HR: 17:00h
AN: H24C-05    [Abstracts]
TI: Full Aquifer Characterization Combining Thermal Data and Long Term Well Monitoring
AU: * Doan, M
EM: doan@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Casier 89 4, place Jussieu, PARIS, 75013 France
AU: Cornet, F H
EM: cornet@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Casier 89 4, place Jussieu, PARIS, 75013 France
AB: In order to study in-situ the coupling between fluid pressure and fault mechanics, a geophysical laboratory was installed in the Corinth Rift, central Greece. A major element of this European project is the 1000 m borehole crossing the Aigio fault. Its permanent instrumentation allows the continuous monitoring of fluid pressure around the fault. The borehole is cased down to 708m. The fault is intersected at 760m. It is impervious and separates two independent aquifers. The upper one is an artesian fractured aquifer, with an overpressure of 0.5MPa. The lower one is a karst. It is also artesian with a larger overpressure of 0.9MPa. During a production test, the outflow from the karst appeared to be limited by the well tubing, so that only a minimal permeability value could be estimated. Also, we need to know the extension of the aquifers and its poroelastic properties in order to interpret correctly the numerous hydraulic anomalies recorded by the instrumentation. To retreive these information, we used (1) tidal analysis (2) thermal profiles (3) long term pressure behavior. The sensors installed in the borehole exhibit excellent resolution on tides and seasonal variations. Tide analysis quantifies the poroelastic response of the medium. We developped an analytical model to interpret phase lag of the pressure response to the oceanic load and constrain the permeability of the karst. A pressure drop arises from the opening of communication between the two aquifers surrounding the fault. We developped special numerical technique to model this transient, and found consistency only with models involving confined aquifers. This is confirmed by the absence of seasonal variations and the persistance of the pressure anomaly. The geometry of the aquifer was further refined using thermal measurements collected inside the borehole. The value of 50±10 mW/m² is one of the first measurements obtained inside the Rift. The consistency of the data also proves that no horizontal flow within the karst screens out the heat flow. We also find evidence of convection in the karst, which allows constraining its vertical extent. All these elements show that the karst appears to be a confined thick aquifer in thermal convection, quite different from the massively flowing feature depicted prior to the borehole monitoring.
DE: 1835 Hydrogeophysics
DE: 1847 Modeling
DE: 1872 Time series analysis (3270, 4277, 4475)
DE: 3653 Fluid flow
DE: 7215 Earthquake source observations (1240)
SC: Hydrology [H]
MN: Fall Meeting 2005