Cetacean Hearing-Damage Zones Around a Seismic Source

Abstract

In deep marine seismic surveying, a vessel tows a large array of hydrophones that measure the echoes from the subsurface resulting from the periodic firing of the seismic source. These echoes are processed to generate images many kilometers deep in the earth so that geologists can try to locate oil and gas. Such surveys typically collect over five terabytes of data per day. Typically, the vessel moves at 2.5 m/s. The seismic source array uses ∼5,000 in.³ (∼80 l) of compressed air at 2,000 psi (∼140 bar) distributed among 18 air guns at a depth of 6 m and emits almost all of its energy below 200 Hz. In three-dimensional (3-D) surveying, there are usually two source arrays fired alternately 25 m to the port and starboard of the sail line. Each one typically has an in-line interval of 37.5 m. In two-dimensional (2-D) surveying, there is a single source array that is normally fired at an in-line interval of 25 m. The source wave field can either be calculated from hydrophone measurements made close to the array following the method of Ziolkowski et al. (1982) be modeled using the method given by Laws et al. (1990). In this paper, I use the former with data that are sampled at 4,000 Hz. I assume reflection coefficients of −1 and +0.3 for sea surface and seabed, respectively, and use an ocean depth of 300 m. I use the method of images to compute the pressure signal in the water.