Seismic surveys in complex environments: effects of environmental variability on sound propagation and mitigation practice

Quantifying and monitoring potential impacts of offshore noise is made possible by the accurate measurements of the sound sources and precise characterisation of how they will be perceived at different ranges, and for different receivers (in particular marine life). Seismic surveys transmit loud sounds and they require the definition of mitigation zones based on sound levels; this was comparatively easier when most surveys occurred in deep waters, but is increasingly difficult as seismic exploration moves to shallow waters and complex coastal settings, looking for example at hydrocarbon deposits or geo-hazards. The variability of geoacoustic seabed parameters, as one moves from the continental break to the shelf and often to complex near-shore geology, is compounded by the high variability of environmental parameters, as water density, salinity and temperature change with geographic location and time, on scales of hours to days. This is a problem increasingly faced by industry and academic research, as summarised in the Oceanoise-2015 report of Erbe et al. (2015), and one of the recommendations was the use of dynamic maps of noise impacts. But what are the best places and times to measure sound? What are the constraints on acoustic propagation? In the case of seismic surveys, what are the implications on mitigation practice? This paper will present the methodology for accurate Sound Source Verification developed by Heath and Wyatt (2014) and Jímenez et al. (2015). Sound propagation scenarios are used to compare field data from several surveys with Parabolic Equation models, highlighting the key parameters to measure in the field, and their effects on sound maps. Potential implications for data acquisition and dynamic sound mapping will also be discussed.