Beam pattern estimation of clicks of a free-ranging Ganges river dolphin

Abstract

The first recordings of bio-sonar clicks of free-ranging Ganges river dolphin (Platanista gangetica) were made in Budhabalanga river in Orissa, India. The system used for recording is a 3.2 meter long array composed of three hydrophones forming an equispaced linear SBL array and another two hydrophones in conjunction with the central hydrophone forming a small SSBL triangular array in a plane perpendicular to the array axis. The array structure was deployed both in horizontal and vertical configurations. The clicks of the dolphin received on each hydrophone were sampled at a rate of 500 kHz per channel. From the sampled clicks data, the sonar transmit beam pattern of the dolphin in both horizontal and vertical plane was estimated using difference of ASL (apparent source level of the click signal) and the absolute angles of the dolphin's location between the central hydrophone and the other two peripheral hydrophones for the on-axis clicks. To estimate more accurate beam pattern, conditions for selection of on-axis clicks were well-considered, and thus we propose a new criteria for determining on-axis clicks. In this method, we assume that the dolphin's beam pointing direction is towards the central hydrophone and it's beam pattern is symmetrical. However, actual measurement of ASL from three hydrophones shows that values at the peripheral hydrophones are not always same. This suggests that the dolphin's beam is not pointing exactly towards the central hydrophone so that the central hydrophone's ASL is not the real peak value of the beam. We propose a refinement in the beam pattern estimation method using interpolation and curve fitting to obtain an improved estimate of the Ganges river dolphin's beam pattern. By analyzing all available clicks, we have estimated the dolphin's beam pattern in both horizontal and vertical plane. The 3-dB beamwidth is found to be approximately 10 degrees in horizontal plane and 14 degrees in vertical plane. In this paper, we propose the new beam pattern estimation method using interpolation and curve fitting.