Broad-Band Source Signature Modelling to Assess the Environmental Impact of Seismic Exploration in Vulnerable Regions

Mitigation measures to protect marine mammals from sound emissions can be implemented more effectively if the sound output of a seismic source is known beforehand. We present a forward modelling workflow to assess the environmental impact of seismic exploration in areas with marine mammal activity at the survey planning stage. The focus is on modeling the output sound exposure level created by the seismic source, typically an airgun array. The employed physical model considers the hydrodynamics of marine airguns, including air bubble oscillations, near-source interactions with other airguns in array-configurations as well as interaction with, and impact of, the surface ghost reflection. Sound pressure and/or sound exposure levels can be obtained as a function of distance from the source through consideration of different geometrical spreading models and a hearing threshold term for different marine mammal species. Combined, these results allow for the estimation of sound output and propagation properties in a number of different scenarios, in line with most recent regulatory requirements. The presented model allows an accurate estimation of sound output of seismic sources up to about 1 kHz, which marks on average the frequency at which airgun sources have decayed by 50 dB from their respective spectral maximum. We present example modeling results that illustrate how the workflow can be employed for the planning of specific mitigation measures, such as exclusion zones, safety radii, and soft-start procedures.