SUBSURFACE CONSTITUTION OF BIKINI ATOLL AS INDICATED BY A SEISMIC-REFRACTION SURVEY*

Abstract

As a part of the fundamental scientific investigation carried out in connection with the atomic bomb tests of 1946, a seismic refraction survey was made by Joint Task Force One to obtain information on the subsurface structure of Bikini Atoll. Naval depth charges were fired on the lagoon bottom along four lines extending across the atoll and the resulting seismic waves were picked up by water-coupled microphones near shore. A total of 126 charges were exploded in the survey. The time-distance curves indicate the existence of three zones with different sound velocity beneath the lagoon floor. For the uppermost layer, averaging about 2500 feet thick, the speed is 7000 ft/sec. Below this is a layer ranging from approximately 5000 to 10,000 feet in thickness with a seismic velocity of 11,000 ft/sec. Underlying the latter is a zone of undetermined thickness having a speed of 17,000 ft/sec. The recent boring, 2556 feet deep, on Bikini Island appears to confirm the conclusion reached from the seismic data alone that the uppermost zone consists of diverse calcareous materials such as are encountered on the sea floor within and around atolls at shallow depths,— i.e., a few hundred feet at the most. Results of a vertical velocity survey to 1800 feet in the same hole show a continuous transition from a velocity of 7000–11,000 ft/sec. It appears probable, therefore, that no essentially different rock materials enter into the composition of the atoll down to the top of the 17,000 ft/sec zone, and that the change in velocity is due mainly to progressive compaction and cementation of the calcareous sediments. In this case the 11,000 ft/sec layer indicated by the refraction data might consist of such sediments in a state of maximum cementation or of finer calcareous material, deposited in deeper water, that might have undergone greater compaction than the overlying coarser sediments. Parts of it, or even most of it, may be dolomitized; it might possibly consist mainly of pyroclastic sediments, although this seems less likely. Only a boring can provide a positive identification. The zone with a velocity of 17,000 ft/sec appears to represent the igneous basement. Its surface has considerable relief, the highest portion being near the center of the atoll about 7000 feet below sea level. From this summit a sharp nose plunges southeastward to the atoll9s edge, the greatest observed depth being about 13,000 feet. The seismic data suggest several possible histories of Bikini Atoll, but all require relative subsidence in the thousands of feet. If the material down to the 17,000 ft/sec zone is calcareous sand, the seismic evidence calls for a minimum of 7000 feet of subsidence. If a terrace on the surface of this zone at 13,000 feet is attributed to subaerial erosion, a minimum of 13,000 feet of subsidence is then required. If, on the other hand, the intermediate zone is identified as volcanic or pelagic material, it is still necessary to assume that there was at least 2000 feet of subsidence. Thus, for Bikini at least, the seismic results tend to favor the conception, first advanced in its simplest form by Darwin, that atolls are formed by long-continued subsidence of initial noncalcareous islands or shallow banks.