Directional frequency and recording (DIFAR) sensors in seafloor recorders to locate calling bowhead whales during their fall migration

Abstract

Bowhead whales, Balaena mysticetus, migrate west during fall approximately 10-75 km off the north coast of Alaska, passing the petroleum developments around Prudhoe Bay. Oil production operations on an artificial island 5 km offshore create sounds heard by some whales. As part of an effort to assess whether migrating whales deflect farther offshore at times with high industrial noise, an acoustical approach was selected for localizing calling whales. The technique incorporated DIFAR (directional frequency and recording) sonobuoy techniques. An array of 11 DASARs (directional autonomous seafloor acoustic recorders) was built and installed with unit-to-unit separation of 5 km. When two or more DASARs detected the same call, the whale location was determined from the bearing intersections. This article describes the acoustic methods used to determine the locations of the calling bowhead whales and shows the types and precision of the data acquired. Calibration transmissions at GPS-measured times and locations provided measures of the individual DASAR clock drift and directional orientation. The standard error of the bearing measurements at distances of 3-4 km was approximately 1.35 degrees after corrections for gain imbalance in the two directional sensors. During 23 days in 2002, 10,587 bowhead calls were detected and 8383 were localized.

Acoustic vector sensor beamforming reduces masking from underwater industrial noise during passive monitoring

“…The suppression methods described above were tested on data collected by custom built Directional Autonomous Seafloor Acoustic Recorders, model C (DASAR-C) (Greene et al, 2004;Thode et al, 2012;Blackwell et al, 2013). DASARs are equipped with an omnidirectional hydrophone and two orthogonal accelerometers that measure the north-south and east-west components of acoustic particle velocity at frequencies between 10 and 500 Hz.…”

“…In the discussion that follows, all acoustic data (industrial noise and whale calls) were well above the instrument's self-noise floor. As explained in Greene et al (2004), the geographic orientation of the sensors on the ocean floor was determined by transmitting frequency-modulated (FM) sweeps, tones, and pseudo-random noise at a set of known locations surrounding the sensor. The derived azimuths relative to the sensors could then be rotated by a correction angle (the azimuth between true north and the sensor y axis) to yield the geographic azimuth of the steering angle h.…”

Software for real-time localization of baleen whale calls using directional sonobuoys: A case study on Antarctic blue whales

“…Directional frequency analysis and recording (DIFAR) sonobuoys have been in use in whale research programs for the past two decades (Blackwell et al, 2012;Blackwell et al, 2013;Gedamke and Robinson, 2010;Greene et al, 2004;Guerra et al, 2011;McDonald and Moore, 2002;McDonald, 2004;McDonald et al, 2001;Miller et al, 2014;Miller et al, 2015;Oleson et al, 2007;Rankin et al, 2005;Rivers, 1997;Sirović and Hildebrand, 2011;Swartz et al, 2003;Thode et al, 2000;Wade et al, 2006;Wade et al, 2011). These studies indicate that sonobuoys comprise an efficient means for synoptic-scale acoustic monitoring of baleen whales allowing for coverage of broad pelagic areas in relatively short time periods.…”

“…Bearings from a single DIFAR sonobuoy allow researchers to home in on vocalising animals from tens to hundreds of kilometres away (McDonald, 2004;Miller et al, 2015;Wade et al, 2006), while bearings to the same source received on multiple DIFAR sensors allow accurate triangulation of the source's geographic position (Blackwell et al, 2012;Greene et al, 2004;Miller et al, 2015). Furthermore, the acoustic bearings and calibrated received pressure levels from a single DIFAR sonobuoy facilitate identification of the sound source as well as estimation of the source level (Greene et al, 2004;McDonald et al, 2001;Rankin et al, 2005;Thode et al, 2000).…”

Listening to the Deep: Live monitoring of ocean noise and cetacean acoustic signals

“…The goal of such PAM systems, is the continuous mapping of presence and distribution of whales over ocean basins (e.g. Greene et al, 2004;Sirovic et al, 2007;Stafford et al, 2007;Mas et al, 2008) and assessing their densities, (e.g. Ko et al, 1986;McDonald and Fox, 1999;Clark and Ellison, 2000), sometimes in quasi real-time (e.g.…”