Backscatter measurements were taken from an irregular rubber layer of uniform thickness, 3 mm, at five discrete frequencies (0.33, 0.54, 0.78, 1.0, and 1.5 MHz). Comparisons were made with data taken from a much thicker rubber target (mean thickness 38 mm) of the same surface roughness. It was found that there were no significant differences in the angular behavior of the return signals at the three lower frequencies. However, at higher frequencies, an oscillatory component appeared in the return from the layer. This implies that at least for the case studied (a rough layer with rms roughness of its interfaces comparable to its mean thickness), a layer must be more than about two wavelengths thick before it can be detected. It also follows that a multifrequency sensor is essential for exploring unknown terrains with possible subsurfaces.