Radar "lensing" by a small river: can a layer of surface water improve the signal?

A radar survey was carried out on a gravel bar between the Hope and Boyle Rivers near the Hanmer Basin in the central South Island of New Zealand. One of the profiles crossed a shallow stream, where the depth of penetration and resolution both appeared to improve. The superior resolution is due in part to the slower average velocity; the water table is at or albove the ground surface in the stream channel. Spectral analysis shows that there is a small increase in the amplitude of the higher frequencies in the stream portion of the profile, so that the improved resolution is real. The apparent increased depth of penetration is partly due to the increased time required for echoes to return at the slower velocity, but the larger twoway travel time does yield a greater depth of penetration.