Wideband Time - Reversal Imaging for Classification of an Elastic Target in an Acoustic

Time-reversal is addressed for imaging and classifying elastic targets situated in an acoustic waveguide. It is assumed that the target-sensor range is large relative to the channel depth. We first investigate the theory of wideband time-reversal imaging of an extended elastic target for which the target dimensions are large relative to the principal wavelengths. When performing time-reversal imaging one requires a forward model for propagation through the channel, and the quality of the resulting image may be used as a measure of the match between the modeled and actual (measured) channel parameters. It is demonstrated that the channel parameters associated with a given measurement may be determined via a genetic-algorithm (GA) search in parameter space employing a cost function based on the time-reversal image quality. Example GA channel-parameter-inversion results are presented for measured data. Target classification based on time-reversal imagery is also considered wherein the main focus is on classification-performance sensitivity to knowledge of the uncertain channel parameters. The classifier is implemented via a relevance-vector machine. We present what we believe to be the first example of time-reversal imaging of extended elastic targets in a water channel based on at-sea experimental data (not a controlled laboratory measurement).