Basics of GPR 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).
There is a common misconception that the energy radiated from a GPR antenna is a pencil-like beam. In fact, GPR waves radiated from standard commercial antennas radiate radar energy into the ground in an elliptical cone with the apex of the cone at the center of the transmitting antenna. The lower the antenna frequency, the broader the transmission cone. Higher frequency antennas, such as the 900 MHz or higher, have quite narrow cones of propagation while the 200 and 300 MHz frequency antennas can spread energy outward a meter or more at depths of only about one or two meters below the ground surface. The higher the RDP of the surface material through which the energy passes, the lower the velocity of the transmitted radar energy, and the more focused (less broad) the conical transmission pattern becomes.
An estimation of this radiation pattern (also called the footprint) is important when designing transect spacing within a grid so that all subsurface features of importance are "illuminated" by the transmitted radar energy, and can therefore generate reflections. In general the angle of the cone, and therefore the size of the footprint, varies as a function of the relative dielectric permittivity of the material through which the waves pass, and the frequency of the radar energy emitted from the antenna.
Focusing and Scattering
Reflection off a buried surface that contains ridges or troughs or any other irregular features can either focus or scatter radar energy, depending on the surface’s orientation and the location of the antenna on the ground surface. If a subsurface plane is slanted away from the surface antenna’s location or is shaped so that the surface is convex upward, most energy will be reflected away from the antenna and no reflection or a very low amplitude reflection will be recorded. This is termed radar scatter. The opposite is true when the buried surface is tipping toward the antenna or the surface is concave upward. Reflected energy in this case will be focused, and a very high amplitude reflection derived from a portion of the buried surface would be recorded.