Thermo-optical parameter acquisition and characterization of geologic properties: a 400-m deep BHE in a karstic alpine marble aquifer

To dimension a geothermal array, it is necessary to explore the geophysical and geologic qualities of the subsoil. At the following example the project engineering of a prospective geothermal array is shown from the investigation up to the execution planning. For the geothermic investigation a 400 m (1312 ft.) deep drilling was established and equipped with 50 mm (1.97 in.) duplex BHE. With the mounting of the BHE a fiberglass hybrid cable was inserted as a loop parallel to the shanks of the BHE. By means of optical frequency domain reflectometry (OFDR) an enhanced geothermal response test has been executed. Due the high local resolution of the resulting profile of conductivities the geological profile can be differentiated in areas with mainly conductive and areas of convective influenced heat transfer. By knowledge of these both parts and its parameters the incident of groundwater flow on the BHE can be calculated (Peclet number analysis/ Darcy velocity). With the help of the ascertained geophysical and hydraulic rock parameters solid rock, cleavages and karst cavity could be identified. Also the undisturbed ground temperature, the effective thermal conductivity and areas with different geothermal gradients and the groundwater velocity in cleaved and caveated rocks could be determined.