Near Field Processes (NF-PRO) Principle of TDR measurements

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 (Febex in situ results).