EGRT (Enhanced Geothermal Response Test)

An EGRT is used for the depth-dependent determination of thermal soil parameters, such as thermal conductivity and borehole resistance. A heatable fiber optic cable is installed along the geothermal probe. Thus, temperature-depth profiles can be measured by distributed fiber optic temperature measurements at any required time.

Test method

By applying an electrical heating voltage, the cable is heated over its entire length. The thermal material parameters are determined along the borehole by evaluating the temperature-time curves. With short heating times, the thermal parameters of the backfill material of boreholes can be determined along the test section and, thus, the quality of the backfill can be controlled. The hybrid cable remains permanently in the subsurface and an EGRT can be repeated as often as desired - even during the operating phase of the system.

The heating cable is connected to a constant voltage power source and the fiber optic is connected to the distributed temperature sensing (DTS) device. Then the resistance of the heating cable is measured and, together with the heating voltage, the delivered heating power can be calculated. The DTS device records the absolute temperatures along the fiber. After the recording of the initial temperature, the heat pulse test can be started.


 For the location of imperfections in the backfill of geothermal probes by means of depth-resolved measurement of thermal conductivities.

The EGRT-Mobil consists of three components:

  • a special EGRT-Mobil cable (fiber-optic-copper hybrid cable) developed for this purpose,
  • a DTS temperature measuring device
  • and a laboratory power supply unit, which keeps the output heating power of the cable constant with a deviation of less than 1%.

The hybrid cable is inserted into a tube of the geothermal probe until the tip of the probe. The copper component of the hybrid cable is used for heating and the fiber optic component is used for measuring the temperature profile. The measurement principle and mathematical description are identical to that of the 'normal' EGRT. Even in existing borehole heat pump systems with several meters of horizontal feed lines, the EGRT-Mobile can be carried out.

Short EGRT

A 'short EGRT' is carried out with the EGRT mobile system. Compared to the measurements of a 'normal EGRT', which last several days, only a few hours are needed for the 'short EGRT'. The 'short EGRT' is used to locate imperfections in the backfill material of geothermal probe - defects in the cementation - and to detect possible hydraulic short circuits between different aquifers.

Test method

The change of the borehole resistance with is determined with three main successive phases.

  • The temperature change of the glass fibre is determined by the thermal properties of the hybrid cable in the first phase (self-heating of the hybrid cable).
  • In the second phase, the thermal properties of the "borehole filling" are determined, which depend on the water-filled PE pipes of the geothermal probe and the backfill material/cementation.
  • The last phase of temperature changes is unlimited in time and is determined by the material surrounding the borehole (damaged zones, rock, groundwater).

Imperfections in the backfill are characterised by the measured thermal conductivity (e.g. air, standing water, flowing water). Very high thermal conductivity contrasts can be caused by strongly flowing groundwater or air-filled voids above the water table.