Time Domain Reflectometry Traces Research Articles

Laboratory Approach to Determine Roller Compacted Concrete Mixture Electrical Characteristics Using a New Time-Domain Reflectometry Interpretation Method

This work deals with using time-domain reflectometry (TDR) to measure the electrical properties of roller compacted concrete (RCC). It is well known that TDR provides a non-destructive method to measure the dielectric constant toward an estimation of moisture content for soil materials. However, few studies have used TDR to determine the moisture content in concrete because of the inability to obtain TDR traces after the concrete hardens. To obtain TDR traces, a transmission circuit is initiated where a wave signal moves through the medium and reflects back in accordance with transmission line theory. In the literature, the TDR waveform has been interpreted empirically to estimate the relative permittivity (or dielectric constant) and electrical conductivity in a given material relative to the determination of associated water content. However, empirical models tend to ignore certain aspects related to the electrical properties of a medium, which has made interpretation of TDR measurements prone to systematic errors. In this paper, a new approach of test configuration and TDR response interpretation has been developed. For the test setup, the approach uses disposable metal probes that can be embedded into the concrete at different depths to obtain the TDR traces. The approach also employs the transmission line equation to estimate the dielectric constant, electrical conductivity, and reflectivity of an instrumented RCC mixture. These properties will affect the understanding of the RCC pavement behavior, especially curling and warping behavior, placement density, and development of long-term distresses.

Application of Spatial Time Domain Reflectometry for Investigating Moisture Content Dynamics in Unsaturated Loamy Sand for Gravitational Drainage

The strength of unsaturated soil is defined by the soil water retention behavior and soil suction acting inside the soil matrix. In order to obtain the suction and moisture profile in the vadose zone, specific measuring techniques are needed. Time domain reflectometry (TDR) conventionally measures moisture at individual points only. Therefore, spatial time domain reflectometry (spatial TDR) was developed for characterizing the moisture content profile along the unsaturated soil strata. This paper introduces an experimental set-up used for measuring dynamic moisture profiles with high spatial and temporal resolution. The moisture measurement method is based on inverse modeling the telegraph equation with a capacitance model of soil/sensor environment using an optimization technique. With the addition of point-wise soil suction measurement using tensiometers, the soil water retention curve (SWRC) can be derived in the transient flow condition instead of the static or steady-state condition usually applied for conventional testing methodologies. The experiment was successfully set up and conducted with thorough validations to demonstrate the functionalities in terms of detecting dynamic moisture profiles, dynamic soil suction, and outflow seepage flux under transient flow condition. Furthermore, some TDR measurements are presented with a discussion referring to the inverse analysis of TDR traces for extracting the dielectric properties of soil. The detected static SWRC is finally compared to the static SWRC measured by the conventional method. The preliminary outcomes underpin the success of applying the spatial TDR technique and also demonstrate several advantages of this platform for investigating the unsaturated soil seepage issue under transient flow conditions.

Feasibility of Centralized Passive Optical Network Monitoring Using Passive Optical Network-Tuned Optical Time-Domain Reflectometry

The increasing deployment of passive optical networks implies an equally increasing need for automatic monitoring systems. This article evaluates the feasibility of obtaining reliable information on fault position and loss by direct inspection of the optical time-domain reflectometry trace and via measurements of end-fiber reflectivity devices. Detailed calculations considering parameters often neglected, such as non-uniformity of splitters, fiber bending losses, as well as different Rayleigh backscattering factors and fiber attenuation coefficients, will be provided and experimentally verified. The limits and reliability of automated solutions for optical time-domain reflectometry passive optical network monitoring will be discussed.

Automation of Pavement Sublayer Moisture Content Determination Using Long-Term Pavement Performance Time Domain Reflectometry Data and Micromechanics

Time domain reflectometry (TDR) is one of the reliable methods to measure in situ moisture content of unbound base and subgrade materials. However, the waveform data obtained from the TDR device do not provide in situ moisture content directly. Rather, the data must be analyzed to determine volumetric moisture content, which is the final product of TDR-related technology; that is, a TDR trace is analyzed to determine the value of the associated dielectric constant, which depends on the soil surrounding the TDR probe affecting the moisture content determination. However, because existing methods to determine the dielectric constant and moisture content are immersed in systematic errors, a new approach was developed; it used the transmission line equation and a three-phase micromechanics approach to improve the accuracy of the interpretation of TDR data for calculating the dielectric constant and the moisture content. Because the system identification method used to calculate parameters in the new approach was an iterative solution scheme, a computational program was required to provide rapid, accurate, and repeatable calculation of parameters. To address this need, a program was developed to facilitate the calculation of dielectric constant, dry density, and moisture content using the micromechanics approach. Details regarding the feature, algorithm, and results of the program are provided.

Robust Filtering for TDR Traces

Time-domain reflectometry (TDR) is a valuable technique for the measurement of dielectric properties of soils. The filtering of noisy TDR traces is treated in this paper. The problem of biased estimation occurs when the permittivity of the soil varies with the depth. In practical issues, only the apparent permittivity of the soil along the TDR line is available. Hence, robust optimal filtering has to be developed to provide a robust and reliable estimation. This filtering step is essential for the estimation of the permittivity with the depth.

Inversion of TDR signals—revisited

AbstractIn this study, we discuss the consistence of measured and calculated TDR traces. The calculated traces are solutions of a time domain reflectometry (TDR) forward solver, an algorithm for computing the TDR trace for a given dielectric profile along a transmission line. An unambiguous and efficient forward solver is a prerequisite for a good solution of the inverse problem, i.e., to extract the spatial distribution of the dielectric properties along the transmission line from a TDR trace. To advance our understanding of TDR inversion, we proceeded in two steps: (1) design of a TDR head section with minimal disturbances on the signal and (2) searching for causes why measured and predicted TDR traces differ. Based on a first experiment with a three‐rod TDR probe of 100 cm length, we demonstrated that our TDR forward solver—like others presented in literature—approximate the measured TDR traces apparently well but not precisely enough for signal inversion. In a second experiment, using a two‐rod TDR probe of 70 cm length, we addressed the problem of non‐parallel transmission lines. We found that the influence of a non‐parallel installation is similar to an increase of the electrical conductivity in soil water but can be distinguished from this property. A third experiment reveals that lateral and longitudinal disturbances in the vicinity of a TDR probe are of minor importance. From the analysis of our experiments, we found that neither lateral disturbances nor non‐parallel rods are responsible for the deviations between calculated and measured traces. This analysis showed us that structure in the sampled medium affects the shape of the TDR traces. Since minor deviations are essential for TDR‐signal inversion, we need new concepts to handle the fuzziness between measurements and calculations.

Determining Moisture Content of Soil Layers with Time Domain Reflectometry and Micromechanics

Although the moisture condition of pavement sublayers can significantly affect pavement performance, accurate interpretation of in situ soil moisture measurements has been difficult to achieve because of the limitations of existing methods. Time domain reflectometry (TDR), originally developed to detect breaks or shorts in electrical conductors, has been used for measuring parameters related to the in situ soil moisture content. However, the apparent length method currently used to determine dielectric constant ignores other electrical properties of the conducting medium that may affect the interpretation of TDR trace to determine soil moisture. Furthermore, the existing methods for computing volumetric water content ignore the variations of dry density and determine the model parameters with assumption or regression analysis. These deficiencies can, in many cases, create a significant systematic error in the final determination of volumetric water content. To minimize these errors and improve the accuracy of moisture content estimate, a new three-step approach was proposed. The approach uses the transmission line equation to calculate the dielectric constant, conductivity, and reflectivity of a soil mixture. A micromechanics and self-consistent scheme was used to determine the volumetric moisture content and dry density on the basis of calibrated values of the solid and water dielectric constants. The system identification method was used iteratively to solve for dielectric parameters, soil moisture content, and dry density values. The validation of the new approach with ground-truth data indicated that the calculated errors were significantly less than those of existing method.

Role of dispersive effects in determining probe and electromagnetic parameters by time domain reflectometry

Time domain reflectometry (TDR) measurements, performed in a cylindrical tube filled with air, glass beads, and water, are analyzed in the time and frequency domains. In the time domain, air and water provide contrasting results of the probe length if the standard tangent line fitting procedure is used. An alternative method, based on the time derivative of the TDR response, is introduced. Its application suggests that the anomalous result provided by the tangent method in water is explained by dispersion effects. To account for the frequency dependence of the permittivity, an analysis in the frequency domain is performed. The problems of the input function choice and of the noise control are investigated. This allows us to correct the experimental scatter function S(ν) and to perform the best fit of its parameterized expression, which is theoretically evaluated from the Debye model of the permittivity. Consistent values of the probe length are derived. It is concluded that the tangent method cannot be applied for determining the geometrical and electromagnetic parameters of a probe filled by dispersive media. As an application, the calibrated probe is used for measuring the TDR trace of glass beads, which simulate sandy soils, and for deriving the frequency‐dependent permittivity within the Debye model.

Efficient reconstruction of dispersive dielectric profiles using time domain reflectometry (TDR)

Abstract. We present a numerical model for time domain reflectometry (TDR) signal propagation in dispersive dielectric materials. The numerical probe model is terminated with a parallel circuit, consisting of an ohmic resistor and an ideal capacitance. We derive analytical approximations for the capacitance, the inductance and the conductance of three-wire probes. We couple the time domain model with global optimization in order to reconstruct water content profiles from TDR traces. For efficiently solving the inverse problem we use genetic algorithms combined with a hierarchical parameterization. We investigate the performance of the method by reconstructing synthetically generated profiles. The algorithm is then applied to retrieve dielectric profiles from TDR traces measured in the field. We succeed in reconstructing dielectric and ohmic profiles where conventional methods, based on travel time extraction, fail.

A new TDR waveform analysis approach for soil moisture profiling using a single probe

Time Domain Reflectometry (TDR) has been widely accepted as a reliable technology for the measurement of volumetric water content ( θ) in soils. Here, we present a new procedure for the graphical interpretation of TDR waveforms to determine the variation of the apparent dielectric constant ( K) along a single TDR probe in non-conducting media and its application for profiling the moisture content in soils. The method is based on the influence of K on the reflection coefficient ( ρ) and travel time ( t) of the TDR signal along a transmission line. A ρ( t) function is initially defined for a length l of 10 cm and plotted together with the TDR waveform for a three-rod probe of length L ( L= nl). The interception point of both lines defines t 11 as the reference time to build a second ρ( t) line that intercepts the waveform at time t 2l. By repeating this process iteratively, a series of ρ( t) lines intercepting the TDR trace at times t 3 l, t 4 l,… and t nl is obtained, making it possible to calculate K for apparent probe lengths equal to l,2l,3l… and nl. To test the consistency of the method, two sand column experiments were conducted with two different TDR probe geometries. A total of 144 values of K were measured during a wetting and draining cycle of the sand column with vertical 10-, 20-, 30-, and 40-cm long three-wire uncoated TDR probes (diameter d: 2.8 mm; spacing of the outer conductors s: 32 mm) and 10-, 20-, 40-, and 60-cm long coated TDR probes ( d: 10 mm; s: 80 mm) using the standard double reflection waveform analysis. A satisfactory relationship ( R 2=0.99) was found between these K values and those obtained for the same depths by the proposed method applied to the 40- and 60-cm long probes, respectively. In this case, a probe-specific correction factor was used for the different probe geometries. Likewise, an excellent match was found between the θ profiles measured in the sand column applying the new approach to the 60-cm long probe and the θ profiles measured with horizontal probes placed at 10, 20, 30 and 50 cm depth. The results show that the proposed method is sound and suitable for determining the variation of K at fixed intervals along a single probe and therefore for soil water content profiling.

Laboratory evaluation of time domain reflectometry for continuous monitoring of stream stage, channel profile, and aqueous conductivity

Time domain reflectometry (TDR) operates by propagating a radar frequency electromagnetic pulse down a transmission line while monitoring the reflected signal. As the electromagnetic pulse propagates along the transmission line, it is subject to impedance by the dielectric properties of the media along the transmission line (e.g., air, water, and sediment), reflection at dielectric discontinuities (e.g., air‐water or water‐sediment interface), and attenuation by electrically conductive materials (e.g., salts and clays). Taken together, these characteristics provide a basis for integrated stream monitoring, specifically, concurrent measurement of stream stage, channel profile, and aqueous conductivity. Requisite for such application is a means of extracting the desired stream parameters from measured TDR traces. Analysis is complicated by the fact that interface location and aqueous conductivity vary concurrently and multiple interfaces may be present at any time. For this reason a physically based multisection model employing the S11 scatter function and Debeye parameters for dielectric dispersion and loss is used to analyze acquired TDR traces. Here we explore the capability of this multisection modeling approach for interpreting TDR data acquired from complex environments, such as encountered in stream monitoring. A series of laboratory tank experiments was performed in which the depth of water, depth of sediment, and conductivity were varied systematically. Comparisons between modeled and independently measured data indicate that TDR measurements can be made with an accuracy of ±3.4 × 10−3 m for sensing the location of an air‐water or water‐sediment interface and ±7.4% of actual for the aqueous conductivity.

Determination of back-scatter coefficient from third-order Rayleigh effect in a Raman amplifier

We propose a precise measurement technique for the Rayleigh back-scattering coefficient based on a single-ended optical time-domain reflectometry trace analysis. By comparing the relative strength of the Rayleigh components-Rayleigh scattering and triple Rayleigh scattering-we show that it is possible to avoid the uncertainties associated with previous measurement techniques, achieving less than 0.1-dB error in the determination of the Rayleigh coefficient. Application of this technique to the noise characterization of the fiber Raman amplifiers will be provided at the end.

Application of TDR to water level measurement

A specialised time domain reflectometry (TDR) probe for measuring water level in tanks collecting surface runoff was developed, calibrated and field-tested. The water level probe — in the form of a slightly modified soil moisture probe — was developed as part of a TDR measuring system designed for continuous monitoring of soil water content and surface runoff in plot studies of water erosion and sediment transport. A computer algorithm for the analysis of TDR traces from the new probe was developed and incorporated into existing software for automated acquisition and analysis of TDR data. Laboratory calibration showed that water level could be measured with sufficient accuracy (standard deviation <2 mm) for a range of applications in hydrology. Soil erosion is typically a short duration process closely linked to soil moisture content and rainfall intensity. A major benefit of integrating time critical measurements of surface runoff and soil moisture into a single system is the synchronisation of measurements. Measurements were made on a regular schedule except during rainfall events when the measuring rate depended on rainfall intensity. In a parallel calibration study it was shown that the performance of the TDR probe was comparable to a commercial ultrasonic liquid level sensor used for measuring runoff at an erosion site not instrumented for automated TDR measurements.

Evaluation of In Situ Moisture Content at Long-Term Pavement Performance Seasonal Monitoring Program Sites

Time domain reflectometry (TDR) has become one of the most reliable nondestructive methods for measuring in situ soil moisture content. TDR sensors developed by the Federal Highway Administration are being used in the Long-Term Pavement Performance (LTPP) Seasonal Monitoring Program (SMP) to monitor the in situ moisture content at 64 LTPP sites. The main goal of this study is to develop procedures to produce good estimates of in situ gravimetric moisture content. All the TDR traces in the LTPP information management system database that were recorded at LTPP SMP test sections were processed using the approach described in this paper. To estimate the in situ gravimetric moisture content, methods were selected to interpret TDR traces. An algorithm and a computer program, Moister, were developed to implement these TDR interpretation methods. Then the apparent length of the TDR trace and the dielectric constant of the unbound material were computed. Models were developed to relate dielectric constant with in situ volumetric moisture content. Finally, gravimetric moisture content was computed using the volumetric moisture content value and dry density of the soil. A diagnostic study of the computed gravimetric moisture content was also conducted to evaluate the reasonableness of the computed moisture content.

MEASUREMENTS OF CUMULATIVE INFILTRATION AND WETTING FRONT LOCATION BY TIME DOMAIN REFLECTOMETRY

Knowledge of wetting front location and cumulative infiltration during irrigation and rainfall events are important to many areas of soil science. We tested a new procedure using time domain reflectometry (TDR) to estimate wetting front location and cumulative infiltration. The method is based on an assumption of a uniform soil water content behind the wetting front. Measurements of wetting front location and cumulative infiltration were made with 0.144-m-long TDR probes vertically inserted into homogeneous loamy sand columns. Unlike previous procedures using TDR, the new procedure could be applied to short TDR probes (≤ 0.3 m) because the procedure did not require determination of a reflection of electromagnetic waves from the wetting front in TDR traces. The method gave satisfactory estimates of both wetting front location and cumulative infiltration.

Comparison of Three Methods to Obtain the Apparent Dielectric Constant from Time Domain Reflectometry Wave Traces

AbstractA calibration of time domain reflectometry (TDR) probes to measure soil water in a particular soil is desirable since no consistent relationship between water content and apparent dielectric constant (Ka) has been found. We compared three procedures to compute Ka from TDR traces: a manual method that uses a plot of the wave trace, a derivative‐based computer algorithm, and fitting a simulated TDR trace to a measured one with a simple multiple reflection model. We added a model of inertia to the multiple reflection model to approximate the rise time of the cable tester. The TDR traces were measured in situ with a Tektronix 1502B cable tester (Tektronix, Inc., Wilsonville, OR) on samples from two soils with contrasting textures, Beltsville silt loam (fine‐loamy, mixed, mesic Typic Fragiudult) and Rumford loamy sand (coarse‐loamy, siliceous, thermic Typic Normudult). Ten‐centimeter probes with three rods were used. The water contents were measured gravimetrically with soil cores. Calibration with apparent dielectric constants obtained from the derivative‐based algorithm had the smallest standard error for both soils, and the manual method was better than the wave simulation method. The intercepts and slopes of the calibration equations for the manual and derivative methods were not significantly different from each other for both soils. The wave simulation method, which gives physically meaningful values for Ka, and characteristic impedances can be a useful tool for theoretical studies.

Ion dynamics of a freezing soil monitored in situ by time domain reflectometry

In situ measurements of the electrical conductivity of the soil solution are cumbersome, especially under frozen conditions, and often involve destruction of the soil volume sampled. The objective of this work was to use time domain reflectometry (TDR) to study the dynamics of ions in the soil solution caused by freezing and thawing. The data set consisted of field‐measured TDR and temperature records in four soil profiles in a loamy soil. Bulk electrical conductivity and electrical conductivity were evaluated from the TDR traces. Freezing caused a temporary twofold to fourfold increase in electrical conductivity. Strong evidence of a downward transport of ions during freezing was found. The transport was attributed to thermally induced regelation. Infiltration resulted in an increased electrical conductivity. TDR was shown to be a promising technique for monitoring the electrical conductivity of the soil solution, in both frozen and unfrozen soils.

Field measurements of soil electrical conductivity and water content by time-domain reflectometry

The ability to easily and simultaneously measure temporal distributions of both soil water content and soil salinity is desirable when conducting experiments designed to evaluate management practices in irrigated agriculture where saline waters are used. Soil salinity is related to soil electrical conductivity. Soil electrical conductivity and water content in a ridge-furrow tilled loamy sand irrigated with a salt solution were measured using time-domain reflectometry (TDR). The salt solution was 0.026 mol kg −1 NaCl which has the electrical conductivity of about 0.3 S m −1. A computer controlled multiplexer was used to acquire electrical conductivity and water content data from a metallic cable tester. Information in TDR traces related to water content and electrical conductivity was not appreciably affected by impedance mismatch within a rotary switch, used as a multiplexer. Soil water content and electrical conductivity of soil solution estimated from TDR agreed well with sampled data.

Simple Baluns in Parallel Probes for Time Domain Reflectometry

Time domain reflectometry (TDR) is used for the simultaneous determination of soil moisture content and electrical conductivity (EC). Limitations of parallel probes compared with coaxial probes have been signal losses in the antenna wire leading to the probes, and the required use of an expensive balun. In addition, signal attenuation in the balun hampered accurate EC determination. We describe a balun that avoids these limitations. It is a 1:1 balun, which means that it converts the electrical field from unbalanced to balanced, but no impedance transformation takes place. Consequently, this balun does not attenuate low-frequency signals, which is inherent to commonly used commerical 1:4 baluns. The balun is inexpensive and easy to fabricate, and is attached directly to a parallel pair of waveguides. This permits a system with coaxial transmission line from the TDR to the parallel waveguides. For soil moisture content determinations, the new balun performs at least as well as a conventional balun. Furthermore, EC of NaCl solutions determined from the TDR trace showed excellent agreement with EC measured with a conductance meter.

Time domain reflectometry for measuring volumetric water content in processed oil shale waste

Time domain reflectometry (TDR) was evaluated and developed to monitor volumetric water content (θυ) in oil shale solid waste retorted and combusted by the Lurgi‐Ruhrgas process. A TDR probe was designed and tested that could be buried and compacted in waste embankments and provide in situ measurements for θυ in the high‐saline and high‐alkaline conditions exhibited by this waste. TDR was found to be accurate for measurement of θυ across a broad range of water contents in the processed oil shale waste. A computer algorithm to automate the analysis of TDR traces to determine θυ, was developed and tested. A sensitivity test was performed to analyze differences between three smoothing algorithms on the measurement. No significant differences were found between smoothing algorithms or between the number of points applied for smoothing.