Effects of reverberation on estimates of synthetic aperture sonar multi-look coherence

Abstract

Multi-look coherence makes use of phase information for target detection and classification by splitting the angle and frequency spectral bandwidth contained in a complex synthetic aperture sonar image into sub-bands and then estimating the coherence between the images formed from these sub-bands. Based on experimental results, it appears to be possible to separate man-made targets from interfering reverberation and clutter using coherence, as targets have features that scatter coherently as a function of angle or frequency. Characteristics of a target object may also be inferred, as sub-band coherence is a sensitive function of both angle and frequency. The expectation operation performed to obtain estimates of the complex correlation coefficient is evaluated using a spatial average, which lowers the spatial resolution of the resulting coherence map. The final resolution of a coherence image will be a function of the original image resolution, the number of sub-look images formed from the original image and the number of samples used in the expectation. In this work, we explore the effects of signal, reverberation, and noise levels on estimates of coherence using experimental data. Trade-offs in terms of signal-to-reverberation, expectation window size, and the number of sub-looks will be presented.