Ambient noise and elastic wave propagation in the deep seafloor

Abstract

In August/September 1989 a borehole seismic array was deployed at DSDP Site 534 in the Blake Bahama Basin. The purpose of the array was to study the physics of acoustic and elastic wave propagation and ambient noise near the seafloor. The array was inserted into the borehole using a wireline re‐entry capability and was recovered by grappling. Each node was individually clamped into the hole. The borehole array consisted of four three‐component seismometers (with a passband from 2–50 Hz) at depths of 10, 40, 70, and 100 m below the seafloor. A borehole hydrophone was also recorded at the top node. Over I Gbyte of data was acquired over a 2‐week period from the array. A shooting program of air‐guns and explosives was carried out. An array of ocean bottom seismometers and a vertical hydrophone array were also deployed. Preliminary observations are (1) the time spread as a result of late‐arriving energy is much less at 10‐m depth than at the seafloor; (2) the borehole hydrophone is susceptible to tube waves for transient signals and ambient noise; (3) ambient noise levels vary by at least 10 dB over time; (4) ambient noise levels decrease by 10 dB between 10‐ and 100‐m depth; and (5) ambient noise levels on a vertical seismometer are 10 dB less than colocated horizontal seismometers.