Quantitative measures of air-gun pulses recorded on sperm whales (<i>Physeter macrocephalus</i>) using acoustic tags during controlled exposure experiments

TLDR

Powerful low‑frequency air‑gun pulses used for seismic seabed exploration raise concerns about negative effects on marine wildlife. This study quantified sound exposure levels recorded on acoustic tags attached to eight sperm whales at distances of 1.4–12.6 km from controlled air‑gun arrays in the Gulf of Mexico. Acoustic tags captured multiple pulses per firing due to multipath propagation, with received levels ranging from 131–167 dB re 1 µPa (pp) and 100–135 dB re 1 µPa (rms) after M‑weighting compensation. The recorded levels varied widely with range and depth, preventing reliable exposure‑zone estimation, and revealed that surface‑proximate whales received most energy between 0.3–3 kHz—frequencies far above those targeted by seismic exploration—highlighting a heightened risk to odontocetes with poor low‑frequency hearing.

Abstract

The widespread use of powerful, low-frequency air-gun pulses for seismic seabed exploration has raised concern about their potential negative effects on marine wildlife. Here, we quantify the sound exposure levels recorded on acoustic tags attached to eight sperm whales at ranges between 1.4 and 12.6km from controlled air-gun array sources operated in the Gulf of Mexico. Due to multipath propagation, the animals were exposed to multiple sound pulses during each firing of the array with received levels of analyzed pulses falling between 131–167dB re. 1μPa (pp) [111–147dB re. 1μPa (rms) and 100–135dB re. 1μPa2s] after compensation for hearing sensitivity using the M-weighting. Received levels varied widely with range and depth of the exposed animal precluding reliable estimation of exposure zones based on simple geometric spreading laws. When whales were close to the surface, the first arrivals of air-gun pulses contained most energy between 0.3 and 3kHz, a frequency range well beyond the normal frequencies of interest in seismic exploration. Therefore air-gun arrays can generate significant sound energy at frequencies many octaves higher than the frequencies of interest for seismic exploration, which increases concern of the potential impact on odontocetes with poor low frequency hearing.