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

Abstract

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 15 m 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 (0.4–0.6 m/day). The second case study is at a polder, where the water level is maintained 2 m below the surrounding lakes, resulting in significant groundwater flow. The heating and fiber‐optic cables are inserted to a depth of 45 m. The specific discharge varies 0.07–0.1 m/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 ∼1.5 m.

A Comparison of Different Methods to Estimate the Effective Spatial Resolution of FO-DTS Measurements Achieved during Sandbox Experiments

“…The active-DTS method was recently developed and consists of continuously monitoring the change in temperature along a heated cable. The elevation in temperature depends on sediments thermal properties but also on groundwater flow velocities that dissipate heat through advection [20,25,[29][30][31]. In the present case, the experiment was based on the electrical heating of the cable through its steel frame injection, meaning that the fiber-optic cable was both the heating source and the measurement point.…”

“…In such case, results should be interpreted cautiously. When it has been taken into account, it has been shown that the limit of identification of hydrogeological structures, such as individual fractures or layer of sediments, is directly dependent on the spatial resolution of the DTS unit [8,20,21].…”

Determining the Relation between Groundwater Flow Velocities and Measured Temperature Differences Using Active Heating-Distributed Temperature Sensing

“…The promise is that using the experimentally determined values of C and m and assuming these to be applicable for the current cable setup, Equations (7), (9) and (12)- (14) can be used to determine groundwater flow velocities from observations of ∆T, from a similarly configured cable under field conditions as authors intend to test. The influence of the cable presence on the groundwater flow is incorporated in the FlexPDE modeling but is not specifically considered in Equation (13).…”

“…They introduced a separate heating element upstream of DTS cables that were installed in the aquifer by direct-push methods. Recently an analytical solution was provided to estimate specific discharge using an AH-DTS setup by solving four constants [14]. Advantage of this solution is that the relative position of the heat source to the temperature measurement location is not needed.…”

Dynamic Consolidation Measurements in a Well Field Using Fiber Bragg Grating Sensors

“…The accuracy of heat tracing methods needs to be discussed because warming up the subsurface changes properties of water and potentially modifies the flow patterns [10]. Recent setups combine the heating element directly with the DTS fiber-optic cable referred to as Active Heating-DTS [11,12].…”