Using cross-spectral phase to characterize individual targets in clutter: Application to broadband echosounders

A method is presented to distinguish single finite targets from acoustic clutter, that is, unresolvable echoes associated with multiple targets. It exploits frequency-dependent interference of multiple targets and the resultant frequency-dependent variation of cross-spectral phase to differentiate between echoes from single targets and coincident echoes from multiple targets. The method is demonstrated, in the context of split-beam echosounders, through mathematical modeling of the cross-spectral phase for an arbitrary number of targets as well as through laboratory and at-sea experiments on elastic spheres. Fresnel zone and resonance effects are used to interpret the findings, providing insights into the connection between the underlying scattering physics and the effects of coincident echoes on cross-spectral phase. False acceptance rates of the method are then calculated and compared with those of existing approaches in the literature. Finally, the method is applied to infer the in situ target strengths of individual gas-bearing organisms.