Novel use of the enhanced thermal response test in crystalline bedrock
Petri Hakala, Sami Vallin, Teppo Arola, Ilkka Martinkauppi
Abstract
The thermophysical properties of bedrock are of primary importance when designing borehole thermal energy systems. We present a novel use of the Enhanced Thermal Response Test (ETRT) to determine bedrock thermal conductivity, natural convection, and drill hole thermal resistance as a function of depth in crystalline bedrock. Bedrock was heated with a 228-m-long hybrid cable containing copper wires and fiber optics for temperature monitoring. A reference fiber optic cable was installed along the whole length of the studied drill hole. For groundwater-filled boreholes, the ETRT offers a means to estimate the magnitude of buoyancy-driven natural convection. We estimated that the heating power in the ETRT should not exceed 20 Wm-1 for the thermal conductivities to be determined with sufficient accuracy. According our results, the accuracy of the ETRT can be significantly improved if the test is performed with a hybrid fiber optic cable combined with a reference fiber optic cable. Thermal resistance can be more accurately determined if a reference fiber optic cable is used. The most important achievement of this method is that compared to other measurement methods, the effective thermal conductivity of bedrock can be simultaneously determined along the entire length of the drill hole.