High-resolution velocity and water-content determination by analysing GPR waves guided along a metallic cylinder
Assertions of the water-content distribution in soils is increasingly getting important for many parts of life on earth. For different fields in hydrogeology as well as agriculture and irrigation management, precise indications of the water-content distribution is necessary. GPR is often considered due to the relationship between the electromagnetic wave velocity and the soil’s water-content. In a special and innovative manner, GPR is used in combination with a waveguide in a borehole to obtain highly resolved water-content distributions. The idea behind this method lies in the usage of guided electromagnetic waves that travel vertically along the waveguide and are reflected at its lower end (fig. 1). Through knowledge of the waveguide’s depth, velocities in each depth interval can be calculated and, via petrophysical relations, converted to water-content.
In this project the method itself is evaluated and improved on its measuring accuracy. Through numerical simulations the sensitive measurement volume, the imaging resolution and the influence of the borehole casing on the measurements is studied. To accurately simulate the coupling of the electromagnetic wave to the waveguide, a 3D model of the GPR antenna is implemented (see fig. 2).
Furthermore the method is optimized on a technical level to be able to capture fast processes like infiltration and thereby derive hydraulic properties of the vadose zone. The method is also studied in combination with borehole NMR measurements, which don’t have the spatial resolution and accuracy of the GPR, but which yield additional information on the different kinds of water bonds in the subsurface. Figure 3 exemplifies the results one would gain from a guided-waves measurement.