A diffraction-limited imaging method in an inhomogeneous medium, which uses phase conjugation and multiple reflectors, is proposed. First, several reflectors with frequency-dependent reflectivities are put in the observation area. Second, ultrasonic signals are acquired for all combinations of transmitter-receiver elements of the array, and data acquisition is repeated for each of the frequencies. Then, signal processing is performed in a computer: (1) The reflected wave in the mth step is phase conjugated and transmitted to get the reflected wave in the (m+1)th step until convergence is attained; (2) the converged phase-conjugated wave is scanned in bearing to get an image; and (3) good image parts obtained for several frequencies are joined together to extend the good image area.
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Osamu Ikeda (1989) studied this question.