Broadband spectra of seismic survey air-gun emissions, with reference to dolphin auditory thresholds

Abstract

Acoustic emissions from a 2120 cubic in air-gun array were recorded through a towed hydrophone assembly during an oil industry 2-D seismic survey off the West Wales Coast of the British Isles. Recorded seismic pulses were sampled, calibrated, and analyzed post-survey to investigate power levels of the pulses in the band 200 Hz-22 kHz at 750-m, 1-km, 2.2-km, and 8-km range from source. At 750-m range from source, seismic pulse power at the 200-Hz end of the spectrum was 140 dB re: 1 microPa2/Hz, and at the 20-kHz end of the spectrum seismic pulse power was 90 dB re: 1 microPa2/Hz. Although the background noise levels of the seismic recordings were far in excess of ambient, due to the proximity of engine, propeller, and flow sources of the ship towing the hydrophone, seismic power dominated the entire recorded bandwidth of 200 Hz-22 kHz at ranges of up to 2 km from the air-gun source. Even at 8-km range seismic power was still clearly in excess of the high background noise levels up to 8 kHz. Acoustic observations of common dolphins during preceding seismic surveys suggest that these animals avoided the immediate vicinity of the air-gun array while firing was in progress, i.e., localized disturbance occurred during seismic surveying. Although a general pattern of localized disturbance is suggested, one specific observation revealed that common dolphins were able to tolerate the seismic pulses at 1-km range from the air-gun array. Given the high broadband seismic pulse power levels across the entire recorded bandwidth, and known auditory thresholds for several dolphin species, we consider such seismic emissions to be clearly audible to dolphins across a bandwidth of tens on kilohertz, and at least out to 8-km range.

The effect of exposure to seismic prospecting on coral reef fishes

“…After being processed, these signals are used by researchers to generate a wealth of information about the topography of the ocean floor and its underlying geological formations. Given that the use of air guns is currently the most important method of seismic prospecting (Wardle et al, 2001;Gausland, 2003), several studies have investigated the environmental impact of the use of air guns, particularly on shrimp fisheries (Andriguetto-Filho et al, 2005), marine turtles (McCauley et al, 2000), and mammals (Richardson et al, 1986;Au et al, 1997;Richardson & Wursig, 1997;Goold & Fish, 1998;Gordon et al, 1998;McCauley, 1998;Schlundt et al, 2000;McCauley & Duncan, 2001). Relatively few studies have been conducted on fish, focusing mostly on how seismic prospecting affects fish abundance (e.g.…”

Environmental Assessment for a Marine Geophysical Survey of Parts of the Arctic Ocean, August-September 2010

“…In general, most of the energy in the sound pulses emitted by airgun arrays is at low frequencies; strongest spectrum levels are below 200 Hz, with considerably lower spectrum levels above 1,000 Hz, and smaller amounts of energy emitted up to ~150 kHz (Goold and Fish, 1998;Sodal, 1999;Goold and Coates, 2006;Potter and others, 2007).…”

Mitigation provisions to be implemented for marine seismic surveying in Latin America: a review based on fish and cetaceans

“…For a single airgun, peak energy is below 200 Hz on average. Although most energy for an array of airguns is between 10 and 200 Hz, it may extend to over 2200 Hz (Goold & Fish 1998). The total acoustic energy of a pulse generated by a single airgun or an array can be measured as the sound exposure level (SEL) in conjunction with other metrics such as rise time, peak amplitude, and crest factor.…”