Double-difference tracking of migrating Bowhead Whales using autonomous vector sensors in the Beaufort Sea

Passive acoustic monitoring has become a standard method for detecting bowhead whale (Balaena mysticetus) activity in Arctic waters. Between 2007 and 2014, over 40 autonomous vector sensors, known as DASARs, were deployed in the Beaufort Sea during the bowhead whale migration season. Individual DASARs can estimate azimuth, allowing calls to be localized by triangulation using multiple DASARs. However, these bearings are subject to calibration biases, and individual sensors were not precisely time-synchronized, making relative time-of-arrival information unreliable for standard localization purposes. Double-difference methods have previously been applied in seismology to obtain high-precision relative positions of earthquakes by measuring changes in relative travel-times between multiple events over widely distributed seismic sensors. This same concept has also been used to track fin whales on a seafloor seismic network. Here, the double-difference method is applied to previously localized bowhead whale calls in order to improve their relative positions. The approach uses changes in both relative call travel-times and bearings, detected at multiple DASARs, to determine high-precision relative locations of these calls despite the presence of systemic timing and bearing errors in the measurements. The resulting positions may allow tracking of individual whales, which would provide insight into the function of these calls.