Abstract:

A four-sonobouy array was deployed 0.25 km offshore in southern California waters. The array was developed and deployed to passively locate coastal bottlenosed dolphins (Tursiops truncatus). Sonobouy mooring sites were selected to overlap areas shown by the Cetacean Behavior Laboratory at San Diego State University to be highly utilized by these coastal dolphins. Arrival-time-difference measurements were used to provide two-dimensional locations of underwater biological sounds such as dolphin vocalizations. Sonobouys were separated by a range of between 367 and 1585 m and the accuracy of source location depended on the relative proximity to the nearest sonobouy. Global Positioning Satellite and theodolite data were used to verify sonobouy locations and, in turn, the acoustic arrival-time differences of on-board calibration noises. Bottlenosed dolphin whistles, including mimics of the calibration sound, were among the biologically identifiable sounds tracked by the array. Other acoustic signals recorded but not located were snapping shrimp (Californiensis dentripes) and sources judged to be fish. Recorded sound levels varied over a considerable range, even on identical hydrophones, and were probably due to the signal transmission distance, shallow-water propagation effects, and diurnal fluctuations in the sources.