Geothermal Measurements 

on Eniwetok and Bikini 

Atolls

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-U
Geothermal Measurements 

on Eniwetok and Bikini 

Atolls

By J. H. SWARTZ

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-U

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1958
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 30 cents (paper cover)
CONTENTS

Page

Abstract_———___________________ 711

Introduction. ________________________________ 711

Purpose of study_____________________ 711

Location_______________________________________ 711

Acknowledgments________________________ 713

Equipment and techniques________________ 713

Thermistors______________________________ 713

Cables__.________________________ 713

Conversion of resistance to temperature _______________ 715

Reliability of data.______________________ 715

Sensitivity of instruments______________________ 715

Precision of measurements_______________________ 715

Accuracy of measurements.____________________ 716

Effects of systematic errors___------_-_-____- 716

Bridge temperature_____________________ 716

Conductor temperature._________________ 716

Thermistor heating_____________________ 717

Incomplete equilibrium._________________ 717

Thermistor calibration_________________ 717

Thermistor stability___________________ 717

Effect of pressure_____________________ 717

Page 

Reliability of data—Continued

Accuracy of measurements—Continued

Accuracy of data__________-___________-____ 718

Temperature____________ _____________ 718

Depth______________________ 718

Temperature observations___________________________ 718

Eniwetok Atoll________ _ ____________ 718

Drill hole F-l, Elugelab island. _ ________ 718

Drill hole E-l, Parry Island____________ 721

Bikini Atoll—drill hole 2B, Bikini island._. ____ . 723

Analysis of observations__________________________-_- 724

Effects of heat of drilling______ ___________ 724

Comparison with Pacific coast thermal profiles. _ _ _ _ 725

Comparison with ocean temperatures in the Marshall

Islands area_______________________ —— _____ 725

Tidal fluctuations in the drill holes.______________ 729

Comparison with Kita-daito-jima.________________ 731

Temperature gradients______________________--_- 734

Forced convection as a method of heat transfer in coral

atolls—.________._________________ 737 

Summary.. _____________________________________ — - 738

Literature cited___________________________________ 739

Index. __________________________———— 741

ILLUSTRATIONS

Page 

FIGURE 235. Map of the west Pacific Ocean showing the location of the Marshall Islands_________________ — _______ 712

236. Map of northwestern Marshall Islands showing the location of Eniwetok and Bikini Atolls and of ocean-temperature observations_________________________________________-___--_____----_---_------------ 713

237. Circuit diagram for a multithermistor cable______________________________--___-_---_------_---------- 714

238. Diagram of measurement setup for measuring temperatures in drill holes____--_-------------_-_~------— 714

239. Circuit for standard resistor unit used in determining bridge-temperature correction—_____________________ 716

240. Map of Eniwetok Atoll showing the location of the island of Elugelab and Parry Island, and of drill holes E-l

and F-l_______________________________________._______.-___.___ — __ 719

241. Temperature profiles in drill hole F-l, Eniwetok Atoll, June and October 1952__________________ —— _____ 720

242. Temperature profiles in drill hole E-l, Eniwetok Atoll, July and August 1952, December 22, 1952, and July

2, 1955.____________-.__.______________________________________-.-_-—— 721

243. Map of Bikini Atoll showing location of Bikini island and drill holes.___________________________________ 723

244. Temperature profiles in drill holes E-l and F-l, Eniwetok Atoll, and drill hole 2B, Bikini Atoll______ —— __ 724

245. Time-temperature graph for a depth of 800 feet, in drill hole E-l, Eniwetok Atoll______________________ 726

246. Temperature profiles in drill hole E-l, Eniwetok Atoll, and in three drill holes on the California coast___----_- 727

247. Temperature profiles in drill holes E-l and F-l, Eniwetok Atoll; drill hole 2B, Bikini Atoll; and in the ocean

in the Marshall Islands area___________-_-_______________________------------_------__----------- 728

248. Water-level fluctuations in drill hole F-l, Eniwetok Atoll; tidal fluctuations on the ocean side of Elugelab

island; and tide-table values for Eniwetok Atoll______________-___------------_----------_-_-------- 729

249. Comparison of water-level fluctuations in drill hole E-l and tide-table values for December 14, 1952._______ 730

250. Comparison of water-level fluctuations in drill hole 2B and tide-table values for November 29, 1952—_____ 731

251. Temperature profiles in drill hole on Kita-daito-jima; drill holes E-l and F-l, Eniwetok Atoll; and drill hole

2B, Bikini Atoll___________________________________________________.______________-____--_----- 733

252. Temperature gradients for each measured interval, Marshall Islands thermal profiles._____________________ 736

253. Vertical section through drill hole E-l, Eniwetok Atoll, perpendicular to atoll flank, showing isogeotherms_ _ 737

m
IV CONTENTS

TABLES

Page 

TABLE 1. Precision of geothermal measurements in drill holes in the Marshall Islands, June 1952 to October 1953--_-_-_ 715

2. Temperature measurements in drill hole F-l, Elugelab island, Eniwetok Atoll, June 1952—__________________ 718

3. Temperature measurements in drill hole F-l, Elugelab island, Eniwetok Atoll, October 1952_________________ 720

4. Temperature measurements in drill hole E-l, Parry Island, Eniwetok Atoll, July and August 1952.__________ 722

5. Temperature measurements in the permanent installation in drill hole E-l, Parry Island, Eniwetok Atoll,

December 1952 to July 1965_________._.__ —— _ — __ — . — — — . — — —— — — — — _-------- 722

6. Temperature measurements in drill hole 2B, Bikini island, Bikini Atoll, November 1952_____-_________----_- 723

7. Ocean-water temperatures in the Marshall Islands area; measured by the Capricorn Expedition, Scripps Institution of Oceanography, October and November 1952_______-_______________-_-__----_---_-------_------ 728

8. Water-level measurements in drill hole F-l, Elugelab island, Eniwetok Atoll, October 1952_____________-_--- 729

9. Water-level measurements in drill hole E-l, Parry Island, Eniwetok Atoll, December 1952_ _________________ 730

10. Water-level fluctuations in drill hole 2B, Bikini island, Bikini Atoll, November 1952________________________ 731

11. Temperature measurements during first Kita-daito-jima drilling, 1934___________________________________ 732

12. Temperature measurements during second Kita-daito-jima drilling, April 1936- ____________________________ 732

13. Tidal fluctuations in drill hole and in old well at Minatoguchi on Kita-daito-jima; measured by T. Sugiyama,

December 1935__._________________________________________________—— 734

14. Average temperature gradients for approximately linear segments of Marshall Islands temperature profiles____ 734

15. Temperature gradient for each measured interval, Marshall Islands temperature profiles._.____________-_---_ 735
BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

GEOTHERMAL MEASUREMENTS ON ENIWETOK AND BIKINI ATOLLS

By J. H. SWARTZ

ABSTRACT

Geothermal measurements in two deep holes drilled in the 

summer of 1952 on Eniwetok Atoll showed that the atoll temperature profiles differ greatly from those in continental areas. In 

continental drill holes, the temperatures below the zone of 

annual change rise steadily with increasing depth. In the atoll 

drill holes, temperatures below the zone of annual change decrease steadily with increasing depth until a minimum is reached 

at a depth of several thousand feet (6.4° C between 3,180 and 

3,380 feet in drill hole E-J, July 1955). Below this depth the 

curve reverses, and temperatures increase with further increase 

in depth, as in continental drill holes.

In both Eniwetok drill holes the temperature decreases rapidly 

to a depth of 1,000 to 1,400 feet, below which the rate of temperature decrease becomes much slower down to the depth of 

minimum temperature.

The temperatures of Marshall Islands ocean water, as determined in October and November 1952 by the Capricorn Expedition of the Scripps Institution of Oceanography, showed the 

same type of temperature profile: a rapid decrease to a depth 

of 800 feet, followed by a continued but much slower decrease 

with increasing depths. The close parallelism between the 

thermal profiles in the drill hole and the ocean leaves little 

question that the character of the thermal profile in the atoll is 

largely a result of cooling by the adjacent ocean.

Additional support for this hypothesis is afforded by measurements of the fluctuations of the water levels in the drill holes. 

In both drill holes there was evidence of tidal motion. In drill 

hole E-l, which was cased to a depth of 4,109 feet, the tidal 

amplitude was small and there was a phase lag of 9J4 hours. 

In drill hole F-l, which was cased to a depth of 1,973 feet, the 

tidal variations agreed in both amplitude and phase with those 

measured simultaneously in the ocean offshore. Such evidence 

of high permeability in the reef sediments emphasizes the 

probability of heat transfer from sediments to ocean.

Further support for these conclusions was found in temperature 

and tidal measurements made in a drill hole on the island of 

Bikini, B ikini Atoll. The temperature curve there showed the same 

continuing temperature drop as in the Eniwetok drill holes. 

Water-level measurements showed a tidal fluctuation approximately one-half the amplitude of the ocean tide and with a 

phase lag of 1 hour.

The rise of temperature with increasing depth below the point 

of minimum temperature was observed only in one drill hole, 

where the rate of rise, 20° C per km, is only about half that 

observed in drill holes along the California coast. As the rise 

begins near the bottom of the drill hole it may be that the curve 

is still affected by the ocean and that a normal gradient would 

be found if measurements extended deep enough.

The average temperature gradient for the upper, steep-gradient part of the thermal profile was —51.5° C per km in the 

Eniwetok drill holes, —35.2° C per km in the Bikini drill hole, 

and —94.2° C per km in the ocean. The average gradient in 

the lower part of the profile was —7.5° C per km in the Eniwetok 

drill holes, -18.1° C per km in the Bikini drill hole, and -6.6° C 

per km in the ocean.

INTRODUCTION

PURPOSE OF STUDY

The drilling of two deep holes on Eniwetok Atoll in 

1952 (Ladd, Ingerson, Townsend, and others, 1953) 

offered an opportunity for a study of geothermal profiles and the thermal regime under a coral atoll. The 

investigation was undertaken by the U. S. Geological 

Survey for the U. S. Atomic Energy Commission in cooperation with the Office of Naval Research and was 

carried out in cooperation with H. S. Ladd, of the Geological Survey, who directed the drilling, and with 

H. K. Stephenson, of the Los Alamos Scientific Laboratory, who had general responsibility for the project.

A hole drilled on Bikini island in 1947 to a depth of 

2,556 feet (Ladd, Tracey, and Lill, 1948) was found to 

be still open in 1952 to a depth of 1,430 feet, and its 

thermal profile was measured for comparison with the 

Eniwetok data.

LOCATION

The Marshall Islands (fig. 235) lie in eastern Micronesia, approximately 2,900 miles west of Hawaii and 

1,200 miles southeast of Guam. Eniwetok and Bikini 

Atolls (fig. 236) lie in the northwest corner of the 

group, Eniwetok at lat 11°30' N., long 162°15/ E., and 

Bikini approximately 165 nautical miles to the east at 

lat 11°35' N., long 165°25' E.

The first Eniwetok drill hole, F-l, was drilled near 

the west end of Elugelab island 1 , at the north end of the 

atoll where it is joined by a guyot or flat-topped seamount extending to the north and northwest. The 

second, E-l, was drilled near the south end of Parry 

Island on the southeast flank of the atoll.

i The island was subsequently destroyed, and this drill hole is no longer available 

for study.

7U
712 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

•3

I
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 713

EXPLANATION

o

Capricorn Expedition, Scripps 

Institution of Oceanography 

therrnal station 

08 

Bathythermograph station

oN 

Nansen-bottle station

oE-1 

Drill hole

10-14-52 

Date

^ TAONGI 

I/ ATOLL

N ?0-15-52

..BIKAR 

(J ATOLL

ENIWETOK 

ATOLL

E-li Parry Island 

Eniwetok Island

BIKINI ATOLL

' Island RONGELAP 

ATOLL UTIRIK

AILINGINAE 

ATOLL

RONGERIK 

ATOLL

p, WOTHO 

N> ATOLL

ATOLL

Jemo Island

AILUK 

ATOLL

Mejit 

Island

LIKIEP 

ATOLL

KWAJALEIN ATOLL

UJAE ATOLL VS

ATOLL

WOTJE 

ATOLL

•\ ERIKU8 

'A ATOLL

Kwajalein Island

MALOELAP 

ATOLL

FIGURE 236.—Map of northwestern Marshall Islands showing the location of Eniwetok and Bikini Atolls and of ocean-temperature observations.

Drill hole 2B, on Bikini Atoll, is near the center of 

the lagoon side of Bikini island. It lies on the northeast side of the road that parallels the lagoon, near the 

native graveyard, and approximately 240 feet inland 

from the break in vegetation at the top of the beach.

ACKNOWLEDGMENTS

The cooperation of C. T. Cooper and H. H. Lewis, 

resident engineers for the U. S. Atomic Energy Commission, and of C. L. Coray and J. M. Lloyd, resident managers of the Holmes and Narver Co., made possible the 

smooth functioning of the program. Rudolph Raspet 

and W. L. Smith, of the Geological Survey, and Alan 

C. Jones, of the Atomic Energy Commission, assisted 

with thermal measurements at Eniwetok and Bikini 

Atolls respectively; and J. L. Moore, R. D. Blenck, Kai 

Hendricksen, V. A. DiTomaso, F. H. Delpino, and L. 

J. Casiano, all of the Holmes and Narver Co., and 

SP/3 C. J. Spengler, U. S. Army Signal Corps, have 

continued the thermal measurements in drill hole E-l 

since the writer's departure from Eniwetok,

EQUIPMENT AND TECHNIQUES 

THERMISTORS

All temperature measurements were made by means 

of thermistors mounted in multiconductor cables. A 

thermistor is a thermally sensitive resistor, a semiconductor, characterized by a high, negative temperature 

coefficient of resistance (Becker, Green, and Pearson, 

1946; Shockley, 1950). At room temperatures the 

thermistors used in this study have a resistance of approximately 1,000 ohms and a rate of change of resistance of approximately 4.4 percent per degree centigrade 

change in temperature.

CABLES

In constructing the cables the thermistors were inserted at regular intervals and vulcanized in place. 

The circuit used (fig. 237) was one designed by the 

writer in 1949 for thermal measurements hi the Arctic 

(Swartz, 1954). Distances in feet from a carefully 

marked zero point were either stamped into the vulcanized joint at each thermistor or shown by carefully
714 BIKINI : AND NEARBY ATOLLS, MARSHALL ISLANDS

Resistor

Soldered joint

FIGURE 237.—Circuit diagram for a multithermistor cable.

positioned rings of vinylite tape. Several feet of free 

cable extended as a lead from the zero mark to a multiconductor connector enclosed in a waterproof cap.

Four cables were used in the present study. Measurements in June, July, and August, 1952, were made with 

a 3-conductor cable, 5,000 feet long, loaned by Prof. 

Francis Birch of Harvard University when delivery on 

other cables was delayed. With this cable only one 

thermistor could be used, as the remaining circuits were 

required for other equipment. The longest of the other 

3 cables, which became available in October 1952, was 

4,200 feet long and contained a steel strength-member 

%2 inch in diameter and carried 15 thermistors. The 

2 shorter cables, 400 and 2,000 feet long respectively, 

carried 17 thermistors each and had cadmium-bronze 

conductors for greater strength, but did not contain a 

strength-member.

In practice, the cable was lowered into the drill hole 

and, except as noted, allowed to remain there until 

thermal equilibrium was attained. Measurements of 

thermistor resistance were then made with a Wheatstone bridge.

Both temporary and permanent cable installations 

were made. In the temporary installations the cables, 

lowered by hand or by winch, were allowed to remain 

in place as long as possible before measurements were 

made and the cable removed. In the permanent installation the cables were lowered to the required depth, the 

zero mark positioned, and the free lead tied in a clove 

hitch to a 24-inch length of 2X-inch-diameter propeller 

shafting resting across the mouth of the casing. These 

cables were left in the hole indefinitely and measurements continued at regular intervals. To protect the 

installation a small hut was placed over the mouth of the 

drill hole.

In making measurements the connector at the upper 

end of the cable was plugged into a multiple-selector 

switchbox (fig. 238) and the selected circuit connected 

to a four-dial Wheatstone bridge. Because of the high 

humidity and salt content of the air, the bridge, the 

switchbox, and the connecting leads were kept in a hot

Multiconductor socket 

Multiconductor cable

- &——Multiconductor cable 

Si

Lead weight

FIGURE 238.—Diagram of measurement setup for measuring temperatures in drill

holes. .
GEOTHEEMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 715

locker when not in use, and the waterproof caps of the 

cable connectors were removed only while measurements 

were being made.

CONVERSION OF RESISTANCE TO TEMPERATURE

The resistance of the thermistor, R, is found by subtracting the resistance of the thermistor circuit leads, 

RL, from the measured resistance, RM, of the thermistor 

plus the measurement circuit. Thus,

R—RM—RL . (1)

The temperature, T, of the thermistor is then computed 

from the equation

*)]-c (2)

where a, b, and c are constants, differing slightly for 

different thermistors, which have been determined by 

careful prior laboratory calibration.

The resistance RL is determined by the resistance of 

the test circuit, RT, in the multithermistor cables. As 

has been shown elsewhere (Swartz, 1954),

where

(3)

(4)

and RLC and RTC are the resistances of thermistor lead 

and test lead, respectively, as measured by prior 

laboratory calibration.

If the cable, as in the single-thermistor cable, 

does not have a test lead, the resistance RL must be 

computed from the equation

where a is the temperature coefficient of resistance of 

the conductor wire, R0 is the resistance of the conductor at 0° C, and TLA is the average, temperature of 

the cable as determined by successive approximations 

from the temperature-measurement data. Inserting 

the values of these constants for the present equipment, 

equation 5 reduces to

#i= 164.6+ 0.62967^ (6) 

for the single-thermistor cable.

RELIABILITY OF DATA

By the reliability of the measurements is meant their 

degree of freedom from error. Several factors affect the 

reliability of the measurements: the sensitivity of the 

instruments, the precision of the measurements, and the 

occurrence of systematic errors.

SENSITIVITY OP INSTRUMENTS

The thermal sensitivity of the instruments may be 

defined quantitatively as the change in temperature 

required to produce a unit deflection of the bridge 

galvanometer. It thus represents in a sense the resolving power of the measuring equipment used.

The sensitivity varied somewhat depending upon the 

particular equipment in use at a particular time. It 

ranged from 0.015° to 0.595°C per millimeter deflection 

with changes in the galvanometer and bridge multiplier. 

Somewhat limited experimental data indicate that with 

the particular equipment and observational techniques 

used, changes of 0.003-0.007°C could be detected in 

the July and August 1952 measurements, changes of 

0.002°C in the October 1952 to March 16, 1953 measurements, and changes of 0.001 °C in later measurements.

PRECISION OF MEASUREMENTS

The precision of the measurements represents the 

closeness with which they can be repeated and is dependent solely on the random errors of measurement. 

It is measured by the magnitude of the median error; 

that is, the one whose magnitude is such that the 

probability of the occurrence of a larger error exactly 

equals that of the occurrence of a smaller.

The precision of the measurements in the present 

survey varied somewhat with the skill and experience 

of the operator and with the particular equipment being 

used. The probable errors computed for the measurements from their beginning in June 1952 to October 

1953 are listed in table 1.

TABLE 1.—Precision of geothermal measurements in drill holes in 

the Marshall Islands, June 1952 to October 1953

Cable

Single 

thermistor.

Multithermistor.

Atoll

Eniwetok 

-__do__-_.

Bikini____ 

Eniwetok

___do____.

_-_do_____ 

Bikini____

Drill 

hole

F-l

E-l

2B F-l

E-l 

E-l 

2B

Date

June 1952. _ .

July and Aug. 

1952. 

Nov. 1952_____

Oct. 1952.______

Dec. 1952-Mar. 

16, 1953. 

Mar. 31, 1953- 

Oct. 1953. 

Nov. 1952_____

Probable error (°C) 

when thermistor 

resistance is —

<1,000 

ohms

±0. 003

i ±.011 

±. 003 

±. 003

>1,000 

ohms

±0. 020

±.020

±. 007 

±. 002

±. 002 

±. 001

1 The majority of the multiple readings on which this figure is based were made 

when the bridge galvanometer was relatively insensitive. This value of the probable 

error is therefore regarded as probably too large.

428688—5£
716 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

It was not feasible, because of operating conditions, 

to make several readings at any one depth, so the 

probable errors given in table 1 are computed from a 

series of pairs of readings clustered about different 

means rather than a similar number of readings clustered about a single mean. It can be shown that the 

probable error so obtained will in general be somewhat 

smaller than that computed from the same number of 

readings clustered about a single mean.

ACCURACY OF MEASUREMENTS

EFFECTS OF SYSTEMATIC ERRORS

Systematic errors are those which affect all readings 

alike and are the major factor in determining the 

accuracy of the measurements. Accuracy is the closeness of approach of the measured value to the true 

value of the measured quantity; that is, the limiting 

value that would be approached by the mean of a 

series of very careful measurements free of all systematic 

error as the number of such measurements is greatly 

increased.

Systematic errors can be eliminated or reduced by 

careful instrumentation and operational techniques. 

When systematic errors can be quantitatively evaluated 

their effects can be removed by corresponding corrections. If they cannot be removed or evaluated it is 

possible only to estimate their probable size.

The following discussion will deal with those sources 

of systematic error found in the present investigation: 

bridge temperature, conductor temperature, thermistor 

heating, insufficient equilibrium time, accuracy of thermistor calibrations, and thermistor stability.

BRIDGE TEMPERATURE

Although the bridge resistors are made of an alloy 

with a low temperature coefficient of resistance, temperature changes may be sufficient to introduce changes 

in the calibration of the bridge. When not in use the 

bridge is kept in a heated locker at a temperature higher 

than the maximum ambient temperature to guard 

against moisture, mildew, and other sources of trouble. 

Prior to use the bridge must be cooled to the ambient 

temperature so that the bridge temperature and the 

calibration will remain constant during the measurement run. If the ambient temperature is not sufficiently 

close to that for which the bridge is calibrated, the actual 

calibration, while constant, will differ from the original 

by a certain factor which may be called the bridgetemperature error. Errors from this source have been 

found to be as much as 0.01° C if uncorrected. They 

may, however, be corrected as discussed below.

To determine both when the bridge temperature has 

become constant and what the bridge-temperature correction factor is, use is made of the standard resistor 

unit whose circuit is shown hi figure 239. The resistors

R, =2OOO ohms

Th= Thermistor

Red

White

-O Th

Common

Common

FIQUEE 239.—Circuit for standard resistor unit used in determining bridgetemperature correction.

RI and R2 are precision resistors of a very low temperature coefficient alloy. Both are very carefully calibrated in the laboratory between the extremes of the 

temperature range to be expected in the field, and then 

mounted in a micarta block, together with a calibrated 

thermistor to measure the temperature of the standard 

resistors. The resistances of the standards are so 

chosen as to fall within the two bridge ranges used for 

the thermistor measurements and to approximate the 

mean values of thejthermistor resistances to be read in 

those ranges.

In operation the unit is attached to the bridge measurement terminals and the resistance of the thermistor 

checked until it has become nearly constant. Because 

the temperature coefficient of the bridge resistors is 

very small compared to that of the thermistor, absolute 

constancy is not required. When a sufficient constancy is reached, the bridge reading of the resistances 

R! and R2 and of the thermistor are recorded. From 

the resistance of the thermistor the temperature of the 

standards is computed and their correct resistances 

determined from the laboratory calibration data. The 

bridge correction factor is then given by the ratio of the 

correct resistance to the reading of the bridge. Seventeen readings gave an average bridge correction of 

0.011° C for one set of measurements.

CONDUCTOE TEMPERATURE

The resistance of the conductors in the cables varied 

considerably with temperature. Errors due to this 

source were eliminated in the multiconductor cables by 

the use of a test circuit described hi a previous paper 

(Swartz, 1954).

This method was not applicable, however, to the 

single-thermistor cable. With this cable the conductor 

resistance had to be computed from the temperature 

coefficient of resistance of the wire, using the measured 

drill-hole temperatures and a method of successive 

approximations to determine the mean temperature of 

the cable. Any residual error from this source is
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 717

believed to be small, as an error of 1° C in the cable 

temperature would mean an error of only 0.63 ohm in 

the conductor resistance, which would correspond to a 

temperature error of only 0.009°-0.016° C in the 

range of temperatures involved in the present study.

THERMISTOR HEATING

Because thermistors are so highly sensitive to temperature change, self-heating by the bridge current can 

be a source of error. To obviate this the bridge battery 

supply was reduced to IK volts and a series resistor of 

1,000 ohms was used to reduce the bridge current still 

further. In addition a special measurement technique 

was employed to minimize the time of bridge-current 

flow. With these precautions a careful operator could 

reduce the error from this source to approximately 

0.001° C. An incautious operator, who permitted the 

bridge current to flow during bridge adjustments, could, 

however, introduce an error of 0.01° C or more from 

this source.

INCOMPLETE EQUILIBRIUM

Measurements in 1952 with the single-thermistor 

cable ordinarily had to be made within 15 to 30 minutes 

after installation and before the cable could reach 

complete thermal equilibrium with the drill hole. 

Errors from this source were found to average 0.1° C.

The multiconductor cables in the temporary installation at Eniwetok drill hole F-l in October 1952 could 

not be left in the hole long enough to reach complete 

equilibrium. However, as the cables were in place for 

3K to 6 hours before readings were taken, the errors 

from this source were much smaller than those for the 

single-thermistor cable. The time-temperature graphs 

from measurements made during the waiting period 

indicate an error on the order of 0.04° C.

THERMISTOR CALIBRATION

All thermistors used were calibrated in a precisioncontrolled temperature bath. Three factors could affect the accuracy of the calibration: stability of the 

bath, stability of the master standard, and accuracy of 

the computed thermistor constants. The bath temperature usually did not change more than 0.001° C 

between two successive readings of the master standard 

during a single set of measurements.

Recalibrations of the master standard through the 

courtesy of the National Bureau of Standards have 

shown a more or less regular decrease in the indicated 

temperature of approximately 0.02° C per year during 

a 5-year period.

Errors in the computed constants of the thermistors 

are small, so that differences between observed and 

computed temperatures are believed in general to 

average less than 0.004° C.

THERMISTOR STABILITY

Extensive measurements in the Arctic on certain 

thermistors permanently installed in the ground at 

points of constant temperature have shown a temperature drift from all causes—calibration drift, climatic 

change, and the like—not exceeding approximately 

0.01° C per year. However, there is a growing body 

of evidence that thermistors that have been subjected 

to sudden large temperature changes, as during vulcanization, exhibit a form of thermal hysteresis. Recent measurements on 56 thermistors vulcanized into 2 

cables have indicated an average initial temperature 

error from this cause of +0.17° C. Following vulcanization and during a period in which the cables were 

subjected to temperatures of 0°-32° C, the error slowly 

decreased to zero and then reversed in sign so that af ter 

2 years the error due to drift was approximately —0.17° 

C.

Occasional thermistors may be found that show much 

greater error. Thus the ice-point resistance of one 

thermistor decreased from 3,209 to 3,197 ohms upon 

calibration at the steam point. During the following 

year it increased in resistance approximately exponentially to 3,290 ohms, a change of 93 ohms or of 

almost 0.5° C from the prehypsometer test value. 

However, such a change after heating seems exceptional 

and its effect can usually be recognized in the field data 

by abnormal and persistent kinks in the thermal 

profiles.

Because the thermistors in drill hole E-l on Parry 

Island are permanently installed at points of essentially 

constant temperature, their calibration drift may be 

expected to be a minimum. It is difficult to differfntiate in the field data between the effects of coolingiiOf 

the drill hole and calibration drift of the thermistors. 

The rather meager evidence as yet available suggests a 

calibration drift of approximately —0.04° C per year 

for these thermistors. Errors from this source may 

therefore be expected to be small and to be less than 

0.2° C for the period of measurement here covered.

EFFECT OF PRESSURE

A moderate amount of experimental evidence indicates that hydrostatic pressure has an effect on the resistance of thermistors. Investigations by Misener 

and Thompson (1952) and by Tavernier and Prache 

(1952) show a small but definite decrease in thermistor 

resistance with increasing pressure. The resultant error would therefore take the form of an apparent increase in temperature, and a correction for the effect of 

pressure would reduce slightly the apparent temperatures.

However, as there is disagreement as to the form of 

the pressure-resistance function and the value of the
718 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

pressure coefficient, no pressure corrections have been 

applied to, the data presented here.

ACCURACY OF DATA

TEMPERATURE

As previously noted, accuracy means the closeness 

of approach of the measured value to the true value of 

the measured quantity. Care must be taken to distinguish between the accuracy of the measured values 

of individual temperatures and that of the values of the 

ehange of temperature with time.

Measurements of individual temperatures are subject 

to a series of systematic errors which may enter in a 

variety of ways. If unconnected, and if the effects 

should be additive (which in general will not be true), 

the total error for the single-thermistor cable, when 

not in equilibrium with the drill hole, could reach or 

slightly exceed 0.3° C. On the other hand the total 

error for the multiconductor cable, which would be in 

thermal equilibrium with the drill hole, probably would 

not exceed 0.2° C.

Measurements of the change of temperature with 

time are, in contrast, much more accurate, because 

systematic errors not due to time cancel out in taking 

the differences. Corrections can be applied for systematic errors that are time dependent, such as calibration drift. Experience in the field has indicated that 

such measurements, when carefully made with multiconductor cables permanently installed at points of 

constant temperature, may be accurate to 0.01° C or 

better. A moderate amount of evidence indicates that 

calibration drifts in such values may be expected to be 

as little as 0.04° C per year.

DEPTH

Each cable when inserted in the drill hole was positioned with its zero point at the top of the casing. 

All depths are therefore referred to the top of the casing 

as their zero point.

Cable 205, the 4,200-foot cable, had a central airplane-steel cable, %z inch in diameter, as a strengthmember. A high degree of dimensional stability is 

claimed for it by the manufacturer. The single-thermistor cable also had a steel strength-member, but of 

much smaller size. The cable, however, was much 

lighter in weight and is believed to have good dimensional stability. Cables 193 and 204, the 400- and 

2,000-foot cables, had no strength-members. However, 

the conductors of these cables are of cadmium bronze, 

which greatly reduces their distensibility.

It is recognized that all of these cables when hanging 

in the drill hole will stretch to some extent under their 

own weight. The amount of stretch will be decreased,

although not entirely eliminated, by the buoyancy of 

the water in the drill hole.

Adequate experimental data are not available for 

computing the stretch of these cables under their own 

weight, so no attempt has been made to correct the 

measured depths for this effect. The depths given 

here are the apparent depths as measured by the 

thermistor positions in the cable.

TEMPERATURE OBSERVATIONS 

ENIWETOK ATOLL

DRILtL HOLE F-l, ELUGELAB ISLAND

Drill hole F-l, near the west end of Elugelab island 

(fig. 240), was begun May 12, 1952, and bottomed 

June 10 at 4,630 feet in a hard, slow-drilling rock that 

was struck at 4,610 feet and is believed to be basalt. 

At that depth the tools were lost by collapse of a part 

of the overlying hole, and the drill pipe was eventuallyshot off at 3,750 feet. The hole was open to a depth 

of approximately 3,500 feet and was cased to a depth 

of 1,973 feet.

The lithologic sequence above the basalt comprised 

largely unconsolidated coral sediments (Ladd, Ingerson, 

Townsend, and others, 1953), with soft beds and cavities forming about 70 percent of the section. Cavities, 

some open, some filled with loose coral sand, totaled 

122 feet in thickness and were found to a depth of more 

than 2,700 feet. Somewhat harder rock was found 

below 3,980 feet. Between 4,300 and 4,600 feet the 

section consisted largely of dolomite or dolomitic 

limestone.

Geothermal measurements were begun with the singlethermistor cable on June 19, 1952, 9 days after completion of the hole. The temperatures are given in table 2 

and figure 241. Because of other equipment mounted in

TABLE 2.—Temperature measurements with single-thermistor cable in 

drill hole F-l, Elugelab island, Eniwetok Atoll, June 19-26, 1952

Temp Day 

(° G) („-

Depth 

(feet)

Temp Day 

(° C) (June)

45___ —— — — 28.7 26

95— — - — — . — — 28.7 26

145_______._ 28.6 26

245—----------- 27.5 26

345___ —--------- 26.9 19

345— _-__--__.__ 26.4 26

445-____.____ 16.5 19

545_______-__ 13.0 19

645—---- —— 11. 1 19

745—-___--__-_. 14.5 19

845_________ 13.2 19

845— __-__--_.-_ 13.3 20

845_-________ 13.3 20

945—— — — — — — 11.5 26

1,045— — — — — — 11.2 20

1,145._______ 10.9 20

l,245---__-__._ 10.5 20

1,345-- — - — — — 10.2 20

l,445--__-___-__- 10.1 20

1,545_------__ 9.8 20

1,645- — — --__ 12.6 20

(feet) (June)

1,745-_________ 12. 6 20

1,845._______ 14.3 20

1,945-_______ 12.0 20

2,045- - ---- 11.1 20

2,145_ —— _______ 9.6 20

2,245—— -_ — — — 10.7 20

2,245_ —- — _— 10.8 23

2,345__._______ 10.9 23

2,445-—______ 11.1 24

2,445_—______ 11.2 24

2,545-- — — _-_.. 11.1 23

2,645—- —_ — __ 11.0 23

2,745 —— -_ — — — 12. 8 23

2,845 — — _ — -___ 13.0 23

2,945—— _- —— _ 13.5 23

3,045— — - — __ 13.5 23

3,145—- — — __ ia4 23

3,245-__— __ 13.8 24

3,345- — — — _'—_ 14.3 24

3,445_ ________ 15.2 24

3,4451—— — — __ 15.3 25
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 719

162° 00' 162° 20'

ELUGELAB

DRILL HOLE F-l

PARRY 1

DRILL HOLE E-l

,—Map Of Eniwetok Atoll showing the location of the island of Elugelab and Parry Island, and of drill holes E-J and F-l,

Water depths in fathoms.
720 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

the cable, only two conductors were available for the 

thermistor circuit, and a test circuit could not be used. 

Circumstances seldom allowed more than 30 minutes 

for the cable to remain in place before measurements 

had to be begun, a period too short for the thermistor 

to reach complete thermal equilibrium with the drill 

hole. A series of tests, however, showed that the 

resultant error averages only 0.1° C.

The top of the casing, which served as zero point for 

the depth measurements, was 4% inches above the 

surrounding ground level and 10.695 feet above mean 

low tide as determined by precise leveling. It was 6.15 

feet below the drilling table which constituted the zero 

point for drill-log depths.

In October 1952 these measurements were repeated 

with two multithermistor cables temporarily installed. 

The results are given in table 3 and figure 241. Although the cables remained in the hole for 3^ to 6 hours

before readings were taken, it was not possible to allow 

enough time for them to reach complete thermal equiTABLE 3.—Temperature measurements with multithermistor cable 

in drill hole F-1, Elugelab island, Eniwetok Atoll, October 1952

Depth

(feet)

Temp (°C)

29. 1 

28. 9 

28.5 

28. 1 

27.9 

27. 7 

27. 6 

198.1._._- 27. 3

24.1-. 

28.1_. 

73.1-- 

98.1.- 

123.1. 

148.1. 

173.1.

27.3 

27.0 

26. 4 

26.0 

25.7 

25.3 

24. 8 

398.1---.- 24. 2 

423.1----- 23.6 

449.8--..- 23.5

223.1. 

248.1. 

273.1. 

298.1. 

323.1. 

348.1. 

373.1.

Depth 

(feet)

500.5

549.8

600.5——-

649.8——-

700.5——-

749.8——-

800.5.—— -

849.8——-

900.5.—— -

949.8——-

1,049.8. _ _

1, 100.5. __

1,149.8,..

1,249.8. . .

1,300

1,400

1,449.8...

1,500

Temp (°C)

22. 0

20.3

19.0

17. 2

14. 8

14.0

13. 2

12.8

11.9

11. 5

10.6

10.4

10. 2

10.0

9.7

9. 6

9.7

9. 7

Depth

(feet)

Temp

1,600_---- 10.0 

1,649.8_.. 10.0 

1,700--.-. 10. 1 

l,800_--_- 9.8 

1,849.8. _. 10. 4 

1,900—___ 9. 8 

2,000.---- 9.7 

2,049.8. _ _ 9. 6 

2,100--.-- 9.3 

2,249.8. __ 8.9 

2,300__--- 8.6 

2,500--.-- 8.7 

2,700.___- 8.7 

2,900.-..- 8.9 

3,100_-._- 8. 8 

3,300.---. 9. 8(?)

4000.

10 15 20 

TEMPERATURE, IN DEGREES CENTIGRADE

25 30

.—Temperature profiles in 4rjll bole F-1, Elugelab island, Eniwetok Atoll, June'and Qctqber"l95?.
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 721

librium with the drill hole. Measurements made during the waiting period indicate that the resultant error 

was on the order of 0.04° C.

Both curves of figure 241 show the same general 

trends, the most striking feature of which is the continuing decrease in temperature to great depths. The 

significance of this and of the differences between the 

two curves is discussed later.

DRILL HOLE E-1, PARRY ISLAND

Eniwetok drill hole E-1 is near the south end of 

Parry Island, approximately 1,200 feet from the south 

end and 175 feet from the west shore of the island (fig. 

240). Drilling was begun on June 24, 1952, and the 

hole was completed in olivine basalt at a depth of 

4,222 feet on July 12, 1952. The hole was cased with

7-inch casing to a depth of 4,109 feet. The lowermost 

113 feet was uncased.

The stratigraphic sequence to a depth of 1,400 feet is 

similar to that of drill hole F-l (Ladd, Ingerson, 

Townsend, and others, 1953). Below that depth, 

although the strata are still reef sediments, there are 

marked differences in lithology and organic constitution. Only 55 percent of the section was soft or cavernous and only 2 small cavities, totaling 8 feet in 

drilling distance, were observed. No dolomite was 

found in this drill hole. Basalt cuttings were first 

observed at 4,154 feet and olivine basalt was cored 

from 4,208 to 4,222 feet.

Thermal measurements were begun on July 24, 1952, 

12 days after drilling was completed, and were continued 

through August 7, 1952. The results are given in table 

4 and figure 242. The measurements were made with

3500

4000 15 20 

TEMPERATURE, IN DEGREES CENTIGRADE

25

FIGURE 242.—Temperature profiles in drill hole E-1, Parry Island, Eniwetok Atoll, July and August 1952, December 22,1952, and July 2,1955,
722 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 4.— Temperature measurements in drill hole E-l, Parry 

Island, Eniwetok Atoll, July and August 1952

Depth 

(feet)

95____-__-

195--_-__-

295__-_-_-

395-__-_--

495---___-

545_-_-__-

595----.--

695___-___

795-______

895-------

995-___-_-

1,095_-----

1.195_____-

1,295-----

1,395- ..._.

Temp (°C)

-- 28.5

_- 26.4

._ 24. 3

.- 23.3

. _ 22. 5

._ 22. 4

._ 22. 3

_- 22. 2

__ 21. 7

_- 20. 3

. - 18. 5

-_ 17. 7

. - 16. 7

_. 15.8

.- 15. 2

Depth 

(feet)

1,495— —

1,595— —

1,695————

1,795- — .-

1,895

1,995———-

2.095. ___- _

2,195— —

2.295_ --__ -

2,395————

2,495————

2,595_ _._- -

2,695————

2,795— —

2.895———-

Temp (°O

— 14. 6

.- 13. 1

_- 13. 2

__ 13. 1

-_ 12. 6

_- 11.6

__ 10.4

-_ 9.9

-_ 9.5

-_ 9. 4

__ 9. 2

__ 9. 1

-_ 8.7

_- 8. 4

-- 8.6

Depth 

(feet)

2,995—--.-

3.095- — —

3,195.--- —

3,295_ -__ __

3,395- -__ __

3,495-.---

3,595-- -__ -

3,695_ -_-_-

3,795—----

3,895__-_--

3.995..----

4.045___-__

4,095— —

4,145— —

4.195—--.-

Temp (°C)

.. 8.7

._ 9. 2

_. 9.3

_. 9. 3

__ 9.5

q 7

._ 9. 9

.. 10. 1

.- 10. 7

__ 10. 8

-. 11. 1

__ 11.4

__ 11.4

-- 12. 2

__ 11. 9

the single-thermistor cable used for the June 1952 

measurements in drill hole F-l. As in the latter, no 

test circuit was available and conductor resistances had 

to be computed from drill-hole temperatures.

All depths were measured from a zero point at the 

top of the casing, 28 inches above the general ground 

level at an elevation of 14.44 feet above mean low 

water. The top of the casing is 4.67 feet lower than 

the drill table (elevation 19.11 feet above mean low 

water), which was used as the zero point for the stratigraphic measurements in the drill log.

On December 18, 1952, three geothermal-measurement cables were permanently installed in drill hole

E-l to permit a continuing study of the thermal 

regime in the drill hole and surrounding strata. Such 

a permanent installation has the added advantage that 

the cable attains and will thereafter remain in thermal 

equilibrium with the drill hole so that the temperature 

of the thermistor is identical with the temperature in 

the drill hole and adjacent beds.

Initial readings were made on the night of December 

18, 3 to 4 hours after the installation of each cable. 

These were taken purely to check the installation, as it 

was too early for the cables to have reached equilibrium 

with the drill hole. The measurements of the permanent sequence were begun 4 days later, on December 22, 

and have been continued at regular intervals since that 

date. Representative readings obtained from December 1952 to July 1955 are given in table 5.

The minimum temperature from 1952 to 1954 

occurred at a depth of 3,380 feet. Since early 1954, 

however, the temperature at a depth of 3,180 feet has 

been dropping somewhat more rapidly than that at 

3,380 feet, with the result that early in 1955 it finally 

became, and since then has remained, slightly less than 

at 3,380 feet. The point of minimum temperature has 

thus migrated a short distance upward in the 2% years 

of observations.

The temperature profiles for December 22, 1952, and 

July 2, 1955, are shown graphically in figure 242, with 

the profile for July and August 1952.

TABLE 5.—Temperature measurements in the permanent installation in drill hole E-l, Parry Island, Eniwetok Atoll, December 1952

to July 1955

Depth (feet)

3.8

27.8___-______-

52.8— __„ . — _

77.8————— —

102.8— —— —

127.8— — ——

152.8——— ———

177.8—— — —

202.8——— ———

227.8--__-_-___

252.8__________

277.8________-_

302.8____-_____

327.8_____-____

352.8— ————

377.8— ————

402.8--. ______

450.3_____.____

500.3— ————

600.3__._______

700.3_. ________

800.3_-_____.__

900.3______.___

Temperature (°C)

Dec. 22, 1952

26. 96 

28. 70 

28.02 

27. 59 

27. 10 

26. 62 

25. 99 

25. 62 

25. 30 

25. 04 

24. 68 

24. 18 

23. 90 

23. 53 

23. 07 

22. 60 

22. 33 

21.80 

21.02 

19. 93 

18. 93 

17. 91 

16.74

July 5, 1953

28. 52 

28.49 

27. 96 

27. 34 

26.88 

26. 42 

25. 99 

25.73 

25.43 

25. 13 

24. 64 

24. 17 

23.80 

23.28 

22. 90 

22.48 

22. 22 

21. 66 

21.04 

19. 67 

18.56 

17. 45 

16. 18

July 15, 1954

27.86

28.41 

27. 94 

27. 30 

26. 82 

26. 37 

25. 82 

25.47 

25.09 

24.80 

24. 26 

23. 94 

23. 66 

23. 12

22. 29 

22. 02 

21.42 

20. 71 

19.45 

18. 37 

17. 21 

15.78

July 2, 1955

27. 31 

28. 20 

27.83 

27. 24 

26. 77 

26.32 

25. 77 

25. 43 

25. 03 

24. 76 

24. 26 

23. 90 

23. 59 

23. 10

22. 30 

22. 04 

21. 40 

20. 66 

19. 46 

18.29 

17. 11 

15. 56

Depth (feet)

1,000.3—- ———

1,100.3__ ————

l,200.3_-__--__

1,300.3.. ------

1,400.3.- ————

l,500.3-_ ————

1,600.3_ _ ————

1,700.3_ _ _ ———

1,800.3__ ——— _

1,900.3. _ ------

2,000.3_ ------

2,080_ ————— _

2,280——————_

2,380__-__-___-

2,580——— ———

2,680_____---_-

2,780____ _ ...

2,980— _ ————

3,180__ —————

3,380_____----_

3,580———.——

3,780__-___ -__

3,980——————

Temperature (°C)

Dec. 22, 1952

15.54 

14. 52 

13. 70 

12.43 

10. 90 

10.87 

10. 20 

9.96 

9.82 

9.47 

9. 16 

8.73 

8.33

7.72

7. 35 

6. 96 

7.08 

6. 94 

7. 16 

7.93 

9.45

July 5, 1953

15. 12 

14. 11 

13. 31 

. 11. 98 

10. 47 • 

10. 40 

9.82

9. 42 

9.08 

8.81 

8.53 

8.21 

7. 98 

7. 66 

7.51 

7.26 

6. 88 

6. 79 

6. 68 

7.63 

8.38 

9.52

July 15, 1954

14.89

13. 90 

13.08 

11. 76 

10. 21 

10. 16 

9. 64 

9. 15 

9. 21 

8.89 

8.65 

8.43 

8.08 

7.88 

7. 53 

7. 34 

7.08 

6.74 

6. 56 

6.52 

6.92 

8. 32 

9.44

July 2, 1955

14.78

13. 81 

12.97 

11. 58 

10. 13 

10.06 

9.54 

8.95 

9. 12 

8.83 

8. 58 

8.40 

8. 05 

7.85 

7.46 

7. 27 

6.99 

6. 54 

6.36 

6. 41 

6. 89 

8. 30 

9.42
MEASUREMENTS, ENIWEffQK AND BIKINI ATOLLS 72$

BIKINI ATOLL—DRILL HOLE 2B, BIKINI ISLAND

Because the thermal profiles observed in the Eniwetok drill holes departed so radically from those observed 

elsewhere, it seemed highly desirable to make similar 

thermal measurements on other atolls. Fortunately, 

this was possible. In 1947 during a resurvey of Bikini 

Atoll, a year after Operation Crossroads, holes had 

been drilled on Bikini island, one (drill hole 2B) to a 

depth of 2,556 feet (Ladd, Tracey, and Lill, 1948; 

Emery, Tracey, and Ladd, 1954). In November 1952 

drill hole 2B was relocated. No maps showing its 

exact position were available, but a description furnished by H. S. Ladd, who had had charge of the 

drilling project, made it possible to find it.

Three drill holes—2, 2A, and 2B—had been drilled 

close together near the center of. the lagoon shore of 

Bikini island (fig. 243). Drill holes 2 and 2A were

165° 30'

I65°30'

FIGURE 243.—Map of Bikini Atoll showing location of Bikini island and 

drill holes.

located 63 feet inland from the break in vegetation at 

the top of the beach, 130 feet seaward from the shore 

road along the lagoon side of the island, 54 feet northwest of the northwest fence of the native graveyard, 

and approximately 800 feet southeast of the Japanese 

monument at the landing near the center of the lagoon 

beach. Drill hole 2B is 179 feet N. 60° E. from drill 

holes 2 and 2A, and 49 feet north of the lagoon shore 

road. The drill hole is marked by its 4-inch casing, 

which projects about 14 inches above the ground, and 

by a protective fence recently placed around it. The 

hole was found to be still open to a depth of 1,430 feet, 

but to be bridged at a point between 1,430 and 1,450 

feet below the top of the casing.

Gable 193 was inserted in drill hole 2B at 1400 hours 

on November 28,'1952, with its zero point 36 inches 

belbw the top-'of the casing and at the surface of the

water table in the casing. It was allowed to remain 

overnight to reach thermal equilibrium and was read 

at 1000 hours on November 29.

No other multiconductor cable was available, so that 

it was necessary to use the single-thermistor cable for 

measurements at depths greater than 403 feet. This 

was installed in the drill hole to a depth of 500 feet at 

1118 hours on November 29. Because there was only 

one thermistor in the cable, the thermistor had to be 

changed to a different depth after each reading. It was 

not possible to leave the thermistor at any given depth 

for more than about half an hour, so it could not reach 

complete thermal equilibrium with the hole. The length 

of time the thermistor remained at each depth was comparable to the length of time in the June, July, and 

August, 1952, measurements at drill holes F-l and E-l 

on Eniwetok. The same considerations of accuracy 

would therefore apply to the measurements in Bikini 

drill hole 2B for depths of 500 feet or more.

Cable 193 remained in place for 18 hours before 

measurements were begun, so that it had reached equilibrium with its surroundings; thus, the measurements 

from 0 to 403 feet may be regarded as equal in accuracy 

to those with the same cable on December 22 and afterward in drill hole E-l on Eniwetok.'

In all the measurements the resistance of the test 

circuit was determined and a correction for conductor 

resistances made. The observed temperatures are 

shown in table 6.

TABLE 6.—Temperature measurements in drill hole 2B, Bikini 

island, Bikini Atoll, November 29, 1952 *

Depth 

(feet)

4

28___ —

53______

78-----.

103___-_

128

153_____

178____.

203_-_._

Temp (°C)

.___ 28.7

.___ 28.0

.___ 27. 6

.___ 27.4

.___ 27. 2

.___ 27. 0

..__ 26.8

. _ 26. 4

._ _ 26. 4

Depth 

(feet)

228_

253-----

278-----

303_.-__

328___-_

353__.__

378-.---

403.____

500___-_

Temp (°C)

.___ 26. 2

. _ 25. 9

.___ 25. 9

. _ 25. 7

.___ 25. 6

.___ 25.5

.___ 25. 3

.___ 25. 2

.___ 24. 6

Depth 

(feet)

600___.__

700_____.

800_____-

1,000 __ .

1,200———

1,400———

1,430———

Temp (°C)

23 1

_._ 22. 1

___ 20.5

___ 18. 0

._. 17. 0

___ 15. 8

.__ 15.7

1 Multiconductor-cable 193 used to depth of 403 feet, the single-thermistor cable 

used below that depth.

The resultant temperature profile is shown in figure 

244 together with those for Eniwetok drill holes E-l 

and F-l for comparison. All three exhibit the same 

continuous drop in temperature with increasing depth, 

although the temperatures for drill hole 2B are uniformly higher below a depth of 350 feet. A sharp 

decrease in the negative gradient for drill Hole 2B occurs 

at a depth of 1,000 feet, but its significance cannot be, 

determined. It may represent the base of the highgradient zone observed at 1,050 feet in F-l and 1,400 

feet in E-l, although it occurs at a higher temperatureand the break is less sKarp. . ..' '

428688—68——8
724 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

1000

S2000 

I

3000

4000 15 20 

TEMPERATURE, IN DEGREES CENTIGRADE

25 30

FIGURE 244.—Temperature profiles in drill holes E-l and F-l, Eniwetok Atoll, and drill hole 2B, Bikini Atoll.

ANALYSIS OF OBSERVATIONS 

EFFECTS OF HEAT OF DRILLING

The operation of a rotary drill, such as that used on 

Eniwetok, develops large amounts of frictional heat. 

In addition the water used on Eniwetok in preparing 

the drilling mud and in drilling the hole was pumped 

from the lagoon at a temperature of approximately 

28° C. The average temperature of the rocks penetrated in drilling was on the order of 12° C. Thus, 

large volumes of water at a temperature approximately 

16° C higher were pumped into the holes. The temperature of the hole and the rocks surrounding it must 

therefore have been considerably increased during the 

drilling. Temperatures in the hole should therefore be 

considerably above normal immediately after the completion of drilling and should gradually decrease with 

time, as the drilling heat is dissipated, until thermal 

equilibrium is reestablished.

That this has occurred in the Eniwetok drilling is 

shown hi table 5 and by figures 241, 242, and 245. 

In figure 242 the thermal profiles in drill hole E-l for 

December 22, 1952, and July 2, 1955, have been superimposed on that for July 24 to August 7, 1952, which 

was measured just after drilling was completed. There 

is a marked drop in temperature between the profile for 

July and August and that for December 22, 1952, and 

the latter curve has become smoother and more regular. 

The temperatures for the July 2, 1955, curve have in 

general decreased still further although the rate of 

decrease has become much less.

In figure 241, where the thermal profile in drill hole 

F-l for October 1952, 4 months after drilling was 

ended, is superimposed on that for June 1952, measured 

a few days after drilling was completed, the divergence 

is even more pronounced than for drill hole E-l. This 

is probably due in part to the longer drilling time, 30
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 725

days for F-l as compared with 19 days for E-1, and 

the greater thermal disturbance that might be expected 

as a result.

Two pronounced anomalies are evident hi the June 

profile. A marked low, culminating at a depth of 650 

feet, extends from 400 to 750 feet. This low had disappeared when the October measurements were made. 

The pronounced thermal high, centered at 1,850 feet 

and extending from 1,550 to 2,150 feet, was still present 

in October but had greatly decreased hi amplitude, and 

concurrently had spread 150 feet farther up and down 

the hole (extending from 1,400 to 2,300 feet hi October). 

The persistence of this anomaly indicates that equilibrium had not been reached 4 months after drilling was 

completed.

Figure 245 shows the change of temperature with 

tune for a depth of 800 feet hi drill hole E-1. It is 

apparent that thermal dissipation was still continuing 

more than a year after drilling was completed.

COMPARISON WITH PACIFIC COAST THERMAL 

PROFILES

It is of interest to compare the characteristics of the 

atoll thermal profiles with those of thermal profiles in 

drill holes in other areas of the Pacific Ocean. Figure 

246 shows the profiles, taken from Van Orstrand (1939, 

figs. 3 and 4, p. 127-128), of three typical California 

coast drill holes: Nesa 11 at Long Beach, and Hobson 

3 and Ferguson 3 at Ventura. Nesa 11 and Hobson 3 

are near the coast, and Ferguson 3 is actually in the 

ocean offshore. All three are, therefore, in places 

similar to the Eniwetok drill holes, with respect to 

their distance from the ocean.

All three are characterized by a steadily rising temperature below the zone of annual change. This 

constant rise in temperature with increasing depth 

below a shallow surface zone is characteristic of continental thermal profiles, whether coastal or interior.

In figure 246 the thermal profile in Eniwetok drill hole 

E-1 for March 16, 1953 is superimposed on the profiles 

of the three California wells. It is at once apparent that 

the profile hi the atoll represents a very different thermal 

condition. Instead of rising, the temperature decreases 

rapidly for the first 1,400 feet, its trend in this portion 

of the curve being nearly at a right angle to the trends of 

the California profiles. Below 1,400 feet it decreases 

less rapidly to a depth of 3,380 feet, where it reaches a 

minimum of approximately 6.7° C. Below 3,380 feet 

the temperature rises and continues to rise to the maximum depth attained.

Because it shows fewer signs of thermal disturbance 

from drilling and has been observed for a longer period

of time, the profile of drill hole E-1 has been used above 

for comparison with the California profiles. However, 

the same general features are evident in drill hole F-l 

(fig. 241); there is an even more rapid initial drop in 

temperature which continues to about 1,050 feet, where 

the slowup in the rate of decrease begins. This slowup 

occurs in both drill holes F-l and E-1 at approximately 

the same temperature rather than at the same depth.

The thermal profile for drill hole F-l belpw this depth 

becomes quite irregular, owing to thermal disturbances 

which in October 1952 had not yet disappeared. As a 

result, the measured minimum temperature of 8.6° C is 

reached at a depth of 2,300 feet. The temperature 

remains nearly constant below this point to a depth of 

3,100 feet. The apparent rise below that point is based 

on a single observation which itself is doubtful.

The thermal profile for Bikini drill hole 2B (fig. 244) 

shows the same long-continued drop in temperature with 

increasing depth, paralleling that in drill hole E-1 quite 

closely below 600 feet although its temperatures are 

uniformly higher below a depth of about 350 feet. If 

the decrease hi the negative gradient at 1,000 feet is the 

same as that occurring at 1,050 feet in F-l and 1,400 

feet in E-1, which cannot be determined without deeper 

measurements, it occurs here at a higher temperature, 

18.0° C, and is much less sharp a change.

All three atoll thermal profiles parallel each other but 

diverge widely from the continental-type profiles illustrated by drill holes on the California coast.

COMPARISON WITH OCEAN TEMPERATURES IN THE 

MARSHALL ISLANDS AREA

As the coral atolls represent isolated land masses of 

quite porous reef sediments, it seemed reasonable to 

believe that the marked divergence of the thermal 

profiles from normal continental profiles could be related 

to the temperatures in the adjacent ocean. An attempt 

to measure the thermal profile immediately off Eniwetok 

was unsuccessful because heavy weather damaged the 

equipment. However, during the 1952-53 Capricorn 

Expedition of the Scripps Institution of Oceanography 

three sets of thermal measurements were made in the 

Marshall Islands waters, from the SS. Horizon, as 

follows (see also fig. 236):

Run Date Location

Nansen-bottle Oct. 15, 1952 Lat 14°16' N., long 169°27' E.

cast. 

Bathythermo- Oct. 14, 1952 Lat 13°45' N., long 168°28' E.

graph.. 

Bathythermo- Nov. 12, 1952 Lat 11°07' N., long 162°22' E.

graph.

The observations, as furnished by Walter H. Munk, 

are given in table 7.
-JJ 3*' JSJaANI^S

400

FIGURE 245.—Time-temperature graph for a depth of 800 feet in drill hole E-1, Eniwetok Atoll.
FEET IN DEPTH, 

x",

/

\L

QKV IME33S STLIOLV l&L
728 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 7.—Ocean-water temperatures in the Marshall Islands area; 

measured by the Capricorn Expedition, Scripps Institution of 

Oceanography, October and November 1952

Temp.

Depth (feet)

0-200 » _____ _

300_ _______ .

400____________

500- ___ _____

600__. ____ ___

700— __ _____

Of

. 82

_ 80

_ 74

_ 65

_ 60

_ 53

°C

27.8 

26.7 

23. 3 

18. 3 

15. 6 

11.7

Temp.

Depth (feet)

800-________ 51 10. 6

1,000________ 50 10. 0

1,500----------- 47.5 8. 6

2,000--._____ 45.5 7. 5

3,000--____.-_ 42.5 5. 8

4,000_________ 40 4.4

1 Isothermal.

In figure 247 the thermal profiles in drill holes E-1, 

F-1, and 2B are shown superimposed on the oceantemperature profile. The similarity of the ocean 

thermal profile to the atoll thermal profiles is immediately apparent. All four show the same rapid decrease 

in temperature in an upper zone (in the ocean to a 

depth of 800 feet) followed by a deeper zone of continued but less rapid decrease in temperature. Although the atoll profiles eventually reverse their trend

and the ocean profile does not, continuing to become 

cooler with increasing depth, the close parallelism of 

all four is striking, leaving little doubt that the ocean 

temperatures are a major factor in controlling atoll 

temperatures, at least in the zone of reef sediments.

The zone of rapid temperature change has been called 

the thermocline. The zone of decreased rate of change 

immediately below it has been called the zone of weak 

gradient (Mao and Yoshida, 1953, p. 12). The term 

asthenocline (from the Greek asthenos, weak, and 

klinein, to incline) is here suggested for the latter.

The temperatures in drill hole F-1 down to depths 

of about 1,400 feet parallel those of the ocean more 

closely than do those in drill hole E-1. This relation 

suggests a closer access in that part of drill hole F-1 

to the cooling effects of the ocean water. Drilling data 

indicated softer beds and more cavernous horizons in 

drill hole F-1 than in drill hole E-1, although nothing 

but a general relationship can be stated.

^2000

3000

4000 10 15 20 25

TEMPERATURE, IN DEGREES CENTIGRADE

FIGURE 247.—Temperature profiles in drill holes E-1 and F-1, Eniwetok Atoll; drill hole 2B, Bikini Atoll; and in the ocean in the Marshall Islands are$.
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 729

TEDAL FLUCTUATIONS IN THE DRILL HOLES

In October 1952, H. K. Stephenson and Walter H. 

Munk made a series of water-level measurements in 

drill hole F-1. At the same tune measurements were 

made of the water level in the ocean offshore on the 

reef. The results, which have very kindly been 

furnished by Dr. Stephenson, are given in table 8.

These observations are shown graphically in figure 

248, where they are compared with tide-table observations (U. S. Coast and Geodetic Survey, 1951). It is 

apparent that the water-level fluctuations in the drill 

hole agree in both phase and amplitude, as closely as 

can be determined, with the tide as measured just offshore and as given in the tide tables. This result was 

surprising, especially so because the drill hole was 

solidly cased to a depth of 1,973 feet. It indicates 

that the reef sediments surrounding drill hole F-1 are 

highly permeable to ocean water.

Similar measurements were made in drill hole E-l by 

the writer in December 1952. At that time water in the

TABLE 8.—Water-level measurements in drill hole F—l, Elugelab 

island, Eniwetok Atoll, October 21, 1952

[Measured by H. K. Stephenson and W. H. Munk]

Time

0458-----______-_-_----------_-

0505_ _ ___________

0730— —— ——— ———— — ————

0810

0915— ———— — — ——— ————

1010______.___-___-____-_______

1055———— ———— —————— ————

1215

1415 ._ _-_ _ _

1520__________.____--_-___---__

1615—— — — ——— ———— ———

1712-__________________________

1715———— ——— — —— —— ——

1800___________________________

2336———— ———— ————— ————

Water level (feet) »

Drill hole

3.70

2. 64

2.08 

1. 42 

1.06 

.98 

1. 46 

i 2. 80 

3. 12 

3.98 

4.48

4. 62 

4.48

Ocean on 

reef

2.35 

1.57 

1. 16 

1.03 

1. 28 

i 2. 80 

3. 16 

3.99 

4. 49

4.57 

4. 41

Tide tables

4.2

.8

12.8

4.8

.8

i Each set of measurements was referred to an individual arbitrary datum. Therefore, they are arbitrarily corrected here to a common midtide measurement of 2.8 feet 

to permit a more satisfactory comparison.

< 3

< Z

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 I I I I III I

2200 0000 0200 0400 0600 0800 1000 1200 1400 1600 1800 2000 2200 2400

OCT20.1952 OCT 21.1952 

LOCAL TIME, IN HOURS

FIGUEE 248,—Water-level fluctuations in drill hole F-1, Eniwetok Atoll; tidal fluctuations on the ocean sjde of Elugelab island; and tide-table values for Eniwetok Atoll.
730 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

drill hole was about 52 inches below the top of the casing. A piece of board with a sharp bottom knifelike 

edge was placed across the mouth of the casing and a 

6-foot folding rule lowered slowly until its metal tip 

touched the surface of the water. The instant of contact was clearly and sharply shown by a slight disturbance wave in the water. The reading on the rule 

at the bottom of the board was recorded as the depth of 

the water surface below the top of the casing. Great 

care was exercised to keep the rule both straight and 

vertical so that contact was made by the entire end of 

the rule at the same time, and readings were immediately repeated one or more times as a check. Despite 

the apparent crudeness of the method, it was found 

possible, because of the sharpness of the disturbance 

signal, to repeat measurements to the nearest thirtysecond of an inch. The measurements are shown in 

table 9.

In figure 249 water-level fluctuations in the drill 

hole are compared with those of the tide as given by 

the tide tables. The fluctuations in the drill hole are 

negligible when both are plotted to the same scale 

(fig. 249A). However, if the amplitude of the morning 

tidal wave is reduced to the amplitude of the fluctuation

TABLE 9.—Water-level measurements in drill hole E-1, Parry 

Island, Eniwetok Atoll, December 14, 1952

Time

0838. — —

0908- — —

0941- __

1007———

1032., ____

1102__ ——

1132. _____

1201-.—

1242_.__._

1302—. _

1333.-...

Water level (inches)

Depth 

below 

top of 

casing

52% 

52%

52%

5218/32

52% 

52% 

52% 

52% 

52% 

52% 

52%

Decimal 

equivalent 

height 

above 

minimum

0 69 

63 

63 

63 

69 

69 

69 

69 

63 

56 

50

Time

1406___.__

1442. _ _ .

1508_ — ___

1541..__._

1609__ __

1639.- —

1709. _____

1739. _____

1809— —

I860— _ .

1938- ——

Water level finches)

Depth below 

top of 

casing

52% 

52% 

523 fa

53^2 

53^2

53%2 

53^2 

53%2 

53^2 

53%2 

53%2

Decimal 

equivalent 

height 

above 

minimum

0 44 

38 

25 

13 

06 

06 

06 

0 

0 

.03 

0

in the drill hole and the phase is shifted so there is a lag 

of 9K hours, quite satisfactory agreement between 

fluctuations of the tide and the water level in the drill 

hole is obtained (fig. 249J?).

Both the small amplitude of the water-level fluctuation in drill hole E-1 and the time lag of 9K hours indicate either a much longer hydraulic path, or a higher

DH E-1 o—o—o-o-o—o—o—o—o -o—o—o—c

0200 0400 0600 0800 1000 1200 1400 1600 1800 2000

LOCAL TIME, IN HOURS

B

FIGURE 249.—Comparison of water-level fluctuations in drill hole E-1, Eniwetok Atoll, with tide-table values, December 14,1952. A, Tide-table values (solid circles) and 

drill-hole values (open circles) plotted to same scale. B. Tide-table values (solid line) reduced to same amplitude as drill-hole values (open circles) and shifted in phase (broken line), with a lag of 9W hours.
GEOTHEEMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 731

hydraulic resistance, or both. Drill hole E-l is solidly 

cased to a depth of 4,109 feet, within 113 feet of the 

bottom of the hole and within 99 feet of the top of the 

basalt. A longer hydraulic path is, therefore, obvious. 

However, this in itself would not seem sufficient to 

account for the very small amplitude and longer time 

lag. A much less pervious rock would seem also to be 

required. Drilling indicated firmer rock and fewer 

cavities. This is also in accord with the temperature 

data, which shows less agreement in the upper 1,400 feet 

with the ocean profile than in drill hole F-l. However, 

the difference between the general conditions in the two 

holes seems less than the small amplitude and large time 

lag in drill hole E-l would demand. It is quite reasonable to believe that the deeper part of the section will 

have in general a lower porosity and permeability. It 

is also quite possible that the longer hydraulic path 

may interpose in its course certain strata more consolidated and of much lower porosity and permeability 

than the average, and that these may cause a disproportionate change in the tidal fluctuations. If the hole 

were not cased below 2,000 feet, it is quite possible that 

a tidal fluctuation of large amplitude and little time 

lag might be observed. No data are available and no 

conclusions can be reached at present concerning these 

different possibilities. It may be safely stated, however, that all the evidence indicates a generally lower 

porosity and hydraulic permeability in the reef sediments around drill hole E-l than around drill hole F-l. 

Measurements were made of the fluctuations of water 

level in the Bikini drill hole at the same time that the 

temperature measurements were being taken (table 10).

TABLE 10.—Water-level fluctuations in drill hole 2B, Bikini 

island, Bikini Atoll, November 1952

Day

Nov. 28

29

29

29

29

29

29

Time

1400

1115

1236

1300

1330

1344

1412

Water level

Depth 

below

top of

casing

(inches)

36.0

47.0

39. 5

37. 0

34. 5

33. 5

32. 5

Height 

above

arbitrary

zero

(feet)

2.40

1.48

2. 10

2. 31

2.52

2.60

2. 69

Day

Nov. 29

29

29

29

29

29

Time

1415

1436

1440

1457

1502

1530

Water level

Depth

below

top of

casing

(inches)

31. 5

30.5

30. 5

29. 5

30. 0

29. 25

Height 

above

arbitrary

zero

(feet)

2. 77

2.85

2. 85

2. 94

2. 90

2.96

These measurements are plotted in figure 250 for comparison with the tide tables (U. S. Coast and Geodetic 

Survey, 1951). It is evident that the water-level fluctuation in the drill hole has a smaller amplitude than the 

ocean tide (fig. 25QA). A comparison of the two sets of 

data indicates that the amplitude in the drill hole is only 

0.45 that of the ocean tide. That it is a true tidal 

fluctuation, however, is shown in figure 2505 where the

1000 1200 1400 1600 1800 2000

NOV 29:1952 

LOCAL TIME, IN HOURS

A LAG 

I 1 HOUR

1400 1600

Nov 29. 1952

LOCAL TIME, IN HOURS

B

FIOURE 250.—Comparison of water-level fluctuations in drill hole 2B, Bikini Atoll, 

with tide-table values, November 29, 1952. A, Tide-table values (solid circles) 

and drill-hole values (open circles) plotted to same scale. B, Tide-table values 

(solid line) reduced to same amplitude as drill-hole values (open circles) and shifted 

in phase (broken line), with a lag of 1 hour.

amplitude of the tidal fluctuations in the ocean (solid 

line) is reduced to 0.45 of normal and referred to the 

same arbitrary zero as that for the fluctuations in the 

drill hole. If the tide curve is shifted in phase (broken 

line) so that there is a phase lag of 1 hour, it falls very 

satisfactorily on the curve of water-level fluctuations 

(open circles) in the drill hole.

The drill hole is solidly cased with 4-inch casing to a 

depth of 804 feet and seems to be bridged at a depth of 

approximately 1,430 feet. It is therefore known to be 

open from 804 to 1,430 feet but it is not known whether 

it is hydraulically blocked below 1,430 feet.

The lag and the decreased amplitude of the fluctuations in the drill hole indicate a lower permeability in 

the surrounding sediments than for Eniwetok drill hole 

F-l. Nevertheless, the presence of a tidal fluctuation 

in the drill hole with an amplitude of 45 percent of that 

in the open ocean indicates quite a high permeability 

for the Bikini sediments.

The evidence from tidal fluctuations of the movement 

of sea water through the coral sediments and the close 

parallelism of the atoll and ocean temperature profiles 

strongly suggests that the temperature of the atolls 

is a function of the temperature of the ocean water 

and its access to the atoll sediments.

COMPARISON WITH KITA-DAITO-JIMA

Since completing the manuscript for this report, the 

writer has received translations of two papers, published in Japanese, by Toshio Sugiyama (1934, 1936)
732 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

describing temperature measurements in a test drill 

hole on Kita-daito-jima, an island in the western 

Pacific Ocean, 200 miles east of the south end of 

Okinawa at lat 25° 57' N., long 131° 18' E., and 

believed to be an elevated coral atoll. To the best 

of the present writer's knowledge this is the only 

other place where systematic measurements have been 

made of inhole temperatures beneath a coral island.

A first test hole was put down to a depth of 686.6 

feet (209.26 meters) in 1934 with a rotary drill. In 

1936 this hole was deepened to a total of 1,416.2 feet 

(431.67 meters).

Temperature measurements were made by means of a 

eopper-eonstantan thermocouple which was tied at 

several points to a wire cable and lowered through the 

drill rods to the desired depth. The thermocouple 

leads were enamel insulated and the inhole junction 

was protected by a very small diameter rubber tube. 

The cold junction of the thermocouple was formed by 

an ice bath at the surface, and the current in the thermocouple circuit was measured by a sensitive galvanometer 

read by telescope. Temperatures were obtained by 

means of a calibration curve previously prepared at the 

Earthquake Research Institute, Tokyo University.

Measurements made at 50-meter intervals between 

1050 and 1150 hours on the 26th day of the first drilling 

are given in table 11.

TABLE 11. — Temperature measurements during first Kita-daitojima drilling, 1934 

Depth —————————————————————— Temp 

Meters Feet (°C) Remarks

TABLE 12.—Temperature measurements during second Kita-daitdjima drilling, April 18, 1936

50.. 

100.

200

5. 1 

164 0 

328. 1 

492. 1 

656.2

20.80 

19.80 

18.00 

15.86 

15.20

Groundwater table.

In the second drilling the hole reached 1,305.8 feet 

(398.00 meters) on April 17, 1936. Temperature 

measurements were made on April 18, between 1400 

and 1700 hours, using a copper-constantan thermocouple as before. The thermocouple leads were 

wrapped with rubber, but this insulation proved 

unsatisfactory and easily damaged. Although no 

statement is made concerning the measurement technique employed it is assumed to be the same as that 

for the first survey in 1934. The temperatures are 

given in table 12 and are shown in figure 251. Measurements were made at 10-meter intervals. After 

completing the temperature run, drilling was resumed 

and the hole finally bottomed at 431.67 meters (1,416.2 

feet) on April 24, 1936.

Both sets of measurements were made during the 

progress of drilling. As described previously, drilling

Depth

Meters

044

10

20

30 _ ______

40_.____.__

60

70

80_________

100

110

120

130

140

150________

170

180

190

200

.Fee*

1.4

32.8

65.6

98.4

131.2

164.0

196.9

229.7

262.5

295.3

328. 1

360.9

393.7

426. 5

459.3

492. 1

5249

557.7

590.6

623.4

656. 2

22.50

21.60

21.00

20. 10

19.85

19.35

19. 10

18. 85

18.38

18.30

17.30

17. 10

16.80

16.30

16.30

16.00

15.80

15.55

15. 05

1480

1420

Depth

Meters Feet

210 _ . 689. 0

220 _ _____ 721. 8

230 754 6

240_.___.__ 787.4

250 _ _ 820. 2

260____ __ 853. 0

270_____ ._ 885. 8

290________ 951.4

300 _ _____ 984.3

310_.______ 1,017.1

320_ _ ___ 1,049. 9

330 _ __ _ 1,082.7

340___.____ 1, 115. 5

350. _ ___ 1, 148. 3

360_. ______ 1, 181. 1

370________ 1,213.9

380________ 1,246.7

390__ ______ 1,279.5

398_ _ _ 1 1,305.8

13.75

13.50

13.20

12.85

12.60

12.00

11.95

11.70

11.40

11. 10

10.85

10. 55

10.30

10. 15

9.95

9.70

9. 50

9.25

9.00

a 75

' Total depth of drill hole on April 18,1936.

introduces a large amount of heat into the hole and the 

rocks surrounding it, especially when a rotary drill is 

used. In addition it is presumed, although not so 

stated by Sugiyama, that water for drilling was pumped 

from one of the ponds nearby. The temperature of 

the surface water of nearby Akaike pond was 26.4° C. 

at the time of the temperature measurements. This 

may be regarded as within a degree or two of the temperature of the drilling water. As this is well above 

the average temperature in the drill hole, a large amount 

of heat must have been added by the drilling water. 

Drill-hole temperatures a few days after the completion of drilling differ by some degrees from the normal 

equilibrium temperatures, and the disturbance would 

be even greater for measurements made during the 

drilling process. It is therefore believed that the Kitadaito-jima measurements are probably a few degrees 

high.

Although error from this cause will differ for different 

depths, the general trends of the thermal profiles are 

not too greatly altered, and it is therefore probable 

that the general trends of the Kita-daito-jima profiles 

are approximately correct.

The curves for Eniwetok drill hole F-l (measured 

4 months after drilling was completed), Eniwetok drill 

hole E-l (measured a year after the completion of the 

drilling), Bikini drill hole 2B (measured a little more 

than 5 years after completion of drilling), and the 

Kita-daito-jima drill hole, are in general much alike 

(fig. 251). All four show the same steady decrease in 

temperature with increasing depth. The trends for the 

Kita-daito-jima profile rather closely parallel those of
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 733

4000 10 15 20 

TEMPERATURE, IN DEGREES CENTIGRADE

30'

FIGUEE 251.—Temperature profiles in drill hole on Kita-daito-jima; drill holes E-1 and F-1, Eniwetok Atoll; and drill hole 2B, Bikini Atoll.

both Eniwetok drill hole E-1 and Bikini drill hole 2B. 

The rate of decrease is more rapid in the upper, thermocline segment of Eniwetok drill hole F-1 than in the 

other three although the F-1 curve closely parallels 

the Kita-daito-jima curve between depths of 900 and 

1,150 feet. As at Bikini, the measurements at Kitadaito-jima were not made at great enough depths to 

determine certainly the sharp break observed in the 

Eniwetok curves at the base of the thermocline segment, 

unless the change in trend in the Bikini curve at a 

depth of 1,000 feet and in the Kita-daito-jima curve 

at a depth of 850 feet can be regarded as the beginning 

of such a change. All four curves show the same sharp 

difference from the trends of the continental curves 

illustrated in figure 246.

In early December 1935 Sugiyama installed a tide 

gage in the original drill hole, then only 686.6 feet 

(209.26 meters) deep, and recorded the water-level

fluctuations (Sugiyama, 1936, tables 8-11). A tidal 

fluctuation ranging from 0.78 to 3.70 feet (0.238 to 

1.127 meters) was observed.

He attempted to make comparable tide measurements for the sea by installing a tide gage at the 

Minatoguchi coast nearby, but the first gage was swept 

into the sea and satisfactory records could not be obtained. An old well approximately 50 meters east of 

Minatoguchi was finally used. Although the well water 

is not in direct connection with the sea, its rise and fall is 

stated by Sugiyama (1936, p. 20) to be nearly the same 

as that of the adjacent sea level. Data from these 

various measurements are given in table 13.

The tidal amplitude in the original drill hole, before 

deepening, was 86 percent of the amplitude in the well 

and the average lag behind the latter was about 1.1 

hours. No tidal measurements were made in the drill 

hole after it was deepened by the second drilling.
734: BIRTtfl AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 13.—Tidal fluctuations in the drill hole and in the old well 

at Minatoguchi, Kita-daito-jima; measured by T. Sugiyama, 

December 1935

[I^low tide? Hr high tide]

Day

Dee. 9 

10

11 

12 

13 

14 

•15 

16

17

18

19

20 

Average.

Tide 

stage

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L 

H 

L

Observed data

Time

Drill 

hole

1244 

1847

0847 

1434 

1939 

0405 

0938 

1506 

2027 

0506 

1034 

1601 

2132 

0545 

1119 

1634 

2214 

0614 

1200 

1739 

2300 

0653 

1227

____

Well

1630 

1636 

2150

1142 

1735 

2257

1222 

1828 

2349 

0705 

1318 

1928 

0052 

0751 

1357 

2121 

0243 

0859 

1505 

2310 

0450

Amplitude (HVL)

Drill hole 

(meters)

0.498 

1.022 

.865 

.471 

.576 

1.127 

.969 

.471 

.603 

1.100 

.943 

.445 

.576 

1 1.074 

.917 

.419 

.550 

.969 

.838 

.419 

.498 

.865 

.760

.394 

.367 

.524 

.603 

.367 

.288 

.445 

.498 

.445 

.288 

.301

0.632

Well 

(meters)

0.550 

.629 

.917 

.786 

.498 

.524 

.812 

.786 

.445 

.419 

.655 

.734 

.445 

.341 

.498 

.576 

.524 

1.288 

.314 

.524 

.576 

.393 

.367?

20.548

Relative amplitude: 

drill bole

well

0.762 

.792 

.943 

.967

.885 

.876 

.800 

.822 

.825 

.845 

.894 

.865 

.849 

1.000 

.958

0.872

Lag of 

drill 

hole 

behind 

well

h m

1 30 

1 3 

1 10

45

1 7

' The value given in Sugiyama's paper (1936) seems to be a typographic error; the 

value given here is probably correct.

2 The amplitudes for the drill hole include, spring-tide values of December 9-13, 

those for the old well at Minatoguchi do not. The average for the drill hole, approximately 0.632 meters (2.07 ft), seems reasonably correct. The average for the old 

well, approximately 0.548 meters, is obviously too low, as the value indicated by the 

average amplitude ratio of 0.872 would be 0.725 meters (2.38 ft).

These data present the same evidence for a high 

permeability of the reef sediments as did the waterlevel measurements in the Eniwetok and Bikini drill 

holes.

Sugiyama reached much the same conclusion for 

Kita-daito-jima that the writer has reached independently for Eniwetok and Bikini Atolls, as indicated by 

the statement that "It is thought that the change of 

temperature is primarily governed by the free permeation of sea water into the limestone which underlies 

Kita-dait5-jima" (1936, p. 20).

TEMPERATURE GRADIENTS

It has been customary, in determining the temperature gradient of a continental drill holer to pass a

straight line through the points of observation of the 

temperature-depth graph by the method of least squares 

and to use the slope of this line as the mean temperature 

gradient for the hole. Although this method possesses 

some significance for drill holes with the continentaltype profile, in which the temperatures increase more or 

less steadily with increasing depth, it would have no 

value for the atoll-type profile with its segments of 

radically different slope and its changes of gradient sign. 

It is possible, however, to subdivide the atoll temperature profiles into a series of segments which do not 

depart too greatly from a straight line and to determine 

by least squares the average gradient for each segment. 

These gradients are given in table 14, which also inTABLE 14. — Average temperature gradients for approximately linear 

segments of Marshall Islands temperature profiles

Depth (ft)

Temperature gradients 1

°C per m Mper°C

Eniwetok drill hole E-l; Apr. 27, 1953

27.7-127.8. __-___-__----_----_-___-_

127.8-227.8. . _______________________

227.8-327.8

327.8-1,400.3- ____ __________________

1,400.3-2, 980.___--_--- ____________ _-

2,980-3,380. ________________________

3,380-3,980. ________________________

-0 068 

- 039 

- 059 

- 039 

- 007 

- 002 

+ 016

-147 

-25. 7 

-16.9 

-25.7 

-138.5 

-406. 4 

+ 61.3

Eniwetok drill hole F-l; Oct. 29, 1952

24.1-123.1. _________________________

123.1-223.1___-_-_____-__ — _______ __

223.1-399.3. ___-____-____ — ________

399.3-700.5_ _______________________ -

700.5-1,050.3_________ — __-_ _________

1,050.3-1,400 »_._______ — ____________

-0. 041 

-.018 

-.054 

-. 101 

-.040 

-. 009

-242 

-543 

— 18. 6 

-9.9 

-25. 1 

-114 2

Bikini drill hole 2B; NOT. 29,1952

4-28____-___-__ — _---- — — _______-

28-253____ — _-_---- — _--- — _---__

253-353

353-500___ ____________________ -__-_

500-1,000_____ ______________________

1,000-1,430. ________________________

-0. 090 

-.029 

-. 014 

-.021 

-.043 

-.018

-11. 1 

-346 

-72. 6 

-46.9 

-23.4 

-55.6

Ocean, Marshall Islands area; Nov. 1952

0-200. ____._. ___-------______-_

200-300. ___________ ---_-_-________-

300-700

700-800_____-_________-__— ________

800-1,500___________ — _ — _-_______ —

1,500-4, 000. __. _ ____________ ___ _

0. 000 

-.036 

-. 124 

-.036 

-. 009 

-. 005

CO

-27.7 

-8. 1 

-27.7 

- 107. 1 

- 183. 1

A negative gradient, as the term is used here, is one in which the temperature is 

decreasing downward, a positive gradient one in which the temperature is increasing 

downward. This convention has been selected so that the normal continental gradient is positive.

a Only the gradients for the upper 1,400 feet of Eniwetok drill hole F-l are shown. 

Below that depth the temperatures were so disturbed by thermal anomalies still 

persisting from the drilling that the gradients have little significance.
GEOTHEBMAL MEASTJREMENTS, ENIWETOK AND BIKINI ATOLLS 735

eludes the gradients for similar segments of the oceantemperature profile.

The selection of points to be included in a linear 

segment involves of necessity a certain subjective element. Moreover, such straight-line gradients are average values that conceal much local detail that may 

be of stratigraphic significance. A more satisfactory 

and objective method is to compute the gradient for 

each interval measured (Birch, 1954). This has 

been done, and the resultant gradients in the drill 

holes on Eniwetok and Bikini, and in the ocean in the 

Marshall Islands area, are given in table 15.

TABLE 15.—Temperature gradient for each measured interval, 

Marshall Islands temperature profiles

Depth interval (ft)

Temperature gradient

°O perm M per °C

Eniwetok drill hole E-l; Sept. 16, 1953

3.8-27.8— _ ——— ——— ——— ——— „

27.8-52.8. _. ________________________

52.8-77.8— ————— — — ——— ———

77.8-102.8. __-----_-__--_-__ --------

102.8-127.8. __--_--_---___----_--- --

127.8-152.8-_------_--- — -_--------

152.8-177.8_-— -_-— _-_--_-__-_--__-

177.8-202.8-_---_--__-_----_----— --

202.8-227.8. ———— ———— —— ——— ___

227.8-252.8— — —— ———— ———— — —

252.8-277.8 — _

277.8-302.8- __ ____ _-__-____ ___

302.8-327.8-. -----------------------

327.8-352.8. ---------------- --------

352.8-377.8. _ . — __________ —— ______

377.8-402.8—. —— ————— —— ————

402.8-450.3... — ——— _ ———— —— „

450.3-500.3- __ __-._______-__---__._

500.3-600.3 _ ----.-_-_---_---____-

600.3-700.3.._ ————— ——— ________

800.3-900.3. -__---_----__--- --------

900.3-1,000.3———— ——— ——— ———

1,000.3-1,100.3.. __.___--.__-__-____.

1,100.3-1,200.3 —— ————— — ————

1,300.3-1.400.3_. -.-- _-___-___---_-_-

1,400.3-1,500.3. _ ——— — ——— ———

1,500.3-1,600.3. .-_-_--_-___-_-___-_ -

1,600.3-1,700.3. ---__-_____------_-

1,700.3-1,800.3. ------------------ -_-

1,800.3-1.900.3- — ---_-____----_--__

1,900.3-2,000.3. _.--_--_--------- --._

2,000.3-2,080___- _--_-.---_._---__-- -

2,080-2,280. _--_--_-___-_-__--- -----

2,280-2,380... —— ———— — ——— ——

2,380-2,580.. _____________ ___„_____

2,580-2,680—— —— ——— ———— —— —

2,680-2,780— _ — _ ————— ————— __

2,780-2,980. __ ———— ———— _ ———— __

2,980-3,180.. ._----_-__-_--_------_.

3,180-3,380.. ---_--_.__--.---------.

3,380-3,580— — — ————— —————

3,580-3,780. _ ——— ————— —— ————

3,780-3. 980- ___ ———— _ ————————

+0. 030

-. 058

-.083

-. 066

-.060

-. 051

-.034

-.038

-. 035

-.070

-.058

-.047

-.070

-. 059

-.047

-.039

-.038

-. 042

-.047

-.035

-.037

-.044

-. 033

-. 033

-. 026

-. 044

-. 050

-.002

-. 018

(-.Oil)

(-. 003)

-.011

-.008

-.Oil

-.005

-.010

-. 003

-.004

-. 008

-.006

-.003

-.002

+.006

+. 022

+.018

+33. 3

-17. 3

-12. 1

-15. 2

-16. 6

-19. 5

-29.3

-26. 3

-28.2

-144

-17. 3

-21. 2

-14 4

-16.9

— 21. 2

-25.4

-26.3

-2a 8

-21. 2

-2a 2

-27.2

-22. 8

-30.2

-30.8

-39. 1

-22. 6

-20. 2

-435. 4

-544

(-95. 3)

(-304 8)

-92.4

-121. 9

-93.4

- 196. 7

-98. 3

-290. 3

-234 5

-121. 9

-169.3

-33a 7

-5oa o

+ 160.4

+ 44 8

+544

TABLE 15. — Temperature gradient for each measured interval 

Marshall Islands temperature profiles — Continued

Depth interval (ft)

Temperature gradient

°C perm M per °C

Eiiwetok drill hole F-l; Oct. 29, 1952

24.1-48.1

48.1-73.1. — —— ——— ——— —————

73.1-99.3— _ ————— _ ———— —— —

99.3-123.1-_-_-_ ------- - -- - --

123.1-148.1- --------------- ---------

148.1-173.1... .______-____-_-_-_----

173.1-199.3- ------------------------

199.3-223.1 _- ——————— _ _-_

223.1-248.1. _ -----------------------

248.1-273.1- ------------------------

273.1-299.3- .___-_-___-______-- — —

299.3-323.1. ___- --------------------

323.1-348.1- ------------------------

348.1-373.1- __--_---___-_--------- __

373.1-399.3. __-_-__---_---___-------

399.3-423.1. _ -----------------------

423.1-450.3- ------------------------

450.3-500.5. _ -----------------------

500.5-550.3-- -----------------------

550.3-600.5. ------------- -----------

600.5-650.3- ___-_-----_ -------------

650.3-700.5. -__-__--_-_------ -------

700.5-750.3- ---------------------- _-

750.3-800.5- ___--___---___-_----_---

800.5-850.3. ------------------------

850.3-900.5 __-_------__------------

900.5-950.3. __--_-----_-----__------

950.3-1,050.3—— -------------------

1,050.3-1,100.5- _---_--------_-------

1,100.5-1,150.3. _ _--_._.__---_.----

1,150.3-1,250.3-. ——— ———— ——— — .

1,250.3-1,300.3. _- — _-_-_ — _____ — --

1,300.3-1,400.0. __-__--__ — ---------

-0. 030

-.046

-.045

-.039

-.025

-.012

-.018

-.023

-.041

-. 075

-.031

-.070

-.047

-.062

-.046

-. 120

-.012

-.096

-. 113

-.086

-. 118

-. 155

-.053

-.056

-. 024

-.059

-.024

-.032

-.010

-.013

-.008

-.011

-.006

-21.8

— 22 2

-25. 9

-40. 1

-847

-57.0

-42. 7

-246 -ia4

-31.9 -142

-21.2

-16.2

-21.6

-8.3

-82. 9

-10.4

-8.9

-11. 7

-8.5

-6. 5

-17,8

-42.2

-17.0

-42.2

-31. 1

-102.0

-79. 9

-127.0

-95.3 -leas

Bikini drill hole 2B; Nov. 29, 1952

4-28_. _.. --------------------------

28-53. ___' ———— ———— --------------

53-78__. ------- --------------------

78-103__ ------------------------- --

103-128. _- -------------------------

128-153.-.. ------------------------

153-178-.. -------------------------

178-203. _____-_---__-- -------

203-228-. --------------------------

228-253. _ _ __----____-____ _ .-

253-278.-- — -------------------- ---

278-303- ------------------- --------

303-328. ----------------- ----------

328-353- __.___--------- ------------

353-378__ _ -------------------------

378-403. _. -------------------------

403-500. _..-_--_------- ------------

500-600... — -•_--- — -------- --

600-700. _. -------------------------

700-800. -. -------------------------

800-1,000--. -------------- _ -------

1,000-1,200. ---------- _ --__-_---__

1,200-1,400... ----------------------

1,400-1.430. ------------------------

-0. 090

-.052

-.025

-.032

-.032

-.020

-.050

-.000

-.026

-.041

-.004

-.021

-.014

-.012

-.024

-.014

-.023

-.049

-.031

-.053

-. 041

-.017

-.019

-.019

— 1L 1

-19. 1

-r40. 1

-31.8

-31.8

-50. 8

-20. 1

00

-38.1 -246

-2540

-47.6

-69. 3

-847

-42.3

-69.3

-44 1

-20.5

-32. 1

-ia9

-244

-53.5 -sas
736 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

TABLE 15.—Temperature gradient for each measured interval, 

Marshall Islands temperature profiles—Continued

Depth interval (ft)

Temperature gradient

°C per m M per °O

Ocean, Marshall Islands area; Nov. 1952

0-200_ ___----_---_-_-_-_----------_

200-300. ____-__--____--___-__--_ ...

300-400____ ._.____._._.___-..__.._-

400-500. _____ ______________________

500-600. -___---------_-------_— ___

600-700.. .._______. ________________

700-800. ___________________ ________

800-1,000... ________________________

1,000-1,500. ___------------_--------

1,500-2,000... _---___-_____- _ _____

2,000-3,000____---_____---___-------

3,000-4,000. __----_--------_------

0 000

- 036

- 112

- 164

- 089

- 128

- 036

- 010

- 009

- 007

- 006

- 005

c

-27.

_ g

-6.

-11.

_ _ n

-27.

-101.

-108. -isa

-179.

-217.

The gradients of table 15 are shown graphically in 

figure 252. Certain general correlations can be noted. 

The higher gradients of the thermocline are apparent 

hi each of the four curves, as well as the sharp decrease 

hi gradient at the base of the thermocline, marked by 

the small arrows. All 3 drill holes show an early 

gradient decrease to a minimum value at depths of 150 

to 200 feet, a correlation especially evident for the 

Eniwetok drill holes E-l and F-l.

Despite these general points of agreement, however, 

the most striking feature of the four curves is not their 

similarity but their marked differences. Eniwetok 

drill hole F-l, with its high gradient maximum, 

matches most closely the general shape of the ocean 

gradient curve. For Eniwetok drill hole E-l, on the 

other hand, the gradients in the thermocline are relatively low and nearly uniform, with no sign of a gradient 

peak. However, the thennocline extends considerably 

deeper than for the other three curves. In Bikini drill 

hole 2B, the gradients are also low, but they are less 

uniform and there is some tendency toward a small 

gradient high. Such differences in gradients represent 

differences in subsurface heat flow. As the climate and 

ocean temperatures are essentially the same for both 

atolls, the sources of the differences must be sought 

within the atolls themselves.

Attention has already been called to the differences 

in the tidal motion hi the three drill holes. It was 

originally presumed that these differences were largely 

a result of differences in casing length or inhole blocknig. The gradient data suggest that the differences 

are more fundamental than this. The best gradient 

correlation between drill hole and ocean is that for 

Eniwetok drill hole F-l, which showed a tidal fluctuation agreeing both hi amplitude and phase with that in 

the ocean offshore. The poorest correlation is that for 

Eniwetok drill hole E-l, which showed tidal motion of

-. 2000

i

DRILLHOLE

THERMAL GRADIENT, IN DEGREES CENTIGRADE PER KILOMETER

FIGURE 252.— Temperature gradients for each measured interval, Marshall Islands 

thermal profiles. Arrows indicate base of thermocline.

very small amplitude and a phase lag of 9J_ hours. 

Bikini drill hole 2B is intermediate both in correlation 

and in tidal amplitude and lag.

These facts suggest that, although tidal fluctuations 

are undoubtedly affected by casing length, both tidal 

variations and thermal gradients are an expression of 

fundamental differences in lithology and permeability 

within the atolls themselves.

If near-surface temperatures are omitted, all three 

drill holes show negative gradients to considerable 

depths. The ocean gradients are negative throughout. 

Omitting anomalous gradients, only Eniwetok drill 

hole E-l shows a positive gradient within the depths 

measured. It seems certain, however, that the positive 

gradient characteristic of continental thermal profiles 

would develop in all three drill holes if they were 

deepened sufficiently.

The thermal gradient for Eniwetok drill hole E-l 

below 3,580 feet averages +0.020° C per meter or 

+49.6 meters per degree C. For 20 drill holes along 

the Pacific coast in California, Oregon, and Washington 

the average is +0.035° C per meter or +28.4 meters 

per degree C (Van Orstrand, 1939, p. 132, 135, 137). 

The positive thermal gradient at the bottom of drill 

hole E-l is thus much lower than that for these Pacific
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 737

coast wells. As the positive gradients in drill hole E-1 

are for the part of the drill hole just below the point 

of curve reversal, it is quite possible that the gradient 

is still influenced by the cooling effect of the adjacent 

ocean water and therefore is less than the normal 

gradient at depth in a coral atoll.

Some idea of the direction of heat flow as well as of 

the general distribution of temperatures underground 

may be obtained through the preparation of an isogeothenn chart for a vertical section through one of the 

drill holes and approximately perpendicular to the atoll 

flank. If it is assumed, as seems reasonable, that the 

temperatures along the flank of the atoll are those of 

the adjacent ocean and that these are essentially those 

given by the Scripps Institution of Oceanography 

measurements, the isogeotherms shown in figure 253 

are obtained for such a section through Eniwetok drill 

hole E-1. The lines of thermal flow are, of course, 

perpendicular to the isothermal lines. The general 

flow is therefore downward and outward above the 

point of minimum temperature and upward and outward below that point.

FORCED CONVECTION AS A METHOD OF HEAT 

TRANSFER IN CORAL ATOLLS

In discussing the temperature profiles for Eniwetok 

drill hole F-l, and in comparing the measurements for 

June with those for October 1952, it was noted that the 

June curve showed a pronounced thermal low of 

abnormal character extending from about 400 to about 

750 feet in depth, with a minimum temperature of 

11.1° C at 645 feet (see fig. 241).

Several features make this thermal low abnormal. 

Even though large amounts of heat are added by the 

drilling process to the drill hole and the strata surrounding it through drilling friction and by the use of warm 

lagoon waters for drilling and for drilling mud, this 

thermal low represents a heat deficiency. Between 

June and October, although there was a drop in temperature in the rest of the profile, the temperature rose in 

the thermal low— at 650 feet as much as 6° C. The 

atoll temperature curves closely parallel the temperature curve in the adjacent ocean and there seems little 

question that the atoll thermal profile owes its character 

to the cooling effect of the adjacent ocean. For this to

34567

HORIZONTAL DISTANCE, IN THOUSANDS OF FEET 

ID

1000

1500

K!

5 2000 

I

PA C I Fl C OCEAN

FIGURE 253.—Vertical section through drill hole E-1, Eniwetok Atoll, perpendicular to atoll flank, showing isogeotherms (in degrees centigrade).
738 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

occur the ocean temperatures must be lower than the 

corresponding atoll temperatures. Yet the temperatures of the atoll profile in this thermal low are lower 

than the ocean temperatures at the same depths. At 

600 feet, for instance, the drill hole is 3.5° C colder than 

the adjacent ocean at that depth. The presence of the 

low immediately after the completion of drilling and 

its disappearance by October, 4 months later, indicate 

that it is an effect associated with the drilling, despite 

the fact that it represents a loss of heat.

The only explanation of these anomalies that seems 

tenable is that the stratigraphic zone from a depth of 

400 to 750 feet is more porous than adjacent beds, or 

in any event gives more easy access to water from the 

drill hole, indicating that the maximum permeability 

is probably at or near a depth of 645 feet, which is the 

point of minimum temperature for the low; that water 

which was colder than the 11.1° C minimum temperature of the thermal low was flushed through the sediments in this zone and cooled them to this temperature; 

and that following the drilling this flushing process was 

reduced or discontinued so that the cooling effect 

disappeared and the beds were then reheated to the 

October level by the influx of heat from, the adjacent 

sediments. This could occur if colder water from lower 

in the hole, perhaps with a temperature of 10° C or 

lower, was forced upward in the hole outside the casing 

and then outward away from the drill hole through this 

permeable horizon, as could happen if, after the casing 

was set at 1,973 feet, water was forced upward between 

the casing and the drill-hole wall. The October measurements showed that temperatures of 10° C and lower 

extended from a depth of 1,200 feet downward to the 

bottom of the hole, and that the temperature at the 

bottom of the casing was approximately 9.7° C. Even 

if the escaping drilling fluid had not been completely 

cooled by its descent to 1,973 feet through the casing, 

it should have been brought to or slightly below 10° C 

by its rise outside the casing through this zone.

This possibility is further supported by drilling 

information. Although 200 bags of cement were used 

in setting the 9% inch casing at 1,973 feet, the seat 

obtained was not as good as had been hoped. The 

casing was later found to turn in the hole. It is therefore quite possible that water was circulated upward 

between the casing and the wall of the drill hole. This 

circulation would cease when the pumping ceased.

These facts all point to a sort of heat-flushing 

process—the removal during drilling of a large amount 

of heat from the sediments in this thermal low by cool 

water moving through the sediments.

The process of heat transfer in which heat is either 

removed from or transferred to the walls of a channel 

by a fluid moving through the channel is known in heat

engineering as forced convection. The occurrence of 

this abnormal low indicates that forced convection can 

be an effective means of heat transfer in coral-reef 

sediments.

Both the effectiveness of the cooling in this case and 

the tidal data indicating a high permeability for the 

coral sediments suggest that the unusual character of 

the atoll thermal profiles and their close parallelism with 

the ocean temperatures are due in large part to cooling 

of the atoll sediments through forced convection by the 

movement of sea water through the pore channels of the 

sediments. Preliminary computations indicate that, 

for sea water moving through the pore channels at the 

average rate of rise and fall of the tide, a temperature 

difference of a few thousandths of a degree between wall 

and water would suffice to remove by forced convection 

all of the heat brought to the channel walls by conduction.

The evidence presented by these various considerations strongly suggests that forced convection in all 

probability constitutes a major factor in the transfer of 

heat from the coral atoll to the sea.

SUMMARY

The temperature profiles in a coral atoll are unusual in 

character, differing in striking fashion from thermal 

profiles in continental areas. The latter increase steadily in temperature with increasing depth below the zone 

of annual change. The atoll curves, on the other hand, 

decrease steadily in temperature below this shallow 

surface zone until a minimum value is reached at a 

depth of more than 3,000 feet.

The atoll curves show typically an upper zone of rapidly dropping temperatures. At or near a depth of 

1,000 feet this trend is sharply altered so that below 

this point temperatures, while still dropping, decrease 

much more slowly.

The temperature profile in the ocean nearby shows a 

close parallelism with the atoll curves. It displays an 

upper zone, the thermocline, having the same rapid 

temperature drop. At a depth of about 800 feet the 

trend is sharply altered to a much slower rate of decrease, the zone of weak gradient or asthenocline. This 

parallelism strongly suggests that the atoll thermal profiles owe their unique character to the cooling effects of 

the adjacent ocean water.

This hypothesis is lent additional support by the 

presence of tidal motion in the water levels in the drill 

holes. This motion is slight with a long phase lag in 

deeply cased Eniwetok drill hole E-l. At Bikini, 

where there is only shallow casing in the hole, the amplitude of the variation is nearly half of the tidal amplitude 

and the lag is only 1 hour. In drill hole F-l, which has 

1,973 feet of casing, the tidal fluctuation agrees in both
GEOTHERMAL MEASUREMENTS, ENIWETOK AND BIKINI ATOLLS 739

amplitude and phase with the tide in the adjacent ocean. 

These facts indicate a high permeability for the reef 

sediments and supports the feasibility of their cooling 

by the ocean water.

Below the point of minimum temperature in the atoll 

profiles the trend is reversed, and the temperatures begin to rise with increasing depth as in the continental 

drill holes. This reversal has been observed with certainty only in Eniwetok drill hole E-l. The positive 

gradient below the reversal point in that drill hole is 

about half the average gradient for 20 wells on the Pacific west coast of the continental United States. It is 

quite possible that the points of measurement in drill 

hole E-l are so little below the point of trend reversal 

that the gradient is still modified by the cooling effect 

of the ocean waters and that at greater depth it may 

attain more nearly a normal value.

The tidal evidence of high permeability in the reef 

sediments and the abnormal cooling effect observed in 

Eniwetok drill hole F-l point to the probability that 

forced convection by water moving through the reef 

sediments plays an important role in heat transfer in 

the atolls.

It is to be hoped that similar measurements may be 

continued in drill holes on other atolls and oceanic 

islands to determine how widespread and how characteristic this type of atoll temperature profile may be, to 

study the factors affecting it, and to determine what 

limiting effects may be present.

LITERATURE CITED

Becker, J. A., Green, C. B., and Pearson, G. L., 1946, Properties 

and uses of thermistors—thermally sensitive resistors: Elec. 

Eng., v. 65, p. 711-725.

Birch, Francis, 1954, The present state of geothermal investigations: Geophysics, v. 19, no. 4, p. 645-659.

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

of Bikini and nearby atolls: U. S. Geol. Survey Prof. Paper 

260-A.

Ladd, H. S., 1952, Foundation of Eniwetok Atoll: Geol. Soc. 

America Bull., v. 63, no. 12, pt. 2, p. 1273.

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

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

Petroleum Geologists Bull., v. 37, no. 10, p. 2257-2280.

Ladd, H. S., Tracey, J. L, Jr., and Lill, G. G., 1948, Drilling on 

Bikini Atoll, Marshall Islands: Sci., v. 107, no. 2768, p. 

51-55.

Mao, Han-Lee, and Yoshida, Kozo, 1953, Physical oceanography 

in the Marshall Islands area: Scripps Inst. of Oceanography SIO Ref. 53-27.

Misener, A. D., and Thompson, L. G. D., 1952, The pressure 

coefficient of resistance of thermistors: Canadian Jour. 

Technology, v. 30, no. 4, p. 89-94.

Shockley, William, 1950, Electrons and holes in semiconductors: 

New York, D. Van Nostrand, 558 p.

Sugiyama, Toshio, 1934, On the test-boring in Kita-daito-jima 

[In Japanese]: Tohoku Imp. Univ. Faculty Sci. Inst. Geology and Paleontology, Contr. in Japanese Language, no. 

11, p. 22-29.

———— 1936, Second test-drilling in Kita-daito-jima [In Japanese]: Tohoku Imp. Univ. Faculty Sci. Inst. Geology and 

Paleontology, Contr. in Japanese Language, no. 25, 34 p.

Swaitz, J. H., 1954, A geothermal measuring circuit: Sci., v. 

120, no. 3119, p. 573-574.

Tavernier, P., and Prache, P., 1952, Influence de la pression 

sur la resistivit6 d'une thermistance: Jour. Physique et 

le Radium, tome 13, nos. 7-9, p. 423-426.

United States Coast and Geodetic Survey, 1951, Tide tables, 

central and western Pacific Ocean, for year 1952: U. S. 

Coast and Geod. Survey Ser. 749.

Van Orstrand, C. E., 1939, Observed temperatures in the earth, 

in B. Gutenberg (editor), in Internal constitution of the 

earth: New York, McGraw Hill Book Co., p. 125-151.

INDEX

Page 

Acknowledgments—....————____—— 713

Area of study—_______________ 711-713 

Asthenocline, denned—.————___——— 728

Basalt, in drill holes..__..._..._.... 718-721 

Bikini Atoll. See Drill hole 2, Drill hole 2A,

Drill hole 2B. 

Bikini island. See Drill hole 2, Drill hole 2A,

Drill hole 2B. 

Bridge calibration——..—————.————— 716

Cables, circuits———.——-—.—._ 713-714 

conductor-resistance variations.,——— 716-717 

construction...—____.„___—— 713-715 

dimensional stability__________._ 718 

effect of incomplete equilibrium____ 717,718 

types used———————————————— 714

Continental thermal profiles_______. 725,739

Drillhole E-l, depth...———.—————— 721 

isogeotherm chart_.___„_...——— 737 

lithologic character____.______-- 721 

location................................ 711,721

temperature measurements—————— 721-722

thermal gradients.__———.——— 734(table),

735(table), 736-737

Page 

Drillhole F-l, depth.....————————— 718

lithologic character. _________—— 718 

location_.—.........——.-....—— 711,718

temperature measurements—__——— 718,720 

thermal gradients._ 734(table), 735(table), 736 

Drill hole and well on KIta-dait8-jima, temperature measurements-_— 731-734 

Drill hole 2, location...._._....._.___ 723

Drill hole 2A, location...—————.———— 723 

Drillhole 2B, depth......——.............. 711,723

location.__....____——........... 713,723

temperature measurements———————— 723 

thermal gradients.... 734(table), 735(table), 736

Drilling, equipment used_________— 724,732 

heat...........————————— 724-725,732

mud and water, effect of temperature— 724,737 

Elugelab island. See Drill hole F-l. 

Eniwetok Atoll. See Drill hole E-l, Drill hole 

F-l.

Forced convection——————.———— 737-738,739

Page 

Kita-daito-jima. See descriptions of drill holes.

Measurements, accuracy...———————— 716-718 

method——————————— 714-715

precision.————.—————————— 715-716 

reliability................................ 715

Ocean water, cooling effect———— 728,734,737,738 

temperature measurements... „—— 725,728,738 

thermal gradients...———— 734 (table), 736

Parry Island. See Drill hole E-l. 

Permeability of reef sediments———————— 728, 

729,731,734,738,739

Thermistors, calibration——————————— 717 

equations————————————————— 715 

effect of hydrostatic pressure—————— 717-718 

definition..——....——....———- 713

resistance————————————————— 715 

self-heating as a source of error_———— 717 

stability—...—.....——————- 717

Thermocllne, defined————.,——————— 728 

Tidal fluctuations in the drill holes———- 729-731,

733-734,738-739

741

o