MODELING EFFECTS OF THERMOGEOLOGY ON BOREHOLES AND ON THE SHALLOW GEOTHERMAL GRADIENT USING DISTRIBUTED TEMPERATURE SENSING

Using fiber-optic distributed temperature sensing (FO-DTS) for studying geothermal exchange system require great precision. The ease of use and capability to determine the temperature of different media (water, air, soil, etc.) at higher temporal and spatial resolutions compared with other technologies benefits its use for a range of application and environments. In this study, FO-DTS was conducted in two adjacent boreholes, one a cased groundwater monitoring 40-m deep and in a grouted, uncased borehole extending 100 m below ground. The fiber optic cable was installed vertically in both boreholes, attached to the PVC pipe in the cased borehole and sealed against the sidewall with bentonite grout in the other. Thermal profiles were measured over a 2-year period from 2015–2017. Following the data collection, heat transfer in the subsurface around both boreholes under both saturated and unsaturated condition were numerically evaluated. According to previous studies in central Illinois, the transient water table in the surficial zone fluctuates within upper 10 to ~15 m. The temperatures measured in this zone are susceptible to seasonal fluctuations in climate conditions at ground surface and below this zone, temperatures are mainly controlled by the local geothermal gradient. In this presentation, we will show how heterogeneities in the thermo-geology and well construction materials impact the natural geothermal gradient in the shallow subsurface.