Quantifying the influence of cable orientation on groundwater flux estimates from Active-Distributed Temperature Sensing

Traditional groundwater monitoring techniques lack the capabilities to measure groundwater fluxes at high spatial resolution over large distances. Breakthrough work by Simon et al. (2021) enabled the quantification of groundwater fluxes at high spatiotemporal resolution using actively heated fibre optic Distributed Temperature Sensing (A-DTS), establishing this as a promising hydrogeophysical method. However, current A-DTS interpretation methods, such as the analytical solution used to process A-DTS data, assume that groundwater flux is perpendicular to the cable. Yet, many field-based applications of A-DTS violate this assumption due to the multi-dimensional nature of groundwater flow. This study aims to characterise the effect of fibre optic cable orientation on the interpretation of groundwater fluxes, and determine the minimum angle for which the method remains applicable.

 

This study presents methodological advancements to A-DTS by characterising how cable orientation relative to flow direction affects groundwater flux estimates in a controlled environment. Estimating groundwater fluxes from A-DTS relies on the Moving Analytical Line Source (MILS) analytical model describing heat dissipation. A key assumption of the MILS model is that the flow is perpendicular to the cable angle, a condition frequently violated in field applications. To establish critical angle thresholds for reliable groundwater flux estimation, a large-scale experiment was conducted at the Site Contrôlé Expérimental de Recherche pour la réhabilitation des Eaux et des Sols (SCERES) platform in Strasbourg, France. This 25×12×3 m experimental tank represents an ~1000 m³ artificial porous aquifer designed to reduce potential boundary effects. This study allowed us to test some of the assumptions underlying the A-DTS method. A hybrid cable containing fibre optic strands and a steel armour for heating, was installed in a configuration with five sections at different orientations relative to the flow direction though the horizontal plane. The cable was buried within the saturated porous medium and different flux rates were imposed to establish the critical angle thresholds for reliable groundwater flux estimation. We will discuss the advantages and limitations of A-DTS for high-resolution groundwater flux monitoring under controlled yet field-representative conditions.