| Abstract |
The University of California, Berkeley, is building a clean, electrified heating and cooling plant utilizing shallow geothermal to reduce greenhouse gas emissions. To explore the feasibility of shallow geothermal as the core of the zero-carbon campus energy system, we conducted ground temperature monitoring, geophysical logging, and a distributed thermal response test (DTRT) for geothermal potential evaluation. The monthly temperature monitoring in four shallow geothermal boreholes using distributed fiber optic sensing (DFOS) shows that the subsurface temperature is influenced by daily and seasonal air temperature variation yet stabilizes below the depth of 50 ft. A 400-ft deep borehole with multiple sensors was drilled and installed. The subsurface of the Berkeley campus comprises a top layer of clay (0 ft to 10 ft) overlying a gravel layer (10 ft to 30 ft). Below 30 ft, the Franciscan Complex forms the primary bedrock. Shear zones are detected at depths of 75 ft, 200 ft, 280 ft, and 330 ft. The thermal conductivity (λ) for the sediment above 30 ft is measured at 1.23 Btu/hr-ft-℉, and for the bedrock from 30 ft to 250 ft, λ is 1.40 Btu/hr-ft-℉. Below 250 ft, λ approaches zero, indicating low heat transfer efficiency. This is probably attributed to the poor grouting quality. This research demonstrates the effectiveness of the DFOS technique in measuring continuous-in-space subsurface temperatures and characterizing geological features, underscoring its value in geothermal investigation. |