Abstract

Broadband systems operating in warm shallow waters are often significantly affected by snapping shrimp (Alpheus & Synalpheus) that are reported to produce signals up to 300 kHz with source levels up to 190 dB re 1uPa @ 1m. Understanding the spatial and temporal distribution of these sources is crucial to their use either as insonifiers for Ambient Noise Imaging or to remove them as interfering noise for conventional sonar. The Acoustic Research Laboratory at the Tropical Marine Science Institute of Singapore has developed a highbandwidth, compact 3-dimensional acoustic array that can localise these sources in time and space. It is a self-contained broadband system with four acoustic recording channels, each sampled at 500kSa/s. The main advantage of the system lies in its compact, portable size, permitting it to be easily deployed in open waters far off shore. The overall array size is about 1m with a cylindrical electronics housing of less than 230mm diameter and 600mm length. The system has been used to map high frequency noise sources within an area of some 20,000 m2 from a single vantage point. An angular resolution of <1 degree has been achieved at the high end of the frequency range. The system has been deployed either as a surface-mounted (with spar buoy) or bottom-mounted (on a tripod) system with remote control capability over cable or as a standalone unit. The initial results show evidence of spatially clustered snapping. This paper presents the experimental results from deployments in 18-20m water depths among sparse reef patches approximately 1km offshore from the nearest islands and the estimated distribution of the snapping shrimp.