Geohydrological and geophysical investigation

Depth-temperature profiles

Measuring temperatures in or around a heatexchanger borehole can reveal information on the specific conductivity of strata. This can be actual thermal conductivity or a groundwater effect. In combination with borehole records or geophysical profiling of a borehole an estimate of the contribution or groundwater effects can be made.

Groundwater effects

The presence, quality and movement of groundwater can have a significant effect on the long term thermal behaviour of a BHE installation.

Dry boreholes, allthough rare, do occur and ensuring long term conductivity between the loop, the borehole filling and the parent material is essential for the operating temperatures and efficiency of the installation.

Most boreholes are to well below the groundwater table or intercept strata that are water bearing (aquifers). Especially where aquifers are a groundwater resource or where strata with different groundwater quality (potable, salt, contaminated) are crossed within the borehole, environmental consideration dictates design.

Groundwater movement whether vertical (seepage, artesian) or lateral (Darcian, fracture flow) can have a very significant effect on long term operational BHE temperatures and therefore on the design and robustness of the BHE.

Especially for larger BHE systems where strata with groundwater movement are anticipated, such aquifers with a groundwater gradient or tidal influence, it is worthwhile to conduct at GRT (Geothermal Response Test) that will quantify flow and to make an inventory of available data from local observation wells.


Moving groundwater

A BHE in strata with groundwater movement will exhibit a dampening effect on the temperature extremes.

Groundwater movement can be lateral, initiated by pressure head differences within an aquifer (water bearing formation) over a certain distance.

Groundwater movement can also be vertical, caused by a pressure differential between water bearing formations in the ground or by water seeping downwards through infiltration by gravity.

Groundwater movement can be a welcome feature by providing a constant in- or outflux of energy. On the other hand it could well be detrimental to energy storage in the ground or it could even be of influence on downstream positioned heatexchangers.

Sometimes groundwater movement should avoided, especially when it involves vertical movement of water of different qualities, as in contaminated topsoils and valuable underlaying groundwater aquifers.

Groundwater movement in a borehole can be detected using a geothermal response test capable of heating and cooling pulses.