Vertical radar profiling: influence of survey geometry on first-arrival traveltimes and amplitudes

In vertical radar profiling (VRP), the receiver antenna is located within a borehole while the transmitter antenna is placed at varying offsets from the borehole. VRP surveys are primarily used to reconstruct subsurface georadar velocities by inverting direct-wave traveltimes. We examine the VRP technique and associated concepts that need to be considered when acquiring, analyzing and interpreting VRP data. Using simple ray-based models, we investigate the occurrence of waves critically refracted at the earth's surface for common near-surface velocity distributions and acquisition geometries. Our analysis demonstrates that these refracted waves are major limiting factors in VRP surveying. They interfere with the direct arrivals and hence reduce the reliability of direct-arrival traveltimes and amplitudes. To avoid waves critically refracted at the earth's surface in the recorded data, near-offset transmitter locations are preferred. We also analyze the amplitude sensitivity of the VRP antenna configuration, which is represented by an infinitesimal horizontal dipole located at the interface between two half-spaces and an infinitesimal vertical dipole in full-space. By combining the far-field radiation patterns of these two dipoles, we determine transmitter-receiver combinations for which maximum sensitivity is expected. A key result is that medium to far offsets between the transmitter and the borehole provide the highest amplitude signals. As a consequence, there is a natural trade-off between the requirement to avoid waves critically refracted at the earth's surface and the quest to record high-amplitude data. Finally, we apply the developed concepts in analyzing a multi-offset VRP field data example. This field data example illustrates how multiple offsets between the transmitter antenna and the borehole allow high-quality data to be measured over a large depth range.