Performance Specs of GPR

The performance of the GPR method depends upon the site-specific surface and subsurface conditions. Performance specifications include requirements for or information about reflections, depth of investigation, resolution, interferences, calibration, quality control, and precision and accuracy.

Reflections

Reflections are created by an abrupt change in the electrical and magnetic properties of the material the electromagnetic waves are traveling through. In most situations, magnetic effects are small. Most GPR reflections are due to changes in the relative permittivity of material. The greater the change in properties the more signal is reflected. In addition to having a sufficient electromagnetic property contrast, the boundary between the two materials needs to be sharp.

Areas with subsurface contamination often have very different permittivities than non-contaminated areas. GPR has been used to map highly conductive contaminated groundwater plumes (Porsani et al. 2004 and Pomposiello et al. 2004). Also studies have shown that weathered fuel releases create a “halo” of conductive soil and groundwater around them that are detectable by GPR (Sauck et al. 1998, Atewanda et al. 2002, and Bradford 2003). Nonaqueous phase non-polar organic contaminants, such as fuels and chlorinated solvents, generally have very low permittivities. In theory, these should provide a good reflectance contrast, and studies have shown that GPR can track their movement in the subsurface during a controlled release (Sneddon et al. 2000 and Brewster et al. 1995); however, in practice, differentiating relatively thin layers of free product from other reflectors where the release area is not known has not been particularly successful. Work has been done (Lane et al. 2004, Patterson 1997, and Bradford 2004,) to suggest that GPR can be used as a remediation monitoring aid by tracking changes in the subsurface conditions.

Depth of Penetration

The principal limiting factor in depth of penetration of the GPR method is attenuation of the electromagnetic wave in the earth materials. The attenuation predominantly results from the conversion of electromagnetic energy to thermal energy due to high conductivities of the soil, rock, and fluids. Scattering of electromagnetic energy may become a dominant factor in attenuation if a large number of inhomogeneities exist on a scale equal to the wavelength of the radar wave.

GPR depth of penetration can be more than 30 meters in materials having a conductivity of a few milliSiemens/m. In certain conditions, such as thick polar ice or salt deposits, penetration depth can be as great as 5,000 meters. However, penetration is commonly less than 10 meters in most soil and rock. Penetration in conductive (e.g., smectites) clays and in materials having conductive pore fluids may be limited to less than one meter.

Interferences

The GPR method is sensitive to unwanted signals (noise) caused by various geologic and cultural factors. Geologic (natural) sources of noise can be caused by boulders, animal burrows, tree roots, and other inhomogeneities that cause unwanted reflections or scattering. Cultural sources of noise can include reflections from nearby vehicles, buildings, fences, power lines, and trees. Shielded antennas can limit these types of reflections. Electromagnetic transmissions from cellular telephones, two-way radios, television, and radio and microwave transmitters may cause noise on GPR records.