Identification and classification of resonant targets with iterative, single-channel time reversal in a noisy environment

The identification and classification of mines resting on and buried beneath the sea floor is an important and challenging problem. Previous numerical results indicate that iterative time reversal using a single-element transducer increases the signal-to-noise ratio of the backscattered return while simultaneously converging to a narrow-band signal characteristic of the scattering target [D. M. Pierson, ‘‘Buried object detection using time-reversed acoustics,’’ Ph.D. dissertation, North Carolina State University 2003]. This technique offers an inexpensive and simple means for maximizing signal level in a noisy or cluttered environment. We investigate this phenomena through experiments performed in a water tank using a 6.35-mm-diam hollow steel sphere and broadband Q∼1.5) piston transducers operating in the 100-kHz to 2-MHz frequency range. Time-reversal convergence of the interrogation pulse was readily achieved using a variety of broadband (0.1–2 MHz) pulses to initiate the process. Results indicate that the spectrum of the echo rapidly converges to the dominant resonant mode of the target, with a significant enhancement in signal level even in the presence of broadband noise and coherent clutter returns. [Work supported by Office of Naval Research Award No. N000140610044.]