Systems and methods process a measured ultrasonic response waveform to determine a well casing thickness and an acoustic impedance of a sealing medium surrounding the well casing. An array of simulated response waveforms corresponding to a set of candidate acoustic impedances for the sealing medium surrounding the well casing and a set of candidate well casing thicknesses is generated. A simulated response waveform from the array of simulated response waveforms is identified that best matches the measured response waveform so as to determine the sealing medium acoustic impedance.