CHARACTERIZING GROUNDWATER FLOW IN FRACTURED ROCK USING FIBER-OPTIC DISTRIBUTED TEMPERATURE SENSING AND NUMERICAL MODELING

Understanding the flow behavior in fractured rocks is of great importance for the development and construction of deep geological engineering projects. Using temperature-depth profiles in the subsurface to characterize groundwater flow is becoming increasingly popular. Not only because the heat transport process is sensitive to groundwater flow(flow rates,preferential flow paths et al.),but also the temperature can be easily measured by Fiber-Optic Distributed Temperature Sensing(FO-DTS)with low costs. In this study,FO-DTS is used to conduct fully distributed space-time measurements of groundwater temperature in two boreholes. These boreholes(BSQ02 and BSQ03)locate at Xinchang Site in Gansu Province,where is the first underground laboratory site in China. The borehole temperature profiles are measured with FO-DTS under pumping conditions to find potential flow path through fractures. Through analyzing these profiles,location of hydraulically active fractures has been determined to be distributed around 40 m depth in BSQ02,and the main flow path is considered to be fracture B2-2. In order to further investigate the groundwater flow fields,measured temperature profiles are then interpreted with inverse numerical modeling of flow-heat transfer processes. Average groundwater flow rate in borehole under pumping condition is estimated to be 0.01 m·s^-1 from inverse models. The temperature of groundwater flowing into the borehole through fracture B2-2 is around 13.65 ℃,which is estimated to be 0.7 ℃lower than the original groundwater(14.35 ℃)in borehole. The velocity of groundwater through B2-2 was estimated to be 1×10^-5 m·s^-1. This study succeeds in characterizing groundwater flow rates and flow paths using temperature data based on FO-DTS technology,which can be a promising method for understanding groundwater flow in fractured rocks.