How DIFAR sensors can enhance detection and 2-D localization of impulsive fish sounds on coral reefs

The impulsive sounds produced by tropical fish are a prominent component of coral reef acoustic environments off Hawaii. The resultant ambient noise field is highly nonstationary, making it difficult to equalize the noise background when implementing standard intensity-based detectors on conventional hydrophones. Here we demonstrate how DIFAR sensors can be used to enhance the contrast between transient fish signals and background ambient noise, permitting simultaneous detection and triangulation of individual pulses. This approach assigns an azimuth to each time-frequency component of a conventional spectrogram, by computing the arctangent of the active intensity measured on two orthogonal axes. The resulting “azigram” can be processed using standard image processing methods to isolate connected regions that share the same azimuth, and to match similar regions on azigrams from nearby DIFAR sensors. The cross-matched bearings can then be used to triangulate the source. The technique is being used to study “hotspots” of fish activity on coral reef pinnacles.