Ground penetrating radar response from voids: A demonstration using a simple model

Much work, some quite complex, has been expended on experiments and modelling for imaging voids using ground penetrating radar (GPR). However, much can be learned from simple models. For example, a simple Ricker wavelet model can be used to illustrate some fundamental properties of the GPR response from voids, both air- and water-filled. In particular, reflections from the top and the bottom of a void can overlap to a significant extent. Thus, one characteristic of the void response is the occurrence of a “bright spot”, analogous to the “bright spot” observed in seismic exploration for gas. For time delays equal to about half the pulsewidth, the reflected wavelets superimpose to yield maximum reflection amplitudes, significantly greater than individual wavelet amplitudes. The wavelet reflected from the top of a void is not completely separated from the wavelet reflected from the bottom of the void until the time delay due to the void exceeds twice the wavelet pulsewidth. However, the two wavelets can be individually identified at earlier time delays, approximately equal to the wavelet pulsewidth. This is still substantial, and explains why it is difficult to image voids, especially air-filled voids, and to use GPR to infer void thicknesses.