Near Field Processes (NF-PRO)

In soil physics, Time Domain Reflectometry (TDR) is widely used to measure the volumetric water content in the unsaturated zone. Due to the robustness of the in-situ probes and the long-term stability of the measuring system the TDR technique is of particular value for long-term monitoring purposes.

Time Domain Reflectometry: Schematic sketch of the main components

The TDR-measuring principle is based on the propagation of guided electromagnetic waves along an electric transmission line (waveguide). The measuring system consists of two components, a Time-Domain-Reflectometer and an in-situ probe (see above). In its simplest form, the probe is built of two parallel electrodes, which may be terminated by well-defined impedance.

The system operates as follows: the TDR-device produces a voltage signal, which is usually a step function. The signal is sent to the probe by a coaxial cable and then guided as an electromagnetic wave along the electrodes. At the end of the probe the signal is reflected and travels back. The graph below shows the signal or so-called reflectogram. The travel time of the wave to the end of the electrodes and back to the injection point can be determined by calculating the temporal derivative of the reflectogram (see below).

Typical reflectogram of a TDR probe and (b) its derivative

Porosity and permittivities of the solid, liquid and gas phase can be determined by laboratory measurements. Consequently, the permittivity of the composite medium is a measure of the volumetric water content of the porous medium.

Some examples of results from in situ TDR measurement can be seen here - Full-scale High Level Waste Engineered Barriers (FEBEX) in situ results.

Near Field Processes (NF-PRO) Experiment

The laboratory programme consists of long-term, controlled saturation/desaturation experiments with the two granite cores. The experimental setup allows continuous recording of the TDR reflectograms and changes in the weight of the granite cores.

The experiments will be carried out under controlled temperature and relative humidity conditions of the air around the cores. Two experiments run in parallel:

1. long-term controlled saturation and desaturation test on the matrix core, and
   
    
2. controlled saturation and desaturation test followed by a tracer test on the shear zone core.
   

Experimental Setup

Near Field Processes (NF-PRO) Experiment

The laboratory tests will be performed on two granite cores from the GTS: (i) tests with a matrix core representing the intact granite at the GTS and (ii) tests with a core which include a shear zone.

The planning of the experimental programme foresees the following steps:

• recovery of two cores
   
• construction of the experimental set-up and detailed test plan
   

Beside controlled saturation and desaturation tests, a tracer test to visualise the flow path will be carried out on the core which includes a shear zone.

The laboratory experiment will feed into the ongoing FEBEX long-term monitoring programme at the GTS by providing new calibration functions for the granite probes. Post-processing of the entire TDR data base will provide an updated TDR data set for the FEBEX master data base.

Time schedule

• Start of the NF Pro in 2005
   
• In-situ measurements (water content evolution of the EBS till 2007)
   
• Laboratory programme 2005 to 2006

Near Field Processes (NF-PRO) Experiment

The Time Domain Reflectometry (TDR) method is known as a technique which is ideally suited for long-term measurements of volumetric water content in geomaterials (mechanical robustness of the TDR sensors, easy emplacement, extraordinary long-term stability).

Typical applications of TDR systems are in the fields of soil physics and agriculture, where reliable long-term monitoring techniques are required for measuring volumetric water content of (high porosity) soils. In the early 1990s the method has been adapted for monitoring water content in low porosity rock formations such as granite with the porosity in the order of ~1%.

A new type of TDR probe was developed for the Full-scale Engineered Barriers Experiment (FEBEX) experiment to measure the water content in the bentonite buffer and the adjacent geosphere (granite). Since 1996, the TDR probes are monitoring the evolution of the EBS system. See the FEBEX section for results obtained to date.

The granite probes were designed such that they allow for spatial resolution of water content in 4 equidistant sections along the probe. Long-term measurements over a period of 8 years suggest that changes of water content in the rock zone around the FEBEX tunnel are very low and in the range of the resolution limit.

The laboratory programme, initiated in 2005, is aimed at optimising both the design of the single hole granite probes and the interpretation of the TDR measurements. Specific objectives of the laboratory task are:

• increase the resolution of the measured water content, and
   
• increase the spatial resolution of measurements.
   

Near Field Processes (NF-PRO) Experiment