Snapping shrimp: measuring their natural distribution in space and time with low frequency arrays

In shallow tropical and subtropical waters, the ambient is often dominated by biological noise from snapping shrimp. This noise can severely limit various acoustic devices that operate between 1 kHz and several hundred kHz. There are other applications that rely on this noise as an incoherent signal for imaging. In either case, there is interest in a better characterization of the fundamental properties of the source of this noise. In particular, it would be of interest to experimentally determine its natural spatial-temporal distribution. There is an opportunity to measure these characteristics with existing horizontal arrays designed and deployed for other purposes. Arrays that are designed for low frequency operation but that record data up to a frequency of 1 kHz or more are well suited for this purpose. Because they are long, they can potentially provide very high-resolution images of sound sources close to the array. We explore the potential of using broadband fully focused beamforming with such arrays to localize nearby shrimp snaps in space and time. Depth of field and cross-range resolution for typical arrays are calculated. For some arrays we expect to be able to achieve a spatial resolution of about 2 m/spl times/10 m over the area within several hundred meters of the array. We believe this is adequate to resolve the locations of individual nearby shrimp. These expectations are compared to data taken in the Timor Sea with a multi-frequency towed array in a water depth of about 100 m. Data were recorded up to a frequency of 1200 Hz. The data clearly contain impulsive noise, highly suggestive of snapping shrimp, which dominate the ambient level above 600 Hz. We find that fully focused beamforming can clearly identify individual snaps and localize them in two dimensions. Achieved resolution is about a factor of two less than theoretical in both the cross-range and range dimensions. A preliminary count yielded a density of about 70 snaps/km/sup 2//sec in one particular region.