1/21

AGU 2007 _Acapulco May 22- 25

Geophysical Characterization Of A Cover Geophysical Characterization Of A Cover Geophysical Characterization Of A Cover Geophysical Characterization Of A Cover With Capillary BarrierWith Capillary BarrierWith Capillary BarrierWith Capillary Barrier EffectEffectEffectEffect

Chouteau, M .Chouteau, M .Chouteau, M .Chouteau, M .1111, , , , Anterrieu, O.Anterrieu, O.Anterrieu, O.Anterrieu, O.1,1,1,1, Aubertin,Aubertin,Aubertin,Aubertin,M. M. M. M. 1111,,,, BussiBussiBussiBussièèèèrererere,,,, B.B.B.B.2,2,2,2, & & & & MaqsoudMaqsoudMaqsoudMaqsoud, A., A., A., A.2222

1École Polytechnique, Montréal, Qc, Canada2UQAT, Rouyn-Noranda, Qc, Canada

Industrial NSERC Polytechnique-UQAT ChairIn Environment and Mine Waste Management

2/21

The Problem

• Covers with Capillary Barrier Effect (CCBE) are used in mine environment & remediation to prevent oxygen flux to reachreactive mine tailings.

• Efficiency depends on water saturation. Suction breaks are constructed when needed to maintain saturation.

• Saturation must be monitored over very large areas (from 0.1km2

to 10km2) by mining compagnies

• Can geophysical techniques provide economic way to monitor saturation with some accuracy?

3/21

What is a CCBE?

capillary barrier principle: When a fine grained material overlies a coarser one, the water retention contrast between the two materials limits the vertical flow of water at the interface.

In the mining industry, a CCBE can be used to reduce the availability of oxygen to the underlying sulphidictailings

slope influences water movement in inclined covers

moisture distribution in the water retention layer is not uniform alongthe slope.

Under specific conditions, progressive desaturation is observed whenapproaching the top of the slope.

4/21

Suction break to control desaturation

slope effect can be reduced by creating a suction break

creation of a localized saturation area in the moisture-retentionlayer (i.e. zero suction).

The influenced zone shows lower suction and thus a higherdegree of saturation and lowergas diffusion characteristics.

GCL: geosynthetic clay liner with a low ksat

5/21

Saturation & Suction Break (SB): Numerical modeling results

•• Slope 3:1, L = 50 m, Moisture retaining layer: MRNSlope 3:1, L = 50 m, Moisture retaining layer: MRN --tailingstailings

0.75

0.80

0.85

0.90

0.95

1.00

0 10 20 30 40 50

Distance along the slope

Sat

ura

tion

ra

tio

7 days 15 days 30 days 60 days

HB-1

Toe Top

0.75

0.80

0.85

0.90

0.95

1.00

0 10 20 30 40 50

Distance along the slope

Sat

urat

ion

ratio

7 days 15 days 30 days 60 days

HB-2

Toe Top

0.75

0.80

0.85

0.90

0.95

1.00

0 10 20 30 40 50

Distance along the slopeS

atu

ratio

n r

atio

7 days 15 days 30 days 60 days

HB-3

Toe Top

Without SBWithout SB

With SBWith SB

6/21

LTA tailings impoundment; cover installed upon closure in 1996, using MRN tailings

Sand & gravel

Silt (non-reactivetailings)

Sand

7/21

Quick test (2005): Estimation of physical parameters

450 MHz

900 MHz

GPR 2D RI

8/21

Estimate of the fluid resistivity in the CCBE

Layer 1 : ρC1~ 400 Ω.m ; φ = 0.36 ; S = 0.4 ;

Archie’s law : ===> ρw= 8.3 Ω.m

Resistivity in the water retention layer

Layer 2 : φ = 0.44 ; ρc2 ~ 20 Ω.m

Schön,1996 : ===> σsurface = 26 mS/m

Formation factor

Resistivities of the CCBE layers

0.75

0.80

0.85

0.90

0.95

1.00

0 10 20 30 40 50

Distance along the slope

Sat

urat

ion

ratio

7 days 15 days 30 days 60 days

HB-1

Toe Top

Evolution of saturation in the water retention layer (Bussière et al., 2000)

)(surfaceF

weff σσσ +=

Smn

wc−−= ϕρρ .1

Smn

F−−

=ϕ

9/21

8608608608600.30.30.30.30.310.310.310.31Mine tailings (layer 4)Mine tailings (layer 4)Mine tailings (layer 4)Mine tailings (layer 4)

4004004004000.40.40.40.40.360.360.360.36sand (layer 3)sand (layer 3)sand (layer 3)sand (layer 3)

2929292927.727.727.727.725.825.825.825.820202020

0.83 [200.83 [200.83 [200.83 [20----30m] top 30m] top 30m] top 30m] top 0.85 [100.85 [100.85 [100.85 [10----20m]20m]20m]20m]0.88 [50.88 [50.88 [50.88 [5----10m]10m]10m]10m]1 [01 [01 [01 [0----5m] base5m] base5m] base5m] base

0.440.440.440.44SiltySiltySiltySilty material (layer 2)material (layer 2)material (layer 2)material (layer 2)

4004004004000.40.40.40.40.360.360.360.36Sand (layer 1)Sand (layer 1)Sand (layer 1)Sand (layer 1)

Estimated Estimated Estimated Estimated resistivityresistivityresistivityresistivityρ ( ( ( ( Ω.m ).m ).m ).m )Saturation Saturation Saturation Saturation

S S S S PorosityPorosityPorosityPorosity

φLayerLayerLayerLayer

Hydrogeological properties and resistivities of CCBE layers

10/21

Resistivity model (CCBE without succion break)

use of Res2Dmod & Res2Dinv

dipole-dipole array; 0.3 m minimum electrode spacing

Resistivity Modelling& Imaging

11/21

Inverted modelled data

0

-1

1

2

3

4

5

6

7

8

9

5 10 20 40 78 155 309 resistivity (Ω.m) 614

Resistivity imaging of a dipping CCBE depth of investigation : 1.5 m

discrimation of the 3 layers

evidence of saturated zones

12/21

CCBE with succion break

Succion break at 10 m from bottom of CCBE (Bussière et al., 2000)

0.75

0.80

0.85

0.90

0.95

1.00

0 10 20 30 40 50

Distance along the slope

Sat

urat

ion

ratio

7 days 15 days 30 days 60 days

HB-2

Toe Top

succion break

13/21

CCBE with succion break

discrimation of the 3 layers

evidence of saturated zones

• succion break

• Base of the CCBE

0

-1

1

2

3

4

5

6

7

8

9

5 10 40 78 155 309 resistivity (Ω.m)61420

Resistivity imaging of a CCBE with succion break

14/21

(Alharthi et al., 1987)( ) kkkk airwatermatrixmediumSS 2

12

12

1.2

1).1(..1 −++−= φφφ

0.1030.1030.1030.1030.00120.00120.00120.00128.58.58.58.55.55.55.55.50.30.30.30.3Mine Mine Mine Mine tailingstailingstailingstailings

(# 4)(# 4)(# 4)(# 4)

5.65.65.65.60.1050.1050.1050.1050.00250.00250.00250.00258.28.28.28.24.54.54.54.50.40.40.40.4Sand (# 3)Sand (# 3)Sand (# 3)Sand (# 3)

21.921.921.921.9

22.5322.5322.5322.53

23.523.523.523.5

28.0728.0728.0728.07

0.0730.0730.0730.073

0.0710.0710.0710.071

0.0680.0680.0680.068

0.0570.0570.0570.057

0.0340.0340.0340.034

0.0360.0360.0360.036

0.0390.0390.0390.039

0.050.050.050.05

16.7 top16.7 top16.7 top16.7 top

17.8 17.8 17.8 17.8

19.419.419.419.4

27.227.227.227.2 basebasebasebase

5.55.55.55.5

0.83 [200.83 [200.83 [200.83 [20----30] top30] top30] top30] top

0.85 [100.85 [100.85 [100.85 [10----20m]20m]20m]20m]

0.88 [50.88 [50.88 [50.88 [5----10m]10m]10m]10m]

1 [01 [01 [01 [0----5m] base5m] base5m] base5m] base

SiltySiltySiltySilty materialmaterialmaterialmaterial

(# 2)(# 2)(# 2)(# 2)

5.75.75.75.70,1050,1050,1050,1050.00250.00250.00250.00258.28.28.28.24.54.54.54.50.40.40.40.4Sand (# 1)Sand (# 1)Sand (# 1)Sand (# 1)

TWTTWTTWTTWT

(ns)(ns)(ns)(ns)

EM EM EM EM velocityvelocityvelocityvelocity

(m/ns)(m/ns)(m/ns)(m/ns)

ElectricalElectricalElectricalElectricalconductivityconductivityconductivityconductivity

( S/m)( S/m)( S/m)( S/m)

DielDielDielDiel. . . . ConstConstConstConst. . . . mediummediummediummedium

(K)(K)(K)(K)

DielDielDielDiel. . . . ConstConstConstConst. . . . matrixmatrixmatrixmatrix

saturationsaturationsaturationsaturation

( S )( S )( S )( S )LayerLayerLayerLayer

2D numerical GPR modelling

15/21

Discrimination of the 3 interfaces

Increasing TWT with saturation

Ambiguity in determining saturation if velocity or th ickness unknown

Modelled GPR response (450 MHz)

ModelModelModelModel

(GPRmax2D)

sandw. ret. layer

sand (drainage)

Mine tailings

K=27.2 K=19.4 K=17.8 K=16.7R1

R2

R3

R1

R2

R3

K=8.2

K=8.2

K=8.5

ResponseResponseResponseResponse

16/21

2D Resistivity Imaging (lines A & B)

Line A

Line B

Succion break

θ ≈ 42.5θ ≈ 38.6

θ ≈ 48,0θ ≈ 38,3

θ ≈ 30,7θ ≈ 37,8

17/21

2D Resistivity Imaging (lines A & C)

Line A

Line C

Succion break

(Away from SB)

18/21

R1 about 8 ns :

interface 1-2

R2about 28 ns :

interface 2-3

R3 about 36 ns :

interface 3-4

GPR reflection profiling at LTA (200 MHz)

Line B (200 Mhz)

R1 ≈ 9 ns

R3 ≈ 36 ns R2 ≈ 28 ns

Line A (200 Mhz)

R1 ≈ 8 ns

R2 ≈ 28 ns

θ ≈ 38.6

θ ≈ 42.5

θ ≈ 38.3θ ≈ 37.8

θ ≈ 48.0

19/21

R1 about 8 ns :

interface 1-2

R2about 28-32 ns :

interface 2-3

R3 (interface 3-4): not detected

High attenuationnear the toe

GPR reflection profiling at LTA (450 MHz)

Line A (450 Mhz)

R1≈ 8 ns

R2 ≈ 28 ns

Line B (450 Mhz)

R1 ≈ 8 ns

R2 ≈ 32 ns

20/21

Discussion• Imaging modelled resistivity data :

• excellent delineation of the 3 CCBE layers• sensitive to saturation variations in the w. retention layer

• Imaging modelled GPR reflection data :• excellent determination of interfaces between layers• TWT increases with saturation in w. retention layer

• Note: Results based on constant thickness homogeneous layers (0.3m, 0.8m, 0.5m)

• Survey resistivity data :– Show 1st layer thin (< 0.3 m) and heterogeneous; 2nd mapped as conductive with

variable thickness (average thickness agrees); 3rd mapped as resistive but lateralvariation.

– 2nd layer resistivities correlated with water content– No easy direct relation between resistivity and saturation– Problem of equivalence (?) knowing that thickness varies

• Survey GPR data:– Show good determination of the two first interfaces (1-2, 2-3); the 3rd is difficult

because of attenuation– Changes in TWT may reflect changes in saturation as well as changes in thickness.

21/21

Conclusion

• Both methods are sensitive to change in water saturation; relative changes but not absolute values of saturation may be tracktable. Alsoboth methods are sensitive to thickness variations in layer 2.

• Further work: – Need better estimation of 2nd layer σeff with σw and σsurf; effect of

T(0C) on interpretation.– Cooperative/joint inversion of both data sets for resistivity,

thickness and velocity.– Use (inversion with constrains) TDR (and suction) data recorded on

sites at few places.– calibration pads on site (thickness and diel. const. known) and use

GPR reflectivity.– A combination of the solutions above.