Study of robustness of single-element-mirror TR method to reverberation and clutter: simulations and tank measurements,
Résumé
Previous results suggest that iterative time reversal (TR) using a single element mirror is a promising, simple and inexpensive means to enhance the backscatter signature of elastic objects and to simultaneously focus on its dominant resonance frequency. Motivated by these findings, this work presents a systematic explanation of the mechanisms thought to be responsible for the TR signal-to-noise ratio (SNR) enhancements under clutter and ambient noise limited conditions. When clutter or reverberation limited, it is shown that in the frequency domain the magnitude of the TR echo spectrum after n iterations is simply a product of successive n+1 clutter and target echo transfer function measurements. Taking the logarithm of this product allows the interpretation of TR as an incoherent average of logarithms of spectral magnitudes and simplifies analysis of its performance as a resonance detector. The calculation of its asymptotic relative efficiency shows that TR suffers only a small loss (~2.2 dB) relative to optimum square-law integration when the signal is weak. In the case when ambient noise limited, the TR amplification and focusing mechanisms and SNR gains are explained by a simple non-linear recurrence equation approximation. The method was also experimentally applied to scattering measurements from a flush buried spherical shell in sandy sediment in presence of clutter. These results motivate that single channel TR is potentially a useful technique for enhancing backscatter signatures in clutter and noise.