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

Abstract

Spatial moisture distribution in natural soil or other material is a valuably information for many applications. Standard measurement techniques give only mean or punctual results. Therefore a new inversion algorithm has been developed to derive moisture profiles along single TDR sensor-probes. The algorithm uses the full information content of TDR reflection data measured from one or both sides of an embedded probe. The system consisting of sensor probe and surrounded soil can be interpreted as a nonuniform transmission-line. The algorithm is based on the telegraph equations for nonuniform transmission-lines and an optimization approach to reconstruct the distribution of the capacitance and effective conductance along the transmission-line with high spatial resolution. The capaci-tance distribution can be converted into permittivity and water content by means of a capacitance model and dielectric mixing rules. Numerical investigations have been carried out to verify the accuracy of the inversion algorithm. Single-and double-sided time-domain reflection data were used to determine the capacitance and effective conductance profiles of lossless and lossy materials. The results show that single-sided reflection data are sufficient for lossless (or low-loss) cases. In case of lossy material two independent reflection measurements are required to reconstruct a reliable capaci-tance profile. The inclusion of an additional effective conductivity profile leads to an improved capacitance profile. The algorithm converges very fast and yields a capacitance profile within a sufficiently short time. The additional transformation to the water content requires no significant calculation time.

Quantifying spatiotemporal dynamics of root-zone soil water in a mixed forest on subtropical coastal sand dune using surface ERT and spatial TDR

“…Spatial TDR data were acquired immediately after each ERT survey. Spatially distributed capacitance (C) was firstly reconstructed with the measured TDR signals using the algorithm of Schlaeger (2005). Each reconstruction process took about three X (m) minutes to complete.…”

Potentials and constraints of different types of soil moisture observations for flood simulations in headwater catchments

“…Robinson et al 2003;Cassiani et al 2006) can be used to estimate the mean soil moisture of the surrounding soil via the reflected travel time of an electromagnetic wave along the waveguide. Several authors have shown (Feng et al 1999;Oswald et al 2003;Schlaeger 2005;Greco 2006) that the reflected signal can deliver information on the water content along the sampled profile. To quantify the spatial distribution of the water content along the wave guide, an inverse signal analysing technology has been introduced by Graeff et al (2010) and called it 'STDR' (spatial time domain reflectometry).…”

Optimiertes Sensordesign zur Bestimmung von Feuchteprofilen in verlustbehafteten Böden (Optimized Sensor Design for the Determination of the Spatial Moisture Distribution in Electrical Lossy Soils)

“…Mit Hilfe eines neu entwickelten Rekonstruktionsalgorithmus ist es möglich, unter Berücksichtigung beider Einflussgrößen Wassergehaltsprofile entlang von Sensoren zu erfassen (vgl. [1][2][3]). Dieses Verfahren zur Erfassung von Wassergehaltsprofilen wurde bereits in verschiedenen Anwendungen mehrfach erfolgreich eingesetzt und ist als Spatial-TDR eingeführt (vgl.…”

“…elektrisch verlustbehafteten Materialien wird allerdings der sich entlang dem Flachbandkabel ausbreitende Spannungsimpuls aufgrund des Energieverlustes stark gedämpft. Bei einem angeschlossenen Koaxialkabel am Sensorendewas elektrisch betrachtet einer 50-Ω-Randbedingung entspricht -können infolgedessen die Reflexionen am Übergang nicht mehr ausreichen, um das Sensorende anhand dieser Reflexion eindeutig zu identifizieren, was allerdings für eine erfolgreiche Bestimmung der exakten Laufzeit des Signals entlang der Sonde unbedingt erforderlich ist [1].…”