REGIONAL OBSERVATION OF A NUCLEAR TEST FROM A VERTICLE HYDROPHONE ARRAY

In order to evaluate the potential of water-column vertical hydrophone arrays for the measurement of continental events at regional distance s, ocean acoustic data bases were examined for arrivals from these typ es of events. In 1987, the nuclear test LOCKNEY (m(b) 5.7, 37.28-degre es-N 116.38-degrees-W) was observed in the deep-ocean water column at a range of 900 km by the Vertical Line Array (VLA) at 35-degrees-N 126 -degrees-W. Although the hydrophone data were contaminated below 10 Hz by array suspension noise and ocean acoustic noise, adaptive array pr ocessing 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 referen ced to 1 muPa in the 4- to 8-Hz band in a 20-sec window, which suggest s a detection threshold of m(b) 5.2 in a deep oceanic environment at t his 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 p ropagated within, the deep ocean sound channel (Aki and Richards, 1980 )] is consistent with a P-wave conversion at the continental margin, a ssuming 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 contin ent to the array.