Positioning accuracy of a large-aperture hydrophone array for sperm whale research

Abstract

Source positioning accuracy of a large-aperture (1 km) hydrophone array was investigated, using 2-D and 3-D algorithms. The array was used for positioning and SL determination of sperm whales off the coast of Northern Norway during the summers of 1997 and 1998 (see Mo/hl, this volume). Positioning accuracy is dependent on mainly four factors: (1) positioning accuracy of the hydrophones, (2) variation in sound velocity of the medium, (3) inaccuracies in the measurements of the time-of-arrival differences, and (4) choice of algorithm (errors associated with geometric over-simplification). Errors introduced by the factors (1) and (3) had small effects compared with errors introduced by the factors (2) and (4). The total estimated error varied considerably in different areas covered by the array and for different array configurations. Sound velocity showed only modest variation with depth, except for a −0.2-m/s per meter gradient in the 0–100-m depth interval caused by summer heating of the surface water. For a sound source close to the surface, this sound velocity gradient causes a shadow zone starting some hundred meters from the source. Such a shadow zone may lead to underestimates of source level estimates from 2-D algorithms. [Work supported by the Danish National Research Foundation.]