Article

Quantifying seismic survey reverberation off the Alaskan North Slope

Marine Physical Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0238, USA.
The Journal of the Acoustical Society of America (Impact Factor: 1.5). 11/2011; 130(5):3046-58. DOI: 10.1121/1.3628326
Source: PubMed

ABSTRACT

Shallow-water airgun survey activities off the North Slope of Alaska generate impulsive sounds that are the focus of much regulatory attention. Reverberation from repetitive airgun shots, however, can also increase background noise levels, which can decrease the detection range of nearby passive acoustic monitoring (PAM) systems. Typical acoustic metrics for impulsive signals provide no quantitative information about reverberation or its relative effect on the ambient acoustic environment. Here, two conservative metrics are defined for quantifying reverberation: a minimum level metric measures reverberation levels that exist between airgun pulse arrivals, while a reverberation metric estimates the relative magnitude of reverberation vs expected ambient levels in the hypothetical absence of airgun activity, using satellite-measured wind data. The metrics are applied to acoustic data measured by autonomous recorders in the Alaskan Beaufort Sea in 2008 and demonstrate how seismic surveys can increase the background noise over natural ambient levels by 30-45 dB within 1 km of the activity, by 10-25 dB within 15 km of the activity, and by a few dB at 128 km range. These results suggest that shallow-water reverberation would reduce the performance of nearby PAM systems when monitoring for marine mammals within a few kilometers of shallow-water seismic surveys.

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    • "The result has been hypothesized to cause acoustic masking and reduce communication space in marine mammals (Clark et al., 2009a,b). However, few studies have provided quantitative measurements and analyses on the sound field characteristics and ambient noise increases resulting from airgun pulse propagation (Guerra et al., 2011). The deficiency of our knowledge on the reverberant field of seismic pulses may be partly due to the lack of a method for quantifying the seismic inter-pulse sound field. "
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    • "Using month-long readings of wind speed, as collected every 10 min by selected offshore buoys (operated by the National Data Buoy Center http://www.ndbc.noaa.gov/) in close proximity to each of the underwater MARUs, we examined the underwater signature of this abiotic source. Since quantification of the relative contributions from wind is still an ongoing area of research (e.g., Guerra et al., 2011), we use here an empirical approach to qualitatively inspect the temporal and spatial variation in wind and its impact on the spectral characteristics of noise, as displayed on the spectrograms. Table 1 MARU deployment information for the geographical locations used in this study. "
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