Range estimation of cetaceans with compact volumetric arrays

Abstract

Passive acoustic monitoring is the method of choice to detect whales and dolphins that are acoustically active and to monitor their underwater behaviour. The NATO Science and Technology Organization Centre for Maritime Research and Experimentation has recently implemented a compact passive acoustic monitor (CPAM), consisting of three arrays of two hydrophones each that are combined in a fixed threedimensional arrangement and that may be towed at depths of more than 100m.With its volumetric configuration, the CPAM is capable of estimating the three-dimensional direction vector of arriving sounds and under certain conditions on relative geometry between the whale and hydrophone array, the CPAM may also estimate the range to echolocating animals. Basic ranging methods assume constant sound speed and apply straightforward geometry to obtain depth and distance to the sound source. Alternatively, ray-tracing based methods may be employed to integrate the information provided by real sound speed profiles. Both ranging methods combine measurements of sound arrival angles and surface reflection delays and are easily implemented in real-time applications, whereby one could promote the raytracing approach as the preferred method because it may integrate real sound speed profiles.