Bikini and 

Nearby Atolls

Part 3. Geophysics

Seismic Studies of Bikini Atoll 

Seismic-Refraction Studies of Bikini and Kwajalein

Atolls and Sylvania Guyot 

Magnetic Structure of Bikini Atoll

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-J, K, L
Bikini and 

Nearby Atolls

Part 3. Geophysics

Seismic Studies of Bikini Atoll

By M. B. DOBRIN and BEAUREGARD PERKINS, Jr.

Seismic Refraction Studies of Bikini and Kwajalein Atolls

By RUSSELL W. RAITT

Magnetic Structure of Bikini Atoll

By L. R. ALLDREDGE, FRED KELLER, Jr., and W. J. DICHTEL 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-J, K, L

UNITED STATES GOVERNMENT PRINTING OFFICE', WASHINGTON : 1954
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - Price 45 cents (paper cover)
CONTENTS

Page 

(J) Seismic studies of Bikini Atoll, by M. B. Dobrin and Beauregard Perkins, Jr_____________________--__-----------_- 487

(K) Seismic refraction studies of Bikini and Kwajalein Atolls, by Russell W. Raitt_________-_________-____-__-_-------- 507

(L) Magnetic structure of Bikini Atoll, by L. R. Alldredge, Fred Keller, Jr., and W. J. DichteL_____.___________---._-__ 529

Seismic Studies of 

Bikini Atoll

By M. B. DOBRIN and BEAUREGARD PERKINS, Jr.

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-J

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1954

CONTENTS

Abstract_ ________________________________________

Introduction. ______________________________________

Vertical velocities_________________________________

Plan and procedure of refraction survey _______________

Sequence of shots____________i______________________

Results.___________________________________________

Determination of velocities_____________________

Determination of depths and thicknesses of layers.-

Page 

487 

487 

487 

489 

492 

492

492

493

Results—Continued

Determination of absorption coefficient._____.___.

Geologic interpretation.____________________________

Possible constitutions of the velocity zones_____-__

Indications of subsidence.______________________

Inferences regarding geologic history of Bikini AtollSelected references-___________________-__----_-_---

Index._____________--_-_-___-__--______-_-__---_.

Page

500

501

501

503

503

504

505

ILLUSTRATIONS

	Page 

Figure 123. Bikini island showing location of deep holes.___________________________________________________________ 487

124. Bikini Atoll showing profiles_______________________-_________________-_-__---_--_--_-_--_----___ 489

125. Sample records-___________________________________-____-_-_____----__-_-__-___-__---_--_----------- 490

126. Graphic log of vertical velocities and the geology.______________________________________________________ 491

127. Time-distance curves Enyu-Namu profile._____________________________---____-__------------------_--- 492

128. Time-distance curves Enyu-Yurochi profile_________________________-_-________---___-_----_---------- 493

129. Time-distance curves Yurochi-Chieerete profile.________________________________________________________ 493

130. Time-distance curves Chieerete-Enyu profile. _____________________________-_______-___-_-__------------ 494

131. Geometry of wave paths.________________________________________-_--_---_------------------'-------__ 495

132. Cross sections along Enyu-Chieerete and Enyu-Namu profiles._____________________________--__-_-----_-- 496

133. Cross sections along Yurochi-Chieerete, Enyu-Yurochi, and "A-B" ______-______---_---_-_-_------_---_--- 496

134. Contour of top of 17,000 fps zone_-_-_-_-_.___._________.____.____-____---__--_---__-_-_-------------- 497

135. Contour of top of 11,000 fps zone_____________,.___-__-______-__________-________________-_----_---_ 498

136. Three alternative interpretations of anomaly in arrival times at eastern end of Chieerete-Enyu profile.________ 499

137. Illustration of group and phase velocities-_-_--________------------------------------------------------ 499

138. Phase velocity and group velocity (of first mode)______________-__________------------_--------------_-- 499

139. Relative amplitudes of pressure versus distance from the shot________________-______--__----_-_---------- 500

TABLE

TABLE 1. Vertical-velocity computations_________________________—-——-,_____________________-_____-__•-_ 488

in

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

SEISMIC STUDIES OF BIKINI ATOLL

By M. B. DOBRIN and BEAUREGAKD PEEKINS

ABSTRACT

During Operation Crossroads in July 1946 a seismic refraction 

survey of Bikini Atoll was made by Joint Task Force I to determine stratification of the subsurface and if possible the thickness 

of the calcareous sediments. One hundred twenty-six depth 

charges were exploded during the survey along 4 profiles extending across the lagoon. One year later a hole was drilled to 2,556 

feet below sea level on Bikini island, and vertical velocities were 

measured from a depth of 1,800 feet to the surface.

The time-distance curves indicate the existence of a surface 

zone with a seismic velocity of 7,000 fps. Below this at a depth 

of about 2,500 feet lies a zone with a velocity of 11,000 fps. A 

third zone in which the seismic velocity is 17,000 fps ranges in 

depth from 7,000 to 13,000 feet below sea level. The vertical 

velocity measurements indicate that the velocity increases with 

depth from the surface velocity to 11,000 fps at about 2,000 feet. 

Thus the calcareous material at the surface extends down to two 

or three thousand feet and may extend to the top of the 17,000 

fps zone. However, since Emery, Tracey, and Ladd have reported dredging basalt and pyroclastics at 1,000 and 1,150 'fathoms, respectively, it is more likely that the base of surface calcareous sediments rests on pyroclastics of lower seismic velocity 

which in turn rest on the 17,000 fps zone. This last zone is 

believed to be igneous rock.

These findings would indicate a minimum subsidence of about 

3,000 feet and a maximum of about 13,000 feet.

INTRODUCTION

In the summer of 1946, during the tests of the Abomb, while the geologic and oceanographic data were 

being gathered at Bikini Atoll, seismic surveys were 

undertaken to determine the stratification and if possible, the thickness of the calcareous sediments which lay 

beneath the lagoon. A reconnaissance refraction survey 

was conducted by the Oceanographic Section of Joint 

Task Force 1 and the Low Frequency Group of the 

Naval Ordnance Laboratory (Dobrin and others, 1949). 

A reflection survey was attempted by another group 

at the same tune, but it was unsuccessful. In the summer of 1947 an expedition was sent to Bikini Atoll to 

study the after effects of the explosion of the A-bombs. 

Advantage was taken of this opportunity to drill several 

holes on Bikini island in one of which vertical velocities 

were measured to a depth of 1,800 feet. The measurements were made by Joseph Chernock of the Geotechnical Corp. In the summer of 1950 geological studies 

and seismic refraction studies were made by Roger R. 

Revelle and Russell Raitt in an expedition sponsored 

by the Office of Naval Research and Scripps Institution 

of Oceanography. The results of these studies are reported elsewhere in this volume (chap. K).

VERTICAL VELOCITIES

Since the vertical velocities from the hole on Bikini 

island have influenced the interpretation of the survey 

data they will be discussed first. The hole (No. 2B 

in figure 123) was drilled to a depth of 2,556 feet on the

165-30'

FIQUEE 123.—Bikini island showing location of drill holes. Hole 2B was drilled 

to 2,556 feet. The others ranged from 150 to 300 feet in depth.

lagoon side of Bikini island. In measuring the velocity 

a geophone was placed in hole 2B at intervals varying 

from 50 to 300 feet between the depths of 1,820 feet and 

150 feet below the surface. Charges were exploded in 

the shallow hole (No. 2A), 178 feet to the west of the 

deep hole. A total of 74 charges were fired, of which 47 

produced usable records. (See table 1.) The depth of 

the shots varied from 35 feet to 16 feet. Each charge

487
488 BIKINI AND NEABBY ATOLLS, MARSHALL ISLANDS

Shot 

No.

73

74

69 

70

71 

72

59 

60 

61

57 

58

55

56

53 

54

50 

51 

52

47

48 

49

44 

45 

46

42

43

41 

39

36 

37 

38

33 

34

27 

28 

29 

30 

32

25 

24

7 

10 

11 

32 

34 

15 

1

f * 

L 21 

PS ylr

1_

Pg

150' 

150«

200" 

200'

300' 

300'

400« 

400' 

400'

450' 

450'

500' 

500'

550' 

550'

600' 

600' 

600'

650' 

650' 

650'

700' 

TOO' 

700'

750' 

750'

800' 

800'

900' 

900' 

900'

1100' 

1100'

1400' 

1400' 

1400' 

1400' 

1400'

1600' 

1600'

leoo'

1800' 

1800' 

1800' 

1800' 

1800' 

1820'

Os

16' 

17'

18' 

18'

17' 

17'

18' 

18' 

18'

20' 

19'

20' 

20«

21'

20'

25' 

21'

21'

25' 

25' 

25'

29' 

29' 

25'

32' 

30'

32'

34'

20« 

19' 

19'

23' 

22'

26' 

26' 

26' 

26' 

26'

28' 

28'

20' 

20' 

20' 

34' 

35' 

35' 

25'

Dgd

326' 

325'

174' 

174'

275' 

275'

374' 

374' 

374'

422' 

423'

472' 

472'

521' 

522'

567' 

571" 

571'

61?' 

617' 

61?'

663' 

663' 

66?'

710" 

732'

760' 

758'

872' 

873' 

873'

1069' 

1070'

1366' 

1366' 

1366' 

1366' 

1366'

1564' 

1564'

1772' 

1772' 

1772' 

1758' 

1757' 

1757' 

1787'

tan^

1.4126 

1.4240

1.0229 

1.0229

.6472 

.6472

.4759 

.4759 

.4759

.4218 

.4208

.3771 

.3771

.3416 

.3409

.3139 

.3117 

.311?

.2384 

.2884 

.2684

.2684 

.2684 

.2668

.2507 

.2500

.2342 

.2348

.2041 

.2038 

.2038

.1665 

.1663

.1303 

.1303 

.1303 

.1303 

.1303

.1138 

.1138

.1004 

.1004 

.1004 

.1012 

.1013 

.1013 

.0996

i

< _________ ns< —— ———— ———— "J

i — -i8' 

Elevation shot hole j,

: :;.

•; Sea level - low tide

_-• __

\ 

\ 

\ 

\ 

\V^

\ \

\

T

.032 

.034

.037 

.039

.049 

.046

.055 

.055 

.055

.059 

.058

.065 

.065

.070 

.070

.075 

.077 

.076

.079 

.079 

.079

.086 

.086 

.085

.089 

.090

.093 

.095

.110 

.112 

.113

.133 

.134

.174 

.175 

.174 

.174 

.173

.195 

.196

.216 

.215 

.214 

.215 

.218 

.216 

.214

cos^

.5779 

.5748

.6990 

.6990

.8395 

.8395

.9030 

.9030 

.9030

.9214 

.9218

.9357 

.9357

.9463 

.9465

.9541 

.9547 

.9547

.9609 

.9609 

.9609

.9658 

.9658 

.9662

.9699 

.9702

.9737 

.9735

.9798 

.9799 

.9799

.9864 

.9864

.9916 

.9916 

.9916 

.9916 

.9916

.9936 

.9936

.9950 

.9950 

.9950 

.9949 

.9949 

.9949 

.9951

Tgd

.0185 

.0195

.0259 

.0273

.0411 

.0386

.0497 

.0497 

.0497

.0544 

.0535

.0608 

.0608

.0662 

.0663

.0716 

.0735 

.0726

.0759 

.0759 

.0759

.0831 

.0831 

.0821

.0863 

.0873

.0906 

.0925

.1078 

.1097 

.1107

.1312 

.1322

.1725 

.1735 

.1725 

.1725 

.1715

.1938 

.1947

.2149 

.2139 

.2129 

.2139 

.2169 

.2149 

.2130

Dgd 

Avg

126'

174'

275'

374'

423'

472'

522'

570'

617'

665'

711'

759'

373'

1070'

3366'

1564'

1768'

Tgd 

Avg

.0190

.0266

.0399

.0497

.0539

.0608

.0663

.0726

.0759

.0824

.0868

.0916

.1094

.1317

.1725

.1943

t2143

Va 

(fps)

6630'

6540'

6890'

7520'

7850'

7760'

7370'

7850'

8130'

8070'

8190'

8290'

7980'

8130'

7920'

8050'

8250'

Dgd

48'

101'

99'

49'

49'

50'

48'

47'

48'

46'

48'

114'

197'

296'

198'

204'

Tgd

.0076

.0133

.0098

.0042

•

.0069

.0055

.0063

.0033

.0065

.0044

.0048

.0178

.0223

.0408'

.0218

.0200

Vi 

(fps)

6320'

7590'

10,100'

11,670'

7100'

9090'

7620'

14,240'

7380'

30,450'

30,000'

6400'

8430'

7250'

9080'

10,200'

Elevation well.

A

" ~~~A

: D

D( ;d

: M \

'

Data by 

The Geotechnical Corp. 

Pg • Geophone depth measured from well elevation

Ds = Depth of shot 

Dgd = Geophone measured from shot depth. 

Tgd m Vertical travel time from shot depth to geophone. 

T = Observed time from shotpoint to geophone. 

Va - Average velocity from shot depth to geophone depth. 

Vj = Interval velocity, 

i = Angle between seismic ray and vertical.

TABLE 1.—Vertical-velocity computations. The travel time for a pulse generated by an explosion at the surface to a geophone located at various 

depths in the hole was measured. The difference in times to the various depths permits the interval velocity to be determined.
SEISMIC STUDIES OF BIKINI ATOLL 489

consisted of one-eighth pound of 60 percent gelatin. 

The charges were tamped with sea water.

In positioning the detector, an attempt was made to 

choose points near the top and the bottom of the various 

cemented layers which had been logged during the drilling of the hole. If successful this would permit the 

determination of the velocities in the cemented material 

separately from those of the unconsolidated sediments.

The velocities are presented in table 1 and are shown 

graphically in figure 126. In the first 1,000 feet below 

the surface the velocity was found to vary widely from 

approximately 6,000 fps in the unconsolidated material 

to about 14,000 fps in the well-cemented layers. From 

a depth of 1,000 feet to 1,800 feet, the velocity increased 

steadily from about 7,000 fps to 10,000 fps. The 

average vertical velocity through the first 1,800 feet 

was approximately 8,000 fps.

165*10'

The seismic-refraction survey indicates only the 

horizontal seismic velocities in the subsurface and those 

discontinuities where the velocity is greater than in any 

overlying strata. Thus, decreases in velocity are completely obscured, and a gradual increase in velocity 

could be marked by a high-speed zone at the base of the 

section in which the increase takes place.

PLAN AND PROCEDURE OF THE REFRACTION SURVEY

The principles of seismic refraction surveying have 

been discussed briefly in Dobrin and others (1949) and 

in more detail in textbooks on geophysical prospecting 

(see Nettleton, 1940; Heiland, 1940; or Dobrin, 1952).

The seismic survey in 1946 was a reconnaissance. It 

consisted of four profiles as shown in figure 124. These 

profiles are designated by the names of the islands at 

each end of the various profiles. All shots were fired

165° 35'

Shot location

K 

Hydrophone location

25'

. 15' 2V 25' 30" 165*35' 

FIGURE 124.—Bikini Atoll showing profiles. Hydrophones were posting on the bottom off Enyu island, Yurochi island, and Chieerete island.
490 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

SHOT-DETECTOR 

DISTANCE=47,IOO'

Avvbwvwvy**»wW«^^

9 SEC 9-1 9.2 93 9,4 

XX X7 \ * ** ^ x

^A^^y

RECORD OF SHOT 92 MADE BY SHELL 

OSCILLOGRAPH ON USS GILLIS ANCHORED OFF YUROCHI ISLAND

The two high - sensitivity traces appear to break shortly after the water - wave arrivals. The water 

waves on the record were weak and were intensified by pencil as far as they could be followed

SHOT-DETECTOR 

DISTANCE=I4,770'

MNf

WATER 

ARRIVAL

FIGURE 125.—Sample records.

RECORD OF SHOT 120 MADE BY

HATHAWAY OSCILLOGRAPH ON ENYU ISLAND

( Time break not shown)

through the water (water arrival). The shot moment Indicates zero time or the time explosive was fired.

on the bottom of the lagoon in water 100 to 180 feet 

deep, on lines between two receiving stations. One 

receiving station (at Enyu island) was stationary. The 

hydrophones were on the lagoon bottom in about 40 

feet of water with cables connecting them to the recording systems on the island about 1 mile distant. A second station was established aboard the U. S. S. Gilliss 

which anchored first off the island of Yurochi to record 

the Enyu-Yurochi and the Yurochi-Chieerete profiles 

and which later anchored off Chieerete to record a portion of the shots on the Chieerete-Enyu profile. At 

both locations hydrophones were lowered to the lagoon 

bottom and connected by cable to the recording equipment aboard the ship. The shot-detector distances 

ranged from 2,000 to 98,000 feet.

The hydrophones and the associated recording systems responded both to the low-frequency pressure 

changes induced in the water by ground waves as well

as to the higher frequencies transmitted directly through 

the water from the explosion. The pressure variations 

were recorded photographically by a Hathaway oscillograph at the Enyu station and by a Shell Oil Co. oscillograph on the U. S. S. Gilliss.

The seismic pulses were generated by detonating 

standard depth charges. The Mark 10 depth charges 

(25 pounds) were used when within about 4,000 feet of 

any of the geophones. The Mark 6 charges (300 

pounds) were used for the more distant shots. The 

charges were lowered to the bottom from a small ship 

which would then back off approximately 1,000 feet. 

The charge was fired by means of a demolition cable 

and an electric detonator which had been inserted in 

place of the usual hydrostatic firing device. A special 

radio time-break transmitter permitted the accurate 

recording of the shot moment on the oscillograph 

records.
SEISMIC STUDIES OF BIKINI ATOLL 491

VELOCITY, IN FEET PER SECOND

WELL LOG

Reef limestone

Alternately hard and 

soft calcareous sand 

and soft coralline 

material. 223 feet

White hard limestone. 

Shell fragments. 

Angular cuttings. 

132 feet

Tan sand interfingered 

with thin hard layers. 

6 to 12 inches thick. 

125 feet

Tan to brown limestone. 

Cuttings firm and 

angular. Some very 

hard streaks 200 feet

Tan fine to coarse sand. 

112 feet

Tan limestone about 

65 feet thick, firm

Limestone 24 feet thick. 

263 feet

Tan to brown calcareous 

sand. Soft. Increasing 

in compactness with 

depth. Highly 

fossiliferous zones 

present throughout.

TIME, IN SECONDS

FIGURE 126.—Graphic chart of vertical velocities from surface to a depth of 1,800 feet. The relation of the velocity to compaction and cementation is shown by the table of

geologic formations derived from the cores.

The sample records (fig. 125) show the shot-moment 

signal and the ground-wave and water-wave arrivals. 

The output of each detector was recorded through two 

channels: one of high sensitivity, the other of low sensitivity. This assured both sharper time breaks and 

clearer records showing details of the recorded wave 

motions. The water-wave arrival time could usually 

be read to within 0.001 second. The ground wave that 

had traveled a short distance could be tuned to within 

0.002 second, while the more distant shots, because of 

the more gradual break, could not be tuned to better 

than within 0.010 second.

The hydrophones from the U. S. S. Gttliss were separated by a few hundred feet while those from the land 

station at Enyu island were 1,000 feet apart. All these

phones responded to frequencies from 1 to 200 cycles per 

second. At the Enyu station in addition there was for 

most of the shots a sonic frequency hydrophone responsive to frequencies from 50 to 10,000 cycles per second. 

This was useful in locating the water-wave arrivals from 

the more distant shots.

The hydrophone and shot locations were determined 

by means of sextant bearings on beacons and buoys and 

tangents to islands. Precise locations were not necessary since the distance from shot to hydrophone was 

determined from the travel time of the sound wave 

through the water from the explosion to the phone. 

The temperature and salinity of the lagoon water had 

been determined by the oceanographers of the Operation 

Crossroads expedition at many stations preceding and
492 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

during the seismic survey. Using the average values 

of temperature 83°±0.5° F. and of salinity 34.45 

±0.35 %0 the theoretical speed of sound is 5,047 fps 

according to tables by Kuwahara (1939). The speed 

of sound was determined experimentally along the 

eastern portion of the profile from Namu to Enyu 

(fig. 124). Here the shot and hydrophone locations had 

been ascertained accurately by the many beacons whose 

coordinates were known to within a few feet. The 

measured speed was 4,980 fps. The discrepancy 

between these values may indicate that Kuwahara's 

tables do not apply to shallow water or that the tables 

are not correct; however, there is the possibility that 

the tuning fork used for timing may have been fast, so 

that the measured speed should not be considered 

definitive. In computing the distances, a sound 

velocity of 5,000 fps was assumed. This is within 1 

percent of either of the above ^alues and will introduce 

an error of less than 1 percent in the calculated seismic 

velocities or in the depth of the observed velocity 

discontinuities.

SEQUENCE OF SHOTS

Of the four profiles across the lagoon the profile from 

Namu to Enyu was shot prior to the first A-bomb test. 

Shots out to 98,000 feet from Enyu island were recorded 

by the Enyu station. The shots fired in the northwest 

end of the profile within 40,000 feet of Namu island 

were recorded by geophones (on the island) operated 

by engineers of the Geotechnical Corp.

The three remaining profiles were shot in the period 

between the two A-bomb tests. The profile from 

Enyu to Yurochi was recorded by the Enyu station 

and by the U. S. S. Gilliss anchored off Yurochi island. 

The eastern two-thirds of this profile could not be shot

owing to the presence of target vessels and personnel 

working in the water preparing for the next bomb test. 

The Yurochi-Chieerete shots were recorded by the 

U. S. S. Gilliss at Chieerete island and by the Enyu 

hydrophones. The profile from Chieerete to Enyu 

was started while the Gilliss was moving from Yurochi 

to a point off Chieerete island. All the shots of this 

profile were recorded by the Enyu station but only 

the shots in the eastern half (shots 111 to 126) were 

recorded by the hydrophones of the U. S. S. Gilliss.

RESULTS 

DETERMINATION OF VELOCITIES

The time-distance curves (figs. 127 to 130) show the 

arrival times of the ground waves plotted against the 

corresponding shot-detector distances on the four 

profiles. The reciprocal of the slope of these lines at 

any point indicates the velocity at which the ground 

wave is travelling in the lower portion of its path. The 

points on the various curves thus appear to indicate 

three zones of different velocities in the first few miles 

below the lagoon floor. The velocities indicated are 

7,000, 11,000 and 17,000 fps. The first of these velocities is very consistent over all four profiles, the deviation 

not exceeding 200 fps. However a study of the well 

log would indicate that the velocity in the first zone 

increases gradually from a little over 6,000 fps at the 

surface to 10,000 fps at about 1,800 feet. It is therefore reasonable to expect the velocity of the nearsurface material to continue to increase with compression of greater depth to the velocity of 11,000 fps shown 

in the time-distance curves. The 7,000 fps horizontal 

velocity can be accounted for as the average velocity of 

the near-surface material since the hard high-velocity 

material is in layers too thin to transmit much energy.

Ti, = 0.60 SEC.

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 

NAMU DISTANCE OF SHOT FROM ENYU HYDROPHONE, IN THOUSANDS OF FEET

FIGURE 127.—Time-distance curve for Enyu-Namu profile. Hydrophones off Enyu Island.

5 0 

ENYU
SEISMIC STUDIES OF BIKINI ATOLL 493

DISTANCE OF SHOT FROM ENYU HYDROPHONES, IN THOUSANDS OF FEET 

75 70 65 60 55 50 45 40 35 30 25 20 15 10

0.43 SEC

I I 

EXPLANATION 

Detectors at Yurochi and Enyu

Times recorded at Yurocha

0 5 

YUROCHI

10 15 20 25 30 35 40 45 50 55 "60 65 70 75 80 

DISTANCE OF SHOT FROM YUROCHI HYDROPHONES, IN THOUSANDS OF FEET

-Ti2 =l.87SEC.

85 

ENYU

FIGURE 128.—Time-distance curves for Enyu-Yurochi profile. Hydrophones off Enyu and Yurochi islands. No shooting was permitted along east part of profile because of

target vessels.

by the Enyu hydrophones through paths which were 

approximately at right angles to the profile. The 

variations in depth of the third layer are indicated by 

the intercept times, as observed for the respective shots 

at Enyu, which are plotted on the upper portion of 

figure 129.

The apparent decrease in slope at the west end of 

the Enyu-Chieerete time-distance curve suggested the 

possibility of a fourth and higher velocity zone below 

the 17,000 fps, layer. Such a zone, with its apparent 

velocity of 25,000 fps, would have a depth of 25,000 feet. 

It was decided, however, that the bend in the curve 

could more reasonably be explained as a rise to the 

west, since the difference between intercept times 

observed at the two ends of the profile from Enyu to 

Chieerete indicated that the western end of the third 

zone was higher than the eastern end.

DETERMINATION OF DEPTHS AND THICKNESSES 

OF LAYERS

To compute the depths of the various beds a method 

was employed which was based on one described by 

Jones (1934) and Gardner (1939). Three of the four 

profiles form a triangle at the vertices of which were 

detectors receiving impulses from shots fired along each 

of the adjacent sides. These stations, as indicated 

previously, were at Enyu (E), Yurochi (Y) and Chieerete (C) islands. The intercept tunes are composed of 

two delay times corresponding, respectively, to the 

shot and detector ends of the least-time trajectory for 

each shot. Since the stations at the vertices are common to two trajectories, equations can be set up, for 

the arrivals from the 17,000 fps marker, that permit

60 55 50 45 40 35 30 25 20 15 10 5 0 

DISTANCE OF SHOT FROM YUROCHI HYDROPHONES, IN THOUSANDS OF FEET

FIGTJBE 129.—Time-distance curves for Yurochi-Chieerete profile. Hydrophones 

off Enyu and Yurochi islands. At top is plotted intercept times for arrivals at 

Enyu from shots along the profile.

The 11,000 fps may represent the velocity of maximum 

compression. The velocity of 11,000 fps was consistent 

throughout the profiles except for a portion of the 

profile from Enyu to Chieerete near the southeastern 

edge of the lagoon where it changes to 9,000 fps 

(fig. 130). This anomaly will be discussed later. The 

apparent velocity indicated by the third segment of the 

curves ranges from 16,500 to 18,500 fps but reverse 

control in the profile from Enyu to Chieerete and the 

distant shots on the Enyu-Yurochi and the profiles 

from Yurochi to Chieerete indicate a true velocity of 

17,000 fps. This value has been used in the calculations 

of the depth of the third layer. Most of the shots fired 

along the Yurochi-Chieerete profile were also received
494 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

O 

O 85

DISTANCE OF SHOT FROM ENYU HYDROPHONE,IN THOUSANDS OF FEET 

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

EXPLANATION 

Detectors at Chieerete and Enyu

Times recorded at Chieerete Ti2=2.00SEC 

Ti|=0.30SEC

0 5 10 15 20 25 30 ^35 40 45 50 55 60 65 70 75 80 85 

CHIEERETE DISTANCE OF SHOT FROM CHIEERETE HYDROPHONE,IN THOUSANDS OF FEET ENYU

FIGUBE 130.—Time-distance curves for Chieerete-Enyu profile. Detectors off Chieerete and Enyu islands. Note anomalous arrival times recorded at Enyu from shots

20,000 to 35,000 feet away.

solving for the separate delay times. Let TE, Tc, and 

TY be the delay times at the respective vertices and 

TiEC be the intercept time for a shot fired at E and 

received at C; similarly, let TtcY and TIEY be the intercept times for shots fired at G and E, respectively, and 

received at Y. Then

the numerical values being obtained from the timedistance curves. Solving, we obtain

TC=Q.6Q 

7V=0.86.

Since on the profile from Enyu to Chieerete, arrivals 

were received from the 17,000 fps marker on both stations for some of the shots, the delay times can be calculated. Thus, for shot at Station 114 we have

= 7W =1.64.

Solving these, we have 7^=1.06 sec. and jPc=0.58 sec.,

which are in good agreement with the preceding values.

The delay times at each shot position in which the

arrivals traversed the deeper interface can now be ascertained by subtracting the computed delay times at the 

receiver from the intercept time for the particular shot.

Since arrivals from the 11,000 fps zone were not 

received at two of the receiving stations from any one 

shot position, it was not possible to determine the 

delay times under the shots. These times were approximated by halving the average intercept time for each 

line. For the delay times under the Enyu receivers 

intercept times were available from the profile from 

Enyu to Namu profile and the profile from Enyu to 

Chieerete. The average value calculated from these 

is 0.26 second. At the Yurochi station, also, two profiles converged, each having an intercept time of 0.44 

second, which gave a delay time of 0.22 second for that 

station. At the Chiereete station no arrivals from the 

11,000 fps horizon were received, so the delay time 

could not be computed, but was assumed to be 0.25 

second.

The thickness of the layers was computed by the 

folio wing formulae :

and

The total depth to the second interface is

In the above Tdi is the delay time associated with the 

depth ZQ to the first interface below some point on any 

of the profiles where arrivals were received after refraction along the first interface. Similarly, T<n is the delay
SEISMIC STUDIES OF BIKINI ATOLL 495

VELOCITY = V0

VELOCITY = V

H,

OFFSET FOR DEPTHS 

TO I Si INTERFACE

VELOCITY = Va

OFFSET FOR DEPTHS 

TO 2.1 INTERFACE

FIGUEE 131.—Geometry of wave paths from shot to detectors. The elastic pulses leaving the shot may travel horizontally to the detectors at velocity Vt or may travel downward and be refracted horizontally thru the second or third zone and travel at a velocity of V\ or Vt, respectively. Vo< Vi< Vs.

time associated with the depth Z to the second interface below some point on any of the profiles where 

arrivals were received after refraction along the second 

interface. The meaning of the other symbols are 

illustrated in figure 131, which shows the geometry of 

the wave paths from the shot to the detectors.

The depths calculated were actually the depths at 

the offset position in each case. The offset distances 

for the two interfaces are indicated in figure 131 and 

are calculated from the following formulae; for the 

first interface

<r

T _ 7 17 /fT/2 T/2\l/2 Li\ — A) v o/ ^ V i — \ o ) ,

and for the second interface

Substituting numerical values for the velocities, the 

formulae for the depths and offsets reduce to

Z0=9080 Tdl

Zx = 14,400 TdZ - 17,200 Tdl

Li^O.83 Z0

^2 = 0.45 Zo+0.85 ZL

In using the above formulae to compute the thickness 

of Zj of the second layer beneath any surface location, 

one must know the thickness Z0 of the uppermost layer 

at the same location. With the type of shooting done 

here there will be no arrivals from the first interface 

along the portions of the profile where there are arrivals 

from the second one. That is, the thickness Z0 cannot 

be calculated for the locations where there are arrivals 

from the second interface, and it therefore must be 

assumed. This assumption will probably be responsible 

for some error.

A more serious source of error lies in the incomplete 

control over the velocity of any given layer within the

area of the survey. Any local variation of speed along 

the wave trajectory would effect the apparent depth. 

With reverse control on the 17,000 fps bed available 

along only a limited portion of a single profile and with 

numerous gaps even in the one way control, there is no 

assurance that such variations do not occur.

Because of these uncertainties, as well as those inherent in the seismic refraction technique even under 

optimum conditions of control (ISTettleton, 1940, p. 

255), the depths indicated in the cross section and contours given below are not exact but represent what is 

thought to be the most reasonable of several possible 

ways in which one can account for the observed time 

data. The depths to the upper interface are probably 

reliable to the nearest 200 feet (except along the 

anomalous zone of the profie from Enyu to Chieerete), 

whereas those to the lower interface may be in error by 

1,000 to 1,500 feet. These uncertainties are not sufficient to affect the fundamental conclusions one can 

draw from the results regarding the internal constitution and geological history of the atoll.

Figures 132 and 133 show vertical cross sections of 

the subsurface below the four shooting profiles and also 

along a synthetic profile A-B which extends almost 

directly south from Romuk island (fig. 134). The last 

profile was chosen mainly as a mode of representing the 

depth data afforded by two shots (107 and 108), displaced somewhat southward from the line from Enyu 

to Chieerete, which indicate a sharp falling off in the 

depth to the 17,000 fps zone as the edge of the reef is 

approached.

It is observed from the sections that the depth to the 

first interface averages about 2,500 feet, whereas that 

to the second is in the neighborhood of 10,000 feet.

The top surface of the 17,000 fps zone is characterized (fig. 134) by a prominent nose plunging to the 

southeast through the center of the atoll. The highest 

observed portion of this surface is under the south-
496 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

NEAR CHIEERETE DISTANCE FROM ENYU HYDROPHONE,IN THOUSANDS OF FEET 

REEF 90 80 70 60 50 40 30 20 10 0 ENYU

^^^r«s.sir***^

—— EN YU-CH EERET!

I.Muifc^fc.JJIUI IIIIJ-JUW^^iU

7000 FT./SEC.

11,000 FT./SEC.

ar^»^sr

——— -'9000 "FT/SEC. ?

*gn»^a^ ——

i 15 <*> &* No 

BOT1 3ELOWLAGON FEET HOUSANDS OF

NAMU 

REEF

DISTANCE FROM ENYU HYDROPHONE, IN THOUSANDS OF FEET 

70 60 50 40 ' 30 0 ENYU

FIGURE 132.—Cross sections showing subsurface structure indicated by seismic data along Enyu-Chieerete and Enya-Namu profiles.

ing seismic velocities as indicated.

Interfaces are between the media havCHIEERETE

DISTANCE FROM YUROCHI HYDROPHONE IN THOUSANDS OF FEET

50 40 30 20 10 YUROCHI

u 

2 

4 

6 

8 

10 

12

\A

*~

YUROCHI--CHIEERE

*»• ' ' ' m "*^*ifc_

17,000

TE

1 "~ 

FT/SEC.

7000

"-•—

( —— i——^— -

FT./SEC.

t-*\

\

^V^

1 •- »

* IIPOO FT/SEC:

L ~

Swo a)<J) -*>roc DEPTH BLOW LAGOONBOTTM IN THOUSANDS OFFEET

NEAR YUROCHI D|STANCE FROM YUROCHI HYDROPHONE, IN THOUSANDS OF FEET 

REEF o 10 20 30 40

u 

2 

4 

6 

8 

10 

12

\A

^

7000FT/SECJ _ —— .

11,000 FT /SEC.

ENYU-YUROCHI

**• —— rrnn

17,000 FT./SEC.

*~— —— - ——— — ——— ^~ ssssaom^. —————

2 

4 

6 

8 

10 

12

14

0 

2 

4 

6 

8 

10 

12

EDGE OF REEF f NORTH —— *- f ROMURIKKU

A-B i ^ROFILE

|

/r~'

S

*-IO,000 FEET-*

"""-— — -*.r— - I7'°°° FT./SEC. """"TM4-

|0 g

FIGURE 133.—Cross sections showing subsurface structure indicated by seismic data along Yurochi-Chieerete, Enyu-Yurochi, and A-B profiles. Interfaces are between

media having seismic velocities as indicated.
165° 15'

SEISMIC STUDIES OF BIKINI ATOLL

20' 25'

497

165° 15' 20' 25' 30' 

FIGURE 134.—Contour map of depths to top surface of 17,000 fps zone. Depths are in feet below sea level.

western portion of the atoll, although there is insufficient control to be certain that the depth does not 

continue to decrease as the western edge of the atoll is 

approached. Low regions are observed under the 

northeast and south central parts of the atoll. It 

should be borne in mind, however, that the greatest 

depth observed on this interface is about a mile higher 

than the ocean floor in the areas between the atolls of 

the Marshall Islands.

The top of the 11,000 fps layer (fig. 135) has a more 

uniform depth, except for the anomalous plunge observed near the southern border of the lagoon beginning 

about 3 miles west of Enyu. The cross section of the 

profile from Enyu to Chieerete and the contour map 

(fig. 135) show only one of three assumptions which 

were made. With such a paucity of data, many assumptions were possible. The three that seemed most 

reasonable were:

1. The velocity of 11,000 fps is constant throughout 

the zone, and at about 3 miles west of Enyu there is a 

sharply cut valley or a fault, downthrown to the west,

in the 11,000 fps material. The increase in depth is 

about 3,000 feet. Two or three miles farther west 

(where there is no control on this horizon) the top of 

this zone must rise by the same amount.

2. The velocity throughout the zone (Yi) changes 

from 11,000 fps to 9,000 fps at the point along the 

profile corresponding to the apparent break in slope on 

the time-distance curve. Farther west, at an undetermined point, the velocity changes back to 11,000 fps,

3. The velocity in the first zone increases with depth 

from a value of 7,000 fps to a maximum of 11,000 fps. 

Everywhere beneath the atoll the base of the first zone 

rests on a second in which the velocity is lower. Along 

the southern edge of the atoll the second zone rises to 

such an elevation that below a portion of the profile 

from Chieerete to Enyu the thickness of the first zone 

is such that velocity increases only to 9,000 fps.

The sections resulting from all three assumptions 

are shown schematically in figure 136. The EnyuChieerete time-distance curves show that there is no 

anomaly in the arrival times from the deepest interface
498 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS 

165°15' 20' 25'

Area of possible 

change in Vj speed 

from 11,000 ft/sec 

to 9000 ft/sec

165°15'

FIGURE 135.—Contour map of depths to top surface of 11,000 fps zone. Depths are In feet below sea level. These contours are based on the assumption that seismic velocity 

in second zone is uniformly 11,000 fps everywhere under the lagoon. Figure 136 shows alternative possibilities.

which corresponds to that in the intermediate speed 

zone above. If the lower interface were horizontal, 

one would expect that any change either in the speed 

or thickness of the intermediate zone should, according 

to the depth-intercept time relations, give rise to some 

anomaly in the arrival times observed from the 17,000 

fps marker. Since there is none, one must postulate 

that a rise in the lower interface just compensates 

either for the depression in the top surface of the intermediate zone or for the change in its velocity, which 

might explain the anomaly in the arrivals from the 

upper interface. The upper and middle diagrams of 

figure 136 illustrate this. The third assumption does 

not require such a conveniently coincidental rise in 

the surface of the 17,000 fps zone, but positive evidence 

for a low-speed intermediate zone is of course lacking. 

The arrival times in the refraction survey was for 

points too widely separated to permit determination of 

the velocity of the material within a few hundred feet 

of the surface by refraction methods. However, by

applying Pekeris' dispersion theory, Dobrin (1950, 

1951) has deduced the velocities in the first 400 feet 

of sediments.

A brief qualitative summary of the dispersion theory 

as given by Dobrin (1950) is quoted below to facilitate 

the discussion to follow.

When an explosion is detonated under water, the initial pressure pulse traveling away from the source can be resolved by 

Fourier analysis into sinusoidal components having all frequencies from zero to infinity. For wave lengths no shorter than 

about one-tenth the water depth, dispersion will be observed, 

and each frequency will travel through the water at a different 

speed. The initial pulse will now spread out into a train of 

almost sinusoidal waves having frequencies continually varying 

with their position in the train.

In such a train, any given wave cycle will change in length as 

the wave moves away from its source. If an observer follows 

the crest of such a wave he finds it has, at any point, an instaneous 

velocity, c, which varies with the frequency. This is defined as 

the phase velocity (fig. 1 [fig. 137, this report]). If, at each 

distance, he selects a pair of crests separated by a given interval— 

that is, constant wave length—he finds that this wave length is
SEISMIC STUDIES OF BIKINI ATOLL 499

DISTANCE FROM ENYU HYDROPHONE IN THOUSANDS OF FEET 

40 30 20

- K),000 g

I. Escorpment or gully in intermediate zone 

No change in speed

— 10,000

Vertical discontinuity in speed of 

intermediate zone-surface horizontal

,^-.*r$i

WW/WM%tW/fM&

^§S?7660ft/>ec'^^

^rtitrr.Zft&X-Z-Z.-X?* .^00

VELOCITY < 9000ft/see

*®®®^^@$0sm !7,OOOff/MC

KS3g=^=3gsSl^3^^^

• ft/sec

l^^*>ii'000ft </sec

—^_i— — ._*— - —— ——

_ 10,000

* Velocity of first zone increasing with depth 

Local uplift of intermediate zone limiting 

the velocity of the first zone

FIGURE 136.—Three alternative interpretations of anomaly in arrival times at eastern 

end of Chieerete-Enyu profile, see figure 130. Dotted lines show postulated configurations where control is missing.

propagated at a characteristic speed depending on frequency. 

This speed, known as the group velocity, U, is obtained directly 

from the record by dividing the shot-detector distance by the 

total travel time of the wave in question.

The phase velocity of a compressional explosion wave transmitted without damping through shallow water will vary inversely as the sine of the angle made with the vertical by the 

multiply reflected wave paths allowing maximum phase reinforcement for each frequency. This angle will be different for 

each wave length and for each allowed mode of transmission at 

this wave length. Thus, for any mode—usually only the first 

need be considered if the proper measurement techniques are 

employed—the phase velocity and group velocity will depend 

on the wave length, and the water waves will exhibit dispersion. 

For a given water depth, the precise form of the dispersion curve 

will depend only on the density of the bottom material and its 

elastic characteristics. It is thus possible to study the bottom 

structure from the dispersion of the compressional waves propagated through the overlying water layer.

Figure 2 [fig. 138, this report] shows a number of curves of c 

and U versus frequency, /, for a water layer with a speed of ct

SOURCE 

X=0

AX, _

GROUP VELOCITY

PHASE VELOC TY

FIGURE 137.—Illustration of group and phase velocities.

.3 .4 .5.6 .8 1.0 34 6 8 10 30 40 60 80 100

FIGURE 138.—Phase velocity and group velocity (of first mode).

and a thickness H underlain by a liquid bottom of speed c2 . 

These were calculated on the basis of Pekeris' normal mode 

theory. The frequencies are expressed in the dimensionless 

form fH/ci, and the velocities in the form c/ci and U/CL Each 

curve represents a different c2/ci ratio, covering the range from 

1.05 to 3. If the upper medium is water with a sound velocity 

of 5,000 ft/sec and the lower medium is sedimentary rock (considered as a liquid), these curves, extending from c2 =5,250 fps 

to c2 = 15,000 fps, should be valid for virtually all homogeneous 

bottom sediments, assumed to be liquid, for which the sound 

velocity is greater than that in water.

A significant feature of the group-velocity curves of figure 

2 [fig. 138, this report] is the minimum observed in each. The 

higher the c2/ct ratio the lower is the frequency at which this 

minimum occurs. Havelock (1924) shows that any maximum 

or minimum in the group-velocity versus frequency curve will 

result in a situation in which components having adjacent 

frequencies will travel at the same velocity and because of 

their reinforcement will give an amplitude maximum. The 

amplitude peak at the frequency of minimum-group velocity is
500 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

readily identified on most oscillograph records of shallow-water 

explosions and is called by Pekeris the Airy wave.

The first "water wave" to arrive after the initial ground wave 

should have, according to figure 2 [fig. 138, this report], a frequency approaching infinity; subsequent water waves should be 

of successively lower frequency. The water-wave arrivals, 

represented by the portion of the group-velocity curve to the 

right of the minimum, should be superimposed on the lowerSHOT-DETECTOR DISTANCE, IN FEET

FIGURE 139.—Relative amplitudes of pressure versus distance from the shot. The 

pressure in the water around the hydrophone is generated by the seismic wave 

upon arrival after refraction through the subsurface beds.

frequency ground waves, represented by the portion of the 

curve left of the minimum and having a frequency which increases with time. At the minimum-group velocity the water 

wave and ground waves coincide and give rise to the Airy 

peak.

Insofar as one can consider the unconsolidated bottom material 

in Bikini lagoon to have the elastic properties of a liquid as well 

as a uniform sound velocity greater than 5,000 fps, the speed of 

sound in it and, hence, its elastic properties should be deducible 

by comparing the observed group velocities with the theoretical 

curves of figure 2 [fig. 138, this report].

Dobrin's study of the data in the light of Pekeris' 

theory shows:

1. The "high frequency" waves, 50 cycles per second and higher, from the shots in shallow water near 

the lagoon's edge indicate that the speed of the seismic 

wave in the first 20 feet below the lagoon bottom averages about 5,250 fps. This speed increases to 6,500 

fps at a depth of about 40 feet.

2. The Airy waves in all parts of the lagoon indicate 

a velocity of 6,500 fps to about 400 feet below the 

bottom.

These indications are in reasonable agreement with 

the velocity measurements made in the hole on Bikini 

island and with the refraction data. High-frequency 

waves from shots in the center of the lagoon give a 

dispersion pattern to which the Pekeris theory does 

not appear to apply. Dobrin attributes this discrepancy to a lateral variation in sedimentary composition of the top 100 feet between the edge and center 

of the lagoon, possibly a downward continuation of 

the surface change from sand to Halimeda debris 

observed by Emery (1948) on going toward the lagoon's 

center.

DETEBMINATION OF ABSORPTION COEFFICIENT

A byproduct of this investigation was a study of the 

attenuation of the elastic pulse in the 11,000 fps zone 

and in the 17,000 fps zone. Perkins (1952) has reported the results briefly. The pulse was assumed to be 

propagated in a cylindrical wave front horizontally 

through the bed corresponding to a particular section of 

the time-distance curve. The relative magnitudes of 

the pressure generated in the water by the compressional wave at each detector was determined. The 

magnitudes of the pressures transmitted by pulses of 

the same period are related by the equation

in which

and r2= two distances at which the firstarriving pulses have been refracted 

horizontally through the same 

horizon;
SEISMIC STUDIES OF BIKINI ATOLL 501

Pi and p2 —pressures at TI and r2 , respectively;

k—absorption coefficient of the medium 

through which the pulse was transmitted horizontally. 

The value of k for each zone was:

fc per 1,000 

feet*

0.2 

.003

Zone (seismic velocity,

in ft/sec) Geologic material

11,000—_________ calcareous (assumed)____

17,000____________ igneous__ _______________

1 For pulses of period 0.095 to 0.11 second.

The suggestion was made that the value of k with the 

seismic velocity can be used to identify subsurface 

sediments from surface observations. A graph of the 

relative amplitudes versus distance is shown in figure 

139.

The determination of k and the seismic velocity in 

media similar to those above and which have had a 

similar history should aid in determining the nature of 

the two horizons in question until such time as a more 

direct determination is possible.

GEOLOGIC INTERPRETATION 

POSSIBLE CONSTITUTIONS OF THE VELOCITY ZONES

Thus far there has been no attempt to identify or 

postulate the compositions of the three zones indicated 

by the seismic-refraction data. The 1947 borings have 

not made definite identification possible because of their 

limited depths and their location on Bikini island, at 

the atoll's edge instead of under the lagoon, where the 

refraction data apply. The well logs on the other hand 

give evidence which, combined with other indications, 

virtually specifies the nature of the uppermost zone 

and also provides us with a strong clue to the identity 

of the intermediate layer. Until a deeper or more 

ideally located hole is drilled, therefore, the internal 

constitution of the atoll can only be inferred from the 

available geological and geophysical evidence.

Cuttings from the 2,556-foot hole on Bikini island 

(Ladd and others, 1948) show only calcareous material 

from the surface to the bottom. The first 1,100 feet 

contains occasional hard streaks of limestone with loose 

calcareous sand in between. From 1,135 feet to the 

bottom, only loose sand (nearly all of Tertiary age) is 

encountered. Vertical seismic velocities measured in 

this hole to a depth of 1,800 feet fluctuated irregularly 

but showed definite trends when smoothed. In the 

first 1,000-foot interval velocities averaged about 8,000 

fps although local values between successive detector 

positions were as high as 14,000 fps. Actually, velocities over intervals of 100 feet or so cannot be determined accurately, and the local fluctuations may indicate 

only the normal uncertainty in reading arrival times. 

However, since the detectors were opposite the known 

interfaces the fluctuations probably are real. In the

remaining 800 feet the velocities were observed to 

increase quite uniformly from about 7,000 to 10,000 fps.

The refraction data indicate that the zone with a 

horizontal velocity of 7,000 fps extends from a depth of 

2,000 or 3,000 feet upward to the lagoon bottom, since 

the time-distance segments representing it intercept 

the origin at zero time. Core samples several feet long 

(Emery, 1946) taken from the lagoon floor show that 

the bottom sediments over much of the lagoon consist 

largely of fragments of the calcareous alga Halimeda. 

Shallow-water sediments of the same lithologic type 

would be expected to continue downward from the 

lagoon bottom to the first seismic discontinuity.

Halimeda does not extend an appreciable distance 

below the surface. But all calcareous materials brought 

to the surface in the drilling are of kinds that would not 

have been laid down in water deeper than a few hundred 

feet. In this sense all are "shallow water" sediments 

as opposed to those ordinarily at the bottom of the deep 

sea. Small irregularities in the refraction arrival times 

from the uppermost zone as well as fluctuations in the 

vertical velocities observed in the well shooting suggest 

that some inhomogeneities exist in the elastic properties 

of these sediments.

The only observations in the literature on the velocity 

of sound in recent calcareous material were made by 

Ewing and others (1948) off Barbados and by Woollard 

and Ewing (1939) at Bermuda. Off Barbados they 

obtained a velocity of 5,620 fps in the uppermost layer 

and at Bermuda, 8,800 fps. The speeds at Barbados, 

Bermuda, and Bikini all lie within the range of 5,600 to 

13,800 fps quoted by Birch (1942) for "soft" limestone.

The second zone, with an apparent horizontal seismic 

velocity of 11,000 fps, is the most difficult to identify. Its constitution, when established, will very 

probably provide the key to the atoll's origin and 

geologic history. If one extrapolates the observed interval velocities in the lower half of the 1,800-foot velocity 

log in the Bikini island boring, one would predict a 

speed of 11,000 fps at a depth of 2,000 feet. This is 

the velocity which, according to the refraction data, 

extends downward without increase (although it may 

decrease) for a considerable depth below an interface 

at 2,000-3,000 feet undersea. The most direct implication to be drawn from all the data taken together is 

that the calcareous deposits continue to the top of the 

17,000 fps zone and that the 11,000 fps marker is 

the top of a zone in which the speed of seismic waves 

no longer increases with depth. Such a velocity distribution would suggest that the calcareous sand 

becomes increasingly compacted, cemented, and possibly replaced by dolomite down to a depth below which 

no further change is possible. The increase hi compac-
502 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

tion and cementation appears to begin at 1,000 feet and 

the limiting depth would be 2,000 to 3,000 feet.

Upon receipt of the data from the vertical-velocity 

survey, some questions arose as to whether the seismicrefraction data, indicating discrete zones of constant 

velocity separated by surfaces of discontinuity, were 

compatible with the vertical-velocity log, which shows 

an increase of velocity with depth below 1,000 feet. A 

calculation was accordingly made to determine the 

time-distance curve that would be observed if the 

horizontal velocity varied with depth in the same way 

as the vertical velocity registered in the up-hole shooting. On the basis of the smoothed log, it was assumed 

that the horizontal velocity is 7,000 fps from the 

surface to 1,000 feet and that it increases downward for 

the next 1,000 feet at a uniform rate of 4 fps for each 

foot, so that at 2,000 feet it would be 11,000 fps, the 

value indicated by refraction for the first interface. 

Below this a uniform velocity of 11,000 fps was assumed. 

A calculation based on curved-path theory showed that 

all arrivals from the zone of increasing velocity would 

be masked by earlier direct-path arrivals through the 

7,000 fps zone and also by arrivals along the top of the 

11,000 fps zone. Hence the observed refraction data 

would not be contradicted by an increase of velocity 

with depth from 1,000 feet to the top of the intermediate 

layer. Such an assumption would require an increase 

of about 20 percent in the computed depth to the first 

interface and about 10 percent to the second, but it 

would not change the significant features of the refraction picture.

Since the suggested interpretation of the 11,000 fps 

zone as "shallow water" calcareous material in a limiting 

condition of compaction or cementation cannot be 

established as correct until a deep boring is put down, 

some alternative possibilities might be suggested. All 

such interpretations are necessarily speculative at this 

stage, and they are proposed mainly to point out the 

range of possibilities allowed by the geophysical 

observations.

In the Funafuti boring, dolomite was encountered at 

600 feet, continuing to the bottom of the 1,114-foot 

hole. No dolomite, on the other hand, was found at 

Bikini down to 2,556 feet. It is not inconceivable that 

there is a thick dolomite zone under Bikini so deep that 

it was not reached by the drill. 1 If its top surface were 

2,000 or 3,000 feet under the lagoon bottom it might 

well constitute the first seismic interface. Available 

literature (Birch, 1942) on the velocity of sound in 

dolomite indicates that it ranges from 16,000 to 19,600 

fps. This is, to be sure, higher than the observed velocity of 11,000 fps, and it may indicate that any dolomite

i The writers are Indebted to Professor W. H. Bucher for calling attention to this 

possibility.

constitutes a relatively small proportion of the calcareous material at this depth. The previously suggested encrustation of higher velocity material at the 

top surface of the intermediate zone might represent a 

layer of greater dolomitization.

Professor K. A. Daly has suggested that the second 

zone might consist of volcanic materials such as pyroclastics. Kevelle (1951) has reported the result of 

dredging on several guyots in the mid-Pacific. He 

found rounded basaltic pebbles, cobbles, and boulders 

at depths of 700 to 1,000 fathoms. In chapter A of this 

professional paper, which deals with the geology of 

Bikini Atoll, Emery, Tracey, and Ladd discuss the 

dredging and core sampling accomplished on the seaward slope of Enyu island. Olivine basalt was found at 

a depth of 1,000 fathoms, and pyroclastics were found 

at 1,150 fathoms. The 1946 aerial magnetic survey 

reported by Alldredge (chapter L) indicates several 

points where igneous rock conies relatively close to th« 

surface. One of these locations is along the southern 

border of the atoll between Chieerete and Enyu islands. 

No data are available on seismic velocities in pyroclastics.

The geologic and magnetic data combined with the 

anomalous seismic data on the Chieerete-Enyu profile 

suggest that the third interpretation of the anomaly in 

arrival times in the Chieerete-Enyu profile (see p. 11) 

is the most reasonable at this time with the calcareous 

sediments extending down from the surface to about 

3,000 feet and resting on volcanics.

Finally, there is the possibility that the zone consists 

of pelagic or other organic limestone deposited on an 

elevated igneous platform or an undersea peak under 

such conditions that it would not dissolve or be swept 

off by ocean currents. The 11,000 fps velocity is well 

within the range of previous measurements on limestone 

of such origin (Birch, 1942).

Identification of the 17,000 fps zone must also be 

based on circumstantial considerations. This velocity 

lies within the range that has been observed both in 

igneous rocks, such as granites and basalts, and in 

compact limestones. While it is conceivable that there 

might be a hard limestone layer at the base of the atoll, 

it would be very difficult to explain the considerable 

thickness that would be indicated for it by the 8-mile 

long segments representing it on the time-distance 

curves unless its velocity were greater than or identical 

to that of the basement. The 6,000-foot relief of its 

top surface would also be peculiar. It is more likely 

that the zone represents the igneous basement, which, 

under the part of the Pacific that lies east of the andesite 

line, is predominantly basaltic. This identification is 

essentially supported by the seismic measurements of 

Brockamp and Wolcken (1929) who obtained a velocity
SEISMIC STUDIES OF BIKINI ATOLL 503

of 18,300 fps for some known basalt formations in 

Germany.

INDICATIONS OP SUBSIDENCE

If it is assumed that calcareous material deposited in 

water not more than a few hundred feet deep extends 

from the surface to the top of the 17,000 fps zone, one 

must conclude that there has been 7,000 to 13,000 feet 

of subsidence, or relative change in sea level, in the 

atoll's geologic history. The broad terrace in the northeast portion of the 17,000 fps surface suggests that the 

topographic relief of this surface is due to subaerial 

erosion. If this could be proved, we would know that 

the total subsidence must have been at least 13,000 feet. 

However, no such proof is possible at present.

If, on the other hand, the intermediate zone has some 

other constitution than compacted coral or algal sand, 

the almost certain identification of the uppermost zone 

as calcareous establishes a minimum subsidence of 

2,000 to 3,000 feet.

The existence of a broad and probably igneous summit under the atoll about 6,500 feet below sea level is 

especially interesting in view of the announcement by 

Hess (1946) that fathometer surveys over a wide belt 

of the central Pacific have revealed the existence of 160 

flat-topped truncated and beveled cones, which he calls 

"guyots," ranging in depth from 3,000 to 6,000 feet. 

Additional seamounts, or guyots, have subsequently 

been charted by Emery (1946). One of these adjoins 

Bikini Atoll on the northwest. Hess explained these as 

volcanic islands that sank too early in geological time 

or too rapidly for reef-building organisms to grow 

substantial accumulations during subsidence.

There is a possibility that the buried igneous mountain under Bikini Atoll coincides in geological, origin 

with the nearby submerged truncated cones. Unfortunately the seismic survey could not give as precise 

data on the depth and shape of the mountain as the 

fathometer has given on the contours of the submerged 

cones, and comparisons are difficult. However, the 

basement surface under Bikini Atoll apparently has 

considerably more relief than the almost flat tops of 

Hess's guyots.

INFERENCES REGARDING GEOLOGIC HISTORY OF 

BIKINI ATOLL

The geophysical data suggest several lines of speculation relative to the origin and geologic history of 

Bikini Atoll. Vaughan (1923) and Hobbs (1923) have 

pointed out the fallacy of assuming that a single theory 

can explain the origin of all coral reefs and atolls everywhere in the world, and it would certainly not be

warranted to assume that all atolls have the composition 

indicated for Bikini Atoll.

In the present state of our knowledge, all theories 

regarding Bikini's geologic history must depend on the 

identity assumed for the 11,000 fps layer indicated 

by the refraction data. If, as seems logical, this layer 

consists of calcareous material, like that now being 

deposited, which has been compacted into a state of 

maximum consolidation, the atoll must have formed 

somewhat in the manner Darwin proposed, with a total 

subsidence of more than 2 miles. If the layer is presumed to be dolomitic, one can postulate essentially the 

same mechanism with the additional stipulation that 

conditions favored dolomitization of the reef limestone 

from the time of its initial deposition until a thickness 

was reached 2,000 to 3,000 feet less than the present 

thickness.

If one assumes that the intermediate zone is volcanic 

and consists of tuffs or pillow lavas, it is logical to propose an eruption from a submerged volcano of igneous 

products that emerged above sea level, forming a volcanic island something like Falcon Island today (Hoffmeister and Ladd, 1944). An island of such soft material would offer small resistance to wave cutting and, 

like Falcon Island, would soon be abraded to sea level. 

At this stage, shallow-water organisms would afix themselves to the platform's surface and upon subsidence 

would build up a calcareous layer several thousand 

feet thick.

If the intermediate layer should consist of pelagic or 

other deep-sea limestone, the mechanism becomes essentially a combination of the mechanism called for by the 

old Rein-Murray theory and that of Darwinian subsidence. Organic matter deposited on the surface of a 

deeply buried igneous platform builds up an increasing 

thickness of pelagic lime until its top is shallow enough 

for calcareous organisms such as corals and algae to take 

over. Subsidence would become the controlling factor 

at this point, and the present structure would be the 

result of a relative rise of several thousand feet in sea 

level. Banks, ridges, and other isolated topographic 

highs at depths as great as 400 fathoms off the California coast have been shown by Revelle and Shepard 

(1939) to have only a sparse and discontinuous veneer 

over rocky bottom. They ascribe this fact to the 

action of turbulence associated with intermittent bottom currents which suspends the sedimentary particles 

momentarily. This allows the component of gravity 

downslope eventually to remove the particles from 

the topographic high. If this process exists on midocean banks, it is not likely that any appreciable amount 

of pelagic material could have accumulated on the 

initial platforms; this theory would thus not be tenable.
504 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

SELECTED REFERENCES

Birch, Francis, ed., 1942, Handbook of physical constants:

Geol. Soc. America Special Papers, no. 36, 325 p. 

Brockamp, B., and Wolcken, K., 1929, Zeitschr. Geophysik,

v. 5, p. 482. 

Daly, R. A., 1915, The Glacial control theory of coral reefs: Am.

Acad. Arts Sci. Proc., v. 51, p. 155-251.

Dana, J. D., 1853, On coral reefs and islands: 143 p. New York. 

Darwin, C. L., 1842, The structure and distribution of coral

reefs: London. 

Davis, W. M., 1928, The coral reef problem: Am. Geog. Soc.

Special Pub., no. 9. 

Dobrin, M. B., 1950, Submarine geology of Bikini Lagoon as

indicated by dispersion of water-borne explosion waves:

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

———— 1951, Dispersion in seismic surface waves: Geophysics, 

v. 16, no. 1.

———— 1952, Introduction to geophysical prospecting: 435 p. 

New York, McGraw Hill.

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

1949, Subsurface constitution of Bikini Atoll as indicated 

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

v. 60, p. 807-828.

Emery, K. O., 1946, Submarine geology of Bikini Atoll (Abstract): 

Geol. Soc. America Bull., v. 57, p. 1191.

Ewing, Maurice, Pekeris, C. L., and Worzel, J. L., 1948, Propagation of sound in the ocean: Geol. Soc. America Mem. 27, 

205 p.

Gardner, L. W., 1939, An areal plan of mapping subsurface 

structure by refraction shooting: Geophysics, v. 4, p. 247- 

259.

Heiland, C. A., 1940, Geophysical exploration: New York, 

Prentice-Hall, Inc.

Hess, H. H., 1946, Drowned ancient islands of the Pacific basin: 

Am. Jour. Sci., v. 244, p. 772-791.

Hobbs, W. H., 1923, Reef formations as an index of mountain 

building processes: Pan-Pacific Sci. Gong. Proc., Australia, 

v. 2, p. 1128-1131.

Hoffmeister, J. E., and Ladd, H. S., 1944, The antecedent platform theory: Jour. Geology, v. 6, p. 388-401.

Jones, J. H., 1934, A seismic method of mapping anticlinal 

structures: World Petroleum Congress Proc., Pt. 1 (1934).

Kuenen, Ph. H., 1947, Two problems of marine geology—atolls 

and canyons: K. Nederl. Akad. Wetensch. Verh. (Tweede 

Sectie) Dl. XLIII, p. 1-69.

Kuwahara, S., 1939, Velocity of sound in sea water and calculation of the velocity in sonic sounding: Hydrographic Rev. 

(Monaco), v. 16, p. 123-140.

Ladd, H. S., and Hoffmeister, J. E., 1936, A criticism of the 

glacial-control theory: Jour. Geology, v. 44, p. 74-92.

Ladd, H. S., Tracey, J. I., and Lill, C. G., 1948, Drilling on 

Bikini Atoll, Marshall Islands: Science, v. 107, p. 51-55.

Matsuyama, Motonori, 1918, Determination of the second 

derivatives of the gravitational potential on the Jaluit Atoll: 

College of Science, Mem., Kyoto, v. 13, p. 17-68.

Nettleton, L. L., 1940, Geophysical prospecting for oil: 444 p., 

New York, McGraw-Hill.

Perkins, Beuaregard, Jr., 1952, Attenuation of seismic waves: 

Geophysics, v. 27, no. 2, p. 411.

Revelle, Roger, 1947, Bikini revisited (Abstract): Science, v. 106, 

p. 512-513.

———— 1951, Paleoecology and deep sea exploration: Committee 

on Treatise on Marine Ecology and Paleoecology, llth 

Ann. Rept., Natl. Research Council, December 1951.

Revelle, Roger, and Shepard, F. P., 1939, Recent marine sediments: Am. Assoc. Petroleum Geologists, Symposium, 

edited by P. D. Trask, p. 245-280.

Royal Society of London, 1904, The Atoll of Funafuti: Rept. 

Coral Comm. of the Royal Society.

Skeats, E. W., 1918, The coral reef problem and the evidence 

of the Funafuti boring: Am. Jour. Sci., 4th ser., v. 45, p. SI90.

Snavely, B. L., Baresford, R., Clarkson, H. N., Erickson, K. W., 

White, G., and Atanasoff, J. V., 1947, Use of a water 

coupled microphone system for seismic surveying (Abstract): 

Geophysics, v. 12, p. 500.

Stearns, Harold, 1946, An integration of coral reef hypotheses: 

Am. Jour. Sci., v. 244, p. 245-262.

Vaughan, T. Wayland, 1923, Coral reefs and submerged platforms^ Pan-Pacific Sci. Gong. Proc., Australia, v. 2, p. 

1128-1131.

Woollard, G. P., and Ewing, Maurice, 1939, Structural geology 

of the Bermuda Islands: Nature, v. 143, p. 898.
INDEX

Page 

Acknowledgments———---------—— —— — —— -.. — -. — —— — — _ 487 Namu island.

Page

. 492

Basement, basaltic...______________..-________.___..__ 502,503

Beds, computation of depths and thicknesses._______.-._---.-_-.. 493-500

determination of depths and thicknesses____------__-______-_ 493-500

Bikini Atoll, profiles.— —— ————.———•————— ————— — ——— .__ 489

Bikini island, location of drill holes._..____--__ — -_.____.____ —_________ 487

Calcareous material___— - — — - — .__ — __ — _— — . — __...... SOI, 502, 503

velocity of soundin——— — — — — — —__ — — — ————______.___ 501

Chieerete island.- ——— __- — __-____--.-- —............ 489,490,492,493,494,502

Chieerete island station—__........_..____........__._....... 493,494

Constitution of atoll, inferred......_...._......_.......__..___-..-__ 501

Determination of absorption coefficient.___.„„.__....„-.--.-__.. 500-501

Dobrin, M. B., quoted—.................................................. 498-499

Dolomite-________.____-._____.___-__....__.__ — — ——__ 501,503

velocity of sound in____--_.-_-.... — __._._..-....._................ 502

Drill holes, Bikini island, location.-.-._....____....__... — ......... 487

Enyu island....-----.._--_____-.___..-................. 489,490,491,492,493,494,502

Enyu island station........._................__......._______....... 493

Geologic history, inferences regarding---..----------.-._................... 503

Geologic interpretation......____..........._......__............... 501-503

Geotechnical Corp.— ....___-..--.........________..... — „____ 488,492

Guyots.---.-_.._-__..----_____-_---.--...____.______.__......... 503

dredging--. _---__ — ......__.-..-_._____.___....__.....___._ 502

Halimeda debris.___... — -.-._.........._____.__.........___ 500,501

Limestone....— -- — — . —-_._- — — - — . — _ —__....._._...... 491,501, 503

Pekeris' dispersion theory, qualitative summary by M. B. Dobrin.-------- 498-500

Profiles.—————--. —— - — - ——— — — —— - —— -- — - — 489

^4-Bprofile— ———— — — — ——— — —— — — —-—-——— 495,496

Chieerete-Enyu....................—...... —— -- 490,492,494,495,496,499,502

Enyu-YurochL—------------------------------------------ 490,492,493,496,497

NamutoEnyu—---——-- — - — - — - — — ------ — -- — - 492,494,496

Yurochi-Chieerete---- - ----- ------- — —------------ 490,492,493,496

Pyroclasticrocks--- —— — - — - ——— — — - —- — - — - — - ——— 502,503

Refraction survey, plan and procedure....——. —— -_____-....-__---.-----__ 489-492 

Relief, topographic._-.. —.... — --_ — — .--.——— - —— - — —— — ———— 503 

Romuk island..__-- — — — . — -------—.——-—.-____...___-- ——..... 495

Salinity of lagoon water—-............ ———. ——— .-.....„.-—————— 491-492

Shots, distance...—— —— — — ———— — — — —----——— 492,493,494

sample of records.-_.------.--------- ——— — - ———..___..-. ——.——— 490

sequence.—_....——.__..- ————— ——— —— -..- — ..-.- —— ———— 492

Shots for determination of vertical velocity... —— —— — . — — . — ————— 488 

Subsidence, indications--------------- —— ....-„-...--..-. — _____---------.. 503

Temperature of lagoon water..------- ————..—-------------------——— 491-492

Velocity zones, possible compositions.--------------- —------------- — --— 501-503

Vertical velocities—— ———————— -------------------- 487-489,501,502

computations.--.----------------- —— ------- —._-_-. —.--__—--- — — .— 488

determination------------------------------------------------------- 492-493

graphic chart-_-————- —— —— ——— ——... ——— —— — -—.—— 491

Well log—-.. — .. — —— —— —— — — — — —— — -—— ——— -—————— 491

Yurochi island------------------------------------------------------- 489,490,493

Yurochi island station-.-——————— — — — ——— — —— —— 493,494

505

Seismic-Refraction 

Studies of Bikini and 

Kwajalein Atolls

By RUSSELL W. RAITT

With a section on Coordination of Seismic Data, Bikini Atoll

By BEAUREGARD PERKINS, Jr.

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-K

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1954

CONTENTS

Page 

Abstract.._________________________________________ 507

Introduction _______________________________________ 507

Acknowledgments__ ______________________________ 508

Operating procedure________________________________ 508

Theory of interpretation of seismic-refraction observations ____________________________________________ 509

Travel-time plots ___________________________________ 513

Velocities. __________________________________________ 514

First layer.__-_--_______-.__-____-___--_-____-_. 514

Second and third layers_______________________ 515

Fourth layer ___________________________________ 516

Page 

Velocities—Continued

Fifth layer__--___---__---____.____------- 516

Sixth layer-.-_-_-____________________ 517

Structure----------------______________________ 517

Interpretation. ___________-_________---_--_---__ 517

Errors of interpretation___„_-_____________-__-_ 522

Discussion _________________________________________ 523

Selected references---------------------------------- 524

Coordination of seismic data, Bikini Atoll, by Beauregard 

Perkins, Jr______-________.______________. 525

Index___.__.___-__-__-----__--_-----__----_------_ 527

ILLUSTRATIONS

	Page 

FIGURE 140. Plan of operations in the Bikini area.______--_____________________-___--__------_-_--------_--------- 509

141. Plan of operations in Kwajalein Lagoon_______-_________-_______-.______-___--_-_--__-----_-----_---_ 510

142. Oscillograms illustrating bottom-refracted waves and water-borne waves-_________________-_---_----_-_-__ 511

143. Model of subsurface structure assumed for interpretation of the travel-time plots__________________________ 511

144. Travel-time plots for section through stations A and B, Bikini Lagoon_____________________.,--_-----__---_ 513

145. Travel-time plots for section through stations B, C, and D, Bikini Lagoon_________________________________ 513

146. Travel-time plots for section through stations D and E, Bikini Lagoon_____-______-___-_--____----_---__ 514

147. Travel-time plots for section through stations E, C, and A, Bikini Lagoon_________________--____---_-_---_ 514

148. Travel-time plots for section through stations A and G, Bikini Lagoon-_-_____-________--_--____-_-------_ 515

149. Travel-time plots for section through stations J and J', Sylvania Guyot_ __________-______-_---_---_-----_ 515

150. Travel-time plots for profile extending southwest of Bikini station J and received at stations J and «7'________ 516

151. Travel-time plots for section through stations K' and K" southwest of Bikini AtolL________________________ 517

152. Travel-time plots for section through stations B, A, and A', extending from Bikini Lagoon into deep water. _ _ 518

153. Travel-time plots for section through stations H and H' northeast of Bikini Atoll-________-__-___-_-_-_---_ 519

154. Travel-time plots for section extending south from station H along the eastern side of Bikini Atoll—_________ 520

155. Travel-time plots for section through stations A and B, Kwajalein Lagoon______-___-____----___---_---_-_ 521

156. Hypothetical cross section of Sylvania Guyot and Bikini Atoll, through stations J', J, E, C, and ^4__________ 521

157. Hypothetical cross section across Bikini Atoll through stations K", K', D, C, B, H, and H'.---------------- 522

158. Hypothetical cross section from Bikini Lagoon south through stations B, A, and -4'______________-______-__ 522

159. Hypothetical cross section of Kwajalein Lagoon through stations A and B_-______________--__- ____________ 523

160. Variation of true velocity in third zone.._.-----.___-_-__-.-____--_-_---_-----_-----------_---_-.-------- ——— _ 526

161. Cross section of Bikini AtolL.........._......._...._.._._.........-._—.... — — ...._-. — ————— — — 526

	m

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS AND SYLVANIA

GUYOT, 1950

By RUSSELL W. RAITT

ABSTRACT

Seismic-refraction studies made at Bikini Atoll, Sylvania 

Guyot, and Kwajalein Atoll on the joint University of California-U. S. Navy Electronics Laboratory Mid-Pacific Expedition of 1950 expanded the scope of the original 1946 studies to 

include a section across Sylvania Guyot and several deepwater 

sections extending from Bikini Atoll. Also Bikini Lagoon was 

surveyed in much greater detail, with particular emphasis on 

more complete control with reversed profiles. In addition, one 

short section was obtained at Kwajalein Atoll.

It was found that the seismic velocities beneath Bikini Lagoon 

vary laterally as well as with depth. They can be grouped into 

six more or less well defined layers averaging approximately 2.5, 

.3, 4, 5.5, 6.5, and 8 kmps. The first four were observed only in 

the lagoon profiles, and the remaining two were found only in 

deep water.

From sections taken on Sylvania Guyot and the flanks of 

Bikini Atoll near regions where samples of volcanic rock were 

obtained from the sea bottom, it was concluded that the third 

layer, of about 4 kmps seismic velocity, represents volcanic rock. 

Its estimated depth under Bikini Lagoon ranges from 600 m to 

2,100 m and averages about 1,300 m. One short section at the 

south end of Kwajalein Lagoon shows somewhat greater depths 

of the third layer than that at Bikini Atoll but the lagoon coverage was insufficient to demonstrate that Kwajalein differs 

significantly from Bikini.

Indications were found that the 6.5 kmps layer is deeper under Bikini Atoll than under the surrounding deep sea. If real, 

this local depression is unlikely to be greater than 3 km.

INTRODUCTION

The seismic-refraction studies of 1950 in the Marshal 

Islands took place during the period September 29 to 

October 9. They formed a concluding phase of the seismic program of the joint University of CaliforniaU. S. Navy Electronics Laboratory Mid-Pacific Expedition (Revelle, foreword of chapter A, Protessional 

Paper 260).

The previous seismic-refraction survey of Bikini in 

1946 (Dobrin, Perkins and Snavely, 1949) indicated

' Scripps Institution of Oceanography, University of California. 

NOTE.—Contribution from the Scripps Institution of Oceanography New Series 

No. 723.

the existence of at least three layers, with velocities of 

the order of 7,000 fps (2.1 kmps), 11,000 fps (3.4 kmps), 

and 17,000 fps (5.2 kmps), respectively. The first 

layer, extending to a depth of 2,000 ft to 3,000 ft (0.6 

to 0.9 km) was inferred to consist of calcareous lagoon 

and reef deposits on the basis of drilling to 2,556 ft on 

Bikini island in 1947.

The zone with a velocity of 17,000 fps was inferred 

to represent igneous basement. Its depth was indicated to be 7,000 ft to 13,000 ft (2.1 to 4.0 km). The 

second zone, with a velocity of 11,000 fps, was more 

difficult to identify. Available circumstantial evidence 

suggested that it was calcareous material similar to 

the first zone with a higher velocity caused by increasing compaction and cementation with depth. However, 

the possibility that it was volcanic material could not 

be excluded.

In planning the seismic operations within Bikini 

Lagoon the spacing of stations was chosen to attain 

an optimum degree of reversed control on the second 

zone. The results of the 1946 survey were very helpful 

in this planning.

In addition to the operations within Bikini Lagoon, 

an approximately equal effort was expended on operations outside the Lagoon, on Sylvania Guyot and extending down the flanks of Bikini Atoll into deep 

water. This work had two major objectives: (1) to 

seek clues for the identification of the second zone by 

means of outcrops and (2) to determine the relationship 

of the subsurface structure of Bikini Atoll and Sylvania 

Guyot to the structure of the deepwater region surrounding it.

A trip to Kwajalein Atoll for supplies gave an opportunity, on October 1, 1950, for one day of seismic 

observations in the southeast end of Kwajalein Lagoon. 

Although there had been no previous work on this atoll 

to guide this operation the principles applied to the 

Bikini Lagoon operation were successfully followed at 

Kwajalein Atoll.

507
508 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

ACKNOWLEDGEMENTS

Seismic-refraction operations at sea, thousands of 

miles from the home base, required careful planning 

and the cooperation of many people, whose help is 

gratefully acknowledged.

The work reported on was sponsored by the Bureau 

of Ships and the Office of Naval Eesearch, the Institute 

of Geophysics of the University of California, and the 

U. S. Geological Survey.

Dr. Roger R. Revelle, director of Scripps Institution 

of Oceanography, and leader of the 1950 Mid-Pacific 

Expedition, was responsible for initiating the project. 

His infectious enthusiasm and inspiring leadership were 

essential to its success.

All shots were fired from USS EPCE(R)-857, whose 

captain, Lt. Comdr. D. J. McMillan, and officers and 

crew skillfully executed the difficult navigation required 

among the coral knolls and reefs of Bikini and Kwajalein Atolls. Major credit for the successful reception 

of the weak refraction signals is due to the sympathetic 

cooperation of Mr. James L. Faughn, captain of the 

receiving ship, the M. V. Horizon, and the officers and 

crew.

The friendly help of the commanding officer of 

Kwajalein Naval Station and his staff in supplying 

explosives, provisions, and fuel to the expedition was 

greatly appreciated.

Mr. Daniel K. Gibson supervised the shooting operations, assisted by Chief Gunner's Mate Albert Obregon, 

who was responsible for firing the shots. Mr. Arthur 

D. Raff was responsible for the receiving-equipment 

installation and for acquiring the vast quantity of 

supplies and spare parts required in its operation. In 

recording the seismic waves on the M. V. Horizon the 

author was assisted by Mr. T. Wayne Runyan and Mr. 

Jeffery D. Frautschy. The travel time data were read, 

plotted, and analyzed by Miss Gloria J. Slaek and Miss 

Gwendolyn A. Roy.

Helpful discussion and criticism have been received 

from B. Perkins, Carl Eckart, R. S. Dietz, H. V. 

Menard, E. M. Hamilton, R. F. Dill, and T. A. 

Magness, who have read this manuscript in rough form.

OPERATING PROCEDURE

The plan of operation for the Bikini area and for 

Kwajalein Atoll is shown in figures 140 and 141. 

Points designated by letters represent receiving stations 

occupied by the receiving vessel, the M. V. Horizon, of 

Scripps Institution of Oceanography. At all stations 

within Bikini and Kwajalein Lagoons, the Horizon 

was at anchor. At all other stations the Horizon was 

either lying to or under way slowly to maintain position. 

Seismic shots were fired by U.S.S. EPCE(R)-857 along

the solid black lines shown in figures 140 and 141. For 

the profiles outside the lagoons standard speed while 

firing was 12 knots. Inside the lagoons the risk of 

navigation among coral knolls required a slower speed, 

usually 8 knots.

With the exception of twenty 20-pound demolition 

charges of tetrytol obtained from Kwajalein Naval 

Station, all charges were made up with one-half pound 

TNT demolition blocks. They were fired with slow 

burning fuse cut to fire about a minute after ignition. 

Standard charge sizes of 1, 4, 20, and 50 pounds were 

used. The time of firing was recorded by picking up 

the direct blast on the firing ship's echo sounder and 

transmitting it by radio to the receiving ship where it 

was recorded by the oscillograph used to record the 

seismic waves.

The refracted ground waves and the direct waterborne waves were picked up by three pressure-sensitive 

crystal hydrophones capable of responding to sound 

frequencies as low as three or four cycles per second. 

The hydrophones were supported by buoys at a depth 

of 120 feet. They trailed out from the receiving ship 

separated by intervals of about 300 feet. It was 

desirable, but not always possible, to lay the line of 

hydrophones in the direction of the line of shots in 

order to obtain a rough estimate of wave velocity from 

a single shot. However, even when wind and current 

caused the hydrophone spread to make a large angle 

with the line of shots, the use of three separate pickups 

gave a great advantage in identifying a weak refracted 

wave in the presence of noise.

Signals from each hydrophone were recorded on a 

Miller oscillograph in three rather broad frequency 

bands centered roughly at 10, 100, and 1,000 cycles per 

second. Frequencies of the refracted ground wave 

were generally of the order of 5 to 10 cycles per second 

and were received only on the lowest frequency channel. 

The higher frequency channels recorded the waterborne waves used to determine distances between shots 

and hydrophones.

Timing lines were flashed on the oscillograph record 

at 0.01 second intervals with a slotted disc driven by 

a fork-controlled synchronous motor. The timing 

accuracy was controlled with a break-second chronometer which was checked daily with WWV time 

signals. Travel times of both water-borne and refracted seismic waves were read to 0.01 second. The 

sharp onset of refracted waves of near shots permitted 

the beginning to be read to this degree of precision. 

However, the more distant shots frequently had very 

gradual beginnings and the error of reading the instant 

of first arrival of these shots is several hundredths of a 

second. Figure 142 gives samples of records at various 

ranges inside and outside of Bikini Lagoon.
40'

SEISMIC-REFRACTION, STUDIES OF BIKINI AND KWAJALEIN ATOLLS

50' 165° 00' 10' * 20' 30' 40'

509

I66°00'

40' 50' 165° 00' 10 20' 30' 40' 

FIGUBE 140.—Plan of operations in the Bikini area. Depth contours are given in fathoms.

166° 00'

For determining the shot-hydrophone distance from 

the travel time of the water-borne sound, the velocity 

tables of Kuwahara (1939) were used. The temperature of 29° C and salinity of 34.5 parts per thousand 

gave a velocity of 1.54 kmps, which was used for all the 

distance determinations.

THEORY OF INTERPRETATION OF SEISMIC-REFRACTION OBSERVATIONS

Travel-time data for seismic waves rarely yield a 

unique picture of the subsurface structure. Not only

is it difficult to determine the nature of subsurface 

rocks from measurements of seismic-wave velocity, it 

is also in many cases impossible to obtain a unique 

solution for subsurface velocity as a function of depth 

and position, from travel-time data alone. In most 

situations it is possible to set up a model of subsurface 

velocities consistent with the curves of observed travel 

time. This model may resemble the true structure to 

a greater or lesser degree dependent on the local conditions and on the control achieved in shooting operations.
510 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

167«35' 40' 45'

167° 35'

FIGURE 141.—Plan of operations in Kwajalein Lagoon.
SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS 511

3-15 c 

High sensitivity

3-15 c 

Low sensitivity

30-200 c

500-3000 c 

High sensitivity

500-3000 c 

High sensitivity

30-3000 c 

Low sensitivity

Rodio receiver

Bottom-refracted 

waves

Water-borne 

waves

Shot instant Time, in seconds 

COMPLETE RECORD OF A I-LB CHARGE, 4.0 KM SW. OF STATION B

f 1

4 Ibs, 34.6 km 

SE of Station E

20 Ibs, 22.8 km 

SW of Station K

LOW-FREQUENCY CHANNELS ONLY

50 Ibs, 61.0 km 

NW of Station J

FIGURE 142,—Oscillograms Illustrating bottom-refracted waves and water-borne waves. In the lower three osclllograms only the first arrivals of the bottom-refracted waves

are shown.

A m n B

FIGURE 143.—Model of subsurface structure assumed for interpretation of the travel-time plots
512 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

The model adopted in the present survey is the conventional one, consisting of successive layers of constant 

velocity, with each deeper layer having an appreciably 

higher velocity than the one above it. This model 

and the interpretation methods involved in its use, have 

been widely discussed in the literature and in geophysics 

textbooks (Nettleton, 1940; Heiland, 1940); however, 

as the procedures used have varied somewhat, a brief 

discussion of the methods employed in this paper is 

in order.

Figure 143 illustrates a subsurface section consisting 

of three layers, with velocities c0, c\, and c2 , respectively. In seismic work at sea the upper layer is 

water. For the example illustrated, the velocities c\ 

and c2 are two and four times as great, respectively, 

as the water velocity, c0 . Two receiving stations, 

A and B, and two shot points, m and n, are shown. 

Sound rays are shown refracted through the third layer. 

The travel time, TAm, from m to A is given by

*-Am=XAm ZAO cos QM , 2Wo cos 9m0

r

cos e

cos e

(1)

where XAm is the horizontal distance from shot to 

hydrophone and the z's and 0's have the meaning 

shown in figure 143. A similar equation is obtained for 

travel time between n and B. The term (TAm—XAmfc2) 

is called the "intercept time," (Gardner, 1939).. The 

four remaining terms of equation (1) are called "delay 

times" and are designated TAO , TOTO, TAI, and Tm\, respectively.

In the typically deep Pacific Ocean, where the depths 

are of the order of 4 to 5 km, the second and third 

terms of equation (1), the "water delays", are each of 

the order of 3 seconds and are generally much larger 

than the delays pertaining to subsurface structure. 

Hence, when the topography is irregular the form of the 

travel-time plot is usually determined by bottom topography to a greater extent than by the underlying 

structure. These water delays are analogous to the 

familiar "weathering corrections" of seismic surveying 

on land. The first step in the interpretation of the 

data is to subtract them from the observed travel times. 

The resultant travel-time data, corrected for water 

delay, effectively represent the travel times that would 

have been observed if the shots and detectors had been 

placed on the sea bottom directly under the shot and 

detector positions.

In the profiles recorded in Bikini and Kwajalein 

Lagoons, the hydrophones and shots were placed close 

to the bottom, the water-delay corrections are very 

small, and the errors in the correction are negligible.

In deep water, where the bottom is flat, the depth of 

shot point and receiver and the angles of incidence at 

the surface can be measured with precision, and the 

error of the water-delay correction is of the order of 

0.01 second. Where the bottom slopes steeply and is 

irregular, such as on the flanks of Bikini Atoll, the 

error is greater, and can be of the order of 0.1 second 

or more.

After correction for water delay the corrected travel 

times T TO and T ra are

Tk=^+rm + rwl , (2)

The velocity c2 can be determined from the slopes of 

the reverse travel-time distance profiles received at A 

and B from shots in the central region between A and 

B. For, if shot points m and n are placed at the same 

point, dzni/dXBn=—(dzmi/dXAm) and

dT'Am dT'Bn 

dXAm dX

( dz,•ml

*= i

C2

(cos 9ml —cos 9wi) \dXj (3)

For gently sloping structures (dz/dX) is small and 

cos Qmi is nearly equal to cos 0ni. Hence, to a sufficient degree of approximation for nearly all cases

(4)

i

dXAm dXBn

Also, when shots m and n are at the same point, rmi 

and rni are nearly equal and may be considered so as 

a first approximation. If the travel time from A to 

B can be measured by placing a shot at B and receiving it at A or vice versa then the delay times at A, 

m and B are determined by three measured intercept 

times,

*• Am -"-Am_ 

C2

C2

C2

^ (5)

Solution of these three equations gives the values of 

TAI and TBI.

The layer thicknesses ZA\, zmi, and ZB\ are given to a 

first approximation by assuming that 6^1, 6OT i, 6Bi and 

QSI are all equal to the critical refraction angle
sin 1 (c!/c2). With this approximation, 

ZAI=

SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS

4

513

(6)

In the interpretation of travel times of waves through 

the c3 layer, of higher velocity beneath the c2 layer, 

corrections for delay in the Ci layer are made in the 

same manner as for the water-delay correction previously described. These corrected times are then 

effectively those that would be observed if shots and 

receivers were placed on the Ci/c2 interface directly beneath their actual positions. This requires knowledge 

of the velocity and thickness of the ci layer under all 

sections of the profile where penetration' to the c3 

layer is achieved. The process outlined can be repeated indefinitely in principle. The practical difficulties and cumulative errors are severe if a large number of layers with small velocity contrast between 

them are present.

TRAVEL-TIME PLOTS

The observed travel times of the bottom-refracted 

waves, corrected for water delay, are plotted in figures 

144 to 155. For the sections outside Bikini Atoll,

.3

_ UJI-

>o

<UJ Oder. Ho: 

o 

o

3.99*.07kmps

——— 3.64*.07kmps

20 2 4 8 10 12

DISTANCE,IN KILOMETERS, FROM STATIONS

FIGURE 144.—Travel-time plots for section through stations A and B, Bikini Lagoon.

where there is a great variation of bottom depth, the 

bottom-depth profiles are shown directly below the 

travel-time data.

It is seen that in most of the profiles the travel-time 

curves can be represented by segments of straight lines. 

The lines drawn on the plot have been fitted by least 

squares to the indicated points, and the apparent 

velocity and standard error of its estimate are shown 

for each segment. Although the values of apparent 

velocity cover a wide range of magnitude there is a 

tendency to group around values of the order of 2.5, 3, 

4, 5.5, 6.5, and 8 kmps, the last value being represented 

by a single determination at the outer end of the .KMine 

southwest of the Bikini Atoll. It therefore appears 

that at least six velocity layers are involved in the 

overall structure. The first four were observed in the 

lagoon profiles "and the last two were identified only 

outside Bikini Atoll.

%

Si.

O CE •!•

££

P S .

K 3 

_J O 

UJ W

^C 

|8 2

S<^S.7e ± .08 kmps

3.69 ± .19

&±

2.63 + .30 kmps

2.39 ±.04kmps

XV 2.53 ± .24 kmps 

/* 2.631 .06 kmps

16 12 6 4 0 4 6 

DISTANCE, IN KILOMETERS, FROM STATION C

FIGURE 145.—Travel-time plots for section through stations B, C, and D, Bikini Lagoon.

12 16
514 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

4.95' 13 kmps

4.541.04 kmps

'

S

\ 2.39* .06 kmps

2.34*. 12 kmpsA

\

2 4 6 8 10 12 

'DISTANCE, IN KILOMETERS, FROM STATION E

14

FIGURE 146.—Travel-time plots for section through stations D and E, Bikini Lagoon. 

VELOCITIES

Not all the travel-time data can be used for velocity 

determination. Except for the initial segment of the 

lagoon profiles where the seismic waves were propagated 

through the topmost layer, the apparent velocities can 

be used to determine "true" velocities of the deep formations only when paired with reversed profiles in the 

manner of equation (4). Even though most of the lines 

in Bikini Lagoon, depicted in figure 140, were traversed 

three times by the firing ship, the distance over any one 

layer in which true reverse control was established was 

necessarily a small fraction of the whole.

FIRST LAYER

Observed only in Bikini and Kwajalein Lagoons, the 

velocity of the first layer was determined by using only 

the data of the first 3 kilometers of each profile. The

waves penetrating the deeper, higher velocity layers are 

first observed at ranges beyond 3 kilometers; this 

insures that a systematic error will not be introduced by 

including some of these higher velocity data. Results 

for Bikini Lagoon are shown in table 1 and for Kwajalein Lagoon in table 2.'

TABLE 1.—First-layer velocities in Bikini Lagoon

Station

A

A

A

A

B 

C

C

0 

D

D

E 

G

Direction

N

S

w

NW

S

sw 

sw

NE

SE

NW 

NE

N 

S

SE 

E

Number 

of shots

6

4

4 

4

7

q

q

3

4 

5

7 

5

Velocity 

(kmps)

2.26±0.09

2. 68±. 16

2.34±.14

2. 50±. 24

9 4.R_t- "I A.

2.63±.06 

2.63±.30

2.53±.24

2.51±.37

2.80±.19

9 SQ-4- fU

2. 34±. 12

2

OQ_i_ n«

2.36±.03 

2.41±.20

Intercept (sec)

-0.034±0.034

.069±.042

-.018±.C50

.015±.060 

.034±.038

.062±.015 

-.006±.072

-.080±.082

-.054±.113

-.019±.052 

-.002±.016

-.014±.043 

.025±.019

.036±.027 

-.023±.065

Harmonic 

mean 

velocity 

(kmps)

2.43

2.53

2.60

2.36

2.38 

2.41

EMS (residuals in 

sec)

0.05

.05

.05

.07 

.03

.02

.05

.05

.08

.03 

.01

.05 

.02

.01 

.08

TABLE 2.—First-layer velocities in Kwajalein Lagoon

Station

A

B

B

Direction

N

N

S

Number 

of shots

7

5

5

Velocity 

(kmps)

2.34±0.09

2. 50±. 15

2.45±.12

Intercept 

(sec)

0.018±0.030

.039±.043

.042±.037

EMS (residuals in 

sec)

0.04

.04

.04

A characteristic feature of all profiles is the large 

scatter from linearity. The magnitudes of the residuals are much greater than the value of 0.01 sec expected 

at short range if the bottom were homogeneous. HowOft 5

.DC. 4 

UJO

uu >iu 

<tr tc.cc. HO

8 12 16 20 24

DISTANCE, IN KILOMETERS, FROM STATION E

FIGURE 147.—Travel-time plots for section through stations E, C, and A, Bikini Lagoon.

26 32 36
SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS 515

» ,6 021 Z9 kmps ^•<1 3.69±.02kmps

29I± 04 kmps

DISTANCE, IN KILOMETERS, FROM STATION G 

FIGURE 148.—Travel-time plots for section through stations A and 0, Bikini Lagoon.

ever, they appear reasonable for a bottom formed of 

irregularly distributed regions of differing seismic 

velocity. This structure is also indicated by the 

irregular bottom topography, the presence of numerous 

coral knolls (Emery, 1948), and by the drilling under 

Bikini island (Ladd and others, 1948).

The highest mean velocity of any of the six stations 

in Bikini Lagoon was observed at station C, the one 

nearest the center of the lagoon. This effect may indicate a systematic increase of velocity from the periphery 

to the center. However, it is so small compared to the 

general variability that it may not be real. Hence, 

there does not appear to be any strong evidence for

systematic variation of velocity of the first layer with 

position. The best estimate of its average velocity 

was taken to be the average for the six positions 

weighted equally. This value is 2.45 kmps. It agrees 

within experimental error with the up-hole times 

measured in the hole drilled under Bikini island (Perkins 

and Lill, 1948). The velocities in Kwajalein Lagoon, 

tabulated in table 2, do not differ significantly from 

the Bikini Lagoon values.

Intercepts, with few exceptions, differ from zero by 

less than the standard errors. There is no positive 

evidence that a layer with a velocity significantly less 

than 2.45 kmps forms the floor of the lagoon. Failure 

to observe a significant intercept indicates that the 2.0 

kmps (6,500 fps) layer indicated in the first survey 

(Dobrin, 1950) has an average thickness less than 50 m. 

A layer having these properties would give an intercept 

of 0.03 sec.

SECOND AND THIRD LAYERS

Velocities determined by equation (4) from the seven 

reversed profile pairs, A-G; A-B; B-C; C-D; C-E; 

D-E; and O-A, figures 144 to 148, ranged from 3.10 

kmps for A-G to 4.74 kmps for D-E. It is concluded 

that velocities beneath the first layer vary with position as well as depth and that velocities 3.10 kmps and 

4.74 kmps probably represent two different layers.

TIME, TRAVEL SECONDS IN 

CORRECTED FORWATER DELAY

o CD ro*o>i\.

• • ? 6.51*. 09 kmps „-••"•""'*

\ .^*"**

• *x.B •*--

"^*->» -•<^***^" 

- ^•^ 4.09*. 05 kmps ' .^•-•'*** "**««^

"**<*^»/ *'*'*** ***^-«,0 5.84*.03'kmos 

•-» ,•"* ***•-«.*/ 

•^. *'*^\3.34-4.ll kmps *••*•

• ^x- -»^

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M

16 0 8 16 24 

DISTANCE, IN KILOMETERS, FROM STATION j'

32 40 48

16 0 8 16 24 

DISTANCE, IN KILOMETERS, FROM STATION j'

32 40 48

FIGURE 149.—Travel-time plots for section through stations ,7 and J', Sylvania Quyot.
516 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

o : '

(C _ 

.UJ »-

CO -I

O UJ

z Q

° „, o 0-

LU UJ 2

VI t

.6I±.IO kmps

4812

DISTANCE, IN KILOMETERS, FROM STATION J

0 4 8 12 

DISTANCE, IN KILOMETERS, FROM STATION J

FIGUKE 150.—Travel-time plots for profile extending southwest of Bikini station J 

and received at stations J and J'. The points in the upper part of the figure represent intercept times TV—-X"/6.15 at station J'.

Thig presents a difficulty of interpretation that was 

also encountered by the first survey (Dobrin and 

others, 1949), in which an unreversed profile in the 

A-Q region also showed an anomalously low apparent 

velocity. Although the problem presented does not 

have a unique solution for the available data, the 

solution which appears to be the simplest and most 

reasonable, while at the same time consistent with the 

travel-time data, requires two layers to include the 

velocity range 3.10 kmps to 4.74 kmps. The upper 

one, designated the second layer, is well expressed only 

in the reversed-profile pair A-Q which is used to 

determine its velocity, 3.10 kmps. The third layer is 

expressed in the second segments of the reversed-profile 

pairs B-C; C-D; D-E; E-C. In these profiles the 

wave refracted through the second layer is suppressed 

entirely by the prior arrival of the waves through the 

'third layer, or its presence is indicated only as a short 

transition between the first and second principal segments of the travel-time curves. The velocities of the 

third layer determined from these four reversed-profile 

pairs are: B-C, 3.98 kmps; C-D, 3.84 kmps; D-E, 

4.74 kmps; E-C, 4.13 kmps. The two remaining 

reversed pairs, A-B and A—C, were not used for velocity 

determinations of the second or third layer, because of 

ambiguity of travel paths for these profiles.

FOURTH LAYER

Each of the long profile pairs B-D and E-A of figures 145 and 147 have outer segments whose apparent 

velocities average around 5.5 kmps. They are assumed 

to represent waves refracted through the fourth layer. 

Systematic departures of the travel time from the 

straight lines determined by least squares occasionally 

amount to about 0.2 to 0.3 sec. This is not an unexpected result, for it could be attributed to variations 

of velocity and thickness of the material above the 

fourth layer. A more serious anomaly is the fact that 

the fourth-layer velocity, determined by equation (4) 

in the overlap region of E-A reversed profile, varies 

with position along the profile. Hence, it was felt to 

be unsafe to base the determination of the fourth-layer 

velocity on the small overlapping central portion of 

A-E reversed-profile pair. A compromise solution 

adopted was to apply equation (4) to the two long segments at the ends of the reversed pair A-E. This 

yielded the value 5.54 kmps, which was assumed for 

the fourth layer throughout Bikini and Kwajalein 

Lagoons.

The long reversed profile pair, J-J', figure 149, extending over Sylvania Guyot northwest of Bikini Atoll 

gives a velocity of 6.15 kmps, a value significantly 

greater than 5.54 kmps, but perhaps not enough greater 

to be regarded as a different material. Comparison of 

reversed slopes at individual points along the profiles 

shows that here too velocity varies with position. The 

systematic difference between the Bikini and Sylvania 

velocities is no greater than the differences indicated 

along this one section and does not necessarily indicate that the fourth layer is significantly different in 

the two areas.

FIFTH LAYER

In the deep-water profiles, K'-K", A-A', and H-H', 

figures 151, 152, and 153, principal segments have 

apparent velocities of the order of 6.5 kmps, a value 

which indicates that this layer corresponds to that observed at all 15 deep-water areas studied in the deep 

Pacific Ocean basin, a layer which has an average velocity of about 6.7 kmps (Kaitt, 1951). Its velocity in the 

Bikini area was determined from the reversed-profile 

pair K'-K" to be 6.53 kmps.
SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS 517

< 

V) u 10 i°

0 <E

Wg 8

Z ^ 

~ £E

liJ O fi 

2 U. o

H Q

-JH 

£ o 4 ^ ^

£l

o 2

*^-. ~<r*

*^~— ••-»_ 9 a30i.2lkmps 5.92±.26kmps ^-•"*"

^^•-^ •"•""'

•— ^.^ 6.70i.08kmps 6.4li.06kmps ^»

^•^. >^ 

•-•^ •-•

«^» 6.6l±.26kmps »^. •-'•" 5.97±.48kmps

*^. .•«<, \-

• •,. 5.26i.22kmps ^ •" 9 * •-•. 3.96i.06kmDs _*^*'*

>• •*-* *Xl _*'• 

L 5.07±.23kmps/ "X^ •'•\364i.l8kmps

V

..... 1 I 1 1 III 1 I 1 1 ! 1 1 1 I 1 1 1 1 1

24 16 0 8 16 24 32 

DISTANCE, IN KILOMETERS, FROM STATION K"

40 48 56

- 2

i"

24 16 8 16 24 32 

DISTANCE, IN KILOMETERS, FROM STATION K'

40 48 56

FIGUEE 151.—Travel-time plots for section through stations K' and K" southwest of Bikini Atoll.

SIXTH LAYER

The outer segment of the 60-km profile extending 

southwest of K' through K" has an apparent velocity 

of 8.30±0.21 kmps. Although a reversed profile for 

this segment was not obtained, the fact that the short 

profile from station K" over the same area gives an 

apparent velocity of 6.61 kmps shows that this high 

velocity cannot represent a steep upward slope of the 

6.53 kmps layer. Correction for the gentle slope indicated by the 6.61 kmps velocity yields a value of 8.2 

km/sec for the sixth layer velocity. This agrees with 

values observed elsewhere in the Pacific Ocean (Raitt, 

1951), and corresponds to the velocities found at the 

Mohorovi£i6 discontinuity at the base of the earth's 

crust (Gutenberg and others, 1951).

STRUCTURE 

INTERPRETATION

Depths to interfaces between the six velocity layers 

have been determined, using the procedure outlined in 

the previous section on the theory of interpretation. 

They are presented in figures 156 to 159, in the form of 

subsurface cross sections. Figure 156 represents a 

section extending the length of the Sylvania-Bikini

axis, through stations J', J, E, C, and A. The section 

on figure 157 extends across Bikini Atoll from southwest to northeast and runs through stations K", K', 

D, C, B, H, and H'. Figure 158 extends southerly from 

Bikini Lagoon through stations B, A, and A' into deep 

water south of Bikini Atoll. It is the only section 

having profiles crossing the edge of the atoll from the 

lagoon to deep water and vice versa. Figure 159 

represents the short section observed in Kwajalein 

Lagoon.

In each figure the stippled portions of the interfaces 

represent sections along which shots have been received 

with velocities corresponding to strata just below the 

stippled interface. The dashed interfaces have been 

assumed in order to calculate the depth to deeper strata.

The velocities used in calculating the layer thicknesses 

are shown on the sections. In sections where the velocity was not directly measured, assumed values based 

on apparent velocities and measurements of the same 

strata elsewhere were used. Wherever this was done, 

the assumed values are enclosed in parentheses.

For example, at station K' the initial segment 

represents the material at or near the bottom surface. 

Its velocity, determined by averaging the two profiles, 

was determined to be 3.79 kmps, a value corresponding 

to that of the third layer. In the other deep-water
518 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

SECONDS TRAVELIM, IN FOR WATER CORRECTED DELAY m * O>O N 6.01 ±07 kmps

••^0 6.07 ± 39 kmps «-^»

**^f^l"* 3.l2±.60kmps

3.64+ Of kmps **x»

-^^w •% 2.04 ±35 kmps

+ ^* ' 2. 45^14 kmps %%

^f-^^ ^ , , , ?

8 0 8 16 

DISTANCE, IN KILOMETERS, FROM STATION A

8 16 24 32 

DISTANCE, IN KILOMETERS, FROM STATION A

40 48

li

oa

1UUJ

"8

8 16 24 32 

DISTANCE, IN KILOMETERS, FROM STATION A

40 48

FIGURE 152.—Travel-time plots for section through stations B, A, and A', extending from Bikini Lagoon into deep water south of the atoll. The data previously shown in

figure 144 are repeated for comparison.
SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS 519

>- 

_J

O O 8

z

X £E 

^H

Z

.tr

LU O

2 u. 

H 4 

_i Q 

%P

IS 

Hg 2

O

o 

0

^^J* 

^^,+*^*^ _

0 4.59± .16 kmps 737±.I6 kmps ^-^-»

•-.. .--•-•

^ ^ 6.11*. 19 kmps _^»^^

.^"•-•^

3.42*. 13 kmps .'^ *"*"•*--•.. 5.I3*.08 kmps .••*

^>S' *•• \«»*

• ^ / * 9

~ •^ 4.81*. II kmps / ~

1 1 1 1 1 1 1 1 1 M 1 1 I

16 24 32 

DISTANCE, IN KILOMETERS, FROM STATION H

40 48

to 

<r

LUiUJ2 

o

2 -

Q. 3

LU 

Q

P 4 _

8 16 24 32

DISTANCE, IN KILOMETERS, FROM STATION H

FIGURE 153.—Travel-time plots for section through stations H and H' northeast of Bikini Atoll.

40

cross sections, however, the velocity of the layer exposed 

at the bottom was not determined. Hence, 3.79 kmps 

was assumed for the upper layer of the lower slopes of 

the atoll in profiles H—Hr and A—A'.

In deep water well away from Bikini Atoll, where 

the bottom is fiat, sediment is being deposited and the 

velocity is lower. The value 3.0 kmps was assumed to 

be reasonable in this area for the average value of the 

upper layer.

Owing to the fragmentary nature of the subsurface 

control, the horizontal variation of velocity, and the 

great changes of depth in the subsurface structure, 

the cross sections shown are necessarily highly schematic. No attempt was made to compute depths shot 

by shot for each of the 617 shots of the survey. Depths

were computed at the receiving station and at a few 

control points between stations. Interfaces between 

between these points were then sketched in with reference to the travel-time data. This procedure yields 

a hypothetical structure whose calculated travel times 

agree on the average within about 0.1 sec with the 

observed travel time. It tends to give some smoothing 

of the interfaces because there is no way of separating 

the relative effects of velocity changes and topographic 

roughness in producing minor irregularities of travel 

time. However, this does not mean that the seismic 

interpretation always tends to show a reduction in the 

magnitude of the relief. An illustration of the reverse 

is the sharp change of depth to the fourth layer between 

stations E and J, which is probably an interpretation
520 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

oo

- 

H

UJ

a:

6.64* 1.54 kmps

8 12 16 

DISTANCE, IN KILOMETERS, FROM STATION H

20

en OL

UJ

i

3

X

fcUJ

a

fe

CD

4 8 12 16

DISTANCE, IN KILOMETERS, FROM STATION H

FIGURE 154.—Travel-time plot for section extending south from station H along the eastern side of Bikini Atoll.

20
SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS 521

7 -

5 6h

z*zo:

O LD c 

Ofc 5

UJS tn>

-O 

ill 1*- 4

Is_jtU) O

3£ 3

eg

6.92 ± .38kmps

3.74 ± .27kmps

7.65 i.26kmps

4.88±.03kmps

2.83+ 23 kmps

^v*«.

3.0I + .05 kmps

~^^^x« *\

•^ *"»

j»"* 3 21 + .07 kmph ^~^***m tr ———————— m^

V®.x* >.45±.l2kmph

2.34 ± .09kmps

32 28 24 20 16 12 8 

DISTANCE, IN KILOMETERS. FROM STATION A

FIGUBE 155.—Travel-tune plots for section through stations A and B, Kwajalein Atoll.

ui I-

£

Q. 

UJ 

0

§

J E

2.45

SiSL^f^^-10 4.13 ' 7777^(3.9i

100 80 60 40 20

DISTANCE, IN KILOMETERS, FROM STATION A

FIGUBE 156.—Hypothetical cross section of Sylvania Guyot and Bikini Atoll through stations J', J, E, C, and A.
522 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

40 20 0 20 4O 60 

DISTANCE, IN KILOMETERS, FROM STATION K 1

FIGUKE 157.—Hypothetical cross section across Bikini Atoll through stations K", K', D, C, B, H, and H'.

80 100

0 20 40 

DISTANCE, IN KILOMETERS, FROM STATION A

FIGUEE 158.—Hypothetical cross section from Bikini Lagoon south through stations 

B, A, and A'.

error caused by the arbitrary change of assumed 

velocity between stations E and J.

ERRORS OF INTERPRETATION

Although it is clear that other structure sections 

differing from those of figures 156 to 159 may agree 

reasonably well with the travel-tune data, it is not 

clear how great a permitted variation from these sections can be. There are many factors affecting the 

error of depth of measurement and they operate in varying degree in different regions of the survey. In general, they can be lumped into three categories: (1) 

errors of measurement of the travel time, (2) errors of 

velocity, and (3) errors in the interpretation of the 

path traveled by the refracted wave.

Errors in category (1) have been discussed in previous 

sections and are rarely greater than 0.10 sec. Errors 

in category (2) depend not only on the accuracy of 

the velocities of the material through which the waves 

can travel but also on the path length. Hence, they are 

greater for distant shots and deep layers and may 

amount effectively to several tenths of seconds, particularly for ca.ses having poor velocity control.
SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS 523

Kwojolein

UJ

i

v 2

LJ 

O

O 

CD

(5.54)

30 20 10

DISTANCE, IN KILOMETERS, FROM STATION A

FIGURE 159.—Hypothetical cross section of Kwajalein Lagoon through stations A and B.

Category (3) is the most difficult to specify because 

it involves departures of the true structure from the 

assumed model in a manner which cannot be determined 

from travel-tune data alone. It is probably least 

important for the Bikini Lagoon profiles, where the 

detailed shot pattern provides the most complete 

control.

The greatest uncertainty in this category occurs in 

those portions of the three deep-water cross sections 

A-A', K'-K", and H-H' which lie under the steep 

flanks of Bikini Atoll. Travel-time data are insufficient 

to determine the continuity of interfaces between the 

lagoon and deep water outside. This is to a great 

extent a fundamental indeterminancy in this complex 

type of structure, which is far from the "ideal" simple 

situation required for a unique solution (Nettleton, 

1940, p. 255). In addition, it is aggravated by sparse 

velocity control.

Although the nebulous character of the errors in 

category (3) renders a precise statement impracticable, 

reasonable estimates of the depth errors would allow the 

order of 100 m to 200 m for the shallower strata under 

Bikini and Kwajalein Lagoons and as much as 3 km 

for the 6.53 kmps layer under the flanks of Bikini 

Atoll. In the relatively horizontal structures in deep 

water far from Bikini Atoll and in the deep layers 

beneath Bikini Lagoon and Sylvania Guyot the depths 

may be in error by the order of %. km to 1 km.

DISCUSSION

Even though many details are missing and much of 

the picture is subject to considerable uncertainty, most 

of the major features of the subsurface structure of 

Bikini Atoll and Sylvania Guyot are shown by the 

seismic cross sections. The geological significance of 

all of these features requires detailed study of the 

palaeontology and petrology of the bottom samples 

and the deep boring in the Bikini area and throughout 

the Marshall Islands as well. Other pertinent geophysical data such as the magnetic studies of Aldredge 

and Dichtel (1949) should also be considered.

The coordination of this great and complex quantity 

of observations is beyond the scope of this report and 

has been treated elsewhere (Emery, Tracey, and Ladd, 

1954). However, it is appropriate to close with a discussion of the most significant contributions of the 

1950 seismic studies to this problem. These involve 

two principal aspects: (1) the form of the ancient volcano now buried by calcareous deposits and (2) the 

local distortion of the crust accompanying the formation of a guyot and an atoll.

Regarding the first aspect, it now appears probable 

that the third layer, whose velocity is of the order of 

4 kmps, represents volcanic rock. There are little 

data in the literature (Birch, 1942; Gutenberg, 1951) 

to support this conclusion. Some data on basalt
524 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

(Brockamp and Wblcken, 1929) give 5.6 kmps, which 

is abou$ that observed for the fourth layer. However, 

this probably represents plateau basalt, whose elastic 

moduli are expected to be higher than for lavas and 

pyroclastic deposits forming a volcano. That velocities of volcanic rock of Pacific islands are variable and 

of the order of 4 kmps is shown by unpublished observations by the writer in shallow water off the northeast coast of Hawaii Island. Reversed profiles in this 

area indicated that velocities of the upper layers are 

variable and of the order of 3 to 4 kmps. Velocities of 

the order of 6 kmps were found only at depths of at 

least 2 km. Similar results were found at Guadalupe 

Island, a recent extinct volcano off the coast of 

Mexico.

Two seismic refraction studies have been made near 

Bermuda, an island which a deep boring has shown to 

be composed of volcanic rock below about 250 ft 

subsea (Pirsson and Vaughan, 1913; Pirsson, 1914; 

Sayles, 1931). Seismic velocity determined by Woollard and Ewing (1934) was 4.9 kmps and by Gaskell 

and Swallow. (1951) was 4.1 kmps.

Furthermore the variability of velocity observed for 

the third layer is quite reasonably explained in terms 

of the great variability of physical structure of the 

various types of rock emitted in volcanic eruption. 

On the other hand, the variations in velocity and topography in the third layer are unreasonable for a structure formed of an orderly succession of calcareous 

shallow-water deposits similar to those now being laid 

down in view of the absence of any systematic variation of velocity of the first layer in Bikini Lagoon.

Direct evidence for the identification of the third 

layer as volcanic is found in the presence of volcanic 

rocks on Sylvania Guyot and on the southern slopes of 

Bikini Atoll, (Emery, Tracey, and Ladd, 1954). In 

both areas the seismic data indicate that the third 

layer crops out. However, on the guyot the outcrop 

is indicated only at the northwest end. Although velocity control is incomplete much of the guyot appears 

to be covered with a layer of undertermined velocity 

significantly less than 3.68 km/sec. This layer may be 

largely composed of calcareous sand, but it may also 

contain a high proportion of volcanic sand and gravel 

similar to that found below the calcareous deposits in 

the deep boring of Bermuda Island.

Beneath Bikini Lagoon the depth of the third layer 

averages about 1.3 km, closely approximating the average depth of Sylvania Guyot. Its relief is considerable, 

being of the order of IK km from greatest to least calculated depth. The highest point determined in 

the survey was found about two-thirds of the distance 

from station C to station A where its estimated 

depth is 0.6 km. At the periphery of the lagoon the

depths are systematically greater. Although no depth 

determinations were made under the islands of Bikini 

Atoll rough estimates from interpolation on the K." to 

Hf cross sections of figure 157 indicates that the depth 

under Chieerete and Bikini islands is about 1.5 km.

The short section under Kwajalein Lagoon shown in 

figure 159 exhibits the same principal features as the 

Bikini sections. Although minor differences are apparent there is no basis for concluding that Kwajalein 

differs significantly in subsurface structure from Bikini.

Regarding the second major aspect—the local depression of the crust beneath Bikini—the sections of the 

profile of greatest interest are also those having the greatest error of depth determination—that is, the sections 

running from deep water to the lagoon. Hence, any 

conclusion concerning them must be qualified by this 

uncertainty.

If it is assumed that prior to the beginning of the 

volcanic eruption the fifth layer, of 6.53 kmps velocity, 

was approximately flat in the Bikini region, then the 

eruption should be accompanied by a down-warping 

of the crust in order to fill the void that would otherwise 

be created by the withdrawal of the material erupted. 

The magnitude of the depression of the crust will depend on such factors as the densities of the volcanic 

rock and of the magma from which it is derived, the 

degree of isostatic compensation, and the depth from 

which the erupting rock is derived. One of the key 

problems in the study of the mechanics of volcanic 

eruption and the related problem of the yielding of the 

earth's crust, is to know whether or not this depression 

takes place locally under the volcano or is regionally 

distributed (see, for example, Daly, 1942; Vening 

Meinesz, 1941; Woollard, 1951).

From the illustrated cross sections the evidence appears to favor a depression of the 6.53 kmps layer. It 

is only slightly indicated in the H-H' and K'-K" 

profiles, and amounts to about 2% km for the A—A' 

section. This is of the same order of magnitude as the 

estimated-depth error and can hardly be regarded as 

significant without further work supporting it; Hence, 

it is concluded that if there is a local depression of the 

crust it is less than 3 km.

SELECTED REFERENCES

Alldredge, L. R. and Dichtel, W. J., 1949, Interpretation of

Bikini magnetic data: Am. Geophys. Un. Trans. v. 30.

p. 831-835. 

Birch, Francis [Editor], 1942, Handbook of physical constants:

Geol. Soc. America Special Papers, no. 36. 

Brokamp, B., and Wolcken, K., 1929, Zietschr furs Geophysik:

v. 5, no. 34, p. 163-171. 

Daly, R. A., 1942, The floor of the ocean: p. 62 ff, Univ. of

North Carolina Press.
SEISMIC-REFRACTION STUDIES OF BIKINI AND KWAJALEIN ATOLLS 525

Dobrin, Milton B., 1950, Submarine geology of Bikini Lagoon

as indicated by dispersion of water-borne explosion waves:

Geol. Soc. Am. Bull. v. 61., p. 1091-1118. 

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

1949, Subsurface constitution of Bikini Atoll as indicated

by a seismic-refraction survey: Geol. Soc. Am. Bull. v. 60,

p. 807-828. 

Emery, K. O., 1948, Submarine geology of Bikini Atoll: Geol.

Soc. Am. Bull., v. 59, p. 855-860. 

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

and nearby atoll; Part 1: U. S. Geol. Survey Professional

Paper 260-A. 

Gardner, L. W., 1939, An areal plan of mapping subsurface

structure by refraction shooting: Geophys., v. 4, p. 247-259. 

Gaskell, T. F., and Swallow, J. C., 1951, Seismic refraction

experiments in the North Atlantic: Nature, v. 167, p.

723-724. 

Gutenberg, Beno and others, 1951, Internal constitution of the

earth: 2d ed., New York, Dover Pub., Inc. 

Heiland, C. A., 1940, Geophysical exploration, New York,

Prentice-Hall.

Kuwahara, S., 1939, Velocity of sound in sea water and calculation of the velocity in sonic sounding. Hydrographic Rev.

(Monaco), vol. 16., p. 123-140. 

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

Bikini Atoll, Marshall Islands. Science Vol. 107. p. 51-55. 

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

McGraw-Hill. 

Perkins, Beauregard, Jr., .and Gordon G. Lill, 1948, Velocity

studies on Bikini island: Office of Naval Research, Monthly

Res. Rept., p. 13-17, July 1. 

Pirsson, L. V., and Vaughan, T. W., 1913, A deep boring on

Bermuda Island. Am. Jour. Sci. v. 36, p. 70-71. 

Pirsson, L. V., 1914, Geology of Bermuda Island. I. The

igneous platform. II. Petrology of the lavas: Am. Jour.

Sci., vol. 38, p. 189-344. 

Raitt, R. W., 1951, Seismic refraction studies of the Pacific

Ocean basin. Paper delivered at Annual Meeting* Seismol.

Soc. America, March 23 and 24. 

Sayles, Robert W., 1931, Bermuda during the ice age: Am.

Acad. Arts, and Sci. Proc., v. 66, p. 381-467. 

Vening Meinesz, F. A., 1941, Gravity over the Hawaiian Archipelago and over the Madeira area: Nederl. Acad. Wetensch.

Proc., v. 44. 

Woollard, George P., 1951, A gravity reconnaissance of the

Island of Oahu: Am. Geophys. Union. Trans., v. 32, p.

358-368. 

Woollard, George P., and Ewing, Maurice, 1939, Structural

Geology of the Bermuda Island: Nature, v. 143, p. 898.

COORDINATION OF SEISMIC DATA, BIKINI ATOLL

By BEAUREGARD PERKINS, JR.

The two seismic surveys of Bikini Atoll differed 

greatly in character. The survey of 1946, which was 

reconnaisance, provided velocity information of the 

various horizons at only a few points. It was without 

reverse control except on one profile and then only for 

the deepest horizon. The 1950 survey, on the other 

hand, provided excellent velocity control and added 

much detail to the subsurface picture. The first survey 

provided velocity information of the first and second 

zone at only two points in the lagoon, one at Enyu

island and the second at Yurochi island. Of these two 

locations only the first corresponds in location to profiles of the second survey. The first survey being so 

sketchy, several pictures of the subsurface could be 

fitted to the data, and actually three possible pictures 

were proposed. Only general features of the results of 

the two surveys can be expected to match.

The following general features are shown by both 

surveys:

a. A surface zone of uniform thickness.

b. The basement (velocity 5.54 kmps) rises from

Enyu toward the northwest.

c. An anomalous velocity situation exists in the 

southeast corner of the lagoon.

In the vicinity of Enyu island where the velocity in 

zones 1 and 2 were determined in both surveys, the 

agreement was very good. In the first survey along 

the Enyu-Chieerete profile the velocity of the second 

zone was 2.74 kmps.

This is in good agreement with 2.95 kmps, the average 

of the velocities of the second zone, shown in figure 148, 

in the second survey, or in reasonable agreement with 

Raitt's calculated value of 3.10 kmps. The velocity of 

3.35 kmps for the second zone near Enyu on the EnyuNamu profile in the first survey agrees with the velocity 

of 3.35 kmps for the same zone along profile A-B and 

compares favorably with the velocity of 3.56 kmps 

along profile A-C in the second survey.

The velocities determined inside the lagoon in the two 

surveys were:

Zone

First..-———————

Third-.——————

Velocities in 

kmps —

1st survey

2.1 

2.74 

3.35

5.2

2d survey

2.45 

2.95 

3.35 

3.56 

4.15 

4.74 

5.54

Nature of zone

Calcareous. 

Assumed calcareous.

Assumed volcanic material. 

Assumed basement.

Velocities of 3.56 kmps to 4.74 kmps were not detected in the first survey. The value 5.2 kmps for the 

deepest horizon of the first paper is probably the average 

of the velocities of the third and fourth zones as defined 

by Raitt.

An areal plot (figure 160) of Raitt's average velocity 

of the third zone within the lagoon shows an increase 

in velocity from the southeast corner of the lagoon 

toward the north and west. This indicates a change 

in elastic properties of the material composing this 

third zone from southeast to northwest. The magnetic 

map (AUdredge, L. R., and Dichtel, W. J., 1949, Interpretation of Bikini magnetic data: Am. Geophys. 

Union Trans., v. 30, p. 831-835) shows sharply defined 

areas of various degrees of intensity. The intensities
526 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

FIGURE 160.—Variation of true velocity in third zone showing gradual increase from 

southeast to northwest. Average velocities calculated from profiles of second 

survey.

increase to the north and west in a similar fashion to 

the velocities. This is probably due to a change in 

magnetic properties of the rock, since the relief of the 

top of the third zone does not match the sharp changes

NAMU 

f*

SE

W 2 45 Calcareous ' \ 

3.35 '-\ *. \

Velocity,expressed in kilometers per second

Horizontal scale 

0 10 Nautical miles

i , , , i l______|

FIGURE 161.—Cross section of Bikini Atoll showing zones of different seismic velocity 

and the change in velocity in the third zone from the vicinity of Enyu on the south 

east to the vicinity of Namu on the northwest, based on data of first and second 

surveys.

indicated by the magnetic map. These changes indicate the possibility of the third zone being composed 

of volcanic material. The possibility of this zone 

being composed of dolomitized limestone cannot be 

ruled out, however, since the velocity change can be 

due to different degrees of dolomitization and the magnetic changes may be due to conditions in the basement. 

Figure 161 is a cross section of Bikini Atoll which incorporates all the above features.
INDEX

Page 

Abstract.—.——..————...—.._...„____....:.. — .....__._... 507

Angles of incidence, measurement__.._______-_______-___- 512 

Anomalous velocity situation__..... ____________. ________ 516, 525 

Atoll, Bikini._.___.___-..-__....„_————— — — —— 507, 512, 524

Basement rock, igneous__——____.______________. ____ 507 

rise to northwest._______________________________ 525

Chronometer_________.___._.._____._______.... 508, 512

Critical angle.——___________________________.____ 512

Cross section, Bikini island and Sylvania Guyot_...-_____...___. 521

hypothetical, Bikini.--------————————————————— 523

Kwajalein——...._„........——__......._._.——.—„... 523

Delay corrections, in second layer____._____..-____--._ —.... 513

Delay times—————.——————_———____....._____....__ 512

Difficulties, interpretation of travel time_._______________. ___ 509

Enyu island__________________.___-______._._____. 525

Errors_________________________________________. 509

in correction, magnitude.—__...——.._____..____——___ 512

in estimate of depth__. ___________________________ 523

in interpretation__________. _______.. _ ____. _ _ 522-523

Fifth layer, depression of______--.--..____....:_________ 524

velocities in———— ————-... ———— __———— — —— — — — — __„ 516

Fourth layer, velocities_________________,____. _______ 516

Frequency, ground waves.._..-.-._--------__.-..____——. —_ 508

Ground Waves, refracted———_..... ———____..—____——_.-. 508 

Hydrophones, recording————_-..-.-...-...___............. —.....-_ 508

Igneous rock——————————————————__—————_.—-—..___ 507 

Intercept times, equation————.—————__...——.____—.___ 512

Lagoon, Bikini__...._......__._...._.___——...._....... 507, 512, 515

Kwajalein——...—— ——-———————————————— 512, 515, 524

Layers, of constant velocity——.....————____———.____—__ 510, 513 

of different velocity______-______________________ 507 

thickness equation———————————..—..————..—————__—— 513

MohorovKie discontinuity——————————_—.———___————__ 517

Objectives———_———————————————__————...— 508

Oscillograms———___...——______.___________.__—._ 510

Page 

Oscillograph...__......____..——..._..-..——_.....——.—————.. 508

Outcrops______._._____...._____——___———.._._——.. 507

Plan of operations-———.—. 

Precision, degree, travel timeRecording, time of shots-____——__--.-....__....——_— ——.. 508

Residuals..___———.——_——.——————..————————.——... 514-515

Sea floor, shape, influence on travel time———_——. —— —————— ----- 512

Second layer, velocity————... ——————.————————————————— 507

Seismic-refraction studies—_...——..————-—————-—-—————— 507

Shots————————————— —————————————— 508

course for __ — —— _.———— —. — ——— —— ———— —— — —— —— — 508

explosive charges—______---..__.———„. — —— --........——.. 508

firing ship._......__.....—.....—..—...—— ————————.. 508

recording time of---—------ — ------------ —— ——— —— ———————— 508

speed of taking-——-_.— — ------------ — —.... — - — — ——— 510

Sixth layer_—————————......—————————————————————... 517

Spacing, hydrophones..___.. ——— ———.——.—————————————. 508

Stations, spacing-———... —— —— ------- —— — —— —. —— —— ————.. 507

Structure.._____——___.. ——— —— ————.——————————-,.-— 507

Subsurface structure, model———— ——————————————— -——————— 511, 512

Subsurface velocities, model_———_———————————————————— 509

Surface zone.--...-——_... —— ——— ——— ———. — —————— ——— 525

Survey of 1946—————————————————————————— 507

Surveys of 1946 and 1950, compared———— — .—.—.....——.——————— 524

common conclusions————— ————— — - — -. ———. —— ———..——— 524 

Sylvania Guyot___-__———————— ——.———————————_ 507, 516

Systematic increase, velocity-------------- — - — ----- —— ——— —————. 515

Third layer, average depth to————————————————————————— 524

not of sedimentary origin—__.—_ — ———. — ————— ————————.. 524 

relief—.————-— ————— — —— —-—————————— — - 524

velocity.——„-.... — —...„—......—.——..„..———.—....— 516

velocity equation——— —.———. — — — - — —. — - —... ———.. 515 

velocity variations, explanation—————....... — — —. —— ———— 525

Time of studies—.-..——-—-—-----———— ———------———————.. 507

Timing, shots—.-—-------—.... ——..'-....-----———————— — .„ 508

Travel time__...___.__—-——-----------—.—..——..———— 508, 509

equations..—————..—————————————————- ———————— — 512 

plots—___—————._———————————— ——————— 513-521

standard error.. ————— —— - — - ———— — — ——... —.. —. —..-- 521

Water delays———————— — —...———————————.———„„ 512

Yurochiisland———————. —————-———— ——————— 512

527

Magnetic Structure of 

Bikini Atoll

By L. R. ALLDREDGE, FRED KELLER, Jr., and W. J. DICHTEL

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-L

UNITED STATES GOVERNMENT PRINTING OFFIQE, WASHINGTON : 1954

CONTENTS

Abstract__--_--- 

Introduction- __. 

Instrumentation. 

Field data_____.

Page 

529 

529 

529 

529

Page 

Method of interpretating results ______________________ 530

Interpretation of results.____________________________ 533

Literature cited.__________________________________ 535

ILLUSTRATIONS

Page 

FIGUBB 162. Total intensity anomaly contour map of Bikini Atoll________________________-___-__--_----_-_---------- 530

163. Total intensity anomaly contour map of Bikini Atoll and its adjacent guyot-__________________-___-----_-_ 531

164. Total magnetic intensity contour map of model of Bikini Atoll____-_-_________----------_-_-_------------ 533

165. Basement relief map of model of Bikini Atoll ______________________________-_____-------------------- 534

m

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

MAGNETIC STRUCTURE OF BIKINI ATOLL

By L. R. ALLDREDGE, FRED KELLER, JR., AND W. J. DICHTEL

ABSTRACT

The magnetic total field intensity was surveyed, at an altitude 

of 1,500 feet above Bikini and the adjacent guyot. The resulting 

contour map shows a broad negative anomaly of 750 gammas 

over Bikini Atoll, with several superimposed more localized 

anomalies. Study based on a magnetic model indicates that 

basement material rises to within 5,000 feet of sea level approximately 1 mile to the northeast of Bikini island. The model 

assumes uniform susceptibility and zero permanent magnetization and conforms to known seismic profiles of the basement.

INTRODUCTION

During June, July, and August of 1947, the Office of 

Naval Research, the U. S. Geological Survey, the Naval 

Ordnance Laboratory, and the U. S. Naval Air Modification Unit at Johnsville, Pa., cooperated on a largescale magnetic survey (Alldredge and Keller, 1949) in 

which 31,000 miles were flown and 8,300 miles of 

magnetic traverse were completed. Nearly 1,700 miles 

were flown in obtaining a detailed magnetic map of 

Bikini Atoll and the adjacent guyot area in the Marshall 

Islands using Kwajalein Atoll as the operating base.

It is of interest to review earlier magnetic work of a 

similar nature done on the Atoll of Funafuti (Creak, 

1904).

INSTRUMENTATION

The survey aircraft was an amphibious PBY-5A 

equipped with an orienting total-field magnetometer 

developed by the Naval Ordnance Laboratory and the 

Bell Telephone Laboratories during the War (Felch and 

others, 1947). The magnetometer sensitive unit was 

housed in a fixed tail cone extending about 4 feet aft 

of the trailing edge of the rudder.

The magnetometer sensitive unit utilizes permalloy 

cores which are driven into saturation with a driving 

current of 1,000 cycles per second. The second harmonic voltage induced in the coil is proportional to the 

magnetic field along the core axis. The sensitive unit 

consists of three such elements mounted mutually perNOTE.—L. R. Alldredge, U. S. Naval Ordnance Laboratory, White Oak, Md.; 

Fred Keller, Jr., U. S. Geological Survey, now at U. S. Naval Air Development 

Center, Johnsville, Pa.; and W. J. Dichtel, U. S. Naval Ordnance Laboratory, 

White Oak, Md.

pendicularly in a gimbal mechanism. Two of these 

elements are used as orientators to keep the third 

element, the detector, in line with the magnetic-field 

vector. The magnetometer measures changes in the 

total magnetic field about an undertermined zero line. 

The zero line is steady with time except for a slow drift 

which depends upon the temperature and the gradual 

decay of a battery which partially nulls the magnetic 

field to be measured. The intensity observed is automatically charted by a recording milliammeter.

The magnetometer is capable of measuring changes 

as small as 1 gamma in the mangetic field while airborne. During this survey, however, the anomalies 

encountered were so large that a reduced sensitivity 

was required. The sensitivity used was such that a 

change of 500 gammas would produce a full-scale deflection of the recording meter. In precise surveys 

such as the one over Bikini Atoll a base line is crossed 

every 15 minutes or less to permit correction for drift 

errors.

FIELD DATA

A detailed survey was made over Bikini and the 

guyot that adjoins it on the northwest. Eleven northtrending and eight east-trending survey lines, spaced 

2 miles apart were flown across Bikini Atoll. Most of 

the lines were continued approximately 10 miles seaward beyond the coral reefs. Figure 162 shows the 

resulting detailed contour map. A few minor positive 

peaks are seen to occur within a broad negative anomaly 

of approximately 750 gammas. This type of anomaly 

would be produced by a broad guyot with rather 

irregular surface features in the basement.

Figure 163 shows a total-field magnetic-contour map 

of Bikini Atoll and its adjacent guyot. The ground 

control for the survey over the guyot was not accurate 

enough to maintain the 50-gamma contour interval of 

figure 162. The entire area was flown using dead 

reckoning with check points taken on Bikini Atoll 

before and after the survey. These check points were 

approximately 8 hours apart.

529
530 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

FIGUEE 162.—Total intensity anomaly contour map of Bikini Atoll. Data taken at 1,500 feet altitude.

METHOD OP INTERPRETING RESULTS

[Based closely on the description given by Alldredge and Dichtel (1949)]

As is well known, there are an infinite number of 

different configurations of magnetic materials which 

could account for the observed data given in figure 162. 

In spite of this ambiguity some useful ideas, based upon 

a few elementary assumptions, may arise from determining a possible magnetic substructure. Model techniques can profitably be employed in such a study.

It is realized that randomly oriented permanent 

magnetization and variations in susceptibility account 

for large anomalies in many places throughout the earth, 

and there is no assurance that these factors are not 

important in the case of Pacific atolls. A model which 

admits these variables and which matches the observed 

magnetic data could be built; in fact, any number of 

such models could be built. Complicated models of 

this type would yield substructures which would be

very difficult to interpret. To simplify the problem it 

was decided to assume zero permanent magnetization 

and uniform susceptibility. Such a simple model must 

have topographic relief to account for the observed 

data. The property of these simple assumptions can 

be judged only after the predictions of such a model 

are checked by other geophysical methods.

It was assumed that a smooth basement surface 

where Bikini Atoll now stands would result in a uniform 

magnetic field of 345 milligauss with a dip angle of 

+ 13.5° and a declination of 5° E. This is the present 

average field. This uniform field was applied to the 

model using a 20-foot three-dimensional coil system 

with the model placed in the center. Bikini Atoll is 

very near the geomagnetic equator having a geomagnetic latitude of approximately 6.5° N. The northtrending total-field gradient is less than 3 gamma per 

mile and was therefore neglected.
MAGNETIC STRUCTURE OF BIKINI ATOLL 531

30'

Doto taken at 1500 fl altitude 

Contour interval 200 gammas

Survey conducted on August II, 1947 

Compiled by L.R. Alldredge

30'

FIGUKE 163.—Total intensity anomaly contour map of Bikini Atoll and its adjacent guyot. Data taken at 1,500 feet altitude.

A mixture of foundry clay, ground magnetite, linseed 

oil, and glycerine was used for the model material. 

For the base of the model very little glycerine was used. 

As the oil dried out, an unyielding base was formed 

to support the upper part of the model. Glycerine was 

mixed with the clay used on the top part of the model 

so it would remain soft for later reshaping. Before 

construction of the model, the clay mixture was thoroughly demagnetized by applying an alternating field 

of diminishing amplitude in zero background field.

The magnetic potential V (x, y, z) at points outside 

the magnetic material is given by

V (x, y, 2)= (1)

__ where / is the magnetic moment per unit volume, r is 

the vector between x, y, z and x' ', y', z' and the integral

extends over the entire volume occupied by the magnetic material. Since zero permanent magnetization 

has been assumed, the magnetic moment per unit 

volume will all result from induced magnetization when 

a uniform magnetizing field Ho is applied to the model.

~* ~* 

We may write I=kH, where k is the magnetic suscep- —»

tibility and H is the actual field applied to each volume 

element. Since k has been assumed uniform throughout 

the model, (1) may be written

r--Adv (2)

It is now evident that the value of k can change the 

magnitude of the potential and the resulting potential 

gradient but can have no effect on the shape of the 

resulting field—that is, the location of maximum and
532 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

minimum. The shape of the resulting field will depend —»

on H and the boundaries of the magnetic material.

—» 

These last two factors are not independent as H

depends not only upon the applied field Ho but also 

upon the demagnetizing factor which depends on the 

geometry of the magnetic material.

With all the simplifying assumptions we have made 

there may still be an infinite number of different 

configurations of magnetic material which can account 

for the known shape of the magnetic-countour map 

shown in figure 162. Fortunately, in the problem 

under consideration something is known about the 

boundary conditions. During Operations Crossroads 

at Bikini Atoll several seismic profiles were obtained 

across Bikini Lagoon (Dobrin, Perkins and Snavely, 

1949) which indicated three distinct layers. The top 

of the deepest layer extended downward from 7,000 

to 13,000 feet below sea level, and it has been assumed 

that this is the basalt layer. To some extent this 

boundary information limits the ambiguity inherent 

in (2) and permits a better determination of the substructure within the original simplifying assumptions of 

the zero permanent magnetization and uniform susceptibility.

The choice of k is not critical, as has been shown. 

The general shape of the substructure can be determined 

using any value of k, which can then be adjusted to 

obtain the correct magnitude.

To determine a likely value of k it was noted that 

the susceptibilities of various magnetic minerals and 

rocks are given (Chapman and Bartels, 1940) as 

basalt (Northumberland), k =0.0002 to 0.01; mean of 

45 dolorites and basalts from the British Isles, k= 

0.0026; lava from Mount Etna, k=0.008.

In addition a simple calculation was made assuming 

that the general features of figure 162 were caused by a 

uniformly magnetized sphere. The data on figure 162 

do not go to seaward far enough, to establish definitely 

the location of the fringe peaks. It is estimated, however, that they occur approximately 30 miles apart for 

a north-trending profile line over the center of the 

lagoon. Using this value for the separation in the 

peaks, assuming that the basement material rises to 

within 7,000 ft. of sea level on the basis of the seismic 

work, substituting 34,000 gamma as the applied field, 

and putting in 700 gamma as the magnitude of the 

central negative peak (see fig. 162), a value of 0.008 

for k was obtained. This value was used throughout 

the model work. No readjustment of k was required 

after the general shape of the substructure was determined. This value of k was obtained by adding the 

proper amount of magnetite to the clay.

The model technique described above avoids the 

erroneous assumption of uniform magnetization which 

is common to many methods where magnetic fields 

are synthesized mathematically by adding uniformly 

magnetized sections of material.

The resulting magnetic anomaly caused by the model 

was recorded using a miniature magnetometer carried 

at the proper height above the model by an automatic 

traverse system. This procedure yields a magneticcontour map which can be compared with the original 

magnetic data.

The miniature magnetometer used in the model 

work was capable of measuring only a single magnetic 

component, whereas the original data were taken using 

an orienting total-field detector. The anomalous field 

of Bikini Atoll has a small value when compared to the 

earth's field. At Bikini Atoll only the component of 

the anomaly in the direction of the earth's field produces 

an appreciable effect on a total-field magnetometer. 

For this reason a single component magnetometer 

may be used in the model work as long as its detection 

axis is maintained in the direction of the earth's field, 

as was done in this work.

The miniature magnetometer has a sensitive core 

element approximately five-eighths of an inch long. 

For the scale used, this represents approximately 2,500 

feet. The field indicated by the magnetometer is 

roughly the average field in this distance, which tends 

to decrease both negative and positive peak values. 

This averaging process is not serious as can be judged 

from figure 162. The miniature magnetometer system 

is accurate to only ± 50 gamma.

Sinclair (1948) has given a rather complete discussion 

of the relationship required between a full-scale system 

and a model. In this model work all linear dimensions 

were scaled, and susceptibility of the model was 

adjusted to be equal to that of the full-scale object. 

Under these conditions equal fields applied to the object 

and model produce the same magnetic states, and the 

resulting model field, when measured at the scaled 

distance, is equal to that of the object.

The size of the model was limited by the dimensions 

of the large coil system used to apply the field found at 

Bikini Atoll, and by the limitations of the automatic 

traverse system used to carry the magnetometer over 

the model. Two nautical miles on the atoll were 

made equivalent to 3 inches on the model. The modelscale factor was therefore approximately 1/48,640.

The general ocean depth in the area surrounding 

Bikini Atoll is 2,500 fathoms. The plywood board 

upon which the model was constructed represented this 

depth below sea level. Copper nails were driven into 

the plywood board so that their heads indicated the 

proper depth below sea level of the upper surface of the
MAGNETIC STRUCTURE OF BIKINI ATOLL 533

basement material as indicated by the siesmic data. 

Magnetic clay was then placed on the board and molded 

flush with the heads of the copper nails. In regions 

void of seismic data, the clay was formed roughly into 

peaks where figure 162 indicates magnetic lows and 

into depressions where magnetic highs are indicated.

A survey with the magnetometer showed where the 

model data differed from the full-scale data. Suggested 

changes were made, always keeping the seismic profiles 

invariant in an attempt to obtain a model which would 

reproduce the full-scale data.

It was soon found that in the first model the seismic 

and magnetic data were incompatible if the uniform 

susceptibility assumption was to be retained. There 

was no way to account for the general decrease in magnetic field in moving from Yurochi island toward Enyu 

island without violating the seismic data in this region. 

Not desiring to abandon the uniform susceptibility 

assumption, since to do so would make any contour 

predictions about the basement in regions not covered

by the seismic data impossible, another solution was 

sought. -

The above incompatibility of data was removed by 

adding an equivalent 7,000-foot thick slab of the magnetic clay to the underside of the model. By cutting 

a rather steep side on the eastern end of the south side 

of the mound and gradually sloping the clay on all the 

other edges, the magnetic data were matched without 

violating the seismic profile lines.

This new model is equivalent to assuming that the 

general basement underlying the ocean floor begins a 

little more than a mile beneath the present ocean floor.

INTERPRETATION OF RESULTS

[Based closely on the description given by Alldredge and Dichtel (1949)]

Figure 164 shows the magnetic contours resulting 

from the final model. A basement relief (topographic) 

map of the final model is shown in figure 165. In most 

places the magnetic-contour lines of the model agree

165°30'E

UNITS IN GAMMAS 

Magnetic declination 13.5

10' 165° 30'E

FIGURE 164.—Total magnetic intensity contour map of model of Bikini Atoll. Data taken at 1,500 feet altitude.
534 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS 

165*10' E 30'

Basement relief map in thousands 

of feet below sea level

Low numbers indicate mountain

peaks 

1 0 4 Miles

165° 10' E 30' 

FIGTIEE 165.—Basement relief map of model of Bikini Atoll, contours in thousands of feet below sea level. Low numbers indicate mountain peaks.

quite well with the original magnetic anomaly map 

(fig. 162). The5 fact that this model shows a string of 

topographic highs around the atoll fringes with major 

peaks near each island adds strength to the simple 

assumption of uniform susceptibility. The only place 

where this is not true is near Chieerete island where a 

topographic low is shown.

The model predicts that the basement rises to its 

highest peak approximately 1 mile to the northeast of 

Bikini island. At this point the basement is only 5,000 

feet below sea level. The water over this peak is 3,000 

feet deep leaving only 2,000 feet of nonmagnetic material. The model predicts 8,000 feet to the basement at 

the northeast edge of Bikini island.

On high peaks, such as occur northeast of Bikini 

island and to the north of Enyu island, changes in 

model height of one-fourth of an inch (1,000 feet) could 

easily be detected magnetically. In the center of the 

large flat area between Yurochi and Enyu islands 

changes in height of three-eighths of an inch (1,500 

feet) were barely detectable magnetically. As would

be expected, the depth resolution was even less precise 

in craters such'as the one to the northeast of Chieerete 

island.

The general location of the seismic profile lines are 

indicated by dashed lines in figure 165. The seismic 

control did not, however, extend along the entire length 

of the dashed lines. The magnetic fit as shown was 

obtained by following the seismic data carefully except 

in the region of Chieerete island. Over a small area in 

this region model depths were 2,000 feet deeper than 

those shown by the seismic data. This deviation from 

the seismic data permitted a better magnetic fit. The 

authors (Dobrin, Perkins and Snavely, 1949) of the 

original seismic work point out that their conclusions 

might be in error by as much as 1,500 feet on the basement. The large crater northeast of Chieerete island 

was necessary to match the magnetic data. The depth 

of the crater was found to make little difference so long 

as it was made greater than 14,000 feet. The exceedingly steep sides were necessary to match the magnetic 

data to the north of the crater.
MAGNETIC STRUCTURE OF BIKINI ATOLL 535

Since the clay was tapered off to zero thickness on 

all sides of the model, neither the magnetic nor the 

topographic maps would be expected to be accurate 

near the northwest corner of the atoll where a guyot is 

known to connect with the atoll.

The magnetic-contour map of the guyot adjacent to 

Bikini Atoll showed a greater magnetic low than Bikini 

atoll itself. If these same model techniques were 

extended into this region it is quite sure that the measurement of the basement would rise to the full height 

of the guyot (4,000 ft. below sea level).

LITERATURE CITED

Alldredge, L. R. and Dichtel, W. J., 1949, Interpretation of 

Bikini magnetic data: Am. Geophys. Union Trans., v. 30, 

p. 831-835.

Alldredge, L. R. and Keller, Fred, Jr., 1949, Preliminary report 

on magnetic anomalies between Adak, Alaska, and Kwajalein, Marshall Islands: Am. Geophys. Union Trans., v. 30, 

p. 494-500.

Chapman, S., and Bartels, V., 1940, Geomagnetism: Oxford, 

Clarendon Press.

Creak, E. W., 1904, The Atoll of Funafuti, Keport of the Coral 

Reef Committee, Royal Soc. London, p. 33-39, 64.

Dobrin, M. B., Perkins, B., Jr., and Snavely, B. L., 1949, Subsurface constitution of Bikini Atoll as indicated by a seismicrefraction survey: Geol. Soc. America Bull., v. 60, p. 807-

828.

Felch, E. P. and others, 1947, Air-borne magnetometers: Elec. 

Eng., v. 66, pp. 680-685.

Sinclair, G., 1948, Theory of models of electromagnetic systems: 

Inst. Radio Eng. Proc., v. 36, p. 1364-1370.

O