136 A new concept of karst development based on hydrogeology and geophysics Downflow and upflow in the borehole To obtain a complete picture of borehole flow, two recording passes are typically performed: – downhole pass: The tool is lowered into the borehole, recording data continuously; – uphole pass: Once at the bottom, the tool is raised while recording data. When the tool moves with the flow, its velocity is added to that of the fluid. Conversely, when the tool moves against the flow, its velocity is subtracted from that of the fluid (or, if the flow is stronger than the tool’s movement, the fluid velocity is subtracted from the tool’s velocity). By knowing the downhole and uphole velocities of the tool (measured in a logger linked with a computer), it is possible to correct the micro-current meter readings to obtain the true fluid velocity and its direction of movement in the borehole. Following the recommendations of Keys (1990), logging was performed during downward passes at a low speed (6 m/min) to minimize disturbance of the water column. The data under study concern the velocities (in m/mn) and the temperatures (in degrees Celsius). Dynamic flow logging The flow logs presented in Figures 3 to 10 allow the identification of water inlets corresponding to productive karst levels. Since the pump is positioned above the probe, only upward flows are measured. The GFTC probe is positioned at the bottom of the borehole and recording is performed by lifting the tool. Water entering the borehole thus causes an increase in velocity measured in rounds per second (rps). Figure 2 Schematic diagram of flow logging.
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