Localizing Snapping Shrimp Noise Using a Small-Aperture Array

Abstract

In warm shallow waters, one can consistently hear the crackling sound due to snapping shrimp. An individual sound or “snap” is highly impulsive. Knowing the source locations of these snaps facilitates the development of some unique passive sensing applications. One common approach in localizing these snaps is to calculate the time difference of arrival (TDoA) of snaps across sensors of an array. For a snap originating from a short distance away, in terms of a few multiples of the array aperture size, the signal from the snap forms a curved wavefront, such that both the direction of arrival (DoA) and the range of the snap can be obtained by studying the TDoA of the wavefront. However, this method is infeasible for long-range 3-D localization due to the need for large-aperture 3-D arrays. Smaller aperture arrays allow estimation of DoA for far-field sounds, but not the range. Since the ocean surface acts like an acoustic mirror that reflects the snaps, we can estimate the range of the snaps by measuring the TDoA between the direct arrivals and their surface reflections, even if the snaps occur in the far-field of the array. Given a set of perfectly associated direct and surface-reflected snaps, we derive a range estimator parameterized by nominal receiver depth and receiver orientation. In practice, we know these parameters only approximately. The problem is further complicated by the fact that snaps are acoustically similar to each other, and so associating a snap with its surface reflection can be difficult. We propose an algorithm that solves the problem of estimation of snap locations, snap associations, and receiver parameters, jointly. We verify the method through numerical simulation and through experimental results.