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

摘要

The spatial sensor characteristics of a 6cm TDR flat band cable sensorsection was simulated with finite element modelling (High Frequency StructureSimulator-HFSS) under certain conditions: (i) in direct contact to thesurrounding material (air, water of different salinities, different syntheticand natural soils (sand-silt-clay mixtures)), (ii) with consideration of adefined gap of different size filled with air or water and (iii) the cablesensor pressed at a borehole-wall. The complex dielectric permittivity orcomplex electrical conductivity of the investigated saturated and unsaturatedsoils was examined in the frequency range 50MHz-20GHz at room temperature andatmospheric pressure with a HP8720D- network analyser. Three soil-specificrelaxation processes are assumed to act in the investigatedfrequency-temperature-pressure range: one primary (main water relaxation) andtwo secondary processes due to clay-water-ion interactions (bound waterrelaxation and the Maxwell-Wagner effect). 3D finite element simulation isperformed with a 1/3 wavelength based adaptive mesh refinement at a solutionfrequency of 1MHz, 10MHz, 0.1GHz, 1GHz and 12.5GHz. The electromagnetic fielddistribution, S-parameter and step responses were examined.