Groundwater Flow Quantification in Fractured Rock Boreholes Using Active Distritbuted Temperature Sensing Under Natural Gradient Conditions

Detection and quantification of groundwater flow in fractures is challenging due to its irregular distribution and fine scale, requiring intensive and depth-discrete field data collection along boreholes. This study presents a new method using fiber optic active distributed temperature sensing (A-DTS) in sealed boreholes to efficiently quantify depth-discrete flow rates along the full length of a bedrock borehole. The method combines field data and numerical modeling to quantify groundwater flow rates under natural gradient conditions, which is important for assessing groundwater flow and contaminant transport. An empirical relationship between enhanced heat dissipation and groundwater flow rates is determined using a numerical model of groundwater flow and heat transport for a system of idealized parallel plate fractures in a homogeneous porous rock with negligible flow through the rock matrix. The empirical relationship is applied to a detailed profile of apparent thermal conductivity measured using A-DTS that combines the effect of rock thermal properties and groundwater flow. In zones with no flow, the A-DTS-derived apparent thermal conductivity matches the laboratory effective rock thermal conductivity values measured independently. Local increases of A-DTS apparent thermal conductivity relative to the rock matrix thermal conductivity can be used to estimate groundwater flow rates using the empirical relationship. The results are in reasonable agreement with straddle pacer tracer dilution tests in the same borehole, which helps to validate the approach. This new approach allows identification of active flow zones and quantification of flow rates and can be efficiently applied in single or multiple boreholes.