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Exploration for unconformity uranium deposits with audiomagnetotellurics

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Title: Exploration for unconformity uranium deposits with audiomagnetotellurics


1
Exploration for unconformity uranium deposits
with audiomagnetotellurics
Martyn Unsworth and Volkan Tuncer University of
Alberta, Canada Weerachai Siripunvaraporn Mahidol
University, Bangkok, Thailand Jim
Craven Natural Resources Canada, Ottawa, Canada
2
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
3
  • Introduction

Crystalline rocks
1000
100
(Wm)
Sedimentary rocks
10
1
Brines
Graphite
After Ruzicka
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
4
2. AMT field techniques
  • Audiomagnetotellurics (AMT)
  • f signal frequency
  • Depth of penetration
  • d 500 sqrt (r/f)
  • Measure resistivity of Earth
  • Zxy / 2pmf
  • Zxy Ex / Hy

2
5
2. AMT field techniques
1980
2000
24-bit A-to-D Low induction coil noise GPS
time synchronized large data storage
capacities low power consumption lower cost
Phoenix Geophysics V5-2000 www.phoenix-geophysics.
com Metronix AMT system www.metronix.de
6
TE-mode Current flow along strike Ex and Hy
Non-linear conjugate gradient inversion 10
noise in rho and phase
7
TM-mode Current flow across strike Ey and Hx
Rho data
Phase data
Non-linear conjugate gradient inversion 10
noise in rho and phase
8
10
TE-mode tipper Current flow along
strike generates a vertical magnetic field
100
1
10000
True model
Wm
10
100
1000
Tipper
Non-linear conjugate gradient inversion 0.01
noise in Tyz (tipper) Cannot determine absolute
resistivity Good horizontal resolution
9
Combined inversions TE, TM and tipper (Tyz)
True model
TETM
TETMTyz
Non-linear conjugate gradient inversion (Rodi
and Mackie, Geophysics, 2000) 10 noise in rho
and phase 0.01 in Tyz
10
3. MacArthur River AMT dataset data processing
  • 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

11
3. MacArthur River AMT dataset data processing
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
12
3. MacArthur River AMT dataset data processing
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

13
3. MacArthur River AMT dataset data processing
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

14
3. MacArthur River AMT dataset data processing
Apparent resistivity and phase curves on Line 224

Above conductor
Away from conductor
  • data rotated to strike direction defined by
    tensor decomposition
  • frequency is a proxy for depth
  • AMT dead band has weak signals

224
15
3. MacArthur River AMT dataset data processing
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
16
3. MacArthur River AMT dataset data processing
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

17
3. MacArthur River AMT dataset data processing
2-D inversion - fit to data
  • Error floor used to give uniform fit
  • Note consistent apparent resistivity and phase

224
18
3. MacArthur River AMT dataset data processing
3-D inversion
Line 304
Line 224
Line 254
Mackie 2D
TETMTzy
Mackie 3D
TETMTzy
Siripunvaraporn 3D
TETM
  • Inverse MT problem is inherently non-unique
  • 3-D inversion much more computationally
    demanding than 2-D

19
3. MacArthur River AMT dataset data processing
3-D inversion
Siripunvaraporn 3D inversion
Mackie 2D inversion
Mackie 2D inversion
20
4. MacArthur River AMT dataset model
verification
Comparison with well logs
21
4. MacArthur River AMT dataset model
verification
Tests to justify a 2-D interpretation
Measured data at 10 Hz
Computed response of 3-D model
  • 3-D effects in induction vectors not due to
    termination of conductors

22
4. MacArthur River AMT dataset model
verification
Tests to justify a 2-D interpretation
Measured data at 10 Hz
Computed response of 3-D model
  • Rose diagram can hide 3-D behaviour
  • Large r.m.s. misfit values can be diagnostic of
    3-D effects

23
4. MacArthur River AMT dataset model
verification
Resolution from 2-D synthetic inversions
  • Resistivity values in ohm-m
  • 10 noise added to synthetic AMT data
  • Invert TE, TM and Tzy data

24
5. Other studies
AMT study in Athabasca Basin by Leppin and Goldak
(2006) Inversion of TE tipper. Apparent
resistivity data at every 4th station Previous
applications in USSR (Olex Ingerov, personal
communication, 2006)
25
NW
SE
  • 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!
  • Future research
  • Evaluate other AMT datasets
  • Integrate various EM methods
  • Sharp bound inversions
  • More objective comparison of the 3D codes

26
  • 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
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