A fast TDR-inversion technique for the reconstruction of spatial soil moisture content

摘要

Spatial moisture distribution in natural soil or other material is avaluably information for many applications. Standard measurement techniquesgive only mean or punctual results. Therefore a new inversion algorithm hasbeen developed to derive moisture profiles along single TDR sensor-probes.The algorithm uses the full information content of TDR reflection datameasured from one or both sides of an embedded probe. The system consistingof sensor probe and surrounded soil can be interpreted as a nonuniformtransmission-line. The algorithm is based on the telegraph equations fornonuniform transmission-lines and an optimization approach to reconstructthe distribution of the capacitance and effective conductance along thetransmission-line with high spatial resolution. The capacitance distributioncan be converted into permittivity and water content by means of acapacitance model and dielectric mixing rules. Numerical investigations havebeen carried out to verify the accuracy of the inversion algorithm. Single-and double-sided time-domain reflection data were used to determine thecapacitance and effective conductance profiles of lossless and lossymaterials. The results show that single-sided reflection data are sufficientfor lossless (or low-loss) cases. In case of lossy material two independentreflection measurements are required to reconstruct a reliable capacitanceprofile. The inclusion of an additional effective conductivity profile leadsto an improved capacitance profile. The algorithm converges very fast andyields a capacitance profile within a sufficiently short time. Theadditional transformation to the water content requires no significantcalculation time.