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Ð O À N Mai Linh Institut de Physique du Globe de Paris
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  1. Étude in-situ des interactions hydromécaniques entre fluides et failles Application au laboratoire du rift de Corinthe ÐOÀN Mai Linh Institut de Physique du Globe de Paris

  2. Fault slip Fluid pressure Build-up Fluid Pressure decrease Fluid-fault interactions Example of fluid-fault hydromechanical coupling: Fault-valve mechanism (Sibson70) Fault closed

  3. Lots of theory and laboratory works Motivations • But field data: • altered outcrops • after slip • dynamical seismics • indirect After Matthai (1992)

  4. Structure of the presentation I Presentation of the Gulf of Corinth and the DGLAB project II Characterization of the hydraulic setting III A peculiar kind of hydraulic transients: Events triggered by far earthquakes I Presentation of the Gulf of Corinth and the DGLAB project II Characterization of the hydraulic setting III A peculiar kind of hydraulic transients: Events triggered by far earthquakes

  5. subduction shear zone extension  Rift of Corinth Pindos Gavrovo- Tripolitza The Corinth Rift Greece Complicated geodynamics Complex geology From Jolivet (2005)

  6. 1.5cm/yr Aigio fault 1-3cm of slip After Koukouvelas (1998) The Corinth Rift After Bernard (1997)

  7. 0.9±0.1MPa karst Deep Geodynamic LABoratory South North 0.5±0.1MPa

  8. Laboratory test on core samples K=0.9-2 10-18m² (Song,2004) • Difference in mineralization Initial hydraulic knowledge of the Aigio fault Impervious fault • Difference in overpressure

  9. (Giurgea, 2004) Double porosity model Results to be taken with caution Bulk properties Matrix properties Initial hydraulic knowledge of upper aquifer Drawdown [m] Hydraulic tests by GFZ – July 2003

  10. Permanent regime  Dupuit formula Q~600m³/h Initial hydraulic knowledge of the karst k=1-1.5 10-5 m/s No storativity

  11. AIG10 permanent sensors

  12. Pressure sensors Tides Log10(Pressure [MPa]) 2 absolute pressure gauges - high precision - low precision 1 relative pressure gauge - hydrophone Log10(Frequency [Hz])

  13. Structure of the presentation I Presentation of the Gulf of Corinth and the DGLAB project II Characterization of the hydraulic setting III A peculiar kind of hydraulic transients: Events triggered by far earthquakes

  14. Quality of the pressure signal Pressure Pressure (Bar) UT Time Resolution better than 1% The pressure is similar to that of the karst The karst dominates the measured pressure

  15. Strategy Tidal calibration Tidal calibration Long-term fluctuations How sensitive is the pressure signal to deformation ? What are the dimensions of the aquifers ? How water flows through the aquifers ? Thermal Regime

  16. Trizonia • Oceanic load • (P. Bernard) (Aigio) Temeni Also • Barometric pressure • (V. Léonardi) Tidal inversion  Triple origine • Earth Tide • (Prediction ETERNA 3.3) Aigion

  17. Analysis of the tidal signal Linear regression on the input data Ouput: pressure in Aigio Input: Tide gage in Trizonia Input: Barometric pressure in Temeni Input: Theoretical tidal strain in Aigio

  18. Analysis of the tidal signal dP=2.748 10-4 dhoc – 1.784 10-4 dter No offset

  19. Barometric effect Bad weather at the end of the year 2003 Observed pressure (detided)Û Atmospheric pressure

  20. Interpretation of the coefficients Poroelastic model (large wavelengths) B : Skempton coefficient Ku : Undrained bulk modulus u : Undrained Poisson ratio  : Barometric efficiency =0.3±0.1 B Ku=17±1GPa

  21. Water flux Oceanic load S Aig10 N ¯¯ Oceanic load ¯¯ Loading profile at a depth of 700m induced by a unit load σxx+σzz/2ρgh AIG10 Distance to southern shore (m) The oceanic load should induce a phase lag !

  22. Influence of boundaries S Aig10 N ¯¯ Oceanic load ¯¯ Aigio fault Helike fault Can the presence of impervious faults explain this absence of phase lag ? Analytical prediction of phase lag for a 1D aquifer with impervious boundaries

  23. S N N Oceanic load Map of semi-diurnal phase lag (°) for a semi-infinite ocean Phase lag [-5 min 5min] ß [-2.5° 2.5°] L x/L

  24. Is Aigio fault impervious at all depths ?

  25. Tidal calibration Long-term fluctuations Tidal information Tidal calibration Tidal calibration Long-term fluctuations How sensitive is the pressure signal to deformation ? Poroelastic parameters → excellent « strain » sensor Karst confined in a NS direction. By Aigio fault ? What are the dimensions of the aquifers ? Storativity → Hydraulic diffusivity How water flows through the aquifers ? Thermal regime

  26. Long-term data Pressure Flow between the two previously independent aquifers No sharp seasonal variations 14 kPa Pressure (bar) Time 1 year

  27. Axisymmetric response for infinite aquifers Pressure (bar) Time (day) Analytical solution Axisymmetric analytical solutions  Finite aquifers  Transients controlled by the radii of the aquifers and borehole radius

  28. Development of the FEM2.1D method 1. Finite Element Method 2D to describe flow in upper and lower aquifers 2. Manual coupling at a well node (0.1D) Same pressure Mass conservation of fluid • Efficient • Keep the characteristic distance of the well radius

  29. Dimensions of the aquifers ? Can the decrease in pressure observed during the first 3 months provide constraints on the dimensions of the aquifers ? Rectangular-shaped aquifers 4 unknowns Hydraulic properties of the upper aquifer 1 unknown (storativity) 2 pieces of information to fit : amplitude and duration of the drop Try to find plausible configurations

  30.  Too small    Too slow    Dimensions of the aquifers ? Upper aquifer: LNS=1000m LWE=200m Lower aquifer: LNS=5000m LWE=? Pressure (bar) Pertinence of The homogeneous Model for the karst ? Time (days)

  31. Long-term fluctuations Thermal regime Long-term information Tidal calibration Tidal calibration Long-term fluctuations Poroelastic parameters → excellent « strain » sensor Karst confined in a NS direction Both aquifers are confined Hydraulic diffusivity (Almost) no flow Storativity → Hydraulic diffusivity Thermal Regime

  32. Thermal profile 1 year after drilling Temperature (°C) Depth (m)

  33. = Heat flow measurement 50±10 mW/m2 ~22°C/km

  34. zt Tt 770m Tmes H>600 m  Gavrovo-Tripolitza nappe Karst convection Fault vertical offset=150m  zt-770m  <150m qb= 70mW/m² qb=100mW/m² qb=200mW/m² Relation Ttzt from extrapolation of Thermal gradient H > 400 m

  35. Temperature (°C) 30 30.2 30.4 30.6 30.8 31 31.2 31.4 31.6 31.8 700 But the introduction of the karst convection does. 710 720 Depth (m) 500 730 Karst in conduction 1000 740 Karst in convection 1500 0 10 20 30 40 50 60 Temperature (°C) Thermal anomaly Heat generated by fault slip does not explain this anomaly

  36. Thermal information Tidal calibration Tidal calibration Long-term fluctuations Poroelastic parameters → excellent « strain » sensor Hydraulic diffusivity Internal advection Hydraulic diffusivity (Almost) no flow Both aquifers are confined Both aquifers are confined Large vertical extension Thermal regime Low heat flow 50±10 mW/m²

  37. Structure of the presentation I Presentation of the Gulf of Corinth and the DGLAB project II Characterization of the hydraulic setting III A peculiar kind of hydraulic transients: Events triggered by far earthquakes

  38. A panel of hydraulic anomalies ~10minute-long ~2minute-long 10-200 Pa 10-200 Pa ~100 events/yr ~20 events/yr Pressure ~2minute-long ~30minute-long 10-400 Pa 50-60 Pa ~200 events/yr 2 events/yr Only associated with teleseismic transients Time

  39. Much earlier than other published triggered events The Mw=7.8 Rat Island Earthquake November, 17th 2003 06:43 UTC Drop of 60 Pa (equivalent to 3.5nstr) BKu~17GPa determined from tidal analysis 30min 5min

  40. Review of triggered hydraulic anomalies Strain>10-8 2003 Rat Island Event Magnitude Strain<10-8 Distance to epicenter (km) After Montgomery and Manga (2003)

  41. Anomalous drop on pressure data only h<5nstr Comparison with other local sensors Trizonia Aigio LF signal 0 10km Sacks-Evertson Strainmeter STS2 broad-band Seismometer (North component)

  42. Validity of the pressure data Comparison of seismic oscillations of both «deformation» sensors Nyquist frequency of the pressure sensor P Frequency h - Strainmeter - Pressure Good correlation of both sensors Time

  43. Response to a dislocation One single wellhead value Average of pressure anomaly along the borehole Poroelastic response HETEROGENEOUS along the borehole Fault movement

  44. Response to a dislocation Average of pressure along the borehole induced by a double-couple located at (x,y) M0=DS Map of Log10(Pressure anomaly) for D×S=1m3 x y S= slip area D= relative displacement Dip direction z x y Distance from borehole ~ √hydraulictrelaxation D×S~1m3 <5000m3 (Trizonia data)

  45. High-frequency hydrophone data Hydrophone Close-up Pressure UTC Time 07:07:02 07:15 07:10 07:05 07:05 07:10 07:15 Hydrophone +0.000 +0.100 07:05 07:10 07:15 Time

  46. Average of pressure along the borehole induced by a double-couple located at (x,y) Map of Log10(Pressure anomaly) for D*S=1m3 Not seen by pressure sensor D*S<0.1-1m3 Angle of slip z x x y y Fault plane Slickensides

  47. Below Nyquist frequency The Mw=9 Sumatra event Data acquisition problem Irregular sampling Pressure in Aigio December, 26th 2004 00:58 UTC P S Strain in Trizonia

  48. Conclusion Hydraulic characterisation of AIG10 • We measure the pressure of the bottom karst • Poroelastic response to both Earth tides and ocean load ÞSensitive “strain” sensor • Aquifers are confined with almost no flow at the boundaries and internal convection within the karst • Aigio fault is impervious at the intersection with the borehole but is it the case below the Pindos nappe • Low heat flow Hydraulic characterisation of AIG10 It is now possible to model the wellhead pressure response to fault movement within an homogeneous poroelastic framework Hydraulic anomalies The DGLAB project provides the opportunity to study dynamic fluid-fault interactions Hydraulic anomalies • A large set of hydraulic anomalies. • An anomalous hydraulic anomaly dynamically triggered by S waves from a teleseism, with a concomitant local microseismic event

  49. Perspectives Interpretation of the remaining hydraulic events • Better knowledge of the surrounding seismicity • Better interpretation of the hydrophone signal • Better understanding of the aquifer and its heterogeneities • But we monitor fluids around a fault • rather than fluids inside a fault • But no independent evaluation • of fluid evolution and fault movement

  50. Perspectives Expected full instrumentation 0 m Hydrophone Installation of the whole instrumentation scheduled in March 2006 Hydrophone 700 m High-precision pressure gage 750 m High-precision pressure gage 870 m 3C Seismometer 1000 m