Quantifying seismic survey reverberation off the Alaskan North Slope

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.

Acoustic tracking of sperm whales in the Gulf of Alaska using a two-element vertical array and tags

“…First, a set of PSD spectra was computed over the 0.2-24 kHz frequency range using a 1024 point FFT size with 50% overlap, then all PSD spectra over a 0.5 s interval were averaged. This relatively short time interval was chosen to increase the odds that a sperm whale click would not be incorporated into the average because the click interval of a single sperm whale is generally greater than 0.5 s. The ambient noise level was then calculated by taking the minimum value of the summed pressure spectral density over the 2-10 kHz frequency band every 4 s (Guerra et al, 2011). By taking the minimum, PSD samples contaminated by sperm whale click spectra could be rejected.…”

Widespread spatial and temporal extent of anthropogenic noise across the northeastern Gulf of Mexico shelf ecosystem

“…Greene & Richardson 1988, Dragoset 1990, 2000, Caldwell & Dragoset 2000. In many instances, we found that the time be tween seismic pulses was occupied by a series of multiple arrivals of the same reverberated pulse immediately following the original (Guerra et al 2011(Guerra et al , 2016, thus inundating the soundscape with near-continuous elevated noise levels. Our study also illustrates that seismic airgun noise in the northern Gulf of Mexico propagated over a large spatial scale of several hundred kilometers, exposing a wide range of species and habitats to chronically elevated noise levels.…”

“…The resultant application of an acute sound exposure metric does not account for the chronic noise characteristics of reverberated and reflected seismic impulses after propagation over many 10s of kilometers or more (Guerra et al 2011(Guerra et al , 2016. Moreover, none of the currently implemented mitigation measures protect nonmammalian marine organisms, despite the evidence that supports the conclusion that noise exposure can both subtly and drastically affect their physiology or behavior (National Research Council 2003).…”

“…Environmental noise generated by precipitation and wind, anthropogenic activities, and biologically 271 Site LF band (10-500 Hz) MF band (500-1000 Hz) Since wind speed has been documented to influence ocean ambient noise spectra at varying depths (Guerra et al 2011), we correlated L eq averaged over 1 h time intervals with wind speed data for each frequency band at each recording site. Historic wind speed measurements were obtained from satellite data collected by NASA between 4 July 2010 and 31 December 2011 (http://opendap.jpl.nasa.…”

Acoustic vector sensor beamforming reduces masking from underwater industrial noise during passive monitoring

“…For example, vessel operations near an oil production island in the Beaufort Sea raised background levels by more than 20 dB over ambient levels (Blackwell and Greene, 2006). Similarly, reverberation from airgun pulses during seismic exploration increased background noise by 30-45 dB within 1 km of the seismic ship (Guerra et al, 2011).…”