Bottom reverberation can be used as a probe-source of opportunity to construct a time-reversal focus that scans along a rough interface. We present results from a tank experiment at ultrasonic frequencies in which reverberation from a sandy bottom was collected on a 64-element source/receiver array. The monostatic reverberation measurements are windowed, time-reversed, and then transmitted. When measureable backscatter is present in the time window, energy is observed to focus at a range corresponding to that window. Two factors contribute to the back-scattered energy level: first, the propagation paths for a particular incident ping will nonuniformally ensonify the interface and, second, the local interface scattering strength will vary with range. A method to capture reverberation at all ranges and an application to realistic shallow water ocean waveguides are also presented.