Ground Penetrating Radar
Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This non-destructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks.
The Method
GPR data are usually collected along closely spaced transects within a grid. It is an active method that transmits electromagnetic pulses from surface antennas into the ground, and then measures the time elapsed between when the pulses are sent and when they are received back at the surface (called two-way travel time). As the radar pulses are transmitted through various materials on their way to the buried target feature, their velocity will change, depending on the physical and chemical properties of the material through which they are traveling. When the travel times of the energy pulses are measured, and their velocity through the ground is known, distance (or depth in the ground) can be accurately measured. Radar travel times are measured in nanoseconds, which are billionths of a second. As the antennas are moved along the ground surface individual reflections are recorded about every 2-10 centimeters along transects, using a variety of collection techniques (See also, Data Collection and Processing).
The depth to which radar energy can penetrate depends largely upon two factors: 1) the frequency of antenna being used, and 2) the characteristics of the soil being surveyed, most specifically its water content (*note .pdf file). This second factor has been shown to be much more decisive in the depth to which an EM pulse can travel and how much energy attenuation occurs. The two major components to affecting energy propagation include the electrical and magnetic permeability.The form of the individual reflected waves (called a waveform) that are received from within the ground are digitized into a reflection trace, and when many traces are stacked next to each other a two-dimensional vertical profile is produced along the transect. Thousands of reflection traces in many profiles within a grid can then be analyzed to produce both two and three-dimensional images of what lies below the surface.
One reflection trace shows the ground surface at about 2.5 nanoseconds, with the waveform losing amplitude with time, as energy is attenuated in the ground. 512 digital samples are collected to define this one reflection trace.
Buried discontinuities where reflections occur are usually created by changes in the electrical or magnetic properties of the rock, sediment or soil, variations in their water content, lithologic changes, or changes in bulk density at stratigraphic interfaces (See also, Variables Affecting a GPR Survey). Reflections also are generated when radar energy passes through interfaces between anomalous archaeological features and the surrounding matrix. Void spaces in the ground, which may be encountered in burials, tombs, tunnels, caches or pipes, will also generate significant radar reflections because of a similar change in radar wave propagation velocity. Many bed boundaries and other discontinuities will reflect a wavelet of energy (a positive and negative amplitude wave) back to the surface to be recorded. A composite of many wavelets are then recorded from many depths in the ground to produce a series of reflections generated at one location, called a reflection trace (see figure above). In order to create a vertical display of the subsurface reflections, all recorded reflection traces (see figure above), no matter what the acquisition method, are displayed in a format where the two-way travel time of the reflected waves is plotted on the vertical axis with the surface location, or trace number, on the horizontal axis. These two-dimenstional profiles are recorded by a computer and appear as black, white, and gray horizontal bands. Strong reflections generate distinct black bands, while medial reflections produce gray bands (see figure below).
GPR Reflection profile
Distance along the profile is measured in meters and two-way radar travel time, measured in nanoseconds, is converted to depth below the surface.
