ASA 124th Meeting New Orleans 1992 October

1aUW11. Coherence measurements of deep-ocean seismoacoustic noise and signals.

L. Dale Bibee

Jeffrey L. Becklehimer

Naval Res. Lab., SSC Detachment, Code 362, Stennis Space Center, MS 39529

In April 1992, an array of six ocean bottom seismometer/hydrophone instruments was deployed on the seafloor in 3800-m-deep water. The instrument site was approximately 300 km west of San Diego, California and was within a few miles of the location of a smaller aperture array used in an experiment conducted in 1987. Initial calculations of the spatial coherence between vertical component seismometer recordings of ambient noise show remarkable similarities to data collected in the earlier experiment. The ratio of vertical to horizontal particle motion at the seafloor during ambient noise recording periods also is consistent with the earlier observations. The combination of coherence structure and particle motion ratios derived from the earlier experiment is used to infer that the ambient noise field at the site is dominated by seismic interface waves. With the new data set, this hypothesis is tested by examining: (1) the behavior of spatial coherence over a larger aperture (~400 m) array, (2) the spatial coherence of the near-bottom pressure field and horizontal particle motion field, (3) the ratios of particle motion to near-bottom pressure, and (4) the particle motion ratios and spatial coherence of signals from near sea-surface and on-bottom explosive shots. [Work funded by ONR and ONT.]