Real time sperm whale depth estimation using passive acoustics

Abstract

Sperm whales (Physeter macrocephalus) make series of transient echolocation sounds (clicks), when foraging. Clicks reflect to the sea surface and the seafloor when propagating towards a receiver. The detection of the once reflected surface and seafloor echoes of a click, and the measurement of the delays of both echoes to the direct path transmitted signal, make possible the localization in depth and in range of the sperm whale which emitted the click, by using a single receiver. Repeating this process click after click then leads to the plotting of the whale depth/range variations while diving. The main difficulty when automating this process is usually to correctly identify direct path signals and echoes. The authors compute the a priori probability density functions of both surface/floor echo delays, improving this click echo identification process. The authors then use an adaptive filter, detecting simultaneously direct path signals with surface echoes. A visual estimation of the range of the whale when starting a dive is used to initiate the detection process. The detection of seafloor echoes then leads to the depth/range estimation of the whale while diving. The detection process works fine (84%, considering a 45-minute dive recording) assuming that a single sperm whale is clicking. The localization process works correctly when seafloor echoes are detected. This method is non-invasive, as it uses neither tags nor active acoustics, and easy to set, as requiring a single hydrophone and a CTD meter, and makes possible an unbiased, inexpensive, automated survey of the diving behavior of single sperm whales in a given area.