Soil Property Variation by Time Domain Reflectometry

Time Domain Reflectometry (TDR) is widely applied for measuring soil water content and electrical conductivity. The method works by analyzing the signal responses under the excitations by a small magnitude electrical pulse, which are used to estimate the properties of the soil sensed by the TDR measurement probe. The commonly used method of TDR signal analysis empirically determines the characteristic reflection points. The soil properties obtained with this method are the average values over length of the TDR probe. The variation of material properties with distance along the probe are generally ignored, which otherwise could provide supplementary information of practical importance. The study of vadose zone is an example where the soil moisture varies with depth. This paper presents an approach for obtaining soil property variation within the probe from TDR measurements. It makes use of a forward model that consists of a frequency domain model for a non-uniform TDR system and a simplified model for soil dielectric permittivity. Inversion analysis was used to determine the geometry and soil properties for each layer within the probe. Both the forward model and the inversion method are first illustrated by numerical examples. They are then applied to analyze data from experimental measurements on layered soils. The preliminary results indicate that the framework provides accurate determination of soil properties, including the thickness of each layer as well as the corresponding soil properties. The capability of obtaining material profiles could further enhance the current TDR-based field monitoring system. It could also be incorporated into the existing cone penetrometers to obtain more information on soil properties.