The measured 3-D primary acoustic field of a seismic airgun array

The Littoral Acoustic Demonstration Center has conducted an experiment to measure the 3-D acoustic field of a seismic airgun array in the Gulf of Mexico. A seismic source vessel shot specified lines to give solid angle and range information. Hydrophone positions were measured by an ultra-short baseline (USBL) acoustic system while the source ship was turning between lines. An acoustic Doppler current profiler measured currents so the positions could be modeled between USBL measurements. The position locations were refined by using information from the acoustic arrival times on the hydrophones. Peak pressures, sound exposure levels, total shot energy spectra, one-third octave band analyses, and source directivity studies are used to characterize the field. One third octave band analysis shows received levels up to 180 dB re 1 µP for emission angles from 0 degrees (vertically down) up to 45 degrees for horizontal ranges up to 200 m at endfire, between 10 Hz and 200 Hz. The levels decrease with increasing frequency above 200 Hz, with increasing horizontal ranges, and for emission angles above 45 degrees. The levels are lower at broadside than at endfire. [Research supported by the Joint Industry Programme through the International Association of Oil and Gas Producers.]