Estimation of the Variation in Specific Discharge Over Large Depth Using Distributed Temperature Sensing (DTS) Measurements of the Heat Pulse Response

An approach is presented to determine groundwater flow in unconsolidated aquifers with a heat pulse response test using a heating cable and a fiber-optic cable. The cables are installed together using direct push so that the cables are in direct contact with the aquifer. The temperature response is measured for multiple days along the fiber-optic cable with Distributed Temperature Sensing (DTS). The new approach fits a two-dimensional analytical solution to the temperature measurements, so that the specific discharge can be estimated without knowledge of the position of the fiber-optic cable relative to the heating cable. Two case studies are presented. The first case study is at a managed aquifer recharge system where fiber-optic cables are inserted 15m deep at various locations to test the fitting procedure. Similar and relatively large specific discharges are found at the different locations with little vertical variation (0.4-0.6m/day). The second case study is at a polder, where the water level is maintained 2m below the surrounding lakes, resulting in significant groundwater flow. The heating and fiber-optic cables are inserted to a depth of 45m. The specific discharge varies 0.07-0.1m/day and is significantly larger in a thin layer at 30-m depth. It is shown with numerical experiments that the estimated specific discharge is smoother than in reality due to vertical conduction, but the peak specific discharge is estimated correctly for layers thicker than similar to 1.5m.