Reverse Time Migration based ultrasonic imaging of rebars embedded in concrete

The Reverse Time Migration (RTM) algorithm has been applied extensively for geophysical subsurface imaging. In this technique, the explored medium is excited at certain locations by surface based acoustic sources. The signals reflected from the interior of the medium are recorded on the surface. These signals are subsequently time reversed and fed back into a simulated medium at the receiver locations. The resulting wave propagation is simulated via a numerical scheme, i.e., Finite Difference in Time Domain (FDTD) method. A second simulation, involving the forward migration of the source field, is also performed by feeding signals at the respective source positions. The fields resulting from the forward and the time reversed simulations are stored at each time step and at each spatial location in the simulated medium. A subsurface image of the medium is generated, using the two migration fields, by applying an Imaging Condition. In the field of Non-Destructive Evaluation (NDE) of concrete, application of the RTM is a relatively new area of research. In the current work, the RTM algorithm is applied on experimental ultrasonic data acquired on a concrete block with embedded rebars. Some of the closely spaced steel rebars are in bonded condition and some of them are artificially debonded from the concrete, thereby simulating a delaminated situation. The ultrasonic data was obtained in a Full Matrix Capture (FMC) scheme and the RTM calculations were performed using the elastic as well as the acoustic wave propagation modality. The RTM based images are able to locate the steel rebars and distinguish between the delaminated and the bonded conditions of the rebars.