PASSIVE SENSING WITH SNAPPING SHRIMP NOISE

Abstract

Snapping shrimp dominate the high frequency soundscape by producing transient impulsive signals known as "snaps". Studies have shown that by estimating the spatial and temporal distribution of snaps we can monitor the health of coral reefs and conduct passive imaging on underwater structures and objects. These sensing applications rely on the ability to estimate the location of individual snaps accurately. Conventional methods estimate location of snaps in the near-field. The detectable range of the snaps is limited by the aperture size of the receiver. In a previous work, researchers have attempted to estimate the 3D location of snaps and their occurrence times by assuming known bathymetric data. However this assumption is not always tenable, and in reality, the exact knowledge of the bathymetry is difficult to obtain.

The goal of this thesis is to explore the use of both direct and surface-reflected snaps to localize far-field snapping shrimp noise for passive sensing, without assuming a priori knowledge of the bathymetry. Typically, a receiver receives signals from multiple snap sources with similar acoustic signatures, therefore it is not feasible to unambiguously distinguish the time difference of arrival (TDoA) in the sensor array data for DoA and range estimation purposes. The situation is further complicated by the fact that the signal propagating path consists of partially unknown parameters. We approach the problem by proposing a robust technique to detect the direction of arrival and time of arrival (DoA-ToA) of impulsive transient signals, by suggesting an algorithm which alternates the arrival association with parameter estimation, in order to estimate the location of snaps based on an ensemble of DoA-ToAs.

In this thesis, the techniques developed allow us to estimate the location of snapping shrimp, together with the time occurrence of their snaps, for a range of up to a few hundred meters away from the receiver. This method was tested in Singapore waters using a receiver measuring 1.3 m in diameter. The estimated locations of snapping shrimp allow us to passively image underwater man-made structures, and the estimated variations in water-depth and receiver orientation are shown to be consistent with tidal variations as reported in the Singapore tide-tables. This method paves the way for using portable sized receivers to conduct large-area passive sensing with snapping shrimp noise in coastal waters.