Airgun array sound measurements in the northeastern Chukchi Sea

Abstract

Underwater acoustic measurements were made during eight 2D/3D seismic surveys with large airgun arrays and nine shallow hazards surveys using small arrays (2–4 airguns) in the shallow (30–50 m) waters of the northeastern Chukchi Sea between 2006 and 2013. The acoustic measurements were made with calibrated seabed-deployed recorders to assess potential noise exposures to marine fauna. Data were acquired from directly under the airgun arrays to over 150 km distance. The seismic pulses contain signals above ambient noise between approximately 5 Hz and 15 kHz. Modal structure develops by about 1 km distance from the sources at frequencies between approximately 30 Hz and 600 Hz, but it is strongest from 100 Hz to 300 Hz. This ocean environment therefore acts as a frequency band-pass filter, selectively passing the intermediate sound frequencies to long distances. The modes are strongly dispersed, with low frequency components arriving up to 2 s later than the higher frequency sounds of the same mode. This causes received signals to have a down-swept spectral shape. We present sound level measurement results from all of the surveys and we discuss the frequency-dependent acoustic transmission loss in context with the modal propagation characteristics of this ocean environment.