DLP Webinar: Near-Field Measurements Versus Far-Field Estimations of Air Gun Array Sound Pressure Levels
|Instructor:||Philip M. Fontana|
|Duration:||40 min + Q&A|
|Main topics:||Measured levels vs published sound pressure (SPL) and sound exposure (SEL) guidelines used by regulators|
During the past 25 years or so there has been steadily increasing concerns about potential impacts from sound levels emitted by seismic survey air gun arrays on marine fauna, cetaceans in particular.
These concerns have spurred increasing activity in research efforts to characterize the sound fields generated by air gun arrays and the propagation of the emitted acoustic energy in the marine environment local to a seismic survey.
In parallel, many government agencies responsible for granting seismic survey permits have included acoustic monitoring and mitigation procedures as part of the permit requirements. Most permit applications require a description of the seismic source and an estimate of the expected acoustic output from the source. Typically, the acoustic source levels submitted with the applications are derived from computer model estimates of the Sound Pressure Levels (SPL) generated in the far-field of a specific air gun array design. These levels are then often used as input to compute monitoring and mitigation parameters.
This approach has an inherent set of problems in that the estimated far-field SPLs are then extrapolated back to a virtual receiver point 1m from a virtual point source. The key word is virtual and the problem is that the quoted acoustic levels for the virtual point source never actually occur in the environment. The reason is that an air gun array, by design, has a spatial distribution of elements that do not behave as a point source emitter at 1m from the geometric center of the array. Therefore, the point source estimate only becomes valid at some distance away from the center of the array where the arrivals from each of the individual array elements add together coherently and appear as if the total energy is emanating from a single point source emitter at a distance. In words that is the definition of the far-field of an array. In general terms the far-field distance can be computed as:
d= (f * a2)/cWhere f is frequency in Hz; a is the greatest spatial dimension of the array; c is the speed of sound in water (Richardson et al, 1995).
Distances of d and greater are referred to as the far-field of the array. Distances less than d are referred to as the near-field of the array.
Notice that the far-field distance is a function of frequency, the higher the frequency the greater the far-field distance criteria. Therefore the actual overall bandwidth of air gun emissions becomes an important component to consider.
In this lecture we will present a set of absolute SPL measurements attained from calibrated near-field hydrophones mounted in proximity to each element in an air gun array. From these measurements we will document the maximum SPL generated by each element in an array and in the near-field of a full array in a production environment. We will then use those values to extrapolate SPL and estimated SEL to the far-field for various source configurations in different shooting scenarios such as sequential, overlapping, and simultaneous.
As an integration of SPL over time, the SEL issue is interesting in addressing questions regarding the environmental and geophysical issues resulting from shooting smaller sources more often.
Specifically interesting for operational geophysicists and HSE advisors in E&P and seismic contractor companies.
About the Lecturer
Phil M. Fontana holds BS and MS degrees in Geological Sciences
from the University of Connecticut in the USA.
Has held the position of Chief Geophysicist at Polarcus, now PXGEO, since December 2008. Prior to that, starting in 1984, he has held senior technical positions in support of marine acquisition at Western Geophysical, WesternGeco, Veritas DGC, and CGGVeritas.