Acoustically mapping (fingerprinting) the main channel, or `Q` routes, of aarbor, over relatively long time periods by using transducers anchored near the harbor or Q route floor. Sonar pulse returns are processed so that echoes from acoustic targets are constantly monitored and integrated over long periods of time. The integrated sonar data are used to establish a characteristic bottom-image map for the presence and location of permanent objects such as debris, underwater formations and the like. The long term, integrated record is obtained to establish a reliable acoustic fingerprint or reference mapping of the bottom. This acoustical record or map, which is quite stable over long periods of time, is used to compare newly detected acoustic variances from the established map. A combination of active and passive acoustic sensors that are installed at fixed positions submerged in the channel permit the determination of both range and bearing when an intruding object has been detected.