Abstract
Snapping shrimp are the dominant biological source of high-frequency (>2 kHz) ambient noise in warm coastal waters. In a recent experiment, the highly impulsive signals produced by shrimp snaps were recorded continually by a large-aperture vertical array (56 m) that was bottom-moored in 100-m deep shallow water. Assuming the array vertical, initial localization of individual snaps based on wavefront curvature along the array indicated that all snaps came from either above or beneath the flat seabed. By constraining all snaps to originate from the seabed, several hundred snaps within a radius of 500 m from the array over a 20-s window were detected successfully and localized in the three-dimensional space of time-of-arrival, range, and array tilt. Since the estimated array tilt for each snap is a projection of the absolute array tilt onto the nominal array-snap plane, the maximal tilt in the range and tilt domain corresponds to the absolute array tilt. Both simulations and data demonstrate that snapping shrimp can be exploited as a source of opportunity for calibration of vertical array tilt.
Highlights
Snapping shrimp are small creatures (e.g., 3–5 cm) that thrive in shallow waters usually less than 60 m deep and warmer than 11 C, forming colonies on or near the seabed in interstices of structures such as rock, debris, and coral.[1,2] A significant feature of snapping shrimp is one enlarged claw, which produces highly impulsive noise, known as snaps, via the collapse of cavitation bubbles during the rapid closure of their claws.[3]
highly impulsive signals produced by shrimp snaps were recorded
that was bottom-moored in 100-m deep shallow water
Summary
Snapping shrimp are small creatures (e.g., 3–5 cm) that thrive in shallow waters usually less than 60 m deep and warmer than 11 C, forming colonies on or near the seabed in interstices of structures such as rock, debris, and coral.[1,2] A significant feature of snapping shrimp is one enlarged claw, which produces highly impulsive noise, known as snaps, via the collapse of cavitation bubbles during the rapid closure of their claws.[3]. A snap reflected off the seabed contains information about sediment properties.[11] These applications require accurate localization of individual snaps or events. In our recent acoustic experiment, unexpected snapping shrimp noise was recorded continuously by a large-aperture vertical array (56 m) that was deployed in about 100-m deep shallow water with a relatively flat sandy bottom.[12]. The wavefront curvature along the vertical array allows for individual snaps within a radius of about 500 m (i.e., in the near field) to be localized in the two-dimensional (2-D) ambiguity surface of range and depth, similar to matched-field processing.[13]. Assuming that snapping shrimp live on the flat seabed, snaps can be localized instead in the 2-D ambiguity surface of range and array tilt, with the primary focus on the array tilt. The objective of this paper is to demonstrate that snapping shrimp can serve as natural sound sources to extract vertical array tilt. IV, the localization results in the range and tilt domain are further enhanced by refinement post-processing using both simulations and data, from which the vertical array tilt is estimated
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