Applying double-difference methods for fine-scale acoustic tracking using a short aperture vertical array

Abstract

Ray tracing can estimate an acoustic source’s depth and range in a waveguide by exploiting multipath arrival information on a vertical array. However, environmental mismatch in the model or array tilt can yield highly scattered trajectories when ray tracing multiple events. “Double-difference” methods have been used to localize earthquakes (Waldhauser and Ellsworth, 2000) and fin whales (Wilcock, 2012) by determining the relative locations of multiple events, rather than their absolute positions. This approach, which exploits changes in relative travel times between events, has recently been reformulated to recover the dive trajectory of a source using a single multi-hydrophone vertical array, whenever three acoustic rays are available for each event. Here, the method is expanded such that changes in ray elevation angles between events can be used to reduce the number of rays required. This technique is tested on data recorded on a short aperture vertical array off the coast of Southern California in 4 km deep water. Trajectories from both a controlled towed source and sperm whale dives are examined. [Work supported by Office of Naval Research—Marine Mammals and Biology and Ocean Acoustics Program.]