Combined fluid temperature and flow logging for the characterization of hydraulic structure in a fractured karst aquifer

SummaryFractured karst aquifers are well known to conceal channeled flow where a few productive-water bearing conduits comprise a relatively small volume within the flow system. This study investigates the ability of combined water temperature measurements and flow logs in wells to reveal the structure of these channeled flows. Temperature and flow logs discussed in this study were performed under various hydrodynamic conditions from ambient flow to radial forced flow at the large scale. Both techniques complement each other since flow logs measure local flow velocities in wells whereas water column temperature profiles delineate inflow depths. For example, temperatures indicate the origin of water (e.g., shallow versus deep) which is not indicated by flow logs. On the other hand, complex temperature profiles due to the interception by a well of several productive layers are better explained when local flow directions are provided by flow logs. The results indicate hydrodynamic conditions affecting the connections between different water-bearing bodies. Pumping the monitored well at a low rate will capture a short-term and local effect related to the convergence of flow into the well bore. When pumping at high discharge rates in a distant well, temperatures and flow logs will capture the long-term re-adjustment of hydraulic heads over a wide portion of the aquifer. In the end, the association of temperature and flow logs reveals a handy and cost-effective tool, well suited to delineate the complex structure of flow in fractured karst aquifers. Knowing this structure would probably improve the ability to design proper scenarios of water resources exploitation and management in complex aquifers.

Research highlights
► Heterogeneous flow structure is characterized by temperature and flow logs.
► Temperature signs the origin of waters and better delineates inflow depths.
► Flow logs validate relative head pressures when anomalous thermal signature exists.
► Both techniques provide useful information about the preferential flow paths.
► Cross-borehole method helps to determine preferential connection between wells.