Adaptive Reverberation Nulling Using a Time Reversal Mirror

The major problem for active sonar systems operating in shallow water is the detection of target in the presence of severe bottom reverberation. The focusing capability of a time reversal mirror (TRM) suggest two different approaches to this problem. First, a TRM focuses acoustic energy on a target enhancing the target echo while shadowing the boundaries below and above the focus in an acoustic waveguide, thereby reducing reverberation. The resulting echo-to-reverberation enhancement has been demonstrated experimentally in the 3-4 kHz band in shallow water. The second approach is reverberation nulling to enhance target detectability since focusing and nulling are complementary. The idea is to minimize the acoustic energy incident on the corresponding scattering interface by applying an excitation weight vector which is orthogonal to the time-gated reverberation focusing vector. Both numerical simulations and experimental results illustrate the potential of reverberation nulling using a time reversal mirror. Sponsored by ONR Contract No. N00014-01-D-0043-D06/D07 Problem Goal Active sonar systems operating in shallow water is to detect targets in the presence of severe bottom reverberation. The focusing capability of time reversal suggests two different approaches to this problem: echo-toreverberation enhancement and reverberation nulling. Demonstrate the concept of the two proposed approaches both numerically and experimentally in a shallow water. FAF-00: Echo enhancement experiment (bi-static) Experimental results including an artificial target show improved target detectability with the time reversal method. An air-filled tube was used as an artificial target. The broad area around 4.2 sec is the direct arrival from the SRA to VRA. An echo is visible around 4.52 sec using time reversal in (b). FAF-00 Reverberation Reduction Experiment Reverberation: Numerical Simulation Normal mode scattering model based on the perturbation method (Tracey and Schmidt, IEEE JOE, 22:317-331, 1887) (a) Experimental configuration and (b) enhancement in the ensonification level at the probe source location of 5 dB using time reversal (TR) over BS transmission filling the water column in (c). (d) confirms the existence of reverberation notch of about 3 dB depth at the probe source range with a TR transmission in addition to an overall reduction of 5 dB over BS transmission. FAF-03 Reverberation Nulling Experiment Reverberation Nulling: Numerical Simulation The TR retransmission produces about a 5 dB notch around 6.4 sec corresponding to the PS range of 4.7 km. The overall level of broadside (BS) transmission is higher than the TR level due to normalization applied at the SRA prior to retransmission. Snapshots: time-gated reverberation † 1 1 : K i i i H H K CSDM = = ∑ R TRM Rough Bottom 15 m, .1 m c L σ = =