The stable range-frequency interference structure （waveguide invariance） of the low frequency monostatic bottom reverberation in shallow water is derived theoretically in this pa- per by using ray-normal mode coherent reverberation model and separable bottom scattering model. And combined the concept of spectral subtraction, a reverberation forecasting and mitigating method based on the waveguide invariance is presented. According to the waveg- uide invariance, the reverberation intensity for a detecting range can be estimated from the reverberation returns scattered from some other range cells nearby the detecting range with frequency shift. Then the reverberation component for the detecting range in the echo can be canceled mostly by using the estimated reverberation intensity, so as to enhance the echo-to- reverberation ratio and improve the detection capability of the active sonar. Simulations in typical horizontal layered shallow water environment show the stability of waveguide invariance structure in the low frequency monostatic bottom reverberation and the efficiency of the reverberation forecasting and mitigating method based on the waveguide invariance.
Combined the decomposition of time reversal operator and the time reversal reverberation nulling, a new time reversal processing approach for echo-to-reverberation ratio enhancement is proposed. In this method, a 2-dimensional signal subspace for the range of the target and two bottom focusing weight vectors for the ranges near the target are obtained by the decomposition of time reversal operator. From the signal subspace and focusing weight vectors, a constrained optimal excitation weight vector of source receiver array can be deduced to null the acoustic energy on the corresponding bottom and maximize the energy at the tar- get. This method remedies the shortages of conventional time reversal processing, time reversal reverberation nulling and time reversal selective focusing method. It focuses sound energy at the target and nulls the energy at the bottom near the target range simultaneously, therefore enhancing the echo-to-reverberation ratio without probe source and prior-knowledge of the relative scattering intensity of target and bottom. Numerical simulations in typical shallow water environments showed the effectiveness of the proposed method and its improved performance for echo-reverberation enhancement than conventional time reversal processing.
