Summary
We have developed a new air-gun that optimizes the useful seismic output while limiting any environmental effects from unnecessary high-frequency emissions. We regulate the output at the air-gun’s ports by controlling the motion of the mechanical components that release air from an internal chamber. Our design process is based on advanced fluid-dynamics simulation to optimize acoustic performance, and mechanical finite-element simulation to ensure structural robustness. This is validated by physical tests to calibrate the models, improve reliability, and characterize the final acoustic performance. Having carried out extensive testing on prototypes, we plan to introduce the air-gun into arrays for a commercial seismic survey.
Introduction
Marine seismic operations are the subject of increasing environmental scrutiny. Several studies have investigated the zones over which marine mammals may be injured (Southall et al., 2007; Breitzke and Bohlen, 2010; Laws, 2010; Laws, 2013; Goertz et al., 2013), leading to the NOAA (2013) draft criteria. As scientific knowledge improves and operational criteria evolve, the precautionary principle dictates that seismic operations take all feasible measures to reduce their acoustic emissions, in particular at the higher frequencies that are of no use to seismic imaging.
Air-guns have, however, historically been designed for a maximally impulsive output (emphasized by performance targets such as peak-to-bubble ratio or peak amplitude), emitting much of their energy above the seismic band. Hopperstad et al. (2012) and Abma and Ross (2013) proposed mitigating this impact by staggering the firing times of the air-guns comprising a source array. However, these methods affect the array directivity and can require additional processing on the array signature.
We instead regulate the acoustic output at the ports of the individual air-gun to limit unnecessary high-frequency emissions while preserving the useful seismic signal. Because almost all of the high-frequency energy is produced by the rising flank of the seismic pulse, the new air-gun is designed to precisely control this event. The airgun may also be combined in arrays to complement the array-tuning methods mentioned above. In this paper, we discuss the implementation and performance of the new airgun that was introduced by Coste et al. (2014).
