Abstract: The analytical method of angular power spectum for averaged field in shallow water is based on the Ray-mode theory. By introducing the concept of the averaged angular power spectrum, some calculation for transmission less, long-range reverberation and spatial coherence will be simplified. The range and depth dependence of the long-range reverberation and echo-reverberation ratio were calculated for the cases of negative gradient and homogeneous layers. It is shown that, for the long-range detection in an inhomogeneous layer, the variable depth sonar is better than the sonar in constant depth. The method and results would be useful for analysing angular and frequency dependence of the bottom (or surface) scattering at low grazing angles.