A Model Based Assessment of Underwater Sound Levels during a Seismic Source Soft-start Operation

Abstract

Abstract The seismic source soft-start or ramp-up procedure involves the progressive activation of individual airguns within a seismic source array over a period of time to gradually increase the emitted sound pressure levels. The intent of the soft-start procedure is to warn nearby marine mammals of pending seismic operations and to allow sufficient time for those animals to leave the immediate vicinity. This is perceived as a ‘common sense’ procedure that assumes animals will move away from lower sound pressure levels and thereby avoid exposures to higher sound levels later in the soft-start process and during full array operations. However, the efficacy of soft-start has not been scientifically evaluated. The approach is commonly used as a mitigation measure during seismic survey operations in order to reduce the risk that marine mammals in close proximity to the source at the time of activation are exposed to levels which might cause auditory or physiological injury. A computer modeling study was performed to estimate sound levels received at various locations in the water column near an industry-standard seismic airgun array during a typical soft-start procedure in 2000 m water depth. The models included an airgun array source model and an acoustic propagation model. Received pressure levels were computed for receiver positions at several distances perpendicular to one side of the seismic vessel tow line and at multiple receiver depths. The computer model generated predictions of per-shot sound exposure level (SEL). These were summed over a number of shot points to compute cumulative SEL at each fixed receiver position. The model predictions indicate that sound levels present during the initial stages of soft-start are below the cumulative SEL thresholds that might lead to Permanent Threshold Shift (PTS) auditory system injury in cetaceans even at receiver locations 100 m off the seismic vessel sail line or 100 m depth when sail line passes directly overhead. The cumulative SEL levels could reach auditory system injury thresholds for pinnipeds at 100 m depth directly beneath the sail line at the third stage of soft-start, but this exposure level would only occur if the animal remained stationary during the approach of the seismic source.