TDR Measurements Research Articles

Quantitative, zerstörungsfreie Feuchtemessung in Baustoffen vor Ort mit der Time Domain Reflectometry / Quantitative, non-destructive moisture measurement in building materials on site by time-domain re ectometry

Abstract A new commercially available dielectric technique for the non-destructive determination of moisture in building materials based on the principle of 'time-domain reflectometry' (TDR) is presented. TDR measurements on samples of sandstone, brick, concrete and floor cover matched very well with results of conventional moisture measuring methods such as oven-drying or calciumcarbide-technique. The new method showed only a low influence of salt content or surface moisture of the material on the results.

Estimating Rainfall Recharge and Soil Water Residence Times in Pukekohe, New Zealand, by Combining Geophysical, Chemical, and Isotopic Methods

AbstractThe combined use of geophysical (time domain reflectometry [TDR]), isotopic (δ18 O and δ2H) and chemical (NO 3 ‐N, Cl) techniques indicates that residence times in the unsaturated zone of a fractured basalt aquifer in the Pukekohe region of the North Island, New Zealand, are at least six months. Each technique provides useful information on specific aspects of recharge and residence times, but combined they provide a basis for determining the mechanisms of water movement in the unsaturated zone. TDR soil moisture measurements provide similar rainfall recharge estimates as model calculations over longer time periods (monthly or seasonal) but may be more accurate for shorter time scales (weekly or daily).The TDR measurements indicate that recharge occurs mainly in the late autumn and winter months. Measurements of nitrate concentrations in the soil profile over this same period, suggest that nitrate is mineralized in the soil during the summer and is flushed past the root zone at the beginning of the recharge period, in early winter. Nitrate concentrations in the soil profile do not increase in concentration later in the recharge period, even after significant recharge events have occurred, because all stored nitrate has already been released. This pattern of nitrate movement indicates that fertilizer applications in spring and early summer will leach less to ground water and applications made in autumn and winter will leach more. Estimations of yearly recharge from mean monthly chloride concentrations (730 mm/yr) are roughly in agreement with the TDR estimates (680 mm/year). However, there is a large range in the chloride estimates (>100%) because monthly chloride concentration measurements in both the rain and soil water vary due to the proximity of the site to the ocean.

Quality assessment of restored soils: combination of classical soil science methods with ground penetrating radar and near infrared aerial photography?

In Switzerland agricultural land is usually restored after gravel exploitation. In order to minimize soil damage, the quality of restored soils should be checked by the authorities. To assess the physical soil properties, a combination of classical soil science methods with ground-penetrating radar (GPR) and near infrared (IR) aerial photography was tested in 1994–1995. GPR profiles were recorded in the reflection mode using 200-MHz, 500-MHz and 900-MHz antennae. IR colour photographs were taken at a scale of 1:3000 from an aircraft using a photogrammetric camera, and at a scale of 1:1000 and 1:500 from a gas balloon using an amateur camera. The tests were accompanied by soil-profile descriptions, TDR and tensiometer measurements and analysis of the soil composition in the laboratory. The experimental site was located in the Swiss Central Plateau. The area was restored in 1994 and mainly covered with ryegrass and clover. The soil structure was poorly developed, with a stone content of more than 20 vol %. Even within the topsoil, the air supply was partly insufficient. The physical soil properties were better higher up on the slope than on the lower areas. Some GPR reflections seem to correspond to water-content changes and stones within the soil profile. But a correlation between the spatial soil property changes and the radar reflections cannot be proved. However, changes of the soil water content over time might be detected with repeated GPR measurements by comparing the signal traces of the same profile. The IR aerial photographs show differences in vegetation density and vitality, which were also detectable by ground checking. A direct relationship between the variations of the vegetation cover and the variations of the physical soil properties cannot be proved, but aerial photography has the advantage of allowing a repeatable documentation of grass conditions over a total area simultaneously, and may therefore be useful in gaining a general overview of the area. For this, photographs taken from an aircraft with a photogrammetric camera are recommended. Investigation of restored areas still has to be based on classical macromorphological soil-profile descriptions. This method is not hindered by a high stone content. Soil pits are recommended in the first year after soil restoration and again a few years later.

Quality assessment of restored soils: combination of classical soil science methods with ground penetrating radar and near infrared aerial photography?

In Switzerland agricultural land is usually restored after gravel exploitation. In order to minimize soil damage, the quality of restored soils should be checked by the authorities. To assess the physical soil properties, a combination of classical soil science methods with ground-penetrating radar (GPR) and near infrared (IR) aerial photography was tested in 1994–1995. GPR profiles were recorded in the reflection mode using 200-MHz, 500-MHz and 900-MHz antennae. IR colour photographs were taken at a scale of 1:3000 from an aircraft using a photogrammetric camera, and at a scale of 1:1000 and 1:500 from a gas balloon using an amateur camera. The tests were accompanied by soil-profile descriptions, TDR and tensiometer measurements and analysis of the soil composition in the laboratory. The experimental site was located in the Swiss Central Plateau. The area was restored in 1994 and mainly covered with ryegrass and clover. The soil structure was poorly developed, with a stone content of more than 20 vol %. Even within the topsoil, the air supply was partly insufficient. The physical soil properties were better higher up on the slope than on the lower areas. Some GPR reflections seem to correspond to water-content changes and stones within the soil profile. But a correlation between the spatial soil property changes and the radar reflections cannot be proved. However, changes of the soil water content over time might be detected with repeated GPR measurements by comparing the signal traces of the same profile. The IR aerial photographs show differences in vegetation density and vitality, which were also detectable by ground checking. A direct relationship between the variations of the vegetation cover and the variations of the physical soil properties cannot be proved, but aerial photography has the advantage of allowing a repeatable documentation of grass conditions over a total area simultaneously, and may therefore be useful in gaining a general overview of the area. For this, photographs taken from an aircraft with a photogrammetric camera are recommended. Investigation of restored areas still has to be based on classical macromorphological soil-profile descriptions. This method is not hindered by a high stone content. Soil pits are recommended in the first year after soil restoration and again a few years later.

Spatial variability of soil water content in the covered catchment at Gårdsjön, Sweden

The spatial variability of soil water content was investigated for a 6300 m2 covered catchment on the Swedish west coast. The catchment podzol soil is developed in a sandy—silty till with a mean depth of 43 cm and the dominant vegetation is Norway spruce. The acid precipitation is removed by a plastic roof and replaced with lake water irrigated under the tree canopies. On two occasions, in April and May 1993, TDR measurements were made at 57–73 points in the catchment using 15 and 30 cm long vertically installed probes. The water content pattern at the two dates, which occurred during a relatively dry period, were similar. The range of water content was large, from 5 to 60%. In May 1993 measurements also were made in areas of 10 × 10 m, 1 × 1 m and 0·2 × 0·2 m. The range and standard deviation for the 10 × 10 m area, which apart from a small-scale variability in soil hydraulic properties and fine root distribution also had a heterogeneous micro- and macro-topography, was similar to the range and standard deviation for the catchment. The 1 × 1 m and 0·2 × 0·2 m areas had considerably lower variability. Semi-variogram models for the water content had a range of influence of about 20 m. If data were paired in the east-–west direction the semi-variance reflected the topography of the central valley and had a maximum for data pairs with internal distances of 20–40 m. The correlation between soil water content and topographic index, especially when averaged for the eight topographically homogeneous subareas, indicated the macro-topography as the cause of a large part of the water content variability.

Water Flow through Intact Soil Columns: Measurement and Simulation Using LEACHM

AbstractAn evaluation of the water flow submodel, LEACHW, of the LEACHM solute transport model was conducted using profiles of volumetric water content, θ(z), and hydraulic head, H(z), measured under steady sated and near‐saturated flow conditions in two large, intact, soil columns. The hydraulic conductivity (K)‐water content (θ)‐pressure head (ψ) relationships, K‐θ‐ψ, required by LEACHW were obtained by fitting the Van Genuchten functions to measurements of sated hydraulic conductivity (Ks) and θ vs. ψ obtained: (i) using in‐situ TDR and tensiometer measurements, with the θ‐ψ data being collected during the initial column wetting; and (ii) using the traditional saturated flow‐desorption technique on intact soil core subsamples collected from the columns at the end of the experiment. The LEACHW model provided reasonably accurate predictions of the θ(z) and H(z) profiles, regardless of whether the in‐situ or soil core‐based K‐θ‐ψ relationships were used, and regardless of whether sated or near‐saturated flow occurred. The LEACHW predictions were substantially more accurate, however, when the K‐θ‐ψ relationships derived from the in‐situ measurements were used. It was concluded that the LEACHW model, coupled with independently measured K‐θ‐ψ functions, could provide reasonably accurate predictions of steady θ(z) and H(z) profiles in large, intact soil columns, regardless of whether the soil macropores are largely water‐conducting (sated flow) or largely nonwater‐conducting (near‐saturated flow). It also appears that in‐situ Ks and θ‐ψ measurements, with the θ‐ψ data collected during initial wetting, may provide more appropriate K‐θ‐ψ relationships for use in water and solute transport models than the traditional saturated flow‐desorption method using intact soil core subsamples.

Assessing Temporal Variations in Soil Water Composition with Time Domain Reflectometry

AbstractTime domain reflectometry (TDR) can be used to study temporal variations in volumetric soil water content (θ) and bulk soil electrical conductivity (σa). The variations in σa are associated with changes in θ and the soil water composition. Laboratory and field experiments were conducted to verify if TDR can be used to monitor the temporal variation in the soil water composition between solution sampling occasions. Effects of cable length and temperature on the σa measurement were evaluated. Including the series resistance of the cable and connectors in the analysis improves measurements at high electrical conductivity levels. The temperature factor of the bulk soil appears to be similar to the temperature factor of soil extracts. Laboratory experiments showed that the theoretical model giving σa as function of θ and the electrical conductivity of the soil solution (σw) combined with the water retention function was capable of describing σw measured on soil solution extracted with ceramic cup solution samplers under static water flow conditions. After optimization of a single parameter, the model was able to describe σw values of the soil solution obtained in the laboratory, whereas literature values were sufficient for field data. Concentrations of a number of solutes in a field data set spanning 3 yr were positively correlated with σw. Site‐specific regressions between solute concentration and σw combined with automated TDR measurements of σa and θ enable a more meaningful interpretation of the temporal variation of the concentration of major solutes present in the soil solution between sampling occasions.

Design of Triple‐Wire Time Domain Reflectometry Probes in Practice and Theory

AbstractTime domain reflectometry (TDR) is accepted as a valuable tool for measuring soil water content and bulk soil electrical conductivity. The accuracy of TDR measurements depends on the quality and type of probes, as well as on the length of cable used. The objective of this study was to test the effects of different triple‐wire TDR probe dimensions and cable lengths on the measurements. Additional measurements were done in order to test the performance of small triple‐wire probes in soils with a wide range of water contents. Measurements in air and in water showed that the position of the first reflection from the connection between cable and probe is influenced by the dielectric medium. This problem was solved by using the zero of the cable tester as a time reference, and calibrating the TDR probes before measurements. The major effect of increasing cable lengths is that the rise time of the TDR voltage pulse increases, spreading each reflection across a larger time interval, which influences the accuracy of the wave form analysis and causes a possible underestimation of the apparent dielectric permittivity in dry soils. As a result, it is not possible to use short probes with long cable lengths. Smaller spacing of the wires results in steeper reflections from the end of the probe. Small triple‐wire probes are convenient in laboratory applications. Large triple‐wire probes are convenient for automated field applications in which long lengths of cable are required.

The application of TDR in laboratory column experiments

An automated TDR system was used together with an automated tensiometer system in laboratory column experiments for the measurement of unsaturated hydraulic properties of soils. The laboratory experiments were a stationary flux (sprinkling infiltrometer) carried out with different fluxes, and a transient (evaporation) method. Simultaneous measurements of water content and pressure head give the water retention characteristic and enable the application of the instantaneous profile analysis for calculating the hydraulic conductivity. This direct analysis is compared with the traditional analysis methods normally used for the laboratory experiments. The application of automated measurement techniques and the use of TDR increases the speed of the measurements, generates more data points and requires less assumptions in the data analysis. Moreover, TDR measurements give insight in the flow process, including effects of hysteresis and heterogeneity within a sample. Performing the sprinkling infiltrometer and evaporation experiments on the same sample enables the determination of the unsaturated hydraulic conductivity at pressure heads ranging from −0.01 m to −6 m.

Monitoring the unfrozen water content of soil and snow using time domain reflectometry

Time domain reflectometry is a technique that can be used to indirectly measure the in situ moisture content of soil. Previously, this method was not used in the field to continuously monitor the liquid water content because of the influence it had on the wetting, drying, freezing and thawing cycles of the soil. The principal objectives of this field investigation were, apply the TDR technique to monitor the unfrozen water content in the soil, utilize this technique to determine snowmelt infiltration into seasonally frozen soils, and explore the feasibility of using the TDR technique to monitor snowmelt percolation in the snowpack. An additional goal of this paper was to explain in a straightforward manner how to use the TDR technique to obtain the liquid water profile in a soil. Various configurations of parallel transmission lines were installed horizontally at various depths in the soil and also in the snowpack. This technique gave a good delineation of the unfrozen water content with depth in frozen soils. Results looked promising in snow if in situ snow density measurements were taken along with the TDR measurements.

Measurements of dielectric properties by time domain spectroscopy

The dielectric response function of severals alcohols is calculated from TDR measurements in the temperature range 0 to 50 °C. Using simple formulas derived by real time analysis, this function was computed from the time integral and self-convolutions up to the second order of the reflections produced by an incident step voltage pulse on a finite sample inserted in a low-loss coaxial line. The calculations take account of finite amplitude of the reflection and the response function in third order approximation and of finite rise time. Precision of the analysis and the experiments are discussed.

Low frequency sound attenuation in the deep ocean : Urick R. J., 1963. J. Acoust. Soc. Amer., 35 (9): 1413–1422.

This study presents an novel Bayesian inversion scheme for high-dimensional undetermined TDR waveform inversion. The methodology quantifies uncertainty in the moisture content distribution, using a Gaussian Markov random field (GMRF) prior as regularization operator. A spatial resolution of 1 cm along a 70-cm long TDR probe is considered for the inferred moisture content. Numerical testing shows that the proposed inversion approach works very well in case of a perfect model and Gaussian measurement errors. Real-world application results are generally satisfying. For a series of TDR measurements made during imbibition and evaporation from a laboratory soil column, the average root-mean-square error (RMSE) between maximum a posteriori (MAP) moisture distribution and reference TDR measurements is 0.04 cm3 cm−3. This RMSE value reduces to less than 0.02 cm3 cm−3 for a field application in a podzol soil. The observed model-data discrepancies are primarily due to model inadequacy, such as our simplified modeling of the bulk soil electrical conductivity profile. Among the important issues that should be addressed in future work are the explicit inference of the soil electrical conductivity profile along with the other sampled variables, the modeling of the temperature-dependence of the coaxial cable properties and the definition of an appropriate statistical model of the residual errors.