Detection of forward scattering from an intruder in a dynamic littoral environment

Forward scattering always occurs when an intruder moves into an underwater area between a source and a receiver. The detection of this forward-scattered signal is a challenge because a strong direct blast overwhelms the forward-scattered signal. Accordingly, an adaptive scheme for detecting the forward scattering introduced by an intruder in dynamic shallow water is proposed in this study. A short pulse was transmitted repeatedly in the water. The recorded direct blast and forward-scattered signal were assumed to be weakly correlated. A segmented adaptive method for the cancellation of the direct blast was applied to different direct arrivals from the transmitter. A cumulative index was defined based on the error sequence of the adaptive filter. The results of the shallow water experiment showed that the cumulative index level increased by up to 10 dB above the background level when the intruder approached the source-receiver line. The time of each crossing event was also determined.