Construction and calibration of a field TDR monitoring station

Time-Domain Reflectometry (TDR) has been used extensively in the past thirty years in order to measure soil water content and bulk electrical conductivity (ECb), both in the laboratory and in the field. TDR can be effectively used in combination with geophysical techniques such as Ground Penetrating Radar (GPR) in order to provide information on relative dielectric permittivity and ECb. As part of the Mapping the Underworld project, a TDR monitoring station was constructed with the aim of monitoring the geophysical parameters of the soil in a field case study. A rigorous methodology, utilizing the latest knowledge for calibration and analysis was followed and is thoroughly elucidated in this paper. The reasons behind the choice of the equipment setup are described, with the intention of providing a reference for similar TDR field installations. The precision and accuracy of TDR and the validation of the calibration procedures were assessed with laboratory and field tests. The standard deviation of several TDR measurements in the laboratory was on average smaller than 2% for both apparent permittivity and ECb. The accuracy, expressed as the mean difference to reference values, was on average smaller than 2% and 3% of apparent permittivity and ECb respectively, although higher errors, up to ≈ 5% and ≈ 7.5% respectively, were measured in media with very low apparent permittivity (i.e., air) and at ECb values smaller than 0.0010 S/m. These results demonstrate that with the chosen methodology and setup, TDR can provide reliable data and can be used for long-term geophysical monitoring. The data provided by TDR monitoring stations could contribute to a data base of geophysical properties for soils. This information may eventually be used to assist the fine tuning of shallow geophysical techniques such as GPR