Acoustic energy attenuation of 3DVSP airgun arrays operating in Long Beach Harbor

Abstract

Increased awareness of the disruptive impact high-power sonars and airguns potentially have on marine mammal populations has resulted in the adoption of flexible survey protocols, which account for movements of these animals through the region. To effectively modulate source transmissions and maintain adequate standoff distances, accurate estimates of range-dependent sound pressure levels (SPLs) are required. To determine safe ranges of airgun operation during a three dimensional vertical seismic profile survey (3DVSP) in Long Beach Harbor, stationary hydrophones were deployed to measure SPLs received from 160 in/sup 3/, 320 in/sup 3/ and 560 in/sup 3/ airgun arrays fired along a 5 km track. Recordings from bow and stern source boat aspects are analyzed for pulse energy, pulse duration, peak instantaneous pressure, and rms pressure over the pulse duration. For each airgun array and aspect combination, range-limited linear regression curve fits are carried out to characterize transmission loss. Distances at which the SPL drops below 190 dB, 180 dB, and 160 dB rms re: 1 /spl mu/Pa are determined, where operation of airguns at distances from marine mammals greater than the 160 dB range is mandated by the National Marine Fisheries Service (NMFS). For the 160 in/sup 3/ array, the bow aspect 160 dB level is estimated at 1279 m. For the 320 in/sup 3/ array, the bow and stern aspect 160 dB levels are estimated at 1712 m and 1916 m respectively. For the 560 in/sup 3/ array, the bow and stern aspect 160 dB levels are estimated at 2057 m and 2672 m respectively. The data collection and analysis techniques described in this paper rapidly and inexpensively determine NMFS compliant airgun operating procedures.