Applying double-difference methods for fine-scale acoustic tracking of controlled sources and sperm whales using a small aperture vertical array

Abstract

Ray propagation modeling can estimate a source’s depth and range in a waveguide by exploiting multipath arrival information on a vertical array. However, environmental mismatch of the model, array tilt, and limited angular resolution of an array can yield highly scattered dive trajectories when ray tracing individual events. “Double-difference” methods have been used to localize earthquakes (Waldhauser and Ellsworth, 2000) and fin whales (Wilcock, 2012) by determining the location of multiple events relative to each other, rather than their absolute position. These same concepts can be reformulated into a “triple-difference” approach to track successive acoustic events on a single multi-hydrophone array. This method examines relative changes in the multipath arrival times and elevation angles over the course of a dive in order to establish a more robust track in terms of relative positions along the trajectory. Presented here are results of applying this new technique on both a towed source and sperm whales, using acoustic data recorded on a short aperture vertical array off the coast of Southern California in 4 km deep water. [Work supported by Office of Naval Research—Marine Mammals and Biology and Ocean Acoustics Program.]