In recent years, ground penetrating radar (GPR) has gained general acceptance in a variety of fields, including geology, engineering, and archaeology as an excellent method for capturing high-resolution imagery of geologic structures and subsurface objects. High-frequency electromagnetic pulses (EM) are emitted from an antenna to probe the Earth. These waves change velocity and are reflected when encountering materials with different dielectric properties like soil, bedrock, water, anthropogenic objects, and other anomalies. The changes in wave velocity are examined to calculate travel times and produce a viable profile image, known as a radargram. Multiple transects or survey lines can then be stitched together to produce a 3-dimensional view for various analyses.
The appeal of GPR is that it provides a non-invasive geophysical method for subsurface exploration that is preferable to more costly excavation or drilling techniques. Current GPR systems are relatively user-friendly and are composed of an antenna to send and receive radio frequency pulses, a laptop or monitor to capture and view data, and a power source.
Depth penetration and resolution varies widely depending on the antenna used. Here in the U.S., common systems range from 100MHz (low-resolution; high penetration) to 1.3GHz (high-resolution; low penetration). The 100Mhz antenna has a maximum depth of ~60ft at a resolution of 1.6ft, while the 1.3GHz can only penetrate to a foot or two, but has a resolution of .035ft. This trade-off between depth and resolution requires investigations to tailor equipment to study parameters and intended purposes.
GPR antennas are either shielded or unshielded. The former are primarily used for medium to high resolution surveys, and are highly suited for urban and residential investigations, which have background noise and interference. Unshielded antennas encounter issues when cultural noise is a factor, so are limited in applications.
Because EM wave velocities change drastically when encountering void spaces and disturbed soils, GPR is particularly suited for grave detection and inventory surveys of old cemeteries and archaeological digs. Many of these sites have absent or crumbling headstones and older graves with unmarked locations. Buried objects and point features produce a reflection hyperbola (looks like an inverted U) on the radargram, while bedrock contacts and soil density changes show up as planar reflectors. Void spaces manifest as a “ringing” reflection. The user can interpret these features to produce a reasonable understanding of near-surface phenomena.
These geophysical indicators of buried objects in the radargram offer enormous potential in locating lost and historical graves, as well as the traditional ability to locate pipes and other man-made objects, without destructive digging, excavation, or drilling. As GPR technology evolves it is becoming more cost-effective and practical for a variety of research, industrial, and educational applications.