Echoes analysis from spherical elastic shells by using iterative time reversal mirror

Abstract

In the shallow water environment, detecting and classifying buried objects is a challenging problem with high operational importance. Acoustical wave becomes an important tool in the detection of objects in underwater systems due to its effectiveness of propagation in the water. Later on, the acoustic scattered field modal for fluid-loaded and thin spherical shell have been determined by the shell's physical properties and appropriate boundary conditions at the fluid interface. These spherical elastic shells produce lamb waves that contain energy within corresponding frequency bandwidth. The iterative time reversal process is then performed to enhance energy of responding lamb waves and makes it possible to distinguish target characteristics. In addition, target distance can be determined from wave propagation time. The study is developing a procedure to distinguish target characteristics such as material and size by observing convergence frequency through iterative time reversal process. The proposed procedure is verified from designed simulations and experiments by using spherical elastic shells as targets. In experiments, interrogating signals are transmitted from an unfocused broadband underwater transducer and recorded received by another same type transducer. Due to the enhancing effect of iterative time reversal, energy of iterative receiving signals converges to certain frequencies. After comparing these frequencies from varying target characteristics such as diameter of target sphere shell and thick of target sphere shell, the result proves that the proposed procedure is capable to distinguished properties of different targets.