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Passive Acoustic Detection of Beaked Whales: Easy Within 1km, Harder Beyond That, and Take Your Time!

Science, Sonar Add comments

The US Navy and others are working hard to find ways to detect elusive beaked whales so that they may be less apt to be triggered into behaviors that seem to cause them to occasionally beach and die.  One “great white hope” is passive acoustic detection, or listening for their calls.  A recent paper from a team of well-known researchers suggests that we should be able to quite easily detect beaked whales while they are foraging (and echolocating)….but with several important caveats.  First, detection is highly likely only within 700 meters, with perhaps 50% likelihood of detection at 2-3km, and virtually no chance of hearing them beyond 4km.  Since there is a good chance that some of the avoidance behaviors occur at longer ranges, it’s unclear how effective listening can be in avoiding exposing beaked whales to sonar.  The other key caveats are that one would have to listen for at least two hours prior to sonar use in order to be sure to hear whales in the area (since they vocalize for only about 30 minutes out of up to two hours).  And, perhaps most important, a sonar vessel is an inappropriate listening platform, since it moves too fast through any one area to listen for long enough: a very slow-moving or stationary listening platform is essential for effective detection.

Here is a full lay summary of the research paper:

Zimmer, Harwood, Tyack, Johnson, Madsen. Passive acoustic detection of deep-diving beaked whales. The Journal of the Acoustical Society of America, November 2008, Volume 124, Issue 5, pp. 2823-2832.
Beaked whales spend very little time at the surface, with foraging dives that last an hour or more, including about 30 minutes of active echolocation at the feeding depth. Visual detection is very difficult, so the possibility of using Passive Acoustic Detection (PAD) is an attractive complement to visual spotting. However, these high-frequency clicks attenuate rapidly. According to this paper, acoustic modeling suggests that in “good conditions,”e.g. wind speed of 2 m/s, a hydrophone close to the surface should detect beaked whales with a high probability within .7km. At the other end of the detection range, no whales would be detected at distances greater than 4km, except in very low ambient noise or unusually good propagation conditions. The detection curve generated by the models, drops steeply as distance increases, with a relative leveling off that suggest roughly 50% detection when whales are 1.5-3.5km distant. Additionally, the researchers substituted some actual dive profile data obtained in D-Tag studies, and found that in these instances, detection probability may rise somewhat, with 80% detection being possible at distances of 1.5-2.5 km. Actual detection of real whales is complicated by the fact that the sound of the clicks is highly directional; only clicks directed nearly directly toward the receiver will be heard at the modeled distances, but echolocating whales do scan in many directions, so at least some clicks from any whale should be detected. Finally, the relatively quiet interval between deep foraging dives can be as long as 110 minutes, meaning that listening time should be roughly 140 minutes to have a high probability of detecting if beaked whales are present. This, in turn, suggests that a slow-moving vessel (such as gliders or drifting buoys) will be more successful than a fast-moving active sonar vessel. (That is, if detections are only going to occur within roughly 4km, the listening platform should not move out of a relatively similar area during the 2-hour-plus PAD session.) Ed. note: While it has still not been confirmed to all observers’ satisfaction, there is some evidence that beaked whales affected by mid-frequency active sonar may well have been tens of km from sonar vessels. Thus, PAD may be useful for avoiding nearby exposure, but may not fully mitigate for effects that could occur at greater distances, including fleeing in ways that disrupt the whales’ post-dive recovery and oxygen/nitrogen rebalancing. 

One Response to “Passive Acoustic Detection of Beaked Whales: Easy Within 1km, Harder Beyond That, and Take Your Time!”

  1. Gianni Pavan Says:

    Passive acoustic detection of beaked whales has been proven feasible with low-noise wide-band towed arrays. In a paper presented at Passive 08 conference last October, Zimmer & Pavan show the results obtained in the Alboran Sea (cruise SIRENA 08) by towing the PAD system developed at the University of Pavia (CIBRA – Italy). Acoustic detection largely outperformed visual observers. SIRENA 08 demonstrated the possibility to map the presence and distribution of Cuvier’s beaked whales in an area. Results match well with the model presented in the JASA paper.

    Context dependent detection/classification of Cuvier’s beaked whale (Ziphius cavirostris)
    Walter ZIMMER, NATO Undersea Research Centre
    Gianni PAVAN, CIBRA / Universita’ di Pavia