Sound levels from a 3D seismic survey in the Porcupine Basin: Validation and Calibration of a Sound Propagation Model using Observed Data

Following Ireland’s 2015 Atlantic Margin Oil and Gas Exploration Licensing Round, the Porcupine Basin continues to be a significant site for seismic exploration activities, which commonly use air guns as an impulsive sound source, to investigate sub-bottom structure. Recent literature has reported on the environmental implications of marine seismic surveys, highlighting alarmed and avoidance behaviour in several marine mammal species as a common response to air gun signals. The European Habitats Directive (EC: 92/43/EEC) orders the protection of all cetacean species in European waters, 23 of which inhabit Irish waters for at least part of the year. As such, underwater acoustic monitoring and mapping has been targeted as a research priority under the European Union's Marine Strategy Framework Directive. The findings will be essential for designing and implementing appropriate regulation regarding underwater noise. A collection of sound propagation codes have been made freely available through the Centre for Marine Science and Technology’s Acoustic Toolbox User Interface and Post-Processor (AcTUP) at Curtin University. In this study, RAMGeo (available in AcTUP), a range-dependent technique which employs the Parabolic Equation (PE), was used to model the transmission loss (TL) of low frequency noise across real, range-dependent 2D source-to-receiver sections from the Porcupine Basin. The TL values computed in the model are validated against their corresponding observed values, which were measured during a research cruise in July 2014, to coincide with 3D seismic operations in the area at locations across the Basin. Based on discrepancies arising from validation results, the geoacoustic environment is adjusted for a closer approximation to the behaviour of the real environment. In doing this, the sound propagation model is refined and then calibrated for the Porcupine Basin. The findings and limitations of the study are evaluated before implications of the research and proposals for subsequent work are outlined.