Wave-produced patterns in rotating sonar images

Banded patterns are observed in sonograph images obtained by a rotating 500 kHz sonar located in shallow water outside the surf zone on a 4° sloping beach. In calmer weather, the bands are caused by acoustic reflections from bottom ripples, but in rough weather the signals are dominated by specular reflections from surface gravity waves approaching shore. The form of the acoustic patterns produced by reflections from these waves is examined and explained in simple geometrical terms.