The GPR, consisting of a transmitter and receiver, is elevated above the surface at a typical height of ~1 m as shown in Figure 24.1. A vehicle-mounted implementation of a surface reflectivity system (Figure 24.2) shows the GPR mounted to the left of the vehicle, with GPS providing positioning information. The transmitter emits a short EM pulse. Part of the energy is transmitted into the soil and part is reflected back into the air. The reflected pulse amplitude is measured in the receiver.
For the incident pulse at the air-soil interface, the reflection coefficient R is determined by the contrast in relative dielectric permittivity (K) between the air (K = 1), and the soil (K = Ks):
It is assumed that the conductivity is sufficiently small to be ignored, that the surface is flat, and that the soil properties are homogeneous. The above relationship can be rearranged to determine the soil permittivity from a measured reflection coefficient. In practice, the GPR system is calibrated with metal plate target with a known reflection coefficient of -1. The reflection coefficient can be measured by calibrating the GPR system with a sufficiently large metal plate (dimensions > Fresnel zone diameter) placed at the surface (Redman et al., 2002). The magnitude of the reflection coefficient is the ratio of the amplitude Ar of the reflected wavelet from the soil surface to the amplitude Am of a wavelet measured at the same elevation over a metal plate target with a reflection coefficient of -1. Using Equation (24.1), Ks can be determined from the ratio of the amplitude of the wavelet (Ar) reflected from the surface and from the amplitude of the wavelet (Am) reflected from a metal plate:
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