Lowest Practicable Source Levels (LPSL)
The idea that seismic surveys can be adjusted to a “lowest practicable source level” by reducing array parameters such as total array air volume has increasingly been touted by environmental organizations as a viable means of reducing sound production. Unfortunately, total volume of sound produced during surveys is misperceived as a simple metric of loudness despite the fact that source volumes do not correspond linearly with source output levels (in fact, it is a cube root relationship, with a 2000 cubic inch array only being about 3 dB lower in amplitude – SPL – or “loudness” than an 8000 cubic inch array).
Other features of a seismic array, such as the number of elements and their arrangement, figure more effectively in setting array amplitude; however, these complex design features are also critical to reducing the percentage of high-frequency sound produced as well as shaping the array output to minimize lateral sound propagation in the water. After more than five decades of experience using existing array designs for seismic surveys, there remains no evidence of injury, death or stranding of marine mammals from exposure to seismic sound pulses, even in the case of larger source arrays. This brings into question whether if array volume or source level were reduced, what would be mitigated and to what degree.
A simple universal solution that would limit or reduce array output without loss of data quality while yielding meaningful benefit to the marine environment is impractical and will likely increase the potential for adverse environmental consequences rather than decrease the potential. Additionally, any suggestion of such a reduction or limitation is not supported by current best available scientific understanding of the relevant issues.
Seismic operators fully consider environmental concerns when designing their acoustic arrays as they consider the geophysical imaging needs and will continue to seek additional win-win solutions for improved geological imaging while minimizing ocean ambient sound. The EnerGeo Alliance cautions against implementation of seemingly simple solutions like striving for lower practicable source levels which can have unintended negative consequences and should be fully thought out when making any decisions about array design.