Identification of a resonant target in the free field and buried in a sediment using iterative, single-channel, time reversal

Numerical studies demonstrate enhanced signal-to-noise ratio and convergence to a narrowband signal using iterative time reversal of the monostatic scattered return from a resonating target [Pierson, David M., “Buried Object Detection Using TimeReversed Acoustics,” PhD Dissertation, North Carolina State University, 2003]. The center frequency of the converged-upon signal is a resonance frequency of the target whose value can be used for classification and identification purposes. This technique could offer a straightforward means for enhancing target return levels in a noisy or cluttered environment using existing sonar systems. We report results from scaled experiments performed in a water tank using broad band (Q~2) piston transducers operating in the 500 kHz – 2 MHz frequency range. The target is a 6.35-mm diameter hollow steel sphere suspended in the free field and buried at different depths in a mediumgrain sand layer. The procedure consists of exciting the target with a broadband pulse, digitizing the echo return, reversing the data stream in time, and using this signal as the source waveform for the next interrogation pulse. Free field results indicate that the spectrum of the echo rapidly converges to a frequency corresponding to a resonant mode of the target. The signal to noise ratios of targets buried in sand are enhanced as the iterations converge to the strongest available target resonance