Spatial Coherence of Seabed Volume Reverberation

Acoustic returns scattered from either surfaces or objects consist of a coherent field and anincoherent field. The coherent field is associated with scattering from points and edges or fromsurfaces smooth relative to a wavelength, and the resulting received field is highly correlated withthe transmitted signal. The incoherent field is typically associated with scattering from surfacesthat are rough relative to a wavelength, and the received field is uncorrelated with the transmittedsignal. For a single transmission, the coherence of the field across frequency or space is dependenton the detailed structure of the ensonified surface. This program extend prior models based on thetheory of partial coherence developed within statistical optics in order to better predict thecoherence of the field scattered from both the natural and manmade objects.The coherence of the acoustic field within synthetic aperture sonar imagery has been studied as amechanism for the detection of underwater objects. This prior work has found that features relatedto coherence may be extracted from Synthetic Aperture Sonar (SAS) imagery and used within anAutomated Target Recognition (ATR) algorithm. The current work in this area has demonstratedthe utility of coherence for the development of ATR features; however, it has not addressed thephysical principles describing how coherence is impacted for near-field geometries. The researchproposed here will develop physical models describing coherence within volumetric imaging SASdata. These models can then be used to provide a physics-based approach to extraction ofcoherence features. This approach may produce more robust performance estimation since thefeatures extracted are based on the physical principles of the underlying data.