Capabilities and Methods for Ground-Penetrating Radar Surveys
Equipment and Software
SIR 4000, 32-bit GPR controller and antennas transmitting at 200, 400, and 900 MHz made by Geophysical Survey Systems Inc. (GSSI) (Figs 1-3). GSSI’s Radan 7 software including the 3D module. Geophysical Archaeometry Laboratory Inc. software GPR-Slice v7.0. For the 400 MHz and 900 MHz antennas, a 4-wheel smart cart and 3 large-diameter wheel smart cart, both with survey-wheel encoder (Figs 4-5). For the 200 MHz, a custom HDPE sled and attachable survey wheel. For road surveys, a 6’x3’, metal-free, wheeled trailer (Fig. 3).
Fig. 3. 400- and 200 MHz antennas on non-metal trailer.
Fig. 1. SIR 4000 single-channel GPR controller.
Fig. 4. GPR 3-wheel smart cart.
Fig. 2. 900 MHz antenna.
Fig. 5. GPR 4-wheel smart cart.
Table 1. Vertical resolution and average penetration depth of GPR antennas assuming average ground velocity.
Penetration Depth, Spatial Resolution, Data Density, and Distance Coverage
Penetration depth and spatial resolution for Geoscy antennas are listed in Table 1. The greater the depth to which a given antenna signal penetrates, the lower is its resolution in the vertical direction.
The exceedingly fast data collection rate of the state-of-the-art SIR 4000 controller means higher data density which increases signal recognition. Alternatively, faster GPR data collection permits GPR profiling over much larger distances than previously possible. Geoscy has the capability to profile at speeds of ~10 mph on highways using our non-metallic trailer, permitting reconnaissance GPR mapping of tens of miles per day.
Once client objectives and site conditions are ascertained, several considerations go into survey design. We are inclined to use antennas at multiple frequencies because the overlap in penetration depths aids interpretation of data from each of the antennas. Given the high data-collection rate and software advances, it is very useful to collect GPR profiles along a grid of lines. Therefore, grid location, spacing of profile lines within the grid, and also orientation of the profiles are part of survey design. Whether or not a grid is appropriate, we use multiple methods to obtain geographic coordinates for the GPR lines and, potentially, topography.
Fig. 6. Three-person GPR profiling with a bistatic 100 MHz antenna.
Field Data Collection
The first phase of field work involves experimental profiling to tune GPR controller parameters and finalize location/orientation/marking of the transects. Preliminary GPR data are analyzed real-time to locate transect lines. If subsurface samples had not already been collected and analyzed, some form of sampling should be accomplished to aid data interpretation. Then follows collection of all GPR survey data (Figs. 6-7). We collect GPS data simultaneously. The geographic coordinates of the transect lines must be precisely surveyed.
Fig. 7. Two-person GPR profiling with a bistatic 100 MHz antenna.