

sw

s

N

S

N

SE

W

E

2.65

2.67

2.96

3.20

3.07

3.20

3 97

3.17

3.20

3.14

E.-— - ————.

G. __________________

F

H— — — — — — _

Harmonic mean

SW

NE

W

NE

SE

NW

3.20

3.27

3.24

2.96

2.96

3.01

3.06

Third layer

4

B__. ___________

0 __

j)

E

G. _________________

SW

SW

s

N

S

N

W

NW

W

NE

3.89

4.04

4.39 

4.59

3.45

4.33

4.10

4.15

4.52

3.84

F.......— ..........

M— -.... —._-__-_.

J£K'

H— -——.——-

SE

NW

W 

E

W

S

N

W

4.21

4.10

3.94 

4.33

4.04

4.39

4.39

4.15

4.15

Fourth layer

B

Q

D

E

S

S

S

N

SE

NW

5.73

5.91

5.69

5.30

5.30

5.80

G_. _-_-_-_._-_-__

J— — — — — — — .

W

SE

W

5.30

5.80

5.69

5.59

Fifth layer

A... -____-___._-___

G______.__-_. _______

!.___________________

NW

E

NW

6.68

7.12

6.83

J W 6.98

6.90

Sixth layer

A_. _________________

I.. __________________

NW

NW

7.88

8.54

J-. — — - — .. — —

Harmonic mean

W 7.84

8.09

SUBSURFACE STRUCTURE

Following the foregoing principles of theoretical interpretation, depths to the interfaces between the 6 principal layers have been estimated for the 2 north-south 

and east-west sections across the atoll and plotted in 

figures 231 and 232.

In these figures hachured interfaces indicate that the 

refracted waves from shots fired above that part of the 

interface have been received with a Velocity corresponding to the layer immediately below. The dashed interfaces are extrapolations and interpolations from the 

areas of direct observations. The necessity of making 

these assumptions in order to calculate depths to 

deeper layers is one of the principal sources of error, 

for an error in estimating these dashed interfaces will 

be reflected in an error in the hachured interfaces beneath them. Because of the preponderance of dashed 

interfaces on the flanks of the atoll and in the deeper 

layers, the structure in these areas is necessarily schematic and the error can be large. For the shallow

layers beneath the lagoon, the concentration of observations gives more complete control and more accurate 

detail.

BEEF STRUCTURE

The detailed profile (fig. 225) across the reef northwest of Elugelab indicates the presence of a thin, lowvelocity layer on the reef surface in which the seismic 

velocity is 1.92 kmps. The thickness of this layer is 

calculated to be 108 in, which puts the depth of its base 

about 50 or 60 m below the average depth of the lagoon. 

It is of interest to note that this depth is of a reasonable 

order of magnitude for a previous glacial lowering of 

sea level (Daly, 1910). It is nearly the same as the 

depth of the volcanic basement beneath Bermuda (Officer, Ewing, and Wuenschel, 1952), at 60 to 100 m, believed to have been eroded to sea level during the Pleistocene epoch. The lower seismic velocity in the material deposited since that time may be a result of 

relatively rapid deposition, or it may merely indicate 

that the relatively cavernous structure of the outer reef 

results in a lower seismic velocity than in the lagoon 

deposits. The properties of this low-velocity layer are 

similar to the layer indicated in the first survey of Bikini 

(Dobrin, 1950) at receiving stations on the periphery 

of the lagoon.

FIRST AND SECOND LAYERS

The first and second layers are considered together 

because drilling has shown that beneath the atoll they 

represent calcareous deposits formed by reef and lagoon 

organisms (Ladd, Ingerson, Townsend, and others, 

1953). Velocities of 2.44 and 3.06 kmps, respectively, 

in these layers are characteristic of partly consolidated 

sediments.

The first layer is probably composed of material very 

similar to that now forming the lagoon floor. Although the short-range travel times show irregularities 

indicating inhomogeneity, there is no consistent tendency for the time-distance curve to have a positive intercept when projected to the time axis, which would be 

expected if there were a lower velocity layer above the 

first zone mapped. The observed irregularity in the 

arrival times of the pulses reduces the possibility of 

detecting a thin overlying layer, and it is possible that 

a layer of 2 kmps velocity and 50 m thickness, for example, would not be observed.

The thickness of the first layer decreases regularly 

from about 600 m at the edge of the atoll to about 200- 

300 in in the center. The discontinuity at its base is 

not definitely established, since the observations could 

also be explained by a moderately rapid increase of 

velocity with depth in the upper few hundred meters
696 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

North South

I AB

I I l

C D E K 

I_____l____l l

10

14

16

18

20

Sea bottom

6.90

8.09

80 60 40 20 0 20 40 60 80

DISTANCE, IN KILOMETERS, FROM STATION A 

FIGURE 231.—Hypothetical subsurface section through Eniwetok Atoll from north to south. Velocities are in kilometers per second.

100

due to compaction caused by pressure of the overlying 

material.

The second layer is about 1 km thick. The irregular 

variation in thickness indicated by the time-distance 

graphs is probably no greater than the experimental 

error in reading arrival times and calculating the water 

delay. There is no systematic change of thickness toward the center of the lagoon. In calculating the 

structure outside the lagoon, the second layer was assumed to be absent since no direct evidence of its presence was obtainable.

THIRD LAYER

The third layer (4.15 kmps) is found at all points 

of the seismic survey to be deeper than the points at 

which it was penetrated by the drilling that identified 

it as volcanic rock (Ladd, Ingerson, Townsend, and 

others, 1953). It is difficult to explain why the seismic 

observations show this layer to be deeper than was 

determined by drilling. It is possible that the horizontal velocity in the first and second layers may

be greater than the vertical velocity, owing to the presence of thin layers of consolidated material in which 

the velocity is higher. For example, if the second 

layer consists of 2 strata, an upper, thin stratum of 

hard rock in which the seismic velocity is 3.06 kmps 

and a lower, thick stratum in which the seismic velocity 

is about 2.44 kmps, the average vertical velocity would 

be such that the calculated depth to the third layer 

would be in agreement with the drilling results. Also, 

the upper part of the volcanic formation may be highly 

fractured or vesicular. Its average compressibility 

could be quite high, hence the velocity in it could be 

low even though the drilling was very hard through 

individual blocks. The velocity in such a formation 

might be little greater than the 3.06 kmps in the second 

layer and would be masked by it.

FOURTH LAYER

A seismic velocity of 5.59 kmps, the velocity in the 

fourth layer, is characteristic of a hard crystalline rock. 

It is in close agreement with one of the few values, 5.6 

kmps, identified with basalt in the literature (Brock-
SEISMIC-REFRACTION STUDIES, ENIWETOK ATOLL 697

~. 8

10

12

14

16

18

West East

6.90

8.09

100 80 60 40 20 0 20 40 60

DISTANCE, IN KILOMETERS, FROM STATION F 

FIGURE 232.—Hypothetical subsurface section through Eniwetok Atoll from west to east. Velocities are in kilometers per second.

80

amp and Wolcken, 1929). The plotted interface between the third and fourth layers does not necessarily 

indicate a structural or lithologic discontinuity. It 

merely indicates the approximate depth at which a 

velocity of 5.59 kmps is reached. It is possible that 

the fourth layer is largely plutonic rock intruded from 

below, while the third layer is mostly extruded. On 

the other hand, it may all have been extruded. The 

higher velocity in the fourth layer could then be attributed to compaction or consolidation produced by 

the pressure of the rocks above.

FIFTH LAYER

The velocity of 6.90 kmps in the fifth layer identifies 

it with the crustal layer in the Pacific basin (Raitt, 

1956) which has a velocity between 6.5 and 7.0 kmps. 

This is higher than field measurements of velocities in 

known rocks, and it may represent a rock type never 

observed at the earth's surface. Current petrologic doctrine indicates that it represents a dense basic rock, probably olivine basalt. Its depth below sea level, 9-10 

km, is greater than the 7- to 9-km depth at Bikini 

(Raitt, 1954), and substantially greater than the 5- to

8-km depth considered to be characteristic of the Pacific 

basin.

Because it was observed at only a few places, its depth 

can only be roughly indicated, and practically nothing 

has been learned about the form of its surface. At the 

southern end of profile A-K' (see fig. 221) the great irregularity of the points, owing to subsurface anomalies 

of unknown character, made it uncertain as to whether 

the pulses received had been refracted through the 6.90 

kmps or the 8.09-kmps layer. The depths are therefore questionable, although consistent with those obtained on other profiles.

SIXTH LAYER

The velocity of 8.09 kmps in the sixth layer is characteristic of the velocity found at the base of the earth's 

crust below the Mohorovicid discontinuity. Its depth, 

16-17 km, is greater than at Bikini (13 km). This is 

due primarily to the great depth of the fifth layer, 

whose thickness, about 6.5 km, is not much greater than 

that observed at Bikini or generally throughout the 

Pacific basin.
698 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

BIBLIOGRAPHY

Birch, Francis (editor), 1942, Handbook of physical constants: 

Geol. Soc. America Special Paper 36, 325 p.

Brockamp, B., and Wolcken, K., 1929, Bemerkungen zu den 

Beobachtungen bei Steinbruchsprengungen: Zeitschr. 

Geophysik, v. 5, p. 163-171.

Daly, R. A., 1910, Pleistocene glaciation and the coral reef problem : Am. Jour. Sci., 4th ser., v. 30, p. 297-308.

Dobrin, M. B., 1950, Submarine geology of Bikini lagoon as indicated by dispersion of water-borne explosion waves: Geol. 

Soc. America Bull., v. 61, p. 1091-1118.

Dobrin, M. B., Perkins, Beauregard, Jr., and Snavely, B. L., 

1949, Subsurface constitution of Bikini Atoll as indicated 

by a seismic refraction survey: Geol. Soc. America Bull., 

v. 60, p. 807-828.

Emery, K. O., Tracey, J. I., Jr., and Ladd, H. S., 1954, Geology 

of Bikini and nearby atolls, Marshall Islands: U. S. Geol. 

Survey Prof. Paper 260-A, 265 p.

Gardner, L. W., 1939, An areal plan of mapping subsurface structure by refraction shooting: Geophysics, v. 4, p. 247-259.

Heiland, 0. A., 1940, Geophysical exploration: New York, Prentice-Hall, Inc.

Ladd, H. S., Ingerson, Earl, Townsend, R. 0., and others, 1953, 

Drilling on Eniwetok Atoll, Marshall Islands: Am. Assoc. 

Petroleum Geologists Bull., v. 37, p. 2257-2280.

Nettleton, L. L., 1940, Geophysical prospecting for oil: New York, 

McGraw-Hill Book Co., Inc.

Officer, C. B., Ewing, Maurice, and Wuenschel, P. O., 1952, Seismic refraction measurements in the Atlantic Ocean—Part 4, 

Bermuda, Bermuda Rise, and Nares Basin: Geol. Soc. 

America Bull., v. 63, p. 777-808.

Raitt, R. W., 1954, Seismic-refraction studies of Bikini and 

Kwajalein Atolls: U. S. Geol. Survey Prof. Paper 260-K, 

p. 506-527.

——— 1956, Seismic-refraction studies of the Pacific Ocean 

basin—Part 1, Crustal thickness of the central equatorial 

Pacific: Geol. Soc. America Bull., v. 67, p. 1623-1640.

V. S. GOVERNMENT PRINTING OFFICE: 1957