CHARACTERIZATION OF HETEROGENEOUS PROPERTIES AND GROUNDWATER FLUXES IN A GRANULAR AQUIFER USING DIRECT PUSH ACTIVE FIBER OPTIC DTS

The development of fiber optic (FO) Distributed Temperature Sensing (DTS) as a tool for hydrogeological measurements with high spatial and temporal resolution has shown potential for characterizing aquifer heterogeneity, which remains a challenge and is needed to predict contaminant transport. Recent studies have shown that groundwater fluxes can be quantified along a vertical profile in granular aquifers by inverting the thermal responses from active heat tracer tests using FO cables. Here, we further investigate the use of active FO-DTS methods and the resulting high resolution profiles for granular aquifer characterization by comparing results with cone penetration tests (CPT), which provide indications of hydrofacies. A multiscale characterization and active heat tracer experiments were performed in a well-studied heterogeneous deltaic aquifer located north of Quebec City, Canada.

Four active FO-DTS heat tracer experiments were conducted by deploying fiber optic cables by direct push at locations with a previous CPT. Interpretation of thermal responses from the active FO-DTS experiments was done with analytical solutions for heat transport, providing independent and accurate estimates of thermal properties and fluxes every 25 centimeters. The resulting profiles of fluxes from DTS measurements correlate well with the response obtained with CPT. Furthermore, as a previous study established a relationship between CPT response and different aquifer materials and their hydraulic properties, the resulting flux profiles from active FO-DTS can be used to obtain a stratigraphy of the different hydrofacies.

Active FO-DTS experiments can thus provide a qualitative or quantitative proxy for hydraulic conductivity and allow the recognition of hydrofacies at a sub-metric vertical scale. At the aquifer scale, the total flux estimated from FO-DTS measurements can also be compared and used as constraints for fluxes obtained from a numerical model. Overall, this study shows that not only does FO-DTS provide coherent results with other characterization methods, but it also adds the key measurement of groundwater flux with great accuracy that cannot be easily obtained by other means. FO-DTS has thus the potential to become a significant addition to existing characterization methods for granular aquifers.