Influence of groundwater flow on the estimation of subsurface thermal parameters

We investigated the influence of groundwater flow on the thermal tests performed in borehole heat exchangers to infer the underground thermal properties. Temperature–time signals were simulated with a moving line source (MLS) model under different hypotheses of Darcy velocity. Periodic and random noise was included in the synthetic data obtained with this model in order to mimic high-frequency disturbances caused by several possible sources (e.g. equipment instability and changes in environmental conditions during the experiment) that often occur in real signals. The subsurface thermal conductivity, the Darcy velocity and the borehole thermal resistance were inferred by minimising the root-mean-square error between the synthetic dataset and the model. The calculated thermal and hydraulic parameters were consistent with the “a priori” values. The optimisation procedure results were then tested with the infinite line source (ILS) model. For a Darcy velocity exceeding 10−7 m s−1, ILS largely overestimates thermal conductivity. The approach relying on the MLS model was finally tested with real temperature–time data and produced reliable estimates of thermal conductivity, Darcy velocity and borehole thermal resistance. The inferred groundwater flow was cross checked by means of an independent method based on the analysis of temperature–depth logs recorded under thermal equilibrium conditions. Such a test validates the Darcy velocity inferred with the MLS approach.