Estimating the location of baleen whale calls using dual streamers to support mitigation procedures in seismic reflection surveys

In order to mitigate against possible impacts of seismic surveys on baleen whales it is important to know as much as possible about the presence of whales within the vicinity of seismic operations. This study expands on previous work that analyzes single seismic streamer data to locate nearby calling baleen whales with a grid search method that utilizes the propagation angles and relative arrival times of received signals along the streamer. Three dimensional seismic reflection surveys use multiple towed hydrophone arrays for imaging the structure beneath the seafloor, providing an opportunity to significantly improve the uncertainty associated with streamer-generated call locations. All seismic surveys utilizing airguns conduct visual marine mammal monitoring surveys concurrent with the experiment, with powering-down of seismic source if a marine mammal is observed within the exposure zone. This study utilizes data from power-down periods of a seismic experiment conducted with two 8-km long seismic hydrophone arrays by the R/V Marcus G. Langseth near Alaska in summer 2011. Simulated and experiment data demonstrate that a single streamer can be utilized to resolve left-right ambiguity because the streamer is rarely perfectly straight in a field setting, but dual streamers provides significantly improved locations. Both methods represent a dramatic improvement over the existing Passive Acoustic Monitoring (PAM) system for detecting low frequency baleen whale calls, with ~60 calls detected utilizing the seismic streamers, zero of which were detected using the current R/V Langseth PAM system. Furthermore, this method has the potential to be utilized not only for improving mitigation processes, but also for studying baleen whale behavior within the vicinity of seismic operations

Summary of the acoustic and visual detections on July 29, 2011.

<p>Summary of the acoustic and visual detections on July 29, 2011.</p

Simulation location error study.

<p>(a) Source-to-array configurations used in the simulations of broadside and oblique sources, (b) Broadside location error, (c) Oblique location error. The solid lines and the dashed lines show the high and low SNR results, respectively.</p

Number of detected baleen whales by visual monitoring, R/V <i>Langseth</i> PAM array, and streamer array.

<p>Number of detected baleen whales by visual monitoring, R/V <i>Langseth</i> PAM array, and streamer array.</p

R/V <i>Langseth</i> towing configuration.

<p>Data is acquired using two seismic streamers which are 8-km long and comprised of 636 hydrophone channels.</p

Summary of the acoustic and visual detections on July 31, 2011.

<p>Summary of the acoustic and visual detections on July 31, 2011.</p

Summary of all the acoustic detections using the streamers data.

<p>Summary of all the acoustic detections using the streamers data.</p

Location results on July 27, 2011.

<p>(a) Spatial distribution in Universal Transverse Mercator (UTM) coordinates of the calls recorded on July 27, 2011 relative to a reference point located in zone 5 at (-44, 6372) km. This figure has the same format as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0171115#pone.0171115.g007" target="_blank">Fig 7A</a>. (b) Spectrogram, in units of decibels re 1 μ<i>Pa</i>, of three whale calls (visible at 2 s at around 60 Hz) recorded on July 27, 2011 at 15:50 UTC concurrent with a full power airgun shot, (c) Spectrogram, in units of decibels re 1 μ<i>Pa</i>, of three whale calls (visible at 4 s, 8 s and 18 s at around 60 Hz) recorded on July 27, 2011 at 15:51 UTC when the airgun array was in a transient state to mitigation gun which started at 15:52 UTC.</p

Location results on July 31, 2011.

<p>(a) Spatial distribution in Universal Transverse Mercator (UTM) coordinates of the calls recorded on July 31, 2011 relative to a reference point located in zone 5 at (-165, 6160) km. This figure has the same format as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0171115#pone.0171115.g007" target="_blank">Fig 7A</a>. (b) Spectrogram in units of decibels (dB) re 1 μ<i>Pa</i> of a whale call (visible at 10 s and around 50 Hz) recorded on July 31, 2011 at 18:35.</p

A block-diagram description of travel time residual localization method.

<p>A block-diagram description of travel time residual localization method.</p