To dimension a geothermal array it is necessary to explore the geophysical and geological conditions of the subsoil. At the following example the project engineering of a prospective geothermal array is shown from the investigation up to the execution design. The investigation was executed in the following steps: - Drilling and recording of the geologic profile. - Mounting of a Duplex BHE and fiberglass hybrid cable into the borehole - Measurement of the rock-physical parameters by means of an enhanced GRT with a spatial depth resolution of 0.5 m (1.64 ft.). - Detection of ground water flow by analyzing the measured geophysical parameters. - Calculation of the Darcy flow in as ground water-leading identified horizons by means of Peclet number analysis. - Use of the measured data in a simulation for the conceptual design of the prospective geothermal array. The geothermal array should be installed in a mountain region of the Austrian Alps. For the geothermal investigation a 400m (1312 ft.) deep wellbore was drilled 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. In the following the drilling was filled with thermally optimized grout. The built in hybrid cable carries along a copper cable as a heating wire beside the fiberglass. The copper cable was connected to an electrical power source and therefore a thermal impulse was generated. The heating power is identical along the heating wire at every place of the hybrid cable. A controller measures the resistivity of the copper leader during heating phase and holds by adaptation of the voltage and the amperage the applied electric power steady. The undisturbed temperature of the subsoil and the temperature rise of the system are recorded by means of Optical-Frequency-Domain-Reflectometry measurement (OFDR). On this application, the fiberglass itself is the temperature sensor. Temperatures of from -200 (-328 °F/73.15 K) to 400 °C (752 °F/ 673.15 K) can be measured with the OFDR. By use of a laser diode a frequency-modulated optical signal is sent into the fiberglass. The optical impulse is scattered and split in a Raman and Raleigh part of the signal. The impulses are reflected by the fiberglass proportionally. A temperature depending phase shift of the optical spectrum of the Raman parts (Stokes and Antistokes) enables the calculation of the temperature at its place of origin. An exactness of 0.02 K is resonable. The analysis of the run time of the back scattered light leads to a spatial resolution up to 10 cm (3.94 in.). The presented installation was measured with a spatial resolution of 0.5 m (1.64 ft.). The evaluation of the recorded temperature curves follows Kelvin’s line source theory. For every measuring point along the hybrid cable the effective thermal conductivity of the surrounding rock can be determined thus. A high local resolution over the whole profile enables to differentiate high conductive sections from convectively influenced heat transfer zones. Applying these parameters in a Peclet-Number-Analysis the groundwater flow patterns adjacent to a particular BHE can be calculated. 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 were determined. The measuring results lead into optimized design procedures

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