135 7. Hydrogeological flow logging and dye tracer tests on the Hydrogeological Experimental Site The value of borehole logging for aquifer characterization has been demonstrated by several authors (Keys, 1990; Paillet, 1993; US National Research Council, 1996; Paillet and Reese, 2000; Muldoon et al., 2001; Schürch and Buckley, 2002; Williams et al., 2002; Audouin et al., 2008). At the HES, geophysical borehole investigations have included caliper, natural gamma, electrical logs, flow logs, borehole imaging (Optical Televiewer - OPTV), and heat-pulse flowmeter surveys. All resulting data are publicly available via the “H+” database, developed within the framework of the Environmental Research Observatory (ERO) program. Two categories of information have been derived from borehole logging: (1) the geological structure of the aquifer, and (2) the structure of the flow paths. Electrical, natural gamma-ray, and borehole imaging logs have been applied both to identify lithologic variations and to correlate stratigraphic units between boreholes (Gaillard et al., 2024). Remote pumping logs revealed both upward and downward flows within the same boreholes during pumping tests. In 2021, tracer tests were performed to confirm these flow patterns in several wells, specifically M02, M05, MP6, M12, M19, and M21. This paper presents the results of experiments conducted in collaboration with the University of Poitiers team in 2021. It builds upon the findings previously obtained at the site, which were used to define the operational methodology for the 2021 investigations. Borehole flow logging acquisition Principle of flow logging The principle of borehole logging consists of lowering probes or measuring devices into the borehole, connected by a cable that ensures both electrical and mechanical linkage to surface instruments (Fig. 2). These tools continuously record physical or chemical parameters as a function of depth. In flow logging, the instruments measure the velocity and/or quality of moving fluids (e.g., water temperature and electrical conductivity). At the HES, logging was conducted using a GFTC probe to measure natural Gamma-ray, Flow, Temperature, and electrical Conductivity. The micro-mill tool, equipped with an impeller, records the vertical velocity of water movement within the borehole. This allows identification and quantification of water inflows by detecting variations in fluid velocity along the borehole depth. Flow logging can be performed under three conditions: (i) without pumping (ambient regime), (ii) with pumping in the borehole being logged (dynamic regime), and (iii) with pumping in an adjacent borehole (cross-dynamic regime), enabling detection of both upward and downward flows. However, logging results may be disturbed by borehole casings or changes in borehole diameter, which can mask certain inflows. For instance, water moving through a reduced-diameter casing produces artificially elevated flow velocities that are not always associated with natural inflows.
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