Observations with grazing and vertical incidence methods of ground impedance estimation

At locations near a ground surface of interest, the ground impedance may be evaluated using a loudspeaker suitably disposed to broadcast toward both the ground and two vertically-separated microphones. By evaluating the complex gain ratio between the pair for a single tone, the usual approximation to the Green function for the monopole field above a locally-reacting ground can be inverted to find the surface impedance. This inversion is somewhat sensitive to noise, but it can also be found to vary according to the placement of microphones and speaker, apart from speaker directivity. Recently the method of Soh et al. [Soh et al. JASA 128:5 EL286 2010] was proposed for measuring ground impedance some distance from the source. Because the method relies more directly on the boundary condition at the ground, it may offer some benefit for use at shorter distances as well. We discuss the comparisons between measurements interpreted by both techniques, consider noise entering the estimation process and placement effects.