Title: Exploration for unconformity uranium deposits with audiomagnetotellurics
1Exploration 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
2Outline 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
3Crystalline 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
42. 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
52. 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
6TE-mode Current flow along strike Ex and Hy
Non-linear conjugate gradient inversion 10
noise in rho and phase
7TM-mode Current flow across strike Ey and Hx
Rho data
Phase data
Non-linear conjugate gradient inversion 10
noise in rho and phase
810
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
9Combined 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
103. 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
113. 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
123. 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
133. 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
143. 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
153. 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
163. 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
173. 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
183. 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
193. MacArthur River AMT dataset data processing
3-D inversion
Siripunvaraporn 3D inversion
Mackie 2D inversion
Mackie 2D inversion
204. MacArthur River AMT dataset model
verification
Comparison with well logs
214. 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
224. 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
234. 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
245. 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)
25NW
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