During a vertical radar profiling survey, a conventional surface GPR transmitting antenna is positioned on the ground surface at a fixed off-set from the borehole, whilst a borehole receiver probe is lowered into the hole. Radar readings are made at set intervals down the hole. The transmitting antenna is then repositioned away from the borehole at a fixed distance, and the receiver probe is once again lowered down the hole. This results in a "half-tomogram", which yields the same information as a cross-hole survey, but over a triangular region extending from the maximum depth of the receiver penetration to the maximum distance from the transmitter to the borehole. Vertical radar profiles may be made at any azimuth, allowing the determination of fault dip angles as well as the construction of conical 3D GPR images of the geology and water content around the borehole.
To perform a GPR survey within a single borehole, both the radar receiver and transmitter probes are lowered into the borehole at a fixed offset distance. The dipole antennas used for borehole surveys radiate and receive radar energy omni-directionally. In this mode of borehole surveying, interpretation is carried out in a similar manner to that for surface GPR investigations. Although the distance from the borehole to a detected target can be accurately measured, the omni-directional nature of the antennas makes a determination of the target's azimuth from the borehole impossible. For the application of fraction detection, the declination of the planar feature can be detected, but not its strike.
Cross-hole survey yields the most accurate image of the interstitial space between two boreholes using GPR. Cross-hole surveys are conducted using separate transmitter and receiver probes that are each inserted into two different boreholes. With the transmitter probe fixed at a certain depth, the receiver probe is lowered down through the depth range of interest, acquiring data at set intervals. The transmitter probe is then lowered one step, and the receiver probe is again lowered through the zone of interest. This process is repeated until the transmitter probe has been lowered past the zone of interest.
This method results in a dense network of raypaths that cover the entire zone of interest. By accurately measuring the one-way travel time for the initial radar energy to arrive at the receiver for each raypath, as well as the amplitude of that arrival, a radar tomogram may be constructed. By comparing the arrival times of the initial radar energy found through experimentation with those of a theoretical homogeneous media, a detailed map of water content distribution between the boreholes may be produced, including water-filled fractures and voids. A common application of cross-hole tomography and vertical radar profiles is the detection of leaks in dams. A variety of additional geological information may be extracted from borehole radar tomography, depending on site conditions.
