Combination of time-reversal focusing and nulling for detection of small targets in strong reverberation environments

Time reversal has attracted considerable research interest because of its potential for signal enhancement. In the past, target focusing and reverberation nulling are separately investigated as two complementary aspects of time-reversal processing. In essence, nulling is regarded as one kind of focusing with minimising the energy incident on a location. When time-reversal operation is regarded as inversed beamforming, it is available to simultaneously realise time-reversal focusing and nulling under an optimum beamforming framework. The feasibility of the combination processing is evaluated in detection of a small target in the presence of strong reverberation. The acoustic energy focused on a target of interest is maintained constant while reverberation is regarded as an interference to be suppressed by the nulling technique. The experimental results have verified the effectiveness of optimum time-reversal processing for active target detection in a shallow water environment.