Cavity Detection Using GPR With Small Offsets

TL;DR: ZOP measurements are inexpensive yet not accurate enough for cavity detection. Small offsets in depth between the receiver and the transmitter improve accuracy.

Abstract: Cross-borehole ground-penetrating radar (GPR) is one of the most effective tools available to detect underground cavities. These cavities are often detected from zero-offset profiling (ZOP) measurements, where the transmitter and the receiver are always at the same depth; the other common alternative is to use multiple-offset gathers (MOGs), such that a measurement is recorded with the transmitter and the receiver at every pair of depths. While the latter strategy sometimes allows one to compute a tomographic inversion and, in general, gives strictly more information, it is often prohibitively expensive to carry out such a survey, and the utility of many of these measurements is limited. ZOP measurements, on the other hand, are comparatively cheap to carry out, yet, it may not be possible to use these measurements alone to detect cavities with the required degree of accuracy. In this article, the use of measurements with a small offset in depth between the receiver and the transmitter is investigated, and it is shown that the use of only these measurements is a very favorable point in the expense-utility tradeoff. Along the way, it is shown that the manner in which traveltime is measured must be appropriate for the noise regime in which the GPR operates.