An outer tubular is imaged by a pad assembly disposed within an inner tubular inserted within the outer tubular. The pad assembly is in contact with the inner tubular, and includes an acoustic pressure source, a backing mounted to a side of the acoustic pressure source, and an intervening layer between the acoustic pressure source and inner tubular. Signals generated by the pad assembly propagate radially outward from the inner tubular and reflect from the outer tubular. The generated and reflected signals travel through a medium between the inner and outer tubulars. An estimate of the distance between the inner and outer tubulars is based on the time from generation of the signal to when the reflected signal is sensed.