Time Domain Reflectometry (TDR)
Adam Pirie, Jacqui Watt, Nathan Odgers
Home
 
Click to enlarge Click to enlarge Click to enlarge
Insertion of waveguides Final configuration of waveguides Using the TDR

Principle/Description

Time domain reflectometry (TDR) is a common technique used (in soil science) to measure soil properties including EC and soil water content.

A TDR device uses an oscilloscope to measure the echo of an electrical pulse (waveform) sent down a coaxial transmission line. At the end of the transmission line is a sensor (waveguide) like the one in the image to the left.

Once the waveform is sent down the transmission line, it is emitted from the end of the sensor and the reflected signal is measured in the TDR using an oscilloscope.

From the time taken for the waveform to move down and back up the waveguide (propagation time), permittivity can be measured. Permittivity is a measure of how much a medium changes to absorb energy when subject to an electric field. The relative permittivity of air is 1. Permittivity is affected by the geometry of the waveguides and the conductivity of the soil medium. As these effects become greater, propagation time increases, hence increasing permittivity.

Permittivity can be directly related to volumetric moisture content using a calibration curve. Higher permittivity relates to higher moisture content.

Methods

TDR was used in the field to measure the soil wetting front. As can be seen here, waveguides were inserted horizontally into a sheer soil face at five depths: 3.5 cm, 6.2 cm, 8.5 cm, 11.4 cm and 15 cm. The pit created to install the waveguides was filled in, and the initial moisture content at each depth was recorded.

Water was applied to the surface using a ponded disc permeameter. The progress of the wetting front was estimated by measuring the water content at the top sensors initially and measuring the lower sensors one after the other as the wetting front progressed down the profile.

When the wetting front had reached the bottom sensor (15 cm depth), the permeameter was taken off the surface and the profile allowed to drain. Drainage of the water from the profile was monitored by measuring the water content at each sensor.

The experiment resulted in a large and complicated data set. The nature of the experiment meant that data were not recorded at equal time intervals, and only one sensor could be recorded at a given time.

The data were graphed as best as possible selecting data at approximately equal time intervals. Because data were not available for every probe (depth) at every desired point in time, splines were fitted to the data for each of the five individual depths to estimate the moisture content at any time.

This new data set of estimated values was used to create a contour plot showing the moisture profile at selected time intervals. The results can be seen in the graph to the right (click to enlarge).

 Click to enlarge
Moisture profiles derived from analysis of the TDR results.

Results

The results of this experiment are shown in the graph to the right. We can see that there are marked differences in the behaviour of the wetting/drying cycle with time at different depths. The top of the profile was initially much dryer than at depth but became wetter than the bottom by the time water application at the surface ceased, at 1118 seconds (18:38 minutes).

The range from initial moisture content to the maximum moisture content tended to decrease with increasing depth. There is often a peak or a trough in each moisture profile at approximately 6 cm depth, which may suggest different layer characteristics.

The surface dried much faster than the bottom of the profile with a large transition zone between 6 cm and 12 cm depth.

Environmental implications of the technique

Installation of the sensors requires excavation of a small pit to the depth of measurement required. The sensors are then inserted into a sheer pit-face at the depths required, causing minimal disruption to the soil structure as each tine of the sensor has a very small cross-section area. After measurement the pit can be back-filled, though soil structure will likely have been destroyed.

Problems with the method

  • Permittivity is influenced by solutes, geometry of the waveguides
  • Simultaneous measurement at multiple depths cannot be achieved without more than one TDR unit
  • Time interval between measurements using one TDR unit is determined by the speed at which sensors can be changed-over to the machine

References

Andrews J. R., 1994. Time domain reflectometry. Symposium and Workshop on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications held at Northwestern University, Evanston, Illinois, September 17-19, 1994 (Washington, DC: U.S. Bureau of Mines, 1994), pp. 4-13. USBM special publication SP 19-94.

Robinson D. A., Jones S. B., Wraith J. M., Or D., Friedman S. P., 2003. A review of advances in dielectric and electrical conductivity measurement in soils using time domain reflectometry. Vadose Zone Journal 2:444-475.