m2 hydrogéophysique – 3 décembre 2008 gpr response and fdtd modeling to water and fuel...

M2 Hydrogéophysique – 3 décembre 2008

GPR response and FDTD modeling GPR response and FDTD modeling to water and fuel infiltration in a to water and fuel infiltration in a

sand box experimentsand box experiment

byMaksim Bano

[email protected]

Ecole et Observatoire des Sciences de la Terre (EOST), 5 Rue René Descartes,

67087 Strasbourg

FRANCE


OutlineOutline• Introduction

What’s GPR? Effect of the frequency and humidity on the GPR data.

• Presentation of GPR experiment in the lab

Experiment set up, data acquisition, comments on the measurements.

• Water content estimation

Comparison with real water volume injected in the box.

• Conclusions on Water Contents

• Influence of the pollution (gasoil) on GPR data

Data acquisition, results, evolution of the pollution and FDTD modeling

• Conclusions and Perspectives on Pollution


IntroductionIntroduction


GPR - GPR - Ground Penetrating RadarGround Penetrating RadarPrincipe of GPR

Lap Top

Wheel

Central Unity

Antennae

Time (ns)

Amplitude


Acquisition. Common Mid-Point DataAcquisition. Common Mid-Point Data

In a Common Mid-Point (CMP) acquisition, antennae separation is increased about some central point.

CMP Acquisition

M


50 MHz Antenna ‘Snake’50 MHz Antenna ‘Snake’

250 MHz Shielded Antenna250 MHz Shielded Antenna

Effect of the frequencies usedEffect of the frequencies used

GPR images obtained with 50 (rough-terrain antenna ; “snake”) and 250 MHz Antennae, Finneidfjord Northern Norway


Effect of the Humidity on GPR dataEffect of the Humidity on GPR data

The same profile acquired with 500 MHz antennae. a) in May 2006 and b) in October 2008

a)

b)

Dry soilDry soil

Humid soilHumid soil


Some important pointsSome important points

• Water Content (volumetric):Water Content (volumetric):

θw=Vw/Vtotal ; θw=φ.Sw

• Relative dielectric permittivityRelative dielectric permittivity (dielectric constant) = /0 with 0 the permittivity of the free space. water=81; dry rocks: = 3-5; humid rocks: = 6-30.

• Propagation velocity:Propagation velocity: v = c / 1/2 m/ns. c = 0,3 m/ns is the free space velocity.


Presentation of GPR experiment Presentation of GPR experiment in the labin the lab


Experiment set upExperiment set up

Sand Box Sand box and injection system

2 m

0,98

m

Steel pipe

PVC Pipes

Steel ball

Clay cake


Data acquisitionData acquisition

Plane view of the sand box with measurement grid and different objects.

Cross section of the sand box with the projection of the objects.

Frequencies used: 900 and 1200 MHz


Four data set of measurementsFour data set of measurements

• Measurements on dry sand• Measurements with water level at 72 cm depth (26 cm thick)• Measurements with water level at 48 cm depth (48 cm thick)• Measurements after draining

72 cm

48 cm


Results (1)Results (1)

SteelP2?

Steel

APVCEPVC

P03 P36 P56

TA T0



Central Profile (P36) with different saturation statesCentral Profile (P36) with different saturation states

Dry sand

Water level at 48 cm depth


After draining



CMP and constant offset profiles (P1) with different CMP and constant offset profiles (P1) with different saturation statessaturation states


Dry sand


Results (4)Results (4)3D GPR data sets3D GPR data sets

a) dry sand and b) the water level at 48 cm depth

a) b)

B


Estimation of water contentsEstimation of water contents


Relative dielectric PermittivityRelative dielectric Permittivity

The determination of the average dielectric constants, for different depth, is performed from the propagation velocities (=c²/v²) :

Depth Dry sand

v (m/ns)

surface 0,14 4,6

38 cm 0,14 4,6

50 cm 0,14 4,6

68 cm 0,14 4,6

bottom 0,116 6,7

Water level at 48 cm

6,3

6,8

9,0

?

16


Water contentsWater contents

Relationships between water content Relationships between water content and relative and relative dielectric permittivity dielectric permittivity ::

Topp RelationshipTopp Relationship (Topp et al., 1980)

= -5,3x10-2 + 2,92x10-2 - 5,5x10-4 2 + 4,3x10-6 3

CRIM CRIM RelationshipRelationship (Mavko et al., 1998)

Hanai-Bruggemann-Sen RelationshipHanai-Bruggemann-Sen Relationship (Hanai, 1968)1

1

1

1m

p

a

w

a

m

wwp S

et

2

2

1

1m

g

p

g

mp

awwws SS 11

wwsw 1


Water QuantitiesWater Quantities

The water quantities (in liters) estimated (in whole box) by using the previous relationships

0

100

200

300

400

500

600

700

800

900

dry sand 72 cm 48 cm drainage

ToppCRIMHBS

Dry sand 72 cm 48 cm Drainage

Dielectric Constant 6,7 11,4 16 9,8

Wat

er q

uant

ity (

liter

)


Variations of water quantitiesVariations of water quantities

TOPP CRIM HBS Injected Volume

V1 284 314 305 340

V2 484 520 506 580

Estimates of the amounts of water (in liter) injected in the sand box (for different saturation cases) as obtained using the Topp, CRIM and HBS equations. V1 is the amount of water for the data set with the water table at 72 cm depth, minus that of the dry sand case; V2 is the amount of water for the data set with the water table at 48 cm depth, minus the amount of water for the dry sand case.


Variations of water quantitiesVariations of water quantities

0

100

200

300

400

500

600

V1 V2

Topp

CRIM

HBS

Réel

Wat

er Q

uant

ity (

liter

)

In each case we underestimated the variation in the amount of water in the sand box using GPR, but the final results are very close to the amount of water injected.


Conclusions on Water ContentsConclusions on Water Contents

GPR is an effective method to assess and monitor water in the case of a vadose zone.

By repeating the same GPR measurements over a controlled vadose zone (sand box experiment), one can compare and calibrate the water content obtained from GPR measurements with the actual water content present in the soil.

The water variations are underestimated (by the three relationships) but the final results were very close to the amount of water injected.


Influence of a pollution (gasoil) on Influence of a pollution (gasoil) on GPR dataGPR data


Data acquisitionData acquisition2nd Experiment:

After drainage we let the sand box resting (two months) and performed measurements in April 2004 (this state is considered as ‘dry’).

Measurements with water level at 72 cm depth (26 cm thick, 240 l) in may 2004

We injected 100 l of fuel (gasoil) and repeated measurements in may 2004 and June 2004 .

Injection point of the gasoil


Influence of the gasoil (1)Influence of the gasoil (1)

Profile T0 before injection Profile T0 after injection The trace 40


Two CMPs acquired after fuel injection. a) CMP16 above the steel ball P1 and b) CMP56 above the steel ball P2. B indicates the reflections from the bottom.

Influence of the gasoil (2)Influence of the gasoil (2)

BB


Laterally extension of the Laterally extension of the plume pollutionplume pollution

Travel time of the reflections from the bottom of the box.

a) Before fuel injection b) After fuel injection

a) b)


Modeling of GPR data by FDTDModeling of GPR data by FDTD

0,98

m

2 m

1,40 m

Basement of the sand box (air, wood and sand)

Sand saturated with water (h=35 cm)

Capillary Fringe (h=32 cm)

Dry sand, = 4,6 (h=32 cm)

Sand saturated with gasoil, = 3,8

Sand mixted with air and gasoil, = 4

Sand mixted with water and gasoil, = 15

Steel pipe

Model used for modeling of profile T0 12 days (in May 2004) after fuel injection.


Modeling by FDTDModeling by FDTD

Modeled profile T0 Real profile T0


Evolution of the pollution in timeEvolution of the pollution in time

Profile T0

May 2004 June 2004

R


Evolution of the pollution in time Evolution of the pollution in time

profile P56.

May 2004 June 2004Trace 19

R

R


Modeling of GPR data by FDTDModeling of GPR data by FDTD

Model used to follow the evolution of the profile T0 45 days after injectionModel used to follow the evolution of the profile T0 45 days after injection

0,98

m

2 m1,4 m

Level of a saturated sand, = 45


Modeling by FDTDModeling by FDTD

Profile T0 modeled Profile T0 real

R

R R


Conclusions and Perspectives Conclusions and Perspectives The GPR data do not show any clear reflections from the plume pollution, however GPR velocities are extremely affected by the presence of the fuel.

The laterally extension of the plume pollution in the vadose zone is shown by plotting the travel times of the reflection from the bottom of the sand box.

It seems that pore water has been replaced by the fuel through a lateral flow by creating a high saturated zone far from the fuel injection point.

The forward FDTD modeling method gave theoretical support to explain the origin of the observed reflections from the contaminated vadose zone.

Perspective:Perspective: To follow the lateral flow of the plume, a joint GPR and lateral flow modeling is necessary.


Thank you for your attention

[email protected]

m2 hydrogéophysique – 3 décembre 2008 gpr response and fdtd modeling to water and fuel...

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