Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 114

radiation is refracted and the stronger the signal received. The largest difference is
between air and water. (See Table 5.1.) There is also an appreciable difference between
soil and both air and water. Originally, GPR was used in areas in which the soil tends to
be dry. Today it is used in both wet and dry conditions [6].

The RDP can be calculated using the equation where k is RDP, C the
speed of light, and V the velocity of radar in the material being investigated. The exact
RDP cannot be accurately calculated when dealing with mixed materials, such as soil and
regolith. Under these field conditions

TABLE 5.1 Relative Dielectric Permitivity (RDP) of Some Common Environmental
Components

Medium RDP

Air 1

Water 80

Dry silt 3-30

Wet silt 10-40

Soil 12

Source: Data from Ref. [6].

it is either necessary or highly desirable to determine k by measurement. Burying a metal
bar at a known depth in the soil or regolith and passing the GPR instrument across the
area in which the rod is buried can easily accomplish this. The locally applicable k is thus
determined, and then used to calibrate the instrument for the medium where the survey is
to be made. At this point the depth to which the GPR survey will be useful can be
calculated, keeping in mind the limitations noted below. The equation used to find this
depth is

where A =approximate long dimension radius of footprint. The footprint is the buried
surface area illuminated by the radar signal. In the equation, λ =center frequency
wavelength or radar energy, D =depth from ground surface to the reflection surface, and
k =average RDP of the material being surveyed. Under the best conditions, the depth of
penetration of radar in soil is 40 meters (approximately 130 ft). Normally, however,
usable penetration is limited to 1 to 5 m (3 to 16 ft). Within this depth, the occurrence of
pipes, power lines, buried objects, caves, differing layers of soil and regolith, and the
depth to the water table can be observed.
Making a survey is done by passing a GPR instrument over an area in a grid pattern