TDR modeling for near-surface moisture profile measurement

Abstract

Conventional time domain reflectometry (TDR) for measurement of water content typically uses a parallel waveguide buried in the material under test, and provides a measure of the mean permittivity and hence water content of a volume surrounding the waveguide. Here we describe an alternative arrangement whereby the waveguide is positioned outside the material. With the altered arrangement, the mean near-surface water content may be deduced using a modeling technique. However, by making measurements at several different positions of the waveguide relative to the material, we show that combining forward modeling with an inversion technique enables the near-surface water content profile to be determined. Using laboratory measurements, we demonstrate that for a coarse-grained soil, an accuracy of 2% volumetric water content is feasible when using an accurate time domain reflectometer, a selfcalibration technique and the Topp equation for translation of real permittivity to soil water content. We then outline two improvements on the modeling and inversion techniques. The first is a simplification of the forward problem that reduces the size of the field matrix and the number of unknown variables. We then describe how combining the simplification with alternative approaches to the inverse calculation can yield improvements in calculation time.