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Regional observation of a nuclear test from a vertical hydrophone array

Lawrence Livermore National Laboratory, Livermore, California 94550
University of California, San Diego, San Diego, California 92106

Abstract

In order to evaluate the potential of water-column vertical hydrophone arrays for the measurement of continental events at regional distances, ocean acoustic data bases were examined for arrivals from these types of events. In 1987, the nuclear test LOCKNEY (m_b 5.7, 37.28°N 116.38°W) was observed in the deep-ocean water column at a range of 900 km by the Vertical Line Array (VLA) at 35° N 126° W. Although the hydrophone data were contaminated below 10 Hz by array suspension noise and ocean acoustic noise, adaptive array processing allows us to extract the P and T phases. The root-mean-square (rms) direct P-wave amplitude in the water column is 100.6 dB referenced to 1 µPa in the 4- to 8-Hz band in a 20-sec window, which suggests a detection threshold of mb 5.2 in a deep oceanic environment at this range for a single hydrophone. The arrival time of the T phase [i.e., the portion of seismic energy that has somehow coupled into, and propagated within, the deep ocean sound channel (Aki and Richards, 1980)] is consistent with a P-wave conversion at the continental margin, assuming a velocity of 6 km/sec from NTS to the continental margin and acoustic propagation at 1.5 km/sec in the water column from the continent to the array.

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