Ground-penetrating radar (GPR) is an active technique that uses an antenna to introduce pulses of electromagnetic waves into the ground (see Conyers 2004, 2006). When a wave encounters a discontinuity in the soil (a change in the electrical properties of the soil), a portion of the wave is reflected back to the antenna. The strength of the reflection is proportional to the degree of contrast that was encountered, and the travel time of the wave is proportional to the distance that the wave traveled. Thus GPR provides information about both the properties and the depth of buried interfaces (Bevan 1998:43).
The radar wave that is emitted by the antenna has both electrical and magnetic components (see Conyers 2004). The key properties of the soil with respect to the movement and reflection of the radar wave are the relative dielectric permittivity (also known simply as the dielectric) and the electrical conductivity. The dielectric permittivity is the ability of material to store and transmit an applied electromagnetic field (Conyers 2004:45). The dielectric determines the speed at which a wave can move through the ground. A high dielectic results in slower wave travel. Wet, clayey soils will generally have a higher dielectric than dry, sandy soils. Highly conductive soils (soils that will readily conduct an electrical current) will remove the electrical portion of the wave, causing it to attenuate. Wet, clayey soils are more conductive than dry, sandy soils.
Portions of the wave are reflected when the wave encounters a material with a different dielectric. The remainder of the wave, now weaker, continues to propagate downward. The strength and travel time of the waves that are reflected back to the antenna are recorded. With some knowledge of the dielectric, the travel time is used to estimate the depth from which the reflection originated. The strength of the reflection is related to the contrast in the dielectrics of the two materials.
The frequency of the antenna (how rapidly the antenna vibrates to produce the wave) determines the wavelength of the wave: slower oscillation frequencies produce longer radar waves. In turn, the wavelength of the radar wave is related to both its depth of penetration (longer wavelengths will penetrate more deeply into a given soil) and the resolution of the survey data (shorter wavelengths are capable of resolving smaller features) (Conyers 2004). Conyers (2004:59) states that “A ‘rule of thumb’ is that the minimum object size that can be resolved is about 25 percent of the downloaded wavelength reaching them in the ground” (downloading refers to the lengthening of the radar wave as it passes through the ground). Cultural features must be large enough, shallow enough, and of sufficiently high contrast to enable detection (Conyers 2004, 2006). As in resistivity survey, there is a trade-off between depth of penetration and resolution.
GPR is the most data-intensive of the three geophysical techniques considered here. Each scan of the radar can produce thousands of samples, and each transect can contain thousands of scans. Radar data can often be viewed as profiles in real time as they are being collected. Multiple profiles can be later combined into datasets that can be “sliced” horizontally to produce radar maps representing different depths of a site. These maps can be further processed to bring out the anomalies of interest and aid in interpretation.
Because of its relative high expense, complexity, and unsuitability at many sites in North America, GPR is probably the least applied of the three techniques described here. When conditions are appropriate, however, GPR has the potential to provide a richness of detail unsurpassed by other techniques (Bevan 1998).
Bevan, Bruce W.
1998 Geophysical Exploration for Archaeology: An Introduction to Geophysical Exploration. Midwest Archaeological Center, Special Report No. 1. United States Department of the Interioir Natioanl Park Service Midwest Archaeological Center, Lincoln Nebraska.
Conyers, Lawrence B.
2004 Ground-Penetrating Radar for Archaeology. AltaMira, Walnut Creek, California.
2006 Ground-Penetrating Radar. In Remote Sensing in Archaeology, edited by Jay K. Johnson, pp. 131-159. University of Alabama Press, Tuscaloosa.