Development of a simplified ray path model for estimating the range and depth of vocalising marine mammals [abstract]

P.A. Lepper, K. Kaschner, P.R. Connelly and A.D. Goodson (1998). Development of a simplified ray path model for estimating the range and depth of vocalising marine mammals [abstract]. Bioacoustics, Volume 9 (3): 231 -232

As part of the EU funded CETASEL project recordings were made of small cetaceans on the edge of the continental shelf between SW Eire, Biscay and North Spain. Various passive acoustic systems were attached to a pelagic trawl operating in relatively deep water (200m). These systems were then used in conjunction with a surface observer program and remotely operated television systems to study small cetacean behaviour around a moving fishing net. A multiple hydrophone system has been developed to track echolocating cetaceans. Comparison of the arrival times of echolocation 'clicks' on a number of spatially separated hydrophones were made allowing estimation of the positioning of the cetacean in relation to the hydrophone array. Additional lower frequency (3-20 kHz) recordings were made on a single hydrophone also attached to the trawl. A number of examples of lower frequency 'whistles' signals including multi-path (bottom and surface reflections) were observed. With knowledge of the hydrophone and water depth, comparison of the arrival time of the various multi-path components of a signal has also allowed the estimation of the range and depth of its source from the receiver. A simplified ray path model has been developed to simulate various source, receiver geometrys. Arrival times of the multi-path signals were calculated and compared with those seen during sea trials. A number of assumptions have been made in initial models including a constant sound velocity depth profile and the treatment of the surface and seabed as a simple reflecting surfaces. Initial results have shown a number of examples with a reasonable correlation between estimated position of a submerged cetacean and the associated surface observations. Examples of multiple (positioning) solutions were however found, these are in the main due to imprecision in the knowledge of the hydrophone and water depth geometry and inaccuracies in the initial timing measurements. The use of correlation techniques and stand alone depth measurement devices is therefore proposed for future measurements and analysis using this technique. It is felt that within certain constraints this technique can provide valuable additional information regarding cetacean behaviour in the wild. The addition of more complex time measurement techniques and better ray path modelling will hopefully provide a useful analysis tool in the study of cetaceans.