Construction and calibration of a field TDR monitoring station

Abstract

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.

Time domain reflectometry (TDR) potential for soil condition monitoring of geotechnical assets

“…The total cable length was kept to a minimum to minimize signal attenuation. The TDR waveforms collected in this study were saved and later analyzed to find K a using the tangent method (Heimovaara 1993;Curioni et al 2012). Temperature sensors (model 107, Campbell Scientific, Logan, USA) were also installed in the field corresponding to each TDR probe so that temperature correction could be applied to the measurements.…”

“…The TDR probes were individually calibrated in the laboratory using the final setup deployed in the field and following procedures reported in the literature (Heimovaara 1993;Robinson et al 2003aRobinson et al , 2003bCurioni et al 2012). The reference materials used for the calibration of the probes were air, acetone, and water.…”

Extending TDR Capability for Measuring Soil Density and Water Content for Field Condition Monitoring

“…The analysis of the waveforms was carried out with scripts developed using the open-source software R (copies can be provided on request). The analysis is very similar to the one reported in Curioni et al (2012). The script used to calculate K a finds the minima and the inflection points occurring in the head of this type of probe and near the reflection at the end of the probe, and intersects the corresponding tangent lines as shown in Fig.…”

“…The TDR equipment used in this study consisted of a TDR100, SDMX50 50 Ω multiplexers, common three-rod TDR probes (model CS635, 150-mm long with either a 5-m or 6-m LMR200 low-loss cable) manufactured by Campbell Scientific. The TDR probes were calibrated individually for both K a in air, acetone, and water, and BEC in potassium chloride solutions, following procedures extensively described in the literature (Heimovaara 1993;Robinson et al 2003;Lin et al , 2008Huisman et al 2008;Bechtold et al 2010;Curioni et al 2012). A separate calibration was conducted for the two different arrangements used, i.e., without multiplexers (mux0) and with two levels of multiplexers (mux2), since it is known that adding attachments can affect the TDR output (Logsdon 2006;Curioni et al 2012).…”

In-pipe GPR configuration and the determination of target depth and ground permittivity

“…Some knowledge can be gained from geological records but with some uncertainty as to whether this accurately records the infill over the first 1m in depth as the records report the natural background. In addition, the permittivity is a function of the weather conditions, especially the rainfall [5]. Delay measurements along the soil surface can provide a measure of the immediate top cover permittivity.…”