PR users invariably ask, “How close to the ground must the GPR be?”. The simplest answer is that the best GPR data is obtained when the antennas are in contact with the ground. This allows the antennas to “couple” to the ground and ensure the maximum amount of energy transmits into the subsurface. The more an antenna is elevated above the ground surface, the more the GPR energy reflects from that ground interface, and the less GPR energy transmits into the ground, greatly reducing the penetration depth. This not only reduces the amplitude of the response of smaller targets, but also decreases spatial resolution. But how high is too high? This will depend on the center frequency of your antenna. Higher center frequency antennas, which are physically shorter in length, need to be closer to the surface, whereas longer, lower center frequency antennas can tolerate more of an air gap. Theoretically, GPR antennas should be kept within 1/10 of the center frequency wavelength (in air) from the surface but our strong recommendation is to keep them even less than that. The table below lists some commonly used antennas and the recommended maximum allowable height:
To illustrate this point, Figure 1 shows NOGGIN® 250 MHz GPR cross-sections along the same line; one with antennas placed on the surface and the others with the antennas raised to heights of 7.5, 15, 22.5 and 30 cm, respectively. As GPR antennas are raised higher above the surface, the amplitude of target responses, the depth of penetration and the lateral resolution all decrease. This is particularly clear where hyperbolic-type responses are present in the GPR record.
The above guidelines are a maximum. Wherever possible, keep the antennas as low to the ground as practically possible; being in contact with the ground is the best. People hate the sound of their GPR system scraping on the ground, but all antennas sold by Sensors & Software are designed to be ground-coupled (i.e., in contact with the surface); antennas with center frequencies 50 MHz and higher have very resistant, high density polyethylene skid plates to protect the antennas from the abrasive ground surface.
There are times when certain applications lend themselves to antennas being placed a little higher off the ground surface. In these cases, refer to the operator’s manual for recommended deployment and setup.
