1 dc slides · 2017. 2. 19. · • dc resistivity is sensitive to: – σ: conductivity [s/m] –...
TRANSCRIPT
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DC Resistivity
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DC Resistivity Survey
Source Data
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Water inflow in mine Oil and Gas
Groundwater
Minerals
Geotechnical
Motivation
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• DC resistivity is sensitive to:– σ: Conductivity [S/m]– ρ: Resistivity [Ωm]– σ = 1/ρ
• Varies over many orders of magnitude
• Depends on many factors:– Rock type– Porosity– Connectivity of pores– Nature of the fluid– Metallic content of the solid matrix
Electrical conductivity
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Outline
• Basic experiment • Currents, charges, potentials and apparent resistivities• Soundings, profiles and arrays• Data, pseudosections and inversion• Sensitivity• Survey Design• Case History – Mt Isa• Effects of background resistivity
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Basic Experiment• Target:
– Ore body. Mineralized regions less resistive than host
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Basic Experiment• Target:
– Ore body. Mineralized regions less resistive than host
• Setup: – Tx: Current electrodes– Rx: Potential electrodes
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Basic Experiment• Target:
– Ore body. Mineralized regions less resistive than host
• Setup: – Tx: Current electrodes– Rx: Potential electrodes
• Currents:– Preferentially flow through conductors
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Basic Experiment• Target:
– Ore body. Mineralized regions less resistive than host
• Setup: – Tx: Current electrodes– Rx: Potential electrodes
• Currents:– Preferentially flow through conductors
• Charges:– Build up at interfaces
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Basic Experiment• Target:
– Ore body. Mineralized regions less resistive than host
• Setup: – Tx: Current electrodes– Rx: Potential electrodes
• Currents:– Preferentially flow through conductors
• Charges:– Build up at interfaces
• Potentials: – Associated with the charges are
measured at the surface 10
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How do we obtain resistivity?
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Steady State Maxwell equations
Faraday
Full Steady State
Ampere
Ohm’s Law
Potential in a homogeneous halfspace
Put it together
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Currents and potentials: halfspace
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Currents and potentials: 4-electrode array
Halfspace (500 Ω')
=VMN
IG
Resistivity
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Currents and Apparent Resistivity
Conductive overburden (100 Ω')
Apparent resistivity
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DC AppWhy interactive apps?
• Visualization aids understanding
• Learn through interaction– ask questions and
investigate
• Open source: – Free to use– Welcome contributions!
16http://em.geosci.xyz/apps.html
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Soundings and Arrays
Wenner
Schlumberger
4 electrode Array
Sounding
Geometry
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Soundings
AB/2 = 5 m
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DC Sounding curve
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Soundings
AB/2 = 10 m
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DC Sounding curve
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Soundings
AB/2 = 30 m
DC Sounding curve
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Summary: soundings
Schlumberger array
100 Ω'500 Ω'
DC Sounding curve
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Scale length of array must be large to see deep
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Inversion
Inversionprocessing
Inversion estimates Earth models based upon data and prior knowledge.
Data
Measurements over the Earth are data.
Model
Dept
h
Resistivity 22
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DCR for a confined body• Useful to formally bring in the concept of charges
J1n = J2n
Conductivity contrast Normal component of current density is continuous
• Electric field discontinuous
• Charge build-up
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Currents, charges, and potentials
Total currents: J Secondary currents: Js
Secondary potential: ()
- +
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Secondary charges: Qs
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Measurements of DC data: gradient array
ρ* = 430MN
ρ* = 430MN
Potential profile Resistivity model
- +
500 Ωm1,Ωm
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Secondary charges: Qs Secondary currents: Js
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Measurements of DC data: gradient array
MN MN
Potential profile Resistivity model
- +
500 Ωm1,Ωm
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Secondary charges: Qs Secondary currents: Js
ρ* = 502 ρ* = 502
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Measurements of DC data: gradient array
MN MN MN MN MN MN
Potential profile Resistivity model
- +
500 Ωm1,Ωm
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Secondary charges: Qs Secondary currents: Js
ρ* = 502 ρ* = 502ρ* = 430 ρ* = 430ρ* = 502 ρ* = 502
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Profiling
MA B Na
Short offset: a = 4 m
Long offset: a = 20 m500 Ωm1,Ωm
Depth of investigation depends upon offset or array length
Fixed geometry: Move laterally
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Long offset, a=20ma a
MA NShort offset, a=4m
B
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Summary: Soundings and ProfilesSounding Profiling
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Expand Translate
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Basic Survey Setups
Wenner
Schulmberger
Gradient
Pole-Pole
Pole-Dipole
Dipole-Dipole
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DC resistivity data
Plotting planePlotting planePlotting plane
IGV
aΔ
=π
ρ2
Each data point is an apparent resistivity:
Source(Amps)
Potential(Volts)
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Example pseudosections
1) A single buried conductive block
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Pole-Dipole• Pole-dipole; n=1,8; a=10m; N=316
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Example pseudosections2) The conductive block with geologic noise.
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Pole-Dipole• Pole-dipole; n=1,8; a=10m; N=316
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Example pseudosections
3) The “UBC-GIF model”
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Pole-Dipole
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Inversion
Inversionprocessing
Model Inversion estimates Earth models based upon data and prior knowledge.
?
DataMeasurements over the Earth are data.
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Example 1: buried prism
• Pole-dipole; n=1,8; a=10m; N=316; (αs, αx, αz)=(.001, 1.0, 1.0)
Resistivity(model
Data(with(5%(Gaussian(noise
Ohm7m Recovered( resistivity
Predicted(data
Ohm7m
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Example 2: prism with geologic noise
Resistivity(model
Data(with(5%(Gaussian(noise
Ohm7m Recovered( resistivity
Predicted(data
Ohm7m
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• Pole-dipole; n=1,8; a=10m; N=316; (αs, αx, αz)=(.001, 1.0, 1.0)
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Example 3: UBC-GIF model
Resistivity model
Data with 5% Gaussian noise
Ohm-m
Need figures
Need figures
Recovered resistivity
Predicted data
Ohm-m
Need figures
Need figures
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• Pole-dipole; n=1,8; a=10m
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The world is 3D • Target
– Size, shape, depth• Background
– Variable resistivity
• Questions– Where to put currents? 2D
acquisition? 3D? – Where to make measurements?– Which measurements? – Effects of topography?
• These are survey design questions
• Crucial element is the sensitivity
Topography
Ore body
Water underground
Host
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Sensitivity
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Sensitivity Function
Is the measured potential sensitive to the target?
G =
d
p=
change in data
change in model
Quantified by the sensitivity
• Collect the data that are sensitive to the target– Need to excite the target– Need to have sensor close to the target
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Exciting the target
Total currents: J Secondary currents: Js
Secondary charges: Qs Secondary potential: ϕ)
- +
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Resistivity model
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Measurements
MN MN MN
MN MN MN
Potential profile
- +
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Resistivity model
ρ* = 430ρ* = 502 ρ* = 502
Secondary charges: Qs
Secondary potential: ϕ) Total potential: ϕMN MN MN
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Coupling
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• Thin plate – different orientations ! different data Total currents: J
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Conductive vs. Resistive Target
Conductive Target Resistive Target
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Total currents: J Total currents: J
Secondary charges: Qs Secondary charges: Qs
Q=3.4x10-11 Q=-3.4x10-11 Q=1.6x10-10 Q=-1.6x10-10
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Summary: Sensitivity• “Excite” the target
– Drive currents to target– Need good coupling with target
• Measuring a datum– Proximity to target– Electrode orientation and
separation
• Background resistivity is important
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- +
Total currents: J
Secondary Charges: Q
Secondary potential: ϕ)Total potential: ϕ
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Survey Design: Questions• What is objective?
– Layered earth (1D) ! do a sounding
– Target body (2D) ! profile, sounding perpendicular to geology
– Target body (3d) ! need 3D coverage
• What is the background resistivity?
• What are the noise sources? fences, power lines, …
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Signal from target
Survey Design: in general
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• Numerical simulation – can we see the target?
• Steps:– Define a geologic model– Assign physical properties– Select a survey – Simulate with (V) and
without (Vp) target
• Best practice– Assign uncertainties to
simulated data– Invert with code you will
use for the field dataNeed
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Outline
• Basic experiment • Currents, charges, potentials and apparent resistivities• Soundings, profiles and arrays• Data, pseudosections and inversion• Sensitivity• Survey Design
• Questions
• Case History – Mt Isa• Effects of background resistivity
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Mt. Isa
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Seven StepsMt. Isa (Cluny prospect)
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SetupMt. Isa (Cluny prospect) Geologic model
Question• Can conductive units, which would be potential targets within the
siltstones, be identified with DC data?51
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PropertiesGeologic model Surface topography
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• Eight survey lines• Two survey configurations.
Survey and Data
Easting (m) Easting (m)
mS/m
Apparent resistivity,pole - dipole.
Data set #1:
Surface topography
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• Eight survey lines• Two survey configurations.
Survey and Data
Easting (m) Easting (m)
mS/m
Apparent resistivity,dipole - pole
Easting (m) Easting (m)
mS/m
Data set #2:
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Surface topography
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Processing and interpretation
3D resistivity model Animation
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Synthesis• Identified a major conductor ! black shale unit• Some indication of a moderate conductor
3D resistivity model Geologic section
Resistivity section
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Outline
• Basic experiment • Currents, charges, potentials and apparent resistivities• Soundings, profiles and arrays• Data, pseudosections and inversion• Sensitivity• Survey Design• Case History – Mt Isa
• Effects of background resistivity
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Effects of background resistivity
ρ* = 430 Ωm ρ* = 1652 Ωm ρ* = 1654 ΩmM N M N M N
500 Ωm1,Ωm
106,Ωm
Resistivity models (thin resistive layer)
Currents and measured data at MN
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Effects of background resistivity
ρ* = 430 Ωm ρ* = 47 Ωm ρ* = 47 ΩmM N M N M N
Resistivity models (thin conductive layer)
Currents and measured data at MN
1,Ωm
500 Ωm1,Ωm
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End of DCR
Lunch: Play with apps
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