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

• Main Authors: Wiecher Bakx, Pieter J. Doornenbal, Rebecca J. van Weesep, Victor F. Bense, Gualbert H. P. Oude Essink, Marc F. P. Bierkens
• Format: Article
• Language: English
• Published: MDPI AG 2019-08-01
• Series: Water
• Subjects: distributed temperature sensing; groundwater monitoring; groundwater velocities
• Online Access: https://www.mdpi.com/2073-4441/11/8/1619

Active Heating-Distributed Temperature Sensing (AH-DTS) has the potential to allow for the measurement of groundwater flow velocities in situ. We placed DTS fiber-optic cables combined with a heating wire in direct contact with aquifer sediments in a laboratory scale groundwater flow simulator. Using this setup, we empirically determined the relationship between <inline-formula> <math display="inline"> <semantics> <mrow> <mo>&#916;</mo> <mi>T</mi> </mrow> </semantics> </math> </inline-formula>, the temperature difference by constant and uniform heating of the DTS cable and the background temperature of the groundwater system, and horizontal groundwater flow velocity. Second, we simulated the observed temperature response of the system using a plan-view heat transfer flow model to calibrate for the thermal properties of the sediment and to optimize cable setup for sensitivity to variation in groundwater flow velocities. Additionally, we derived an analytical solution based on the heat flow equation that can be used to explicitly calculate flow velocity from measured <inline-formula> <math display="inline"> <semantics> <mrow> <mo>&#916;</mo> <mi>T</mi> </mrow> </semantics> </math> </inline-formula> for this specific AH-DTS cable setup. We expect that this equation, after calibration for cable constitution, is valid for estimating groundwater flow velocity based on absolute temperature differences measured in field applications using this cable setup.