Temperature Logging to Characterize Flow Patterns in a Highly Permeable Fractured Karstic Aquifer, Poitiers (France)
Abstract
In heterogeneous media, such as higly fractured rocks or karstic aquifers, the characterization of the main flow paths is required for further flow modeling but rather difficult to achieve due to the complexity of the flow field. Here we compare the information provided by temperature logs with flow logs measurements in a fractured karstic aquifer at the Hydrogeological Experimental Site (HES) of Poitiers (France). Temperature logs have been widely used to address flow in fractured rocks. Basically, the method tracks eventual distortions of the geothermic gradient due to groundwater motion. It is considered as inexpensive since the point is simply to monitor water temperatures along a borehole profile and then to translate the measures in terms of inflow-outflow locations and incidentally of vertical flows along the borehole. Flow logs were obtained from an heat-pulse flowmeter that allow to measure downward or upward vertical groundwater velocities larger than a mm/s. The HES is set up over a 100m thickness confined Jurassic limestone aquifer. The HES includes 32 fully penetrating wells which geometry is inspired from the "five spot" borehole configuration. The inter- comparisons between previous data from flowmeter testing, borehole imaging and 3D seismic imaging indicate that main flow-paths are mostly concealed within sub-horizontal layers riddled with karstic channels and along sub-vertical fractures. Temperature and flow measurements have been carried out under both ambient and forced flow conditions. Two forced flow conditions are available yielding short pumping time data with pumping at the monitored well, large pumping time data with pumping at a distant well. Most boreholes show quite constant vertical profiles of temperature indicating the existence of flow along boreholes. In addition, a few wells may show drop of temperature at 115 m depth and anomalous warm-water arrivals at 65m. These features associated with non equilibrated temperatures highlight rapid motion of water and inter-connections between karstic layers due to vertical fractures and the boreholes themselves. These temperature anomalies could serve as indicator of eventual connections between wells. On the other hand, the rapid flows and the non equilibrated water temperatures jeopardize a precise evaluation of both inlet- outlet points and vertical flow directions along boreholes. Compared with flowmeter data, important discrepancies are observed. Even though temperature data are able to qualify features of flow patterns, in the present case, measures are not sufficiently reliable to evaluate, for instance, the relative hydraulic properties of productive levels in an aquifer subject to local rapid (karstic) flow. In the end, for this type of medium, temperature logging should be viewed as complementary information of more accurate but also more heavy methods of investigation such as flow-logs.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H31B0838C
- Keywords:
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- 1828 Groundwater hydraulics;
- 1829 Groundwater hydrology;
- 1895 Instruments and techniques: monitoring