Airgun inter-pulse noise field during a seismic survey in an Arctic ultra shallow marine environment

Abstract

Offshore oil and gas exploration using seismic airguns generates intense underwater pulses that could cause marine mammal hearing impairment and/or behavioral disturbances. However, few studies have investigated the resulting multipath propagation and reverberation from airgun pulses. This research uses continuous acoustic recordings collected in the Arctic during a low-level open-water shallow marine seismic survey, to measure noise levels between airgun pulses. Two methods were used to quantify noise levels during these inter-pulse intervals. The first, based on calculating the root-mean-square sound pressure level in various sub-intervals, is referred to as the increment computation method, and the second, which employs the Hilbert transform to calculate instantaneous acoustic amplitudes, is referred to as the Hilbert transform method. Analyses using both methods yield similar results, showing that the inter-pulse sound field exceeds ambient noise levels by as much as 9 dB during relatively quiet conditions. Inter-pulse noise levels are also related to the source distance, probably due to the higher reverberant conditions of the very shallow water environment. These methods can be used to quantify acoustic environment impacts from anthropogenic transient noises (e.g., seismic pulses, impact pile driving, and sonar pings) and to address potential acoustic masking affecting marine mammals.

Source level measurements for harbor seals and implications for estimating communication space

“…More individuals may therefore experience masking from impulsive noise sources than experience noise-induced hearing loss. While impulsive noise is characterized by a sharp onset with rapid rise and fall times, propagation and reverberation of low-frequency components of air gun pulses across tens to thousands of kilometers can increase background noise levels even during the intervals between pulses (Greene and Richardson, 1988;Guerra et al, 2011;Nieukirk et al, 2012;Guan et al, 2015;Nowacek et al, 2015). Accurate predictions of auditory masking for seals, who are sensitive to such low-frequency sound, are thus necessary to inform effective noise management practices in an increasingly industrialized Arctic.…”

The influence of temporally varying noise from seismic air guns on the detection of underwater sounds by seals

“…Based on the particular air gun samples used in this example, 100 Hz noise levels returned to ambient within $7 s of the pulse received at 1 km, and within $10 s of the pulse received at 30 km. It is important to note that the time at which air gun received levels fall below background noise will vary greatly based on the source array, distance to the receiver, environment, current ambient conditions, and other factors, and in some cases there may not be a complete return to background levels between pulses (Guerra et al, 2011;Nieukirk et al, 2012;Guan et al, 2015;Nowacek et al, 2015). In this example, ambient noise levels were defined based on received levels $1 s before the impulse; if reverberation consistently elevated noise levels between consecutive pulses, these values provide an overestimate of ambient noise and a resulting underestimate of the extent of masking caused by the impulse.…”

“…Sounds received from air gun operations associated with oil and gas exploration vary dramatically (in both the frequency and the time domains) depending on characteristics of the seismic array, distance from the source, and a range of environmental parameters (Greene and Richardson, 1988). Seismic air guns are typically considered to be broadband, transient noise sources (Richardson et al, 1995), but at distances of tens to hundreds (and in some cases, thousands) of kilometers seismic noise can influence ambient noise levels, even during the intervals between pulses (Greene and Richardson, 1988;Guerra et al, 2011;Nieukirk et al, 2012;Guan et al, 2015;Nowacek et al, 2015). Despite awareness of the masking potential of impulsive noise sources, it remains unclear how masking probabilities should be estimated for animals exposed to seismic air gun surveys.…”

Multi-frequency Seafloor Characterization Using Seismic Sources of Opportunity

“…Depending on source array design, the number of sources and their relative geometries, seismic sources are known to emit sound at frequencies up to 20 kHz (e.g. Tashmukhambetov et al, 2008;Guan et al, 2015;Banda and Blondel, 2016). The large range of frequencies occurring induces other challenges in interpreting reflections of seismic airguns, used as sources of opportunities.…”