Post on 14-Jan-2016
description
ÐOÀN Mai LinhInstitut de Physique du Globe de Paris
Étude in-situ des interactions hydromécaniques
entre fluides et failles
Application au laboratoire du rift de Corinthe
Fluid-fault interactions
Fault closed
Fluid pressureBuild-up
Fault slip
Fluid Pressuredecrease
Fault-valve mechanism (Sibson70)Example of fluid-fault hydromechanical coupling:
Motivations
Lots of theory andlaboratory
works
But field data:• altered outcrops after slip• dynamical seismics indirect
After Matthai (1992)
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
Structure of the presentation
The Corinth Rift
From Jolivet (2005)
Greece
Complicated geodynamics
subduction
extension
Complexgeology
Pindos
Gavrovo-Tripolitza
shear zone
Rift of Corinth
The Corinth Rift
1.5cm/yr
Aigio fault
1-3cm of slip
After Koukouvelas (1998)
After Bernard (1997)
Deep Geodynamic LABoratory
0.5±0.1MPa
0.9±0.1MPa
karst
South North
Initial hydraulic knowledge of the Aigio fault
Impervious fault
• Difference in overpressure
K=0.9-2 10-18m²(Song,2004)
• Laboratory test on core samples
• Difference in mineralization
Initial hydraulic knowledge of upper aquifer
Hydraulic tests by GFZ – July 2003
(Giurgea, 2004)
Dra
wdo
wn
[m]
Double porosity model
Bulk properties
Matrix properties
Results to be taken
with caution
Initial hydraulic knowledge of the karst
Q~600m³/hk=1-1.5 10-5 m/s
No storativity
k
lnRr b
2 HPg
Q
Permanent regime Dupuit formula
AIG10 permanent sensors
Pressure sensors
2 absolute pressure gauges- high precision- low precision1 relative pressure gauge- hydrophone
Log10
(Frequency [Hz])
Log
10(P
ress
ure
[MP
a])
Tides
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
Quality of the pressure signalPressure
Pre
ssur
e (B
ar)
UT Time
Resolution better than 1%
The pressure is similar to that of the karst The karst dominates the measured pressure
Strategy
Long-termfluctuations
Tidal calibration
ThermalRegime
Tidal calibration
How sensitive is the pressure signal to deformation ?
What are the dimensions
of the aquifers ?
How waterflows through the aquifers ?
Tidal inversion
Triple origine
Earth Tide (Prediction ETERNA 3.3)
Aigion
Oceanic load (P. Bernard)
Trizonia
(Aigio)
Also
Barometric pressure (V. Léonardi)
Temeni
Input: Theoretical tidal strain in Aigio
Input: Barometric pressure in Temeni
Input: Tide gage in Trizonia
Ouput: pressure in Aigio
Linear regression on the input data
Analysis of the tidal signal
dP=2.748 10-4 dhoc
– 1.784 10-4 dter No offset
Analysis of the tidal signal
Barometric effectBad weather at the end of the year 2003
Observed pressure (detided) Atmospheric pressure
Interpretation of the coefficients
Poroelastic model (large wavelengths)
P
ter
B K u
PPatm
B 1 u
3 1 u
P
ter
B K u
P
ter
B K u
B Ku=17±1GPa =0.3±0.1
B : Skempton coefficientK
u : Undrained bulk modulus
u : Undrained Poisson ratio
: Barometric efficiency
Oceanic loadN SAig10Oceanic load
Loading profile at a depth of 700m induced by a unit load
Water flux
The oceanic load should induce a phase lag !
Distance to southern shore (m)
σ xx+σ zz
/2ρg
h
AIG10
Influence of boundariesN SAig10Oceanic load
Aigiofault
Helikefault
Can the presence of impervious faults explainthis absence of phase lag ?
Analytical prediction of phase lag for a 1D aquifer with impervious boundaries
Oceanic load
Map of semi-diurnal phase lag (°) for a semi-infinite ocean
L
x/L
Phase lag[-5 min 5min]
[-2.5° 2.5°]
L
x
N
S
L
x
N
Is Aigio fault impervious at all depths ?
Tidal information
Long-termfluctuations
Tidal calibration
Thermalregime
How sensitive is the pressure signal to deformation ?
What are the dimensions
of the aquifers ?
How waterflows through the aquifers ?
Poroelastic parameters → excellent « strain » sensor
Tidal calibration
Karst confinedin a NS direction.By Aigio fault ?
Storativity → Hydraulic
diffusivity
Tidal calibration
Long-termfluctuations
Long-term dataPressure
Time
Pre
ssur
e (b
ar)
1 year
14 kPa
Flow betweenthe two previouslyindependent aquifers
Nosharpseasonal variations
Analytical solutionAxisymmetric response for infinite aquifers
Pre
ssur
e (b
ar)
Time (day)
Axisymmetric analytical solutions Finite aquifers
Transients controlled by the radii of the aquifers and borehole radius
Development of the FEM2.1D method
1. Finite Element Method 2D to describe flow
in upper and lower aquifers
Efficient Keep the characteristic distance of the well radius
2. Manual coupling at a well node (0.1D) Same pressure Mass conservation of fluid
Dimensions of the aquifers ?
Can the decrease in pressure observed during the first 3 monthsprovide constraints on the dimensions of the aquifers ?
Try to find plausible configurations
Rectangular-shaped aquifers 4 unknownsHydraulic properties of the upper aquifer 1 unknown (storativity)
2 pieces of information to fit : amplitude and duration of the drop
Dimensions of the aquifers ?
Time (days)
Pre
ssur
e (b
ar)
Upper aquifer: LNS=1000m LWE=200m
Lower aquifer: LNS=5000m LWE=?
Too small
Too slow
Pertinence ofThe homogeneous
Model for the karst ?
Long-termfluctuations
Storativity → Hydraulic
diffusivity
Karst confinedin a NS
direction
Poroelastic parameters → excellent « strain » sensor
Long-term information
Tidal calibration
ThermalRegime
Tidal calibration
Long-termfluctuations
Thermalregime
Hydraulicdiffusivity
(Almost) no flow
Both aquifersare confined
Thermal profile
Depth (m)
Tem
pera
ture
(°C
) 1 year after drilling
Heat flow measurement
=
50±10 mW/m2~22°C/km
H > 400 m
ρ
ρ
pf
P
C
HTKgaR
24 CP aRaR
Relation Ttzt
from extrapolation ofThermal gradient
Karst convection
qb= 70mW/m² qb=100mW/m² qb=200mW/m²
770m
zt
Tt
Tmes
Fault vertical offset=150m
zt-770m <150m
H>600 m Gavrovo-Tripolitza nappe
Thermal anomaly
Heat generated by fault slip does not explain this anomaly
Temperature (°C)30 30.2 30.4 30.6 30.8 31 31.4 31.6 31.831.2
700
710
740
Dep
th (
m)
720
730
But the introduction of thekarst convection does.
Karst in conduction
Karst in convection
500
1000
1500
Temperature (°C)3020100 40 50 60
Hydraulicdiffusivity
(Almost) no flow
Long-termfluctuations
Poroelastic parameters → excellent « strain » sensor
Thermal information
Tidal calibration
Both aquifersare confined
Thermalregime
Tidal calibration
Hydraulicdiffusivity
Internal advection
Both aquifersare confined
Large vertical extension
Low heat flow50±10 mW/m²
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
A panel of hydraulic anomalies
~2minute-long
~2minute-long ~30minute-long
~10minute-long
Only associated withteleseismic transients
~200 events/yr
~100 events/yr~20 events/yr
2 events/yr
10-400 Pa
10-200 Pa
10-200 Pa
50-60 Pa
Time
Pre
ssur
e
The Mw=7.8 Rat Island Earthquake
November, 17th 2003 06:43 UTC
Drop of 60 Pa(equivalent to 3.5nstr)
5min 30min
Much earlier than other publishedtriggered events
BKu~17GPa
determined from tidal analysis
Review of triggered hydraulic anomalies
Distance to epicenter (km)
Mag
nitu
de
Strain<10-8
Strain>10-8
2003Rat Island
Event
After Montgomery and Manga (2003)
Comparison with other local sensors
Anomalous drop on pressure data only
h<5nstr
Aigio
Trizonia
0 10km
Sacks-EvertsonStrainmeter
STS2broad-band
Seismometer(North component)
LF signal
Validity of the pressure dataNyquist frequency
of the pressure sensor
Fre
quen
cy
Time
Good correlation of both sensors
P
h
Comparison of seismic oscillations of both «deformation» sensors
- Strainmeter- Pressure
Response to a dislocation
Fault movement
Average of pressureanomaly
along the borehole
Poroelastic responseHETEROGENEOUS
along the borehole
One single wellhead value
Response to a dislocation
yx
y
zx
Dip direction
Average of pressure along the borehole induced by a double-couple located at (x,y)
Map of Log10
(Pressure anomaly) for D×S=1m3
Distance from borehole
~ √hydraulic
trelaxation
D×S~1m3
S= slip areaD= relative displacement
M0=DS
<5000m3 (Trizonia data)
High-frequency hydrophone data
Pressure HydrophoneClose-up
Time
+0.000
07:15
07:05 07:10 07:15
Hydrophone
UTC Time 07:07:02
+0.100
07:1007:05
07:05 07:1507:10
Angle of slip
yx
y
zx
Fault plane
Slickensides
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
The Mw=9 Sumatra event
Data acquisition problem Irregular sampling
P S
Pressurein Aigio
Strainin Trizonia
Below Nyquist frequency
December, 26th 2004 00:58 UTC
Hydraulic characterisation of AIG10● We measure the pressure of the bottom karst
● Poroelastic response to both Earth tides and ocean load Sensitive “strain” sensor
● Aquifers are confined with almost no flow at the boundariesand internal convection within the karst
● Aigio fault is impervious at the intersection with the boreholebut 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
Conclusion
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
Hydraulic anomalies
The DGLAB project provides the opportunityto study
dynamic fluid-fault interactions
Perspectives
Interpretation of the remaining hydraulic events
• But we monitor fluids around a fault rather than fluids inside a fault
• But no independent evaluation of fluid evolution and fault movement
Better knowledge of the surrounding seismicity Better interpretation of the hydrophone signal
Better understanding of the aquifer and its heterogeneities
PerspectivesExpected full instrumentation
0 m
700 m
750 m
870 m
1000 m
Hydrophone
Hydrophone
3C Seismometer
High-precisionpressure gage
High-precisionpressure gage
Installation of thewhole instrumentation
scheduled in March 2006
Link between storativities
S = uniaxial storativity 11= 22=0, dσ33=0
S = unstrained storativity =0Sσ = strained storativity dσ=0
S S Sσ
S = (1-αB) Sσ
The AIG10 borehole
0.5±0.1MPa
0.9±0.1MPa
karst
Age of the karst waterSimple optimistic model :
Čermák model
T>1000 yr(In accordance
with the absence of Tritium in water)
The lower aquifer is karstic
800
900
Are the aquifers well confined ?
Long-termfluctuations
All the three studies were necessary
Tidal calibration
Thermalregime
Tidal calibration
Pressure sensors
2 absolute pressure gauges- high precision- low precision1 relative pressure gauge- hydrophone
Log10
(Frequency [Hz])
Log
10(P
ress
ure
[MP
a])
Tides
Development of the FEM2.1D method
1. Finite Element Method 2D on each aquifer2. Manual coupling at a well node (0.1D)
Efficient Keep the characteristic distance of the well radius
Analytical axisymmetric solutions shows that the transitory regime
is partly controlled by the borehole radius
Conclusion
Hydraulic anomalies● A large set of hydraulic anomalies.
● A anomalous hydraulic anomaly dynamically triggered by S waves from a teleseism,
with concomitant a local seismic event
Borehole instrumentation provides tools to understand the triggering mechanism
Hydraulic characterisation of AIG10● We measure the pressure of the bottom karst
● Poroelastic response to both Earth tides and ocean load Sensitive “strain” sensor
● Aquifers are confined with almost no flow at the boundariesbut internal convection within the karst
● Aigio fault is impervious at the intersection with the boreholebut is it the case below ?
● Low heat flow and rigid block. Not a process zone.