Pole contribution to the field reflected by sand layers

Measurements of the reflection coefficient at oblique incidence and for inhomogeneous waves are presented for layers of sand having a large flow resistivity and different thicknesses. The dependence of the reflection coefficient on the angle of incidence is different for layers of small thickness and semi-infinite layers, due to the influence of a pole which is related to an ordinary guided wave in the first case, and the Zenneck wave in the second case. Simulations indicate that the amplitude of the Zenneck wave can be measured for small source receiver distances, allowing a simple evaluation in situ of the surface impedance. The same method can be used with the ordinary surface wave for thin layers.