Paper Summary
The pressure wavefields emitted by air gun sources in a marine seismic survey are formally measured as a variety of received sound levels for observation points proximal and distal to the source array. Depending upon the local regulations specific to a survey area, the modelled source output, and sometimes the measured sound levels, the operation of seismic sources may be subject to threshold-based operating restrictions in terms of seasonal windows, survey exclusion areas, or marine mammal reduced power / shutdown radii from the source location. This document attempts to explain how seismic sound levels are modelled, measured, understood, and managed. For those familiar with the basic mechanics of seismic surveys, I provide a general technical reference for the nomenclature and physical fundamentals relevant to the propagation of sound through water, and how sound levels are measured in the context of how they may impact marine fauna. The three-dimensional sound wavefield emitted by an array of air guns is explained for different locations of received sound as a function of distance from the center of the source array, azimuth with respect to the vessel sailing direction, and vertical emission angle with respect to the horizontal plane. These fundamentals are then briefly used to explain the key considerations required by pre-survey modeling of received sound levels appropriate to specific survey conditions and stakeholder sensitivities. This document is much longer than my typical general interest articles and is designed as a white paper type of reference for future articles of environmental significance of marine seismic surveys. A key takeaway is that received sound levels can be accurately modelled for location-specific survey conditions (water depth, bathymetry, etc.), and such methods build upon several decades of calibrated measurements. A reliable platform to describe and understand the acoustic properties of seismic surveys is essential for any transparent dialogue with stakeholders.