Time Domain Reflectometry Reflection Research Articles

Guidelines from direct shear modeling in centrifuge for TDR landslide monitoring

The suitability of using time-domain reflectometry (TDR) for landslide monitoring and assessment has been validated by field studies. This method can be used for detecting shear surfaces and monitoring shear displacements at depth, which can provide warning of potential slope failures or landslide reactivations. Although laboratory tests have suggested using the TDR reflection coefficient to estimate shear displacement, quantifying the relationship between these parameters is challenging because it is affected by various cable–grout–ground interactions. Investigating these interactions is challenging because of the limited artificial overburden pressure that can be applied in a laboratory. Therefore, this study improved the TDR applicability through high-gravity centrifuge modeling to simulate sliding at depth. Furthermore, a flexible coaxial cable was modified to improve its sensitivity, which was evaluated using the simple direct shear test. The modified coaxial cable with stiff grout had sufficient sensitivity to detect a small-scale shear displacement of 0.5 mm. Reversed-fault centrifuge modeling was conducted for sandy soil to simulate the shear plane under a gravity level of 55 g, and the shear plane was realistically generated and detected by a cable–grout–ground composite and cables located adjacent to an inclinometer casing. Notably, only the model-scale fault movements directly correlated with TDR reflection peaks and integration areas. Finally, guidelines for conducting TDR-based slope monitoring and directions for further research regarding TDR-based landslide centrifuge modeling are suggested.

Automatic TDR Monitoring System for Rock Mass Deformation in a Landslide Area

This study employs time domain reflectometry (TDR) technology for the automatic monitoring of a landslide area to explore rock deformation mechanism in comparison with rock cores and to estimate the location of a potential sliding surface and the amount of the sliding. Comparing to the data from the TDR monitoring and rock cores in Lushan landslide area in Taiwan, sliding surfaces of the landslide area occurred mainly at two locations of rock formations. One was located at an interface between colluvium and weathered rock; the other’s location was a buckling type of rock structures associated with slope creeping. Using a regression equation of the relation between TDR reflection coefficients and shear displacements, the magnitude of a TDR-cable deformation in the field can be determined. Resulting from field TDR monitoring, rock mass creep increased from 0 to 0.67 cm at 13.4-, 57.9- and 70.6-m depths of T3 borehole in the landslide area from September 2018 to May 2019. Overall, the TDR technology is practically used for a real-time and automatic slope monitoring system to detect the location and magnitude of sliding surfaces in the landslide area.

Applying time domain reflectometry to quantification of slope deformation by shear failure in a landslide

This study employed time domain reflectometry (TDR) to monitor the deformation of grouted coaxial cables in a landslide over an extended period. Laboratory experiments were performed to quantify the magnitude of the deformation by shear failure to simulate types of cable deformation in the field when two localized shear types had been detected using TDR. The study applied two qualification methods to measure the magnitude of the laboratory shear deformation. The integration method of the TDR reflection coefficients demonstrated a stronger significant positive correlation with the shear displacement than did the linear regression method of reflection coefficients in relation to the deformation magnitude of the cables. The integration method indicated increases of localized shear displacements from 15 to 47 mm over time at 10.33- and 13.36-m depths in the landslide. The average maximum magnitude of the cable deformation by shearing was 47 mm corresponding to a 210-mρ reflection coefficient when the cable ruptured in the case study. Overall, the location and magnitude of shear deformation in the landslide can be determined using changes in the TDR waveforms and calculations of the integration method of reflection coefficients.

Power Cable with Two Joints Experimental Analysis for Defect Assessments

<span>Time domain reflectometry (TDR) is an easy technique that provides quick response which is ideal for power utility companies to conduct cable diagnostics on-site. Electricity disruption due to power cable failure is a major challenge for power utility companies due to the long length of cable installed with joints. The long time taken to diagnose the defect along the cable before electricity can be restored has not only jeopardized the reputation of power utility companies but also brings losses to the economy. Hence, this study conducts experimental analysis on cable with two joints with the application of TDR technique to reduce the electricity disruption time. This research is divided into two stages where stage 1 conducts experiments on cable with one degraded section while stage 2 looks into cable with two degraded sections. The TDR reflection characteristics are studied from stage 1 experiments and applied to stage 2 experiments to verify the consistency of the TDR reflection characteristics. The cable conditions of stage 2 experiments are predicted using the reflection characteristics from stage 1 observations and are then validated by comparing these predictions with the actual cable configuration. Results obtained from these experiments have proven that the TDR reflection characteristics are consistent and accurate which can be used to sectionalize the degraded cable section. Detail findings of all experiments conducted with the TDR application are discussed in part three of this paper.</span>

Nondestructive Sensing of Interconnect Failure Mechanisms Using Time-Domain Reflectometry

This paper presents time-domain reflectometry (TDR) as a nondestructive sensing method for interconnect failure mechanisms. Two competing interconnect failure mechanisms of electronics were considered: solder joint cracking and solder pad cratering. A simple theoretical analysis is presented to explain the effect of each failure mechanism on the TDR reflection coefficient. Mechanical fatigue tests have been conducted to confirm the theoretical analysis. The test results consistently demonstrated that the TDR reflection coefficient gradually decreased as the solder pad separated from the circuit board, whereas it increased during solder joint cracking. Traditional test methods based on electrical resistance monitoring cannot distinguish between failure mechanisms and do not detect degradation until an open circuit has been created. In contrast, the TDR reflection coefficient can be used as a sensing method for the determination of interconnect failure mechanisms as well as for early detection of the degradation associated with those mechanisms.

Effective frequency and attenuation measurements of glass beads/magnetite mixtures by time‐domain reflectometry

ABSTRACTSamples with different percentages (5–30%) of magnetite and different ranges of grain size are analysed by time‐domain reflectometry (TDR). The passband, the attenuation factor and the effective frequency are derived from the TDR measurements. The passband was obtained from the second edge of the sample by eliminating the DC component and applying a Fourier transform to the residual signal. The bandwidth of the resulting spectral shape decreases as the magnetite percentage increases. The second edge was also used to evaluate the effective attenuation factor , using multiple‐reflection theory, terminated at the second reflection. The attenuation values can be estimated independently by means of the electromagnetic parameters obtained using an LCR meter. To compare and , an effective TDR frequency is introduced. The agreement between the values obtained from TDR and LCR‐meter techniques supports the reliability of the attenuation derived from the second TDR reflection, and that computed from the effective frequency. The shapes of the passbands and the and values for magnetite/glass‐beads mixtures, which simulate dry soils with increasing iron oxides content, are reported and discussed.

Time domain reflectometry of glass beads/magnetite mixtures: A time and frequency domain study

The measurements of the time domain reflectometry (TDR) signal propagating along a coaxial probe, filled with dry mixtures of glass beads and magnetite, are analyzed for deriving the attenuation factor α and the bandwidth. Samples with different percentages (5%–25%) of magnetite are considered. The α values are obtained by two different methods: (i) wave amplitude at the second reflection, (ii) electromagnetic parameters and widths of the passing bands. The two methods provide consistent α values within the experimental uncertainties. The agreement supports the possibility of measuring the attenuation factor from the second TDR reflection, for anhydrous samples similar to the investigated mixtures.

TDR calibration of organic forest floor media

We carried out a time domain reflectometry (TDR) calibration for 25 forest floor samples of five different forest stands. Linear regression was used to estimate the volumetric water content from the refraction index. Due to the presence of bound water, it was impossible to predict the calibration line parameters from theoretical values with a refractive mixing model. However, when the apparent dielectric constant of the water was considered, it was possible to predict the offset parameter because the low forest floor bulk density caused the calibration line slope and offset to be almost reversely proportional. Calibration parameters for different forest floor materials are presented. Allowing a somewhat higher margin of error we could also establish general calibration curves which were in close agreement with a published calibration curve (Malicki et al., 1994). An error analysis showed that decomposition of organic matter, residual water and temperature effects have negligible effects on the calibration parameters. Shrinkage of the organic material significantly influenced both the volumetric water content and the TDR reflection times. If not corrected, both effects yielded systematic errors of approximately 0.02 cm 3 cm −3 for a strongly shrinking H horizon.

Errors in Converting Time Domain Reflectometry Measurements of Propagation Velocity to Estimates of Soil Water Content

AbstractThe determination of soil water content by time domain reflectometry (TDR) involves the measurement of the propagation velocity of a radio frequency (RF) pulse in soil and the conversion of this measurement to an estimate of soil water content. The objective of this study was to quantify errors in this conversion. A comprehensive error analysis can only be carried out if a well‐defined measurement procedure is followed. We define and recommend such a procedure and identify the physical origin and the magnitude of each of the error sources in the procedure. Total measurement errors are calculated as the root mean square sum of each of the individual errors. We express propagation velocity measurements in terms of time intervals, that is, the travel time of an RF pulse in soil (T) relative to that in air (Tair). Our measurement and error calculation procedure is based on an analysis of our own and published data for nonclay soils, which reveals that in all cases, there is a linear relation between T/Tair and volumetric water content (θv) with a mutually independent slope and intercept. A theoretical explanation for this observation is presented. We show that the dominant measurement error source is the transition time of the TDR reflection, which in turn is a function of θv. Absolute measurement errors range from 0.015 to 0.028 m3 m‐3 if the T/Tair vs. θv intercept is known and increase to 0.023 to 0.034 m3 m‐3 if a nominal value (1.55) is used. We recommend that a T/Tair vs. θv slope value of 0.1193 (5% less than theory) be used for measurements in nonclay agricultural soils. This leads to little loss in accuracy. Measurement resolution is primarily a function of the time base error of the TDR instrument and can be as good as 0.0018 m3 m‐3.

Fracture monitoring within concrete structure by time domain reflectometry

Time Domain Reflectometry (TDR) is an electromagnetic pluse testing technique using Radar theory. Electrical engineers use it to locate breaks in power transmission lines. Recently, this technique has been applied to monitor rock movement induced by underground mining. Coaxial cables are grouted into the rock mass which is expected to move. Rock movement deforms the cable and produces changes in the TDR reflection signature. These changes can be quantitatively related to the location and magnitude of rock mass movement. While the coaxial cable is embedded in a concrete structure, the fracture or damage within the structure could be detected by sending an electromagnetic pulse down the cable. Observation of the reflected pulse on the oscilloscope would show the change. This forms a very good tool, and could be used to detect and monitor fracture or damage within a large concrete structure. Experimental data will be presented to demonstrate the applicability of this technique. Results show positive. Fracture and relative block movement can be measured quantitatively. This technique can be applied in building warning and monitoring systems for the safety of large concrete structures.