Source range and depth estimation of propeller cavitation bubble collapse transients in a multipath environment

Abstract

An experiment is conducted in which a wide aperture array, which is proximate to the sea floor in water 20 m deep, records transits of two motorboats. Occasionally, acoustic transients, which are attributed to collapsing propeller cavitation bubbles, are observed to be superimposed on the continuous broadband radiated noise component of each of the array’s three widely spaced hydrophones. By measuring the time differences of arrival of the transient signals propagating via the direct path at adjacent pairs of hydrophones, passive ranging by wavefront curvature is used to estimate the range and bearing of the transient source. Next, the source depth is estimated by measuring the multipath delay of the surface reflected transient signal. Also, the spectrogram of a single hydrophone’s output is observed to display a Lloyd’s mirror interference pattern due to the direct propagation path of the broadband radiated noise combining with the indirect propagation path signal reflected from the sea floor. The reciprocal of the frequency difference between adjacent destructive interference fringe minima is equal to the multipath time delay. This multipath delay, which equates to the quefrency of the cepstrum’s first rahmonic, contains source range information. The results of the two passive ranging methods are presented.