Use of 3D electrical resistivity tomography to improve the design of low enthalpy geothermal systems

In designing low enthalpy geothermal systems, the ideal location and length of the boreholes in the well-field is the key to improve the performance and reduce the costs of the installation. The correct assessment of the heat conductivity of the ground (lambda) plays also a very important role in estimating the amount of energy that we are going to be able to obtain from the subsoil and the ideal pace of the process. In low enthalpy geothermal installations based on granite type environments is especially important to improve the information we have from the subsoil at a small scale. This is due to the great horizontal variation we can find on this kind of terrain. Electrical conductivity (C = 1/rho, rho = resistivity in ohm meters) can be related to thermal conductivity (lambda) of many rock types (Directive (EU), 2019) (see Robertson, 1988). We show that a 3D electrical resistivity survey can be used as a proxy for lambda in terrain with weathered and solid granitic rock. Knowledge of lambda is essential for the design of efficient ground source heat pump systems that use vertical wells for closed-loop systems. Shorter well lengths are accomplished if wells are in solid granite with high lambda. Furthermore the electrical resistivity survey identifies low density, clayey subsurface materials that may require specialized drilling methods. Project cost savings can result from shorter borehole lengths, number of holes, and correct drilling methods.