Ð O À N Mai Linh Institut de Physique du Globe de Paris Étude in-situ des interactions...

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




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

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


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-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 ?


Storativity → Hydraulic

diffusivity

Karst confinedin a NS

direction


Long-term information

Tidal calibration

ThermalRegime

Tidal calibration


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


(Almost) no flow



Thermal information

Tidal calibration


Thermalregime

Tidal calibration


Internal advection


Large vertical extension

Low heat flow50±10 mW/m²

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 ?


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.

Ð O À N Mai Linh Institut de Physique du Globe de Paris Étude in-situ des interactions...

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