Abstract

The normal-mode theory of Pekeris has been applied to the water-wave dispersion recorded from the under-water explosions set off in the 1946 seismic-refraction shooting in Bikini lagoon. The aim was to ascertain from the dispersion data the velocity of sound in the sediments down to several hundred feet below the lagoon bottom and thereby gain information about the recent sedimentary history of the lagoon. Because of the lack of short shots this information could not be obtained from the refraction results. The principal merit of the dispersion method here is that the long refraction shots supplied some information about the superficial layers.

Information on the first 100 feet or so of sediments below the lagoon bottom was sought from the “high-frequency” dispersion of water waves 50 cps and higher. The data from shots along the edge of the lagoon within 1-2 miles of the reefs and islands appear to be compatible both with normal-mode predictions for a liquid bottom and with refraction indications. From these data it is concluded that the speed within 20 feet of the water bottom averages 1.05 times that in water, with a rapid increase (probably from compaction) to a speed 1.3 times that in water at a depth of about 40 feet. Shots fired somewhat farther from the edge of the lagoon give dispersion curves indicating bottom speeds (on the basis of the Pekeris theory for liquid bottoms) much higher than the refraction data allow. Shots fired in the more central portion of the lagoon give dispersion patterns to which the Pekeris theory does not appear to apply. The author attributes the discrepancy to a lateral variation in sedimentary composition between the edge and center of the lagoon, possibly a downward continuation of the surface change from sand to Halimeda debris observed by Emery on going toward the lagoon's center.

The Airy waves (high-amplitude, low-frequency waves at minimum group velocity) give information on the bottom structure to about 400 feet. In virtually all parts of the lagoon their group velocity and frequency are as predicted by the Pekeris theory for a bottom velocity of 6500 ft/sec. This agrees with the first branch of the refraction travel-time curves, which were drawn from very limited data. That the Airy waves fit the theory for shots fired near the center of the lagoon while the higher-frequency waves do not suggests that the lateral change in sediment type within the uppermost layer disappears within a depth of about 100 feet. The change comes at a depth which can only be estimated roughly as 300 feet below sea level and was detected only in the central portion of the atoll.

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