Estimating received sound levels at the seafloor beneath seismic survey sources
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The offshore seismic survey industry typically uses arrays of devices called airguns to generate the intense, lowfrequency sounds required for imaging the seabed geology. Concerns have arisen about the impacts that these high intensity sounds may have on benthic species, such as shellfish, site attached fish and crustaceans, that have little or no capacity to move out of the way of an approaching seismic vessel. In order to assess the impacts a survey may have on these species it is essential to have a means of predicting the sound levels they are likely to be subjected to. This paper discusses the characteristics of the sound field beneath a typical seismic airgun array and compares results obtained using some simplistic formulae for predicting sound exposure level and peak sound pressure level, that are applicable in the acoustic far-field of the array, to those obtained using a more accurate model that includes array near-field effects. For the typical medium-sized 49.2 l (3000 in3) airgun array considered here the predicted near-field to far-field transition distance was 14.5 m and a simple equivalent point source model was found to over predict the sound exposure level by 1.1 dB and to over predict the peak sound pressure level by 3.3 dB at this distance below the array. At double this distance these errors had reduced to 0.6 dB and 1.4 dB respectively.
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