Imaging a defect in layered media with different shaped interfaces using reverse time migration without velocity model known a priori

In this work, the highly accurate image of a defect in the layered media using a reverse time migration (RTM) is obtained without priori information of the interface geometry and acoustic velocity of the lower medium. The interface of layered media is reconstructed by RTM, and its profile is determined using the reconstructed image within the range of -6 to 0 dB. At the assumed acoustic velocity of the lower medium, the minimum variance of the possible position distribution of a defect is evaluated to be an acoustic velocity of the lower medium by processing the travel times of the defect scattered echoes. The velocity model of the layered media is reconstructed by the mapped interface and the calibrated acoustic velocity of the lower medium, and a defect in the layered media is imaged by RTM. The proposed method is verified by utilizing the silicone rubber-water layered media with horizontal, titled, and circular arc interfaces. The obtained results for the interface geometry, acoustic velocity of the lower medium, and location of defect are consistent with the actual values. Therefore, a defect in the layered media can be accurately located and imaged without the velocity model of layered media known a priori via the RTM method.