Based on the fact that the transfer function vector between a source receiver array and the dominant scatterer of boundary reverberation at a range can be obtained from the corresponding reverberations scattered from this range cell,a reverberation nulling concept using time reversal processing has been proposed.However,current reverberation nulling methods have certain limitations when applied into practice,which would null boundary reverberation and target echo simultaneously.As a solution,a passive reverberation nulling and echo enhancement method at low frequency using waveguide invariance is proposed in this paper.In this method,the reverberation subspace for the target range cell is not obtained directly from the return signals scattered by the target range cell but from the return signals scattered by a range cell located before the target using waveguide invariance,so as to suppress the reverberation embodied in the target echo by passive reverberation nulling.Besides,a range-dependent optimal weighting vector rather than conventional projector matrix is deduced to null the reverberation component meanwhile maximizing the target echo,thereby enhancing the echo-to-reverberation ratio furthest.Numerical simulations in typical range-independent shallow water environment demonstrate the efficacy and the improved performance of the proposed method for echo-to-reverberation enhancement.
