Ground penetrating radar for determining volumetric soil water content; results of comparative measurements at two test sites

Ground penetrating radar (GPR) can provide information on the soil water content of the unsaturated zone in sandy deposits via measurements from the surface, and so avoids drilling. Proof of this was found from measurements of radar wave velocities carried out ten times over 13 months at two test sites in the Netherlands. At these same locations and on the same dates, soil water content was measured in access robes using a capacitance probe. Comparison of GPR and capacitance probe observations revealed that: 1. The inferred absolute values of soil water content agree well. This is remarkable because the soil water content is deduced entirely differently for the two methods. 2. Seasonal fluctuations in soil water content established for different (general) depth zones of radar waves correlate well with the fluctuations observed in the access tubes. 3. The various methods to determine the propagation velocities of radar waves are complementary; together they produce a realistic and reasonably complete image of the vertical distribution of the soil water content of the entire unsaturated zone. High-frequency (200 MHz) direct groundwaves and refracted waves constitute a particularly attractive complementary combination, which provides information on consecutive shallow zones in the underground, i.e. the zones of major soil water content fluctuations. 4. Lateral variations established at one of the test sites where several access tubes have been placed in a transect also follow from the GPR measurements along that profile; this non-destructive determination of soil water content in practically continuous and detailed sections is one of the great assets of GPR, opening the way to mapping preferential soil water flow paths.