Gravimetric and leveling measurements for basement ... · K1= 10-6 m/s Hard Rock K3 ?? Fracture...
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Gravimetric and leveling measurements for basement aquifers in West Africa Gravity monitoring of aquifers ΔZ, specific yield : φ Large experience in West Africa on sedimentary as well on basement aquifers was obtained thanks to the GHYRAF project (2008-2011, Hinderer et al., 2011) Δg(nms -2 )=400 φΔx(m) gravimeter CG5 relative FG5 SG good nms -2 φ=0.1 Presently, a gravimeter can be considered as a poor tool for measuring water levels (0.5 m for 10 % porosity), but this will change with the new generation of portable superconducting gravimeters (iGrav) (better than 0.02 m) CG5 FG5 SG Drawdown after 7 days (m) X (m) -100 -50 0 50 100 -100 -80 -60 -40 -20 0 20 40 60 80 100 0.5 1 1.5 2 2.5 3 3.5 X (m) -100 -50 0 50 100 00 80 60 40 20 0 20 40 60 80 00 0.5 1 1.5 2 2.5 3 X (m) -100 -50 0 50 100 -100 -80 -60 -40 -20 0 20 40 60 80 100 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 X (m) -100 -50 0 50 100 -100 -80 -60 -40 -20 0 20 40 60 80 100 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 K2=2e-5; K3= 6e-6 K2=2e-5; K3= 2e-5 K2=2e-5; K3= 1e-4 K2=6e-6; K3= 2e-5 Drawdown decreases with increasing permeabilities K2 and K3 Anisotropy increases with increasing K3 THE MODEL AQUIFER TESTING IN GRANITE AQUIFERS Water level changes are computed in a simplified aquifer using the finite difference Modflow package and a variable grid (Δx = 0.4-2500 m). A constant pumping rate of 55 m 3 /h is imposed during 7 days. The geometry of the fissured layer inside the fault and outside the fault are supposed to be known from exploration geophysics. The width of the fracture zone is 40 m. 0 m 30 m 40 m 80 m Saprolites K1= 10 -6 m/s K3 ?? Hard Rock Fracture Subsidence Clay present in the saprolite layer deforms when the water pressure decreases in the aquifer. The volume change is ΔV/V= ΔP/E=ρg Δh/E, where ΔP is the pressure change induced by the drawdown Δh. With E in the range [10 6 ; 10 7 ] Pa and a 2 m total clay content in the profile, the subsidence is in the range [2×10 -3 ; 2×10 -2 ] m / m. Gravity effects Direct effect -400 nms -2 /m for each drawdown meter Indirect effect [+6 ; +60] nms -2 for each drawdown meter (using free air gradient of 3000 nms -2 /m) Fissured layer K2 = ?? Hard Rock DRAWDOWN RESULTS Experimental design It consists of 6 gravity points and 12 leveling points distributed along the fault and parallel to it. These points can be measured in one day by an experienced team. The objective function It is defined as the RMS of the differences of actual observations with those of a reference case (K2=8×10 -6 m/s, K3=4×10 -5 m/s, E=3×10 6 Pa) normalized by the standard error of each measurement (Δg=20nms -2 , Δl=1 mm, Δh= 0.5 m for the central borehole, and 0.02 m otherwise). = 1 × 1 ( − 0 ) 2 ∆ 2 + 1 ( − 0 ) 2 ∆ 2 + 1 ℎ (ℎ −ℎ 0 ) 2 ∆ℎ 2 Where 0 indices refer to the reference case, g is the gravity signal, l refer to leveling, and h to the water level in the aquifer. An objective function > 1 only can be achieved with current experimental methods. A n g ,n l ,n h are the number of gravity, leveling and water level points and n= n g +n l +n h INVERSION WITH GRAVITY DATA INVERSION RESULTS (objective function OF) Gravity + leveling K2+K3 is well defined, however K2-K3 is poorly resolved Hyd. Cond. outside fault Hyd. Cond. inside fault 0.5 1 1.5 2 2.5 3 x 10 -5 0.5 1 1.5 2 2.5 3 x 10 -5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 Hyd. Cond. outside fault Hyd. Cond. inside fault 1 2 3 4 5 6 7 8 9 10 x 10 -5 1 2 3 4 5 6 7 8 9 10 x 10 -5 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 Hyd. Cond. outside fault Hyd. Cond. inside fault 0.5 1 1.5 2 2.5 3 x 10 -5 0.5 1 1.5 2 2.5 3 x 10 -5 1 2 3 4 5 6 7 8 9 10 + 2 boreholes 55 m away (Δh=0.02 m) The two additional boreholes provide a strong constraint on the mean value of K2 and K3, but poorly resolve the difference between K2 and K3 (probably due to the poor definition of anisotropy of drawdown with only 2 boreholes) CONCLUSION • Gravity data provide information on water level fluctuations, but only a few points can be measured in one day. For weathering profiles including highly deformable clays, we propose to use combine gravity and high precision leveling. • With current gravimeter precision, a fair constraint can by brought on the mean value of the high permeability fractured layer with a 7 days aquifer testing experiment. However the geometry of this layer must be known (for example by exploration geophysics). • The difference of permeability in the fault zone and outside the fault zone (K2-K3) is poorly constrained. • 2 additional borehole provide a stronger constraint on the mean permeability, but not on the K2-K3 difference. • With 2 nm/s 2 precise gravity data, both K2 and K3 could be resolved. This order of magnitude of precision fall in the range of future portable gravimeters. • These results stand for an idealized granitic aquifer including heterogeneities relying on an idealized weathering profile and should be therefore confirmed by field experiments. References Hinderer et al., 2011. Land water storage from ground and space geodesy; first results from the GHYRAF (Gravity and Hydrology in Africa) experiment, Pure Appl. Geophys., doi: 10.1007/s00024-011-0417-9 Damiata, B.N., Lee, T.C., 2006. Simulated gravitational response to hydraulic testing of unconfined aquifers, J. of Hydrol., 318, 348-359. Herckenrath, D., Aucken, E., Christiansen, L., Behroozmand, A.A., Bauer-Gottwein, P., 2012. Coupled hydrogeophysical inversion using time-lapse magnetic resonance sounding and time-lapse gravity data for hydraulic aquifer testing: Will it work in practice? Wat. Res. Res., doi: 10.1029/2011WR010411. ! Hydraulic conductivity at the base of the weathering profile ? Ferhat Gilbert 1 , Genthon Pierre 2 , Mouhouyouddine Ali Houmadi 2-3 , Hinderer Jacques 1 , Hector Basile 5 , Yameogo Suzanne 4 1 EOST- IPG Strasbourg-INSA 2 HydroSciences Montpellier 3 Comores University 4 Hydrogeology and Remote Sensing, Univ. Ouagadougou, Burkina Faso 5 LTHE Grenoble Note the change in the color scale
Transcript of Gravimetric and leveling measurements for basement ... · K1= 10-6 m/s Hard Rock K3 ?? Fracture...
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Gravimetric and leveling measurements for basement aquifers in West Africa
Gravity monitoring of aquifers
ΔZ, specific yield : φ
Large experience in West Africa on sedimentary as well on basement aquifers was obtained thanks to the GHYRAF project (2008-2011, Hinderer et al., 2011)
Δg(nms-2)=400 φΔx(m)
gravimeter CG5relative
FG5 SG
good
nms-2
φ=0.1
Presently, a gravimeter can be considered as a poor tool for measuring water levels (0.5 m for 10 % porosity), but this will change with the new generation of portable superconducting gravimeters (iGrav) (better than 0.02 m)
CG5FG5
SG
Drawdown after 7 days (m)
X (m)
Y (
m)
-100 -50 0 50 100-100
-80
-60
-40
-20
0
20
40
60
80
100
0.5
1
1.5
2
2.5
3
3.5
X (m)
Y (
m)
-100 -50 0 50 100-100
-80
-60
-40
-20
0
20
40
60
80
100
0.5
1
1.5
2
2.5
3
X (m)
Y (
m)
-100 -50 0 50 100-100
-80
-60
-40
-20
0
20
40
60
80
100
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
X (m)
Y (
m)
-100 -50 0 50 100-100
-80
-60
-40
-20
0
20
40
60
80
100
0.3
0.4
0.5
0.6
0.7
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0.9
1
1.1
1.2
1.3
K2=2e-5; K3= 6e-6 K2=2e-5; K3= 2e-5 K2=2e-5; K3= 1e-4 K2=6e-6; K3= 2e-5
Drawdown decreases with increasing permeabilities K2 and K3 Anisotropy increases with increasing K3
THE MODELAQUIFER TESTING IN GRANITE AQUIFERS
Water level changes are computed ina simplified aquifer using the finitedifference Modflow package and avariable grid (Δx = 0.4-2500 m).A constant pumping rate of 55 m3/his imposed during 7 days.
The geometry of the fissured layerinside the fault and outside the faultare supposed to be known fromexploration geophysics. The width ofthe fracture zone is 40 m.
0 m
30 m
40 m
80 m
SaprolitesK1= 10-6 m/s
K3 ??Hard Rock
Fracture
Subsidence
Clay present in the saprolite layer deforms when the water pressure decreases in theaquifer. The volume change is ΔV/V= ΔP/E=ρg Δh/E, where ΔP is the pressure changeinduced by the drawdown Δh. With E in the range [106 ; 107] Pa and a 2 m total claycontent in the profile, the subsidence is in the range [2×10-3 ; 2×10-2] m / m.
Gravity effects
Direct effect -400 nms-2/m for each drawdown meter Indirect effect [+6 ; +60] nms-2 for each drawdown meter
(using free air gradient of 3000 nms-2/m)
Fissured layer K2 = ??
Hard Rock
DRAWDOWN RESULTS
Experimental designIt consists of 6 gravity points and 12 leveling points distributed along the fault andparallel to it. These points can be measured in one day by an experienced team.
The objective functionIt is defined as the RMS of the differences of actual observations with those of areference case (K2=8×10-6 m/s, K3=4×10-5 m/s, E=3×106 Pa) normalized by thestandard error of each measurement (Δg=20nms-2, Δl=1 mm, Δh= 0.5 m for the centralborehole, and 0.02 m otherwise).
𝑂𝐹 =1
𝑛× 𝐒𝐐𝐑𝐓
1
𝑛𝑔(𝑔𝑖−𝑔0𝑖)
2
∆𝑔2+
1
𝑛𝑙(𝑙𝑖−𝑙0𝑖)
2
∆𝑙2+
1
𝑛ℎ(ℎ𝑖−ℎ0𝑖)
2
∆ℎ2
Where 0 indices refer to the reference case, g is the gravity signal, l refer to leveling,and h to the water level in the aquifer. An objective function > 1 only can be achievedwith current experimental methods. A ng, nl, nh are the number of gravity, leveling andwater level points and n= ng+nl +nh
INVERSION WITH GRAVITY DATA
INVERSION RESULTS (objective function OF)
Gravity + leveling
K2+K3 is well defined, however K2-K3 is poorly resolved
Hyd. Cond. outside fault
Hyd.
Cond.
insid
e f
ault
0.5 1 1.5 2 2.5 3
x 10-5
0.5
1
1.5
2
2.5
3x 10
-5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Hyd. Cond. outside fault
Hyd.
Cond.
insid
e f
ault
1 2 3 4 5 6 7 8 9 10
x 10-5
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7
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10x 10
-5
0.6
0.8
1
1.2
1.4
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1.8
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2.2
2.4
2.6
Hyd. Cond. outside fault
Hyd.
Cond.
insid
e f
ault
0.5 1 1.5 2 2.5 3
x 10-5
0.5
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2
2.5
3x 10
-5
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10
+ 2 boreholes 55 m away (Δh=0.02 m)
The two additional boreholes provide astrong constraint on the mean value of K2and K3, but poorly resolve the differencebetween K2 and K3 (probably due to thepoor definition of anisotropy of drawdownwith only 2 boreholes)
CONCLUSION
• Gravity data provide information on water level fluctuations, but only a few points can be measured in one day. For weathering profiles including highly deformable clays, we propose to use combine gravity and high precision leveling.
• With current gravimeter precision, a fair constraint can by brought on the mean value of the high permeability fractured layer with a 7 days aquifer testing experiment. However the geometry of this layer must be known (for example by exploration geophysics).
• The difference of permeability in the fault zone and outside the fault zone (K2-K3) is poorly constrained.• 2 additional borehole provide a stronger constraint on the mean permeability, but not on the K2-K3 difference.• With 2 nm/s2 precise gravity data, both K2 and K3 could be resolved. This order of magnitude of precision fall in the range of future portable gravimeters.• These results stand for an idealized granitic aquifer including heterogeneities relying on an idealized weathering profile and should be therefore confirmed
by field experiments. ReferencesHinderer et al., 2011. Land water storage from ground and space geodesy; first results from the GHYRAF (Gravity and Hydrology in Africa) experiment, Pure Appl. Geophys., doi: 10.1007/s00024-011-0417-9Damiata, B.N., Lee, T.C., 2006. Simulated gravitational response to hydraulic testing of unconfined aquifers, J. of Hydrol., 318, 348-359.Herckenrath, D., Aucken, E., Christiansen, L., Behroozmand, A.A., Bauer-Gottwein, P., 2012. Coupled hydrogeophysical inversion using time-lapse magnetic resonance sounding and time-lapse gravity data for hydraulic aquifer testing: Will it work in practice? Wat. Res. Res., doi: 10.1029/2011WR010411.
!
Hydraulic conductivity at the base of the weathering profile ? Ferhat Gilbert1, Genthon Pierre2, Mouhouyouddine Ali Houmadi2-3,
Hinderer Jacques1, Hector Basile5, Yameogo Suzanne4
1 EOST- IPG Strasbourg-INSA 2 HydroSciences Montpellier3 Comores University 4 Hydrogeology and Remote Sensing, Univ. Ouagadougou, Burkina Faso5 LTHE Grenoble
Note the change in the color scale
