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Spatial TDR

Work Package III.IV.

Time-domain reflectometry (TDR) is a recognized electromagnetic (EM) method for determining the average material moisture content along the TDR probe. Spatial TDR is an innovative extension of the method that enables the determination of moisture profiles along the probe. A TDR is a cable radar with a guided electromagnetic (EM) wave. It transmits EM pulses along a waveguide (“probe”) embedded in the soil, which may consist of a special three-wire ribbon cable or parallel steel rods. Typical probe lengths are a few meters for the ribbon cable and a few decimetres for the rod probe. The length limitation is primarily due to the installation method. During pulse propagation along the unshielded waveguide, portions of the wave are repeatedly reflected back, depending on the change in the so-called characteristic impedance, which is largely determined by the soil moisture distribution along the probe. Spatial TDR makes it possible to estimate the moisture content along the waveguide from the temporal progression of these measured signal reflections using inverse modeling – a complex and computationally intensive mathematical algorithm.

Preliminary work on this innovation at the University of Karlsruhe dates back several years. The practical use of spatial TDR has been demonstrated in numerous applications. These include, for example, soil moisture profile monitoring in agricultural areas (agricultural hydrology) and monitoring the seepage of river dikes (civil engineering, earth dam construction). Compaction horizons (“plough bottom”) and seepage rates (infiltration) are reflected in the temporal progression of the soil moisture profiles. This also allows for conclusions to be drawn about deep infiltration and groundwater recharge. In addition to determining soil or earth moisture, other industrial applications can be served by this technology. These include, for example, the material moisture content of bulk materials (grain silos, plastic granules in the production process) or even fill level indicators in silos. However, spatial TDR has not yet been tested in these applications. In the two aforementioned application areas for determining soil moisture, a TRL of 5 was achieved. 

However, it has not been possible to further increase the TRL because of two fundamental challenges:

(1) Conventional TDR instruments are too expensive and large, making them unsuitable for widespread use.

(2) The inverse modelling required to derive soil moisture profiles from the measured data is very time-consuming and complicated. Both of these factors severely limit the cost-effective application of many systems across large areas.

We want to eliminate these grievances in the project and implement a number of innovations to achieve this:

  • Highly integrated probe head with energy-saving measurement electronics based on fast microcontrollers or FPGA equivalent time sampling unit with 20 picosecond time resolution (RF frontend, HSRW)
  • LoRa-based data radio module (RF Frontend, HSRW) Ground-mountable waveguide with adaptation to the measurement electronics (RF Frontend, HSRW)
  • Increasing the accuracy, robustness, and acceleration of inverse soil moisture modelling through improved numerical algorithms supported by AI-based signal analysis running in the probe head (edge computing) (RF Frontend, HSRW, KnowH2O). Validation and calibration through comparative measurements with existing products (RF Frontend, HSRW).
  • Preparation of a certification taking into account the standards used so far (RF Frontend, HSRW)
  • Integration of the TDR into the SHAPE Fieldlab (RF Frontend, HSRW)