Parameters affecting seismic survey sound levels versus distance and marine mammal safety

Abstract

Measurements of airgun sounds were made from 1996 through 1999 in the coastal waters north of Alaska (the Beaufort Sea). Sound levels versus distance (typically 100 m to ca. 6 km) from the source were measured for two single airguns and nine airgun arrays. Parameters of the measurements were number and size of airguns, array geometry and depth, receiver and water depths, and aspect. The total volume ranged from 10 to 120 in.3 for the single airguns and from 56 to 1500 in.3 for airgun arrays. The number of airguns ranged from 4 to 16. The U.S. National Marine Fisheries Service (NMFS) presently states that pinnipeds and cetaceans should not be exposed to received levels of pulsed sounds above 190 and 180 dB re: 1 μPa (rms over duration of pulse), respectively. NMFS often expresses concern that the disturbance may occur above 160 dB re: 1 μPa (rms). For each airgun source, distances within which animals might receive these three levels were determined. An effort is being made to relate the physical parameters of airgun operation to azimuthally radiated sound levels. [Work supported by BP Exploration (Alaska), Inc. and WesternGeco.]