Active Time-Domain Reflectometry for Unattended Safeguards Systems FY15 Report

The International Atomic Energy Agency (IAEA) continues to expand its use of unattended measurement systems. An increasing number of systems and an expanding family of instruments create challenges in terms of deployment efficiency and the implementation of data authentication measures. Traditional data security measures, for example tamper-indicating (TI) conduit, are impractical for the long separation distances (often 100 meters or more) between unattended monitoring system (UMS) components. Pacific Northwest National Laboratory (PNNL) is studying the viability of active time-domain reflectometry (TDR) for the detection of cable tampering in unattended radiation detection systems. The instrument concept under investigation would allow for unmanned cable integrity measurements, remote surveillance reporting and locating of cable faults and/or tampers. This report describes PNNL’s FY16 progress and includes: an overview of the TDR methods under investigation; description of the TDR evaluation testbed developed by PNNL; development and testing of advanced signal processing algorithms to extract weak signals from relatively high noise levels; and initial testing of a laboratory prototype intended for IAEA UMS applications and based on a commercially available TDR module. Preliminary viability findings and recommendations for the next stage of development and testing are provided.

Time domain reflectometry (TDR) and the heat pulse method are both used to measure soil water content. Changes in ambient temperature have been shown to affect TDR measurements, but less is known about the behavior of heat pulse sensors in response to changes in temperature. This study directly measured and compared the temperature sensitivity of the TDR and heat pulse methods. Both methods were used to estimate water content in silt loam and sand at two fixed water contents across a wide temperature range. An increase in temperature led to an increase in measured water content in most cases. Across the 40°C temperature range, changes in measured water content were generally 0.04 m 3 m −3 or less for both methods. Weighted linear regression showed that in these soils the heat pulse method exhibited greater temperature sensitivity than the TDR method, although the differences were not statistically significant. A previously proposed correction for the temperature sensitivity of the TDR method produced mixed results. The temperature sensitivity of the heat pulse method was attributed to the changes in the density and specific heat of water and specific heat of soil with respect to temperature. When the changes in these parameters were accounted for, the temperature sensitivity was eliminated in three out of four cases.

Comparison of TDR and X-ray method for determining moisture transport parameters

Significant quantities of agricultural chemicals can be transported rapidly through preferential flow pathways in soil. Time domain reflectometry (TDR) has been used to characterize solute transport in soil. However, previous TDR studies have not fully addressed the preferential flow of reactive solutes. A TDR method was tested for its ability to measure the preferential flow of nitrate and phosphate in soil. Saturated miscible displacement experiments were conducted using three undisturbed soil cores and a tracer solution containing chloride, phosphate, and nitrate. An inverse curve fitting method (CXTFIT) was used to estimate mobile-immobile model (MIM) parameters using the TDR and observed effluent data. The parameters fitted to the time-varying TDR-determined relative resident concentration were similar to the estimates from measured effluent chloride breakthrough curves (BTCs). Predicted BTCs were obtained from the parameters fitted to the TDR data. The predicted BTCs were comparable to the measured effluent nitrate BTCs, with a root mean square error (RMSE) of 0.0054. The peak times were 0.18, 0.16, and 0.12 pore volumes for the predicted BTCs and 0.26, 0.19, and 0.21 for the effluent nitrate BTCs. Phosphate BTCs differed distinctly from chloride and nitrate BTCs in our study; thus, the TDR method did not work for phosphate. However, the vertical TDR probe technique proved to be a practical method for a first approximation of nitrate preferential flow in soil.

Sorption is a well-known phenomenon that may cause the retardation effect of zinc in the subsurface environment. In this study, the governing process for zinc sorption during transport was investigated by conducting 2-D plume tests in a laboratory scale sand tank model using the time domain reflectometry (TDR) method. Tracer solutions of NaNO3 and ZnSO4 were applied at a constant flow rate as a pulse type to capture the plumes of both solutes based on TDR-measured resistance. It was revealed that the observed zinc sulfate plume showed no retardation relative to sodium nitrate with a retardation factor of R ≈ 1. Instead of retardation, a prominent reduction of zinc sulfate mass occurred during transport through the tank model due to the irreversible sorption as well as longitudinal dispersion. This indicates that the controlling factor for the sorption process of zinc sulfate in the sand tank model is kinetic rather than equilibrium. These hydrogeological parameters would provide valuable information on the prediction of the fate of zinc in sandy aquifer materials.

A series of infiltration and tracer experiments was conducted in unsaturated sand and gravel deposits on Cape Cod, Massachusetts. A network of 112 porous cup lysimeters and 168 time domain reflectometry (TDR) probes was deployed at depths from 0.25 to 2.0 m below ground surface along the centerline of a 2‐m by 10‐m test plot. The test plot was irrigated at rates ranging from 7.9 to 37.0 cm h−1 through a sprinkler system. Transient and steady state water content distributions were monitored with the TDR probes and spatial properties of water content distributions were determined from the TDR data. The spatial variance of the water content tended to increase as the average water content increased. In addition, estimated horizontal correlation length scales for water content were significantly smaller than those estimated by previous investigators for saturated hydraulic conductivity. Under steady state flow conditions at each irrigation rate, a sodium chloride solution was released as a tracer at ground surface and tracked with both the lysimeter and TDR networks. Transect‐averaged breakthrough curves at each monitoring depth were constructed both from solute concentrations measured in the water samples and flux concentrations inferred from the TDR measurements. Transport properties, including apparent solute velocities, dispersion coefficients, and total mass balances, were determined independently from both sets of breakthrough curves. The dispersion coefficients tended to increase with depth, reaching a constant value with the lysimeter data and appearing to increase continually with the TDR data. The variations with depth of the solute transport parameters, along with observations of water and solute mass balance and spatial distributions of water content, provide evidence of significant three‐dimensional flow during the irrigation experiments. The TDR methods are shown to efficiently provide dense spatial and temporal data sets for both flow and solute transport in unsaturated sediments with minimal sediment and flow field disturbance. Combined implementation of lysimeters and TDR probes can enhance data interpretation particularly when three‐dimensional flow conditions are anticipated.

아스팔트 콘크리트 포장의 하부구조는 보조기층, 동상방지층 및 노상층으로 구성되어 있으며, 노상층의 경우 겨울철을 지나 봄이되면 동상효과에 따른 포장 파손과 여름철 장마 기간 동안의 수분 침투로 인한 지지력 감소와 같은 문제점이 발생할 수 있다. 따라서 도로하부의 함수비를 계측하기 위하여 시간이력 반사방식(Time Domain Reflectometry, TDR)의 함수센서를 통하여 실시간 함수비를 계측하는 기술이 국내에 도입되었다. TDR 방식의 함수센서는 흙과 물의 유전율을 통하여 함수비를 측정하기 때문에 온도의 영향을 받을 수 있다. 특히 국내는 계절 영향을 받아서 겨울철 대기 온도가 지역적으로 영하 <TEX>$20^{\circ}C$</TEX> 보다 낮은 온도를 보여줄 수 있기 때문에 영하의 측정 온도 범위에 대한 함수센서의 정확도를 파악할 필요가 있다. 따라서 본 연구에서는 흙의 온도 범위를 영하 <TEX>$2^{\circ}C$</TEX> 미만까지 확장하여 TDR 방식의 함수비 센서에 대한 온도 민감도 시험을 수행하였다. 체적함수비 20%와 30%에 대한 시험 결과 공시체 표면 온도가 영하로 떨어지면서 센서부근의 수분이 감소하고, 공시체 상부의 함수비가 증가하는 경향을 보여주었다. 그러나 함수센서에 대한 온도의 영향은 함수비가 낮을 경우 미소한 것으로 나타났다. The infrastructure of flexible pavement is composed of aggregate subbase, anti-frost layer, and subgrade. In particular, the subgrade performance is affected by climates such as frost action and precipitation. The method of TDR(Time Domain Reflectometry) sensors to measure moisture contents in subgrade layer has been used in the research. Due to the TDR method using dielectric permitivity of soil and water, the sensors can be affected by the low subgrade temperatures. The air temperatures frequently drops below <TEX>$-20^{\circ}C$</TEX> in the winter in Korea. As a result, it is necessary to estimate the accuracy of the TDR moisture sensors in the range of below zero temperatures. In this study, the subgrade temperatures of lower than <TEX>$-2^{\circ}C$</TEX> were extended to evaluate temperature sensitivity of the TDR moisture sensors. The test results revealed that the moisture contents around the sensors were reduced while those of the upper part of specimen showed a tendency to increase as the specimen surface temperature drops below zero under the volumetric moisture contents(VMC) of 20% and 30%. However, the impact of temperature on the function of the sensor at lower water contents was found to be negligible if any.

Time and frequency domain reflectometry for the measurement of tree stem water content: A review, evaluation, and future perspectives

Time domain reflectometry (TDR) is a technique that allows simultaneous estimates of apparent permittivity (ε a ) and hence volumetric water content (θ) and bulk electrical conductivity (σ a ). Difficulties arise for θ and σ a determination, however, when uncoated TDR probes (UP) are used in highly conductive media. This work shows that σ a can be estimated in highly conductive media using a TDR probe coated with a high‐dielectric insulator (CP). To this end, the Dalton method for σ a estimations was applied to a 10‐cm‐long three‐rod TDR probe insulated with a 0.2‐mm‐thick epoxy‐ceramic composite coating with a relative permittivity, ε r , of 32.3. This method was calibrated on different NaCl–water solutions (0–15 dS m −1 ) and compared with the standard long‐time TDR method for σ a estimations using an UP. The method was subsequently used for determining σ a in four different soils with different values of θ and σ a (0–6 dS m −1 ) and again compared with the standard TDR procedure. The low error (RMSE = 1.5) for the comparison between the ε a measured with the CP and that calculated with the analytical solution for coaxial probes indicates that the CP is accurate enough for ε a estimations. For σ a values &lt;4 dS m −1 , the UP allows determinations of ε a and the most accurate estimations of σ a using the standard TDR method ( R 2 = 0.99). For higher σ a values, however, estimations of ε a were only possible using the CP, where σ a was satisfactorily determined ( R 2 = 0.99) using the Dalton method. To this end, a previous calibration between the real and the apparent bulk electrical conductivity estimated with the CP was required.

The freezing temperature of water in soil is not constant but varies over a range determined by soil texture. Consequently, the amounts of unfrozen water and ice change with temperature in frozen soil, which in turn affects hydraulic, thermal, and mechanical properties of frozen soil. In this paper, an Am‐241 gamma ray source and time‐domain reflectometry (TDR) were combined to measure unfrozen water content and ice content in frozen soil simultaneously. The gamma ray attenuation was used to determine total water content. The TDR was used to determine the dielectric constant of the frozen soil. Based on a four‐phase mixing model, the amount of unfrozen water content in the frozen soil could be determined. The ice content was inferred by the difference between total water content and unfrozen water content. The gamma ray attenuation and the TDR were both calibrated by a gravimetric method. Water contents measured by gamma ray attenuation and TDR in an unfrozen silt column under infiltration were compared and showed that the two methods have the same accuracy and response to changes of water content. Unidirectional column freezing experiments were performed to apply the combined method of gamma ray attenuation and TDR for measuring unfrozen water content and ice content. The measurement error of the gamma ray attenuation and TDR was around 0.02 and 0.01 m3/m3, respectively. The overestimation of unfrozen water in frozen soil by TDR alone was quantified and found to depend on the amount of ice content. The higher the ice content, the larger the overestimation. The study confirmed that the combined method could accurately determine unfrozen water content and ice content in frozen soil. The results of soil column freezing experiments indicate that total water content distribution is affected by available pore space and the freezing front advance rate. It was found that there is similarity between the soil water characteristic and the soil freezing characteristic of variably saturated soil. Unfrozen water content is independent of total water content and affected only by temperature when the freezing point is reached.

Time domain reflectometry (TDR) was applied to dielectric relaxation measurements and relaxation spectra of four kinds of phospholipids (dipalmitoyl phosphatidylcholine, egg lecithin, phosphatidyl ethanolamine, and sphingomyelin) and were evaluated in chloroform solutions. Fast measurements covering a wide frequency range from 1 MHz to 10 GHz could be done easily for materials having multiple relaxations and dc conductivity. A procedure of extracting individual spectra from the complicated complex permittivity has been presented. Reasonable assignment of molecular motion could be given to each spectrum. The first spectrum observed at about 1.5 ns is caused by an overall rotation of the whole molecule in the solution. The second spectrum at 0.2–0.4 ns is related to a motion of the zwitterionic head group. The third spectrum observed in lecithin at 50–80 ps is mainly caused by internal motions of the ester groups. It is concluded that the TDR method is quite suitable for the investigation of molecular dynamics of biomaterials.

Daily actual evapotranspiration measured with TDR technique in Mediterranean conditions

Accurate soil profile water content monitoring at multiple depths has heretofore been possible only using the neutron probe (NP) but with great effort and at infrequent time intervals. Despite the existence of several frequency domain electromagnetic (EM) sensor systems for profile water content measurements, accuracy and spatial representativeness has been precluded by fundamental problems related to soil conductivity and structure effects on the volume explored by the EM field of these sensors, which causes nonrealistic spatial variation in reported profile water contents. Time domain reflectometry (TDR) methods have the distinct advantage of employing a moving EM field that must pass through and be affected by both the drier and wetter soil structures in which the TDR electrodes are embedded. This article describes a profiling water content system based on TDR. The design, laboratory calibration, and field testing is detailed. The sensor system provides unattended, real-time, data acquisition. And, it can be installed without disturbing the soil around the access tube on the outside of which the TDR electrodes are embedded. The correlation coefficient between neutron probe and TDR measured soil water content was 0.94 with a slope of 1.40 and an intercept of -0.08 m3m-3. Bias between TDR and NP readings (TDR-NP) was positive at all depths below 10 cm, ranging from 0.021 and 0.096 m3 m-3. Uncertainty in data of ~0.012 m3 m-3 for soil water, and uncertainty in bulk electrical conductivity of 0.030 S m-1 (both partly due to unreliable mechanical electrical connections) shows that improvements must be made before such a system is acceptable for widespread use.

Studies have been made of the complex dielectric permittivities of mixtures of cholesteric liquid crystals and of a homologous series of normal alcohols. The dielectric spectroscopy was done by the method of time-domain reflectometry (TDR). The liquid crystals were cholesteryl alkanoates with cholesteric-isotropic transition temperatures of 19 degrees C and 72 degrees C. The 50 weight percent mixture has T/sub CI/=32 degrees C. This transition was investigated by TDR. In the case of the alcohols a simple quantum model has been developed to explain a systematic variation of the dielectric loss peaks versus molecular length. A comparison with experimental data indicates that all of the alcohols studied (up to C/sub 12/H/sub 25/OH, dodecanol) behave as fully extended molecules at the temperature at which librational motion goes over into rotation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Identifying the location of an electrical fault (mainly short-circuit or circuit break) in close systems, such as superconducting magnet, is very difficult, especially when a problem appears only at cold conditions (no resistance). The time domain reflectometry (TDR) method could be useful notwithstanding that there is a commonly known limitation on the use of TDR to test the inductive circuit results of the high attenuation of incident and reflected pulses. This paper describes a method for locating electrical faults in superconducting magnets by means of measurements of the pulse response of magnet coil circuits (TDR principle). As a result of adapting TDR to test a circuit characterized by high values of equivalent capacitance and inductance, which is a winding of superconducting magnet, a study of the relationship between the frequency response of magnets and the injected pulse transient parameters is presented. The cutoff frequency of the investigated circuit is crucial information for pulse shaping. A TDR reflectometer with a pulse shaping circuit is described. The instrument consists of commonly available laboratory-measuring equipment: a pulse generator and a 50-MHz oscilloscope, which significantly facilitates the performance of the measurements. Test measurements were carried out for both the undamaged magnets and the magnets with various locations of coil shorts. Advantages and limitations of the proposed method are discussed.