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

Date Published: February 15, 2017

Publisher: Public Library of Science

Author(s): Shima H. Abadi, Maya Tolstoy, William S. D. Wilcock, Songhai Li.

http://doi.org/10.1371/journal.pone.0171115

Abstract

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.

Partial Text

Marine mammals use sound for their important life functions such as communicating, navigating, and finding food or a mate. Ocean noise pollution has increased greatly in recent years due to human activities in the ocean [1]. Seismic surveys are one of the more common high source-level anthropogenic sounds in the ocean. The high sound intensity of the airguns involved in seismic surveys has led to concerns over their effects on marine life [2]. Since seismic surveys use low frequency sound to image structure beneath the seafloor, the potential impact on baleen whales that communicate in the same frequency range may be particularly significant. It has been shown that bowhead whales exposed to seismic sources interrupt their normal activities and move away [3]. One study shows that bowhead whale calling rates near seismic operations increase initially at cumulative sound exposure level of ~94 dB re 1 μPa2-s, but then begin decreasing for values above ~127 dB re 1 μPa2-s until ~160 dB re 1 μPa2-s when whales are silent [4]. Avoidance behavior has also been seen in humpback whales [5] and possibly in blue whales [6].

The data utilized in this paper are from the Alaska Langseth Experiment to Understand the megaThrust (ALEUT) seismic reflection experiment [21] conducted by the R/V Marcus G. Langseth on cruise MGL1110, from 11 July 2011 until 5 August 2011. The aim of this cruise was to characterize the megathrust, the overriding and down-going plates, and other fault systems associated with the Alaska-Aleutian subduction zone. The survey plan included multi-channel seismic (MCS) survey lines, which were oriented both north to south and east to west in a wide range of water depths from <25 meters to >6000 m (Fig 1). Survey lines varied in length from approximately 20 to 400 kilometers.

To assess the performance of the sound source localization technique, we consider a 90-m-deep water channel that mimics the shallow water environment of Alaska. The normal mode propagation algorithm KRAKEN [23] is used to propagate an 800 ms chirp through a simple two-layer waveguide shown in Fig 4. Different bandwidths are selected to mimic the whale calls we discuss below in Section V.

The visual monitoring effort produced a total of 52 baleen whale detections while data was being recorded from the streamers. The focus of this study is on a subset of 25 baleen whale detections (Table 2) where the visual monitoring led to a mitigation action: powering down to the mitigation gun or completely powering off. The R/V Langseth PAM short array did not detect calls from any of the observed animals. It is likely that interference from low frequency ship noise limits the sensitivity of the PAM array for detecting baleen whale calls. To search for baleen whale calls in the streamer data, spectrograms were visually inspected for several elements at different positions along each streamer, for all the time periods that PSO’s reported whale sighting in their visual observations report and the airguns were either powered down or shut down. Table 2 summarizes the results of these efforts; calls were detected by the streamers for 40% of the visual detections indicating that individual elements in the seismic streamers are more sensitive than the PAM array, presumably because they are towed further away from the ship (Fig 2). Moreover, some of the calls recorded by the streamers have frequencies below the cutoff frequency of the PAM hydrophones.

The first event occurred on July 23, 2011 and comprised a prolonged sighting of a pair of fin whales followed by the detection of the extremely rare north pacific right whale and a humpback whale (Table 4). The pair of fin whales observed at 15:43 was initially sighted traveling antiparallel to the vessel while on a survey line with the source firing on full power. A power-down was implemented and shortly after the whales changed course to approach the vessel, following alongside the vessel, crossing back and forth under the vessel from side to side and approaching as close as 40 m to the side of the ship. The pair remained in the area of the vessel for over three hours. The north pacific right whale was observed at 17:23 blowing close to the airgun and triggered a complete shutdown. The north pacific right whale traveled parallel to the vessel for 47 minutes during which time several blows were observed in addition to the animal fluking and diving twice and pectoral fin slapping once. Over an hour later, a humpback whale appeared under the port streamer.

More than 50 baleen whale calls are localized using dual seismic streamers during a seismic reflection survey in Alaska. In general, there is not enough information documented by PSOs to examine the accuracy of the acoustic results. Implications of the observations are discussed below, along with areas that require more supporting information or further analysis for verification:

Dual streamer data are used to detect baleen whale calls during a seismic reflection survey in Alaska. The low frequency localization method developed in a previous study [20] was used to estimate the location of the recorded baleen whale calls during the mitigation process. The locations of ~60 whale calls were estimated in four days, all during the mitigation procedures. Overall, three main conclusions can be drawn from this study: 1) seismic streamers are more reliable tools than the current R/V Langseth PAM system for locating and monitoring vocalizing baleen whales in the vicinity of seismic operations; 2) a single seismic streamer can usually resolve left/right ambiguity because it is rarely perfectly straight, but dual streamers significantly improve the location results; and 3) the data set used in this study is not sufficiently large to be used to infer animal behavior study, although no striking behavioral changes are observed. However, streamer data have the potential to be used for evaluating mitigation processes and studying baleen whale responses to airguns.

 

Source:

http://doi.org/10.1371/journal.pone.0171115

 

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