The central questions in sonar system design concern the limits of coherent processing and are: What is the optimum frequency range and bandwidth?; How large of a receiving aperture is required?; What is the achievable detection range? These questions are determined by the propagation and scattering of sound. In the shallow water channel, under downward refraction, the bottom is the determinant factor. A parametric sonar equation under reverberation limited conditions shows two factors are important, the effective bottom back scatter strength and the coherent signal gain. A parametric analysis is presented that focuses on the frequency-dependent characteristics of the bottom backscattering strength and the effect of coherence on receiving array aperture. Several recent reverberation and signal coherence measurements are presented and the needs for future studies are delineated. The backscattering strength is shown to be strongly frequency dependent and to depend on the depositional sediment layer. The coherence is also found to depend on frequency and range as determined by signal gain measurements.