1 Exploration for unconformity uranium deposits with audiomagnetotellurics Martyn Unsworth and Volkan Tuncer University of Alberta, Canada Weerachai Siripunvaraporn

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Exploration for unconformity uranium deposits with audiomagnetotellurics

Martyn Unsworth and Volkan TuncerUniversity of Alberta, Canada

Weerachai SiripunvarapornMahidol University, Bangkok, Thailand

Jim CravenNatural Resources Canada, Ottawa, Canada

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Outline

1. Introduction

2. AMT field techniques

3. MacArthur River AMT dataset – data processing

4. MacArthur River AMT dataset – model verification

5. Other studies

6. Conclusions

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1

10

100

1000

(m

)

Crystalline

rocks

Sedimentary

rocks

Brines

Graphite

1. Introduction

Why use audiomagnetotellurics (AMT) for uranium exploration?

Graphitic conductors are strong targets. Can also resolve structures above the unconformity

Logistically simple – no TX loops, small receiver

Good depth of penetration

Plane wave signal allow full 3-D inversion with modest computation

After Ruzicka

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Audiomagnetotellurics (AMT)

f = signal frequency

Depth of penetration

d = 500 * sqrt (/f)

Measure resistivity of Earth

= Zxy / 2f

Zxy = Ex / Hy

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Phoenix Geophysics V5-2000www.phoenix-geophysics.com

Metronix AMT systemwww.metronix.de

• 24-bit A-to-D• Low induction coil noise• GPS time synchronized• large data storage capacities• low power consumption• lower cost

1980 2000


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m

101001000

TE-modeCurrent flow along strikeEx and Hy

Non-linear conjugate gradient inversion 10 % noise in rho and phase

0 km

5 kmTrue model

100

10

1 10000

Rho data

1000 Hz

0.001 Hz

1 Hz

Phase data

1000 Hz

0.001 Hz

1 Hz

Rho fit

1000 Hz

0.001 Hz

1 Hz

Phase fit

1000 Hz

0.001 Hz

1 Hz

Inversion model

0 km

5 km

7

m

101001000

Non-linear conjugate gradient inversion 10 % noise in rho and phase

0 km

5 kmTrue model

100

10

1 10000

Rho data

1000 Hz

0.001 Hz

1 Hz

Phase data

1000 Hz

0.001 Hz

1 Hz

Rho fit

1000 Hz

0.001 Hz

1 Hz

Phase fit

1000 Hz

0.001 Hz

1 Hz

Inversion model

0 km

5 km

TM-modeCurrent flow across strikeEy and Hx

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0.20.0-0.2

TE-mode tipperCurrent flow along strikegenerates a vertical magnetic field

Non-linear conjugate gradient inversion 0.01 noise in Tyz (tipper)

Cannot determine absolute resistivity

Good horizontal resolution

0 km

5 kmTrue model

100

10

1 10000

1000 Hz

0.001 Hz

1 HzReal data

Quad data

1000 Hz

0.001 Hz

1 Hz

Real fit

1000 Hz

0.001 Hz

1 Hz

Quad fit

1000 Hz

0.001 Hz

1 Hz

Inversion model

0 km

5 km

m101001000

Tipper

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TE+TM

TE+TM+Tyz

0 km

5 kmTrue model

0 km

5 km0 km

5 kmNon-linear conjugate gradient inversion (Rodi and Mackie, Geophysics, 2000)

10 % noise in rho and phase0.01 in Tyz

m

101001000

Combined inversionsTE, TM and tipper (Tyz)

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• EXTECH IV was a cooperation between the Canadian government, industry and universities• tested a range of geophysical and geological techniques above a known deposit• Full tensor AMT data and vertical magnetic field recorded at all sites

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Dimensionality - tensor decomposition

Forward problem

Measured electric fields = regional electric fields + distortion


Undistorted electric fields

Tensor decomposition

Regional electric fields = measured electric fields - distortion

• assumes a 2-D regional structure with local 3-D distortion

• assumes no EM induction occurs in the distorter

• computes strike angle and distortion (twist and shear angles)

• r.m.s. misfit gives a measure of how well the above assumptions are satisfied at each MT station

• static shift still unknown

Electric fields distorted by shallow structure

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Dimensionality - tensor decomposition

• used multi-site, multi-frequency algorithm of Gary McNeice and Alan Jones • plot best fitting geoelectric strike direction as map and rose diagram• r.m.s. misfit shows if assumptions are valid (should be in range 0.5 – 1.5 )• inherent ambiguity of 90 degrees in strike direction


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Dimensionality – induction vectors

•Projection of the real component of the vertical magnetic field•In the Parkinson convention, these vectors point at conductors. •Direction reverses above the conductor (as in VLF)•More sensitive than apparent resistivity data to structures to the side of AMT station

?


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Apparent resistivity and phase curves on Line 224

•data rotated to strike direction defined by tensor decomposition

•frequency is a proxy for depth

•AMT dead band has weak signals

Above conductor Away from conductor


224

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Pseudosection displays – Line 224

• 1-D analysis not appropriate since major lateral changes

• Note the sign reversal in the tipper (Tzy)

• Need to convert frequency to true depth


224

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2-D inversion – Line 224

• Inverted with NLCG6 algorithm developed by Randy Mackie

• Inverse MT problem is inherently non-unique

• Overcome this issue by imposing extra conditions on solution (e.g. smooth model, discontinuity at known location etc). Note that smoothing broadens the basement conductor

• Full imaging requires both modes and tipper


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2-D inversion - fit to data

• Error floor used to give uniform fit

• Note consistent apparent resistivity and phase


224

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Mackie 2D

Mackie 3D

Siripunvaraporn 3D

Line 304 Line 224Line 2540 km

1 km

2 km0 km

1 km

2 km0 km

1 km

2 km

304

224

254

• Inverse MT problem is inherently non-unique

• 3-D inversion much more computationally demanding than 2-D

3-D inversion

TE+TM+Tzy

TE+TM+Tzy

TE+TM

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3-D inversion


Siripunvaraporn 3Dinversion

Mackie 2Dinversion

Mackie 2D inversion

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Comparison with well logs


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•3-D effects in induction vectors not due to termination of conductors

Tests to justify a 2-D interpretation

Measured data at 10 Hz Computed response of 3-D model


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Tests to justify a 2-D interpretation

•Rose diagram can hide 3-D behaviour

•Large r.m.s. misfit values can be diagnostic of 3-D effects

Measured data at 10 Hz

Computed response of 3-D model


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Resolution from 2-D synthetic inversions


0

2

1

depth (km

)

0

2

1

depth (km

)

0 21

distance (km)

3 0 21

distance (km)

3

•Resistivity values in ohm-m•10% noise added to synthetic AMT data•Invert TE, TM and Tzy data

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5. Other studies

AMT study in Athabasca Basin by Leppin and Goldak (2006)Inversion of TE tipper. Apparent resistivity data at every 4 th station

Previous applications in USSR (Olex Ingerov, personal communication, 2006)

GEOTEM channel 12 Vertical magnetic field

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6.Conclusions

• Depth and dip of the basement conductor can be reliably mapped to 2 km

• Vertical magnetic fields very useful

• 2D inversions validated by 3D inversions (likely not true for all deposits)

•Features above the unconformity may be artifacts of the inversion – beware!

NW SE

Future research

•Evaluate other AMT datasets

•Integrate various EM methods

•Sharp bound inversions

•More objective comparison of the 3D codes

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Acknowledgements

• Grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Alberta Ingenuity Fund to Martyn Unsworth are gratefully acknowledged

• AMT data collection was made possible by the financial support of Cameco, Cogema, Geosystem and the Geological Survey of Canada

• Charlie Jefferson (GSC) is thanked for his enthusiasm and initiative during the EXTECH-IV project

• The Geosystem field crew are thanked for the high quality of the AMT data

• Alan Jones and Gary McNeice are thanked for the use of their tensor decomposition code (STRIKE)

• We thank Randy Mackie for use of his 2D inversion and for the 3D AMT inversion of the EXTECH-IV dataset

Documents

1 Exploration for unconformity uranium deposits with audiomagnetotellurics Martyn Unsworth and Volkan Tuncer University of Alberta, Canada Weerachai Siripunvaraporn