GPR study of pore water content and salinity in sand

High-resolution studies of hydrological problems of the near-surface zone c an be better accomplished by applying ground-probing radar (GPR) and geoele ctrical techniques. We report on GPR measurements (500 and 900 MHz antennae ) which were carried out on a sorted, clean sand, both in the laboratory an d at outdoor experimental sites. The outdoor sites include a full-scale mod el measuring 5 x 3 x 2.4 m(3) and a salinity site measuring 7.0 x 1.0 x 0.9 m(3) with three buried sand bodies saturated with water of various salinit ies. Our studies investigate the capability of GPR to determine the pore wa ter content and to estimate the salinity. These parameters are important fo r quantifying and evaluating the water quality of vadose zones and aquifers . The radar technique is increasingly applied in quantifying soil moisture but is still rarely used in studying the problems of water salinity and qua lity. The reflection coefficient at interfaces is obtained from the amplitude spe ctrum in the frequency and time domains and is confirmed by 1D wavelet mode lling. In addition, the GPR velocity to a target at a known depth is determ ined using techniques of two-way traveltime, CMP semblance analysis and fit ting an asymptotic diffraction curve. The results demonstrate that the reflection coefficient increases with incr easing salinity of the moisture. These results may open up a new approach f or applications in environmental problems and groundwater prospecting, e.g. mapping and monitoring of contamination and evaluating of aquifer salinity , especially in coastal areas with a time-varying fresh-water lens. In addi tion, the relationship between GPR velocity and water content is establishe d for the sand. Using this relationship, a subsurface velocity distribution for a full-scale model of this sand is deduced and applied for migrated ra dargrams. Well-focused diffractions separate single small targets (diameter of 2-3 cm, at a depth of 20-180 cm and a vertical interval of 20 cm). The results underscore the high potential of GPR for determining moisture conte nt and its variation, flow processes and water quality, and even very small bodies inside the sand or soil.