Transmission of air gun array signals in Australian waters

Abstract

Received air gun array signal parameters from 15 seismic surveys in Australian waters have been analyzed. Measurements include sources from 1115 to 4900 cui (18.3 to 80.3 L), ranges of hundreds of meters to thousands of kilometers along travel paths of 15 m to full ocean depth. For any array there may be large (±9 dB) difference in received level between consecutive signals, especially if two 3‐D arrays are operated alternatively. All arrays demonstrate horizontal directivity, being typically omnidirectional below nominally 50 Hz but focusing higher frequency energy about their longer axis. Three‐dimensional arrays tend to be square and 2‐D arrays rectangular with the long axis across track; thus 2‐D arrays are more focused. At a nominal range, differences of up to 40 dB were found when comparing small arrays operating in shallow water with large arrays in deeper water. For example, a 2500‐cui array transmitting in 70 m water had no waterborne energy at 20‐km range whereas a 4900‐cui array operating on the shelf slope was detectable via a deep sound channel path at 2000‐km. This large variation in received levels under different scenarios implies similar differences in environmental implications.