Quantification of time-varying groundwater flow in boreholes in fractured crystalline rock using long-term distributed temperature sensing

Quantification of groundwater flow is an important factor for several applications, such as water supply, boreholes for energy extraction/storage and drainage and flood prevention projects. In this study, distributed temperature sensing (DTS) with fibre-optics has been combined with energy calculations to estimate the time-varying groundwater flow in fractures in four stand-alone boreholes at Åkneset in Norway. The method captures the natural, undisturbed time-variation of the groundwater flow as no tracers or pumps were used. Compared with temperature profile measurements using a probe, long-term distributed temperature sensing (from several weeks) gives a profound understanding of the hydrogeological conditions for a site. One example of how long-time measurements enhance this understanding is that they provide information about the sources of the groundwater flow: For some fractures, the groundwater estimations showed no correlation with meteorological data, indicating that these fractures are fed from deeper regional flow, with relatively large response times. In other fractures, the temporal variations in estimated groundwater flow showed high correlation (>0.60) with precipitation or temperature, with 1.4–9.0 days delay. This indicates that these fractures are fed mainly from precipitation and snow melting. The correlation with weather conditions at the surface also indicates that the method gives a true time-variation of groundwater flow. The results from the study show that DTS can be a useful tool to quantify groundwater flow in boreholes made for energy and monitoring (e.g., in tunnels). The method could be further improved by injection of heat along the entire borehole length, which has been done before. This would be similar to a thermal response test, which is an important pre-investigation for borehole thermal energy storage.