The ground wave in GPR can be used for reconnaissance studies of moisture in the subsurface. Wave velocity depends on moisture via the dielectric properties of the material. When using a small antenna separation, the ground wave may be superimposed by the air wave. On the other hand, the ground wave decays fast with distance. As a consequence, the ground wave can be observed in the radargram only within a limited antenna separation. We begin the field studies with a wide angle reflection and refraction 0NARR) measurement to identify the ground wave, to determine its velocity and to fmd the optimal antenna separation. This separation is kept throughout a profile when tracing out the lateral variation of ground wave velocity. The free volumetric water content is determined from the dielectric properties of subsurface matter by an empirical relation of Topp et al. (1980). Field data are usually influenced by several factors, such as antenna frequency, antenna separation and small inhomogeneties near the surface. Moisture in the subsurface can be determined more accurately using lower antenna frequencies in a humid subsurface than vice versa. We used the Pulse EKKO IV radar system with 50 and 200 MHz antennas for the field measurements. Reconnaissance studies of moisture with GPR ground wave are non-destructive, free of radiation hazard, fast and suitable for both soft and hard subsurface.


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