Determination of the spatial TDR-sensor characteristics in strong dispersive subsoil using 3D-FEM frequency domain simulations in combination with microwave dielectric spectroscopy

The spatial sensor characteristics of a 6cm TDR flat band cable sensor section was simulated with finite element modelling (High Frequency Structure Simulator-HFSS) under certain conditions: (i) in direct contact to the surrounding material (air, water of different salinities, different synthetic and natural soils (sand-silt-clay mixtures)), (ii) with consideration of a defined gap of different size filled with air or water and (iii) the cable sensor pressed at a borehole-wall. The complex dielectric permittivity or complex electrical conductivity of the investigated saturated and unsaturated soils was examined in the frequency range 50MHz-20GHz at room temperature and atmospheric pressure with a HP8720D- network analyser. Three soil-specific relaxation processes are assumed to act in the investigated frequency-temperature-pressure range: one primary (main water relaxation) and two secondary processes due to clay-water-ion interactions (bound water relaxation and the Maxwell-Wagner effect). 3D finite element simulation is performed with a 1/3 wavelength based adaptive mesh refinement at a solution frequency of 1MHz, 10MHz, 0.1GHz, 1GHz and 12.5GHz. The electromagnetic field distribution, S-parameter and step responses were examined.