Abstract: A method of filtering an acoustic B-scan (500) for passive detection of an object (106, 108) underwater comprising ensonifying (400) a region (104) of an underwater environment (102) and receiving (402) acoustic signals from the ensonified region (104) of the underwater environment (102), the received acoustic signals corresponding to a plurality of sonar beams. The method also comprises generating (404) the acoustic B-scan (500) from the received acoustic signals and pre-processing (406-414) the acoustic B-scan (500) to remove historic artefacts and mitigate influence of reverberant energy. Energy content is then scored (416) in respect of the pre-processed acoustic B-scan (524) to provide a plurality of energy scores and at least one local maximum (200) of the plurality of energy scores is; identified (418). A predetermined criterion is then applied (808) to a local maximum (700) of the at least one local maximum (700) identified.

Abstract: A method of pre-processing acoustic signals received from an ensonified region (104) of an underwater environment (102) for passive detection of an object (106, 108) in the underwater environment (102). The method comprises generating (404) an acoustic B-scan (500) in respect of a predetermined time frame, and selecting (406) a temporal portion (502) of the acoustic B-scan (500) in accordance with a predetermined selection criterion. The method also comprises compressing (408) the selected temporal portion (502) of the acoustic B-scan (500) to provide a compressed acoustic B-scan (510), and applying (410) a B-scan background mask (512) to the compressed acoustic B-scan (510) to suppress any historic reflectivity samples of the compressed acoustic B-scan (510) and to provide an extant compressed acoustic B-scan (524), A dynamic range of the extant compressed acoustic B-scan (524, 600, 602, 604) is then suppressed (412).