Figure 4: Echo signatures of natural and experimental surfaces.

In the first row a comparison of a natural smooth (water) (a) and a natural textured surface (sand) (b) is given. Below, the echo signatures of our three experimental materials (metal, wood and plastic) are compared for smooth and textured plates. Smooth plates are depicted on the left (c, e, g) and textured surfaces on the right (d, f, h). The white scale bar in g corresponds to 10 ms. The colour bar codes for the amplitude of the signal in a relative dB scale. Smooth (left side): After the outgoing signal (S), there is a time delay until the first echo returns; this is the echo front reflected perpendicularly from the ground (G). All other parts of the signal are reflected away and thus do not reach the microphone (see Fig. 2 for a schematic representation). In the water sonogram (a), an additional echo from the back edge (E) of the water pool shows up. Textured (right side): After the perpendicular ground echo (G), a series of many overlapping echoes from the uneven surface structures follows (U). Overall, the echo reflections of the smooth experimental plates strongly resemble those of a water surface, whereas the reflections of the textured plates mimic those of uneven ground.