UBC-GIF: Capabilities for EM Modelling Modelling and ... · – Conductivity (indicative of water)...

© UBC-GIF 2003

The University of British ColumbiaThe University of British Columbia Geophysical Inversion FacilityGeophysical Inversion Facility

UBCUBC--GIF: Capabilities for EM GIF: Capabilities for EM ModellingModelling and and Inversion of LSBB dataInversion of LSBB data

http://http://www.eos.ubc.ca/ubcgifwww.eos.ubc.ca/ubcgif

Douglas W. OldenburgDouglas W. Oldenburg

Department of Earth and Ocean SciencesDepartment of Earth and Ocean Sciences

June 12, 2009

© UBC-GIF 2003LSBB: Karst Aquifer Characterization

• Principle Questions:

– What is the underground matrix– Hydrogeologic properties – Storage capacity – Production capacity– Potential for pollution – Sustainability

• What is the role of geophysics?

© UBC-GIF 2003

Framework for Applied Geophysics• What is the question to be answered?

• What are the diagnostic physical properties?

DensityMagnetic susceptibilityElectrical conductivity

Electrical permittivityElastic parameters

• Choose survey type• Collect data• Invert data to get physical property model• Interpret model and synthesize with other data

© UBC-GIF 2003Geophysical Experiment

DC surveyData: ”pseudosection”

Data images cannot be directly interpreted in terms of geologyData must be inverted

• Physical property: Electrical conductivity• Survey: DC Resistivity• Collect data

© UBC-GIF 2003What is Inversion?

Inversionprocessing

Model Inversion estimates Earth models based upon data and prior knowledge.

??

Data

Measurements over the Earth are data.

© UBC-GIF 2003

The inverse problem

• Geophysical data are: F[m] +

= d– m: model --- unknown– F: forward mapping operator– : errors– d: observations (data)

• Given:– data, errors, a forward modelling method

• Find:– the model that generated measurements.

• Major Difficulty: Nonuniquenes

??F-1

© UBC-GIF 2003

Inversion as optimization: 3 parts

Inversion as optimization:

= d

+ m

.

0 < <

is a constant

Choose such that d

< Tolerance

A priori information: reference model, structural detail...

...)()()(2

020

dvxmmdvmmm

Sx

Ssm

Model objective function:

• s , x … constants• m0 : reference model

Misfit: 2

1

][

N

i i

obsii

ddmF

• i : standard deviation

© UBC-GIF 2003Numerical solution

• Discretize: Divide the earth into M cells of constant physical property (M>>N).

2

0

2)()][( mmWdmFW m

obsd

md

• Minimize

• Use the Gauss-Newton method for solution.

• Solving for :- Discrepancy principle.- GCV.- L-Curve.

1

M

© UBC-GIF 2003Numerical solution (Gauss-Newton method)

• Minimize

• set

20

2)(][ mmWdmF obs

md

0)()][()()( 0

mmWWdmFmJm

mg TobsT

j

iij m

dJ

J is the sensitivity matrix:


• Minimize

• set

20

2)(][ mmWdmF obs

md

0)()][()()( 0

mmWWdmFmJm

mg TobsT

j

iij m

dJ


mmJmFmmF )(][][ • Expand forward operator (dropping higher order terms):


)()()( kT

kT

k mgmWWmJmJ • where mk

is the model at the kth iteration.• This is an M x M system of equations.

(M = # model parameters, or cells)

• Solve:

0)()][()()( 0

mmWWdmFmJm

mg TobsT

mmJmFmmF )(][][ • Expand forward operator (dropping higher order terms):

• Minimize

• set

20

2)(][ mmWdmF obs

md

j

iij m

dJ


© UBC-GIF 2003Inversion Capabilities

• Gravity (3D)

• Magnetics (3D)

• DC resistivity and IP (2D and 3D)

• Frequency domain EM (1D and 3D)

• Time domain EM (1D and 3D)

Software for inversion is distributed world wide through UBC and third party vendors

3D UTEM

--2150 -1550

0

200

400

600

800

© UBC-GIF 2003

“Keel”

Tertiary BrecciaMafic Volcanics

Quartz Rhyolit

e

Massive Sulphide

Mafic Volcanics

Ele

vatio

n (m

) 2000

1600

-2000 -1100Easting (m)

LocationGeologic cross section

Physical properties

Field Example: San Nicolas Deposit

Unit Density Susceptibility Resistivity Chargeability(g/cc) (S.I. x10) (ohmm) (msec)

Qal 2 0 50 5Tv 2.3 0 20-30 10Mst./Lst. 2.4 0 150 20Mafic Vol. 2.7 0 80 30Mafic/IntVol. 2.7 0 80 30Sulphide 3.5 10 20 200Qtz. Rhyolite 2.4 0 100 10Graphitic Mst. 2.4 0 100+ 30

Unit Density Susceptibility Resistivity Chargeability(g/cc) (S.I. x103) (ohm- m) (msec)

Qal 2 0 50 5Tv 2.3 0 20 -30 10Mst./Lst. 2.4 0 150 20Mafic Vol. 2.7 0 80 30Mafic/IntVol. 2.7 0 80 30Sulphide 3.5 10 20 200Qtz. Rhyolite 2.4 0 100 10Graphitic Mst. 2.4 0 100+ 30

- 10(20)

- 5- 5

- 10- 5

- 30- 40- 50- 50

- 20- 70

© UBC-GIF 2003Gravity

Nor

thin

g (m

)

Observed Data

-2900 -600

600

-120

0

Easting (m)

San Nicolas

2.3

1

-0.4

Predicted Data

-2900 -600

600

-120

0

Easting (m)

Nor

thin

g (m

)

2.3

1

-0.4

Gravity data collected at the San Nicholas deposit.

© UBC-GIF 2003Gravity Inversion Results

-0.22

0.07

0.35

g/cc

Dep

th (m

)

Easting (m)

Cross-section of Density Contrast Model with Geology

-800-2200

0

200

400

600

800

Density contrast

© UBC-GIF 2003Magnetics

-100

-400

-700

-2200 -1000

Observed Data

Predicted Data60 22 -16

nT

Easting (m)

Nor

thin

g (m

)-1

00-4

00-7

00

-2200 -1000Easting (m)

Nor

thin

g (m

)

Magnetic data collected at the San Nicholas

deposit.

© UBC-GIF 2003Magnetic Inversion Results

0

5

10

Dep

th (m

)

O

-2200 -800

800

Easting (m)

Cross-section of Magnetic Susceptibility Model with

Geology

© UBC-GIF 2003

“Keel”


Quartz Rhyolit

e

Massive Sulphide

Mafic Volcanics

Ele

vatio

n (m

) 2000

1600

-2000 -1100Easting (m)

Geologic cross section


Physical propertiesUnit Density Susceptibility Resistivity Chargeability

(g/cc) (S.I. x10) (ohmm) (msec)




- 10(20)

- 5- 5

- 10- 5

- 30- 40- 50- 50

- 20- 70

Density Magnetic Susceptibility• Electrical Conductivity• Chargeability

© UBC-GIF 2003Electrical Conductivity: Different Surveys

Sources:• Galvanic (grounded electrodes)• Inductive (current loops)

Waveforms• Sinusoidal (Frequency domain

• Time waveforms (Time domain)

Waveform

Time

I

Time

I

© UBC-GIF 2003

3D EM: Frequency Domain

Borehole Data

Depth

data

(E, H)

Source: Loop or grounded electrode

Waveform

Time

I

eiωσiω

JEHHE

)(0

Data (E,H)

© UBC-GIF 2003CSEM Survey

1.6 km

3.5 km

transmitter

receivers

• 15 Frequencies between 0.5Hz, 8192 Hz

• 3 lines, 1.6km long, 200m apart

• 25 meter station spacing

• single transmitter

• data are scalar impedances (Ex/Hy)

• data collected with the goal of MT interpretation

© UBC-GIF 2003

Iso-surface cutoff 10 Ohm-m

Frequencies0.5, 8, 64, 256 Hz

3D CSEM Inversion

© UBC-GIF 2003

(Loop or grounded electrode)Source

Waveform

Time

I (half sine, step…)

Surface Data

(E, H, dB/dt)

Borehole Data

Depth

data

(E, H, dB/dt)

3D TEM Setup

© UBC-GIF 2003

Introduction to UTEM data at San Nicolas

• 3 large loop transmitters– 2 km by 1.5 km

• dB/dt receivers– mainly z component

• transmitter waveform– 30 Hz sawtooth wave– dI/dt constant over

half cycle

I(t)

dIdt

dBdt

15 ms

© UBC-GIF 2003

Loop 1Loop 2

Loop 9

San Nicolas UTEM data

dBz /dt

nT/s

UTEM channel 4 (1.513ms)

easting

1075

-1300-3000 -220

nort

hing

© UBC-GIF 2003Fitting the data

View from SW

Observed 15 m iso-surface 1000.0

31.0

1.0

observedpredicteddBz /dt

nT/s

log10(t)

One decay curve: Observed and predicted

Observed

Predicted

© UBC-GIF 2003San Nicolas inversion results:

easting-2500 -500

1000

5

m

Recovered cross section at 450 S

easting-2500 -500

1000

5

m

Resistivity from drilling at 450 S

© UBC-GIF 2003Density, Magnetic susceptibility, Conductivity

3D CSEM

3D UTEM-950-2150 -1550 -950-2150 -1550

-950-2150 -1550

0

200

400

600

800

0

200

400

600

800

0

200

400

600

800

-800-2200

0

200

400

600

800

Density contrast

-800-2200

0

200

400

600

800

Magnetic susceptibility

© UBC-GIF 2003

“Keel”


Quartz Rhyolit

e

Massive Sulphide

Mafic Volcanics

Ele

vatio

n (m

) 2000

1600

-2000 -1100Easting (m)

Geologic cross section


Physical propertiesUnit Density Susceptibility Resistivity Chargeability

(g/cc) (S.I. x10) (ohmm) (msec)




- 10(20)

- 5- 5

- 10- 5

- 30- 40- 50- 50

- 20- 70

Density Magnetic Susceptibility Electrical Conductivity• Chargeability

© UBC-GIF 2003Induced Polarization

Source Current (Amps)

Measured Voltage (Volts)

V Vs

(t)

VIP datum: Vs/Vn

Non-zero area occurs because charges took time to equilibrate.

Collect IP data along with DC resistivity data

© UBC-GIF 2003DC/IP data at San Nicolas

• Pole-dipole • Real Section

Pseudo-section

-800-2200

0

200

400

600

800

Chargeability

3D Inversion

© UBC-GIF 2003

Summary of physical property inversion at San Nicolás

Sulfide: dense, chargeable, susceptible, conductive.

-800-2200

0

200

400

600

800

Density contrast

-800-2200

0

200

400

600

800

Chargeability

-800-2200

0

200

400

600

800

Magnetic susceptibility

-950-2150 -1550

0

200

400

600

800

3D UTEM

© UBC-GIF 2003Back to Karst Aquifers: LSBB

• What scale? • Which physical properties?

•Soil•Epikarst•Unsaturated•Saturate

© UBC-GIF 2003Back to Karst Aquifers: LSBB

• Tunnel Scale (meters to km):– Conductivity (indicative of water)

(DC resistivity, FEM or TEM)

– IP might be useful for clay layers if they are chargeable

– Time-lapse DC (EM) resistivity can provide information about hydraulic conductivity

– Electrical permittivity (GPR)

© UBC-GIF 2003Geophysics for large scale aquifer

• Conductivity– Airborne EM for 200 meters (Epikarst delineation)– ZTEM or MT for deeper structure (large voids/conduits,

depth of saturated zone)

© UBC-GIF 2003Geophysics for large scale aquifer

• Density: (Change in water volume)• Magnetics ??• Self-potential (from fluid motion)• Chargeability ??• Self-potential, MRI, Seismic

Good news from geophysics side: We can invert most types of survey data to recover 3D distribution of physical properties.

UBC-GIF: Capabilities for EM Modelling Modelling and ... · – Conductivity (indicative of water)...

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