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Long-range detection and location of some shallow-depth explosions in the Northwest Pacific

NAVAL UNDERSEA RESEARCH AND DEVELOPMENT CENTER, SAN DIEGO, CALIFORNIA 92132

Abstract

During a survey of underwater explosions, a set of 382 closely-spaced events was located in the Northwest Pacific near the Emperor Seamounts. Later, it was learned that the shots were from a joint U.S.-Japan science cooperation seismicrefraction program. After-the-fact correlations were then made between the SOund Fixing And Ranging (SOFAR) fixes and shot-log data.

Shots were composed of 3 to 288-lb Therytol or Seismogel demolition charges. Detonation depths varied from 60 to 420 ft. Results are that: (1) If only shots of 8.6 lb or greater are considered, a larger percentage (87.7) of shots composed of Seismogel was successfully located than of those composed of Tetrytol (59.5); (2) the threshold of detection was lower (9 lb at 90 ft as compared to 12 lb at 110 ft) in the northern part of the profile (37°N) than in the southern part (32°N); and (3) signals from shots detonated over the southern edge of the Shatsky Rise were received at a higher level than those detonated off the Rise. The reason for (1) is the higher detonating velocity of Seismogel; for (2) it is the shoaling of the SOFAR channel with increasing latitude, and for (3) it is the effect of a highly reflective bottom which sloped toward the hydrophones.