4c Resistivity CPL

8/3/2019 4c Resistivity CPL

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DC Resistivity Method

[email protected]

DC Resistivity Method

Resistivity Basics

Resistivity Surveys and Geology

Resistivity Equipment and FieldProcedures

Interpretation of Resistivity Measurements

Case Study


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Resistivity Basics

Current flow and Ohms law

Resistivity vs. Resistance

Resistivity for common earth materials

Current density and electrical field

Measurement of resistivity

DC resistivity basics

Current Flow and Ohms Law



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Resistivity vs. Resistance


Resistivity of Earth MaterialsMaterial Resistivity (Ohm-meter)

Air Infinite

Pyrite 3 x 10^-1

Galena 2 x 10^-3

Quartz 4 x 10^10 - 2 x 10^14

Calcite 1 x 10^12 - 1 x 10^13

Rock Salt 30 - 1 x 10^13Mica 9 x 10^12 - 1 x 10^14

Granite 100 - 1 x 10^6

Gabbro 1 x 10^3 - 1 x 10^6

Basalt 10 - 1 x 10^7

Limestones 50 - 1 x 10^7

Sandstones 1 - 1 x 10^8

Shales 20 - 2 x 10^3

Dolomite 100 - 10,000

Sand 1 - 1,000

Clay 1 - 100

Ground Water 0.5 - 300

Sea Water 0.2

Archies Law

=aw-m



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Current Densities and Equipotential


A First Estimate of Resistivity



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Two closely spread electrodes

Current Path % of Total Current

1 17

2 32

3 43

4 49

5 51

6 57


A practical way of measuring resistivity



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A practical way of measuring resistivity

I

VK

rrrr

I

VV pPa

=

+

=

4

1

3

1

2

1

1

1

221


Resistivity Survey and Geology

Sources of Noise

Depth of Current Penetration vs. CurrentElectrode Spacing

Current Flow in Layered Media

Variation in Apparent Resistivity: Layeredvs. Homogeneous Media

Resistivity survey and geology


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Sources of Noise

Electrode polarization

Use nonpolarizing electrodes

Use a slowly varying AC current

Telluric currents

Presence of nearby conductors

Low resistivity at the near surface

Near-electrode geology and topography

Current induction in measurement cables.


Depth of current penetration vs.current electrode spacing



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Current flow in two layer media

1

2

2

1

2

1

tan

tan

==


Current flow in two layer media



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Current distribution


Layered vs. Homogeneous Media

IVK

rrrr

I

VVpP

a

=

+

=

4

1

3

1

2

1

1

1

221


ApparentResistivity


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Current flow in layered media-Case 1


Resistivity Surveys and GeologyCurrent flow in layered media-Case 1



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Resistivity Equipment and Field Procedure

Equipment

Survey Types Overview

Soundings

Profiles

Tomography

Choice of Best Array

Field Work

Resistivity equipment and field procedure

DC Resistivity Equipment

Current source

Ammeter

Voltmeter

Electrodes

Cables



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Survey Types Overview

Resistivity Soundings

To look for variations in resistivity with depth

Resistivity Profiles

To detect lateral variations in resistivity

Resistivity Tomography

2-D resistivity tomogram


Resistivity Soundings-Pole-Pole Array

Pole-Pole sounding data is plotted as

apparent resistivity vs. a



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Resistivity Soundings-Pole-Dipole Array

Pole-Dipole sounding data is plotted asapparent resistivity vs. a


Resistivity SoundingsWenner Array

Wenner sounding data is plotted asapparent resistivity vs. a on a log-log plot



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Resistivity SoundingsSchlumberger Array

Schlumberger sounding data is plotted asapparent resistivity vs. s (AB/2) on a log-log plot


Resistivity SoundingsDipole-Dipole Array

Dipole-Dipole sounding data is plotted asapparent resistivity vs. s (AB/2) on a log-log plot



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Resistivity SoundingsElectrode Spacings and Apparent Resistivity Plots


Resistivity Profiles



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Pseudosection


CHOICE OF THE BEST ARRAY

Depends on:1) type of structure to be mapped

2) sensitivity of the resistivity meter3) background noise level

Things to be considered:

1) depth of investigation2) sensitivity of the array to vertical and horizontal structures3) horizontal data coverage

4) signal strength.



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Field Work


Interpretation of Resistivity Measurements

Apparent resistivity curves for soundingsover one-layered media

Apparent resistivity curves in two-layeredmedia

Resistivity Modeling and Inversion

Interpretation of resistivity measurements


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Apparent Resistivity CurveOne-layered media case 1


Apparent Resistivity CurveOne-layered media case 2



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Apparent Resistivity CurveTwo-layered media


Apparent Resistivity CurveTwo-layered media



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1D inversion

1D inversion


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Resistivity Modeling





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2D inversion



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Seismic and Electrical Tomography

Case Study 1

Case Study 1

ERT - along tunnel axis(Pole-Pole Array)


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ERT Simulation

Case Study 1

ERT Simulation

Case Study 1


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ERT Simulation

Case Study 1

Site condition

199

1

Case Study 2


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Logging Data

Case Study 2

DH5 DH1 DH2 DH4 DH3

Line1

Line2

Line3

Line3

cross

Measurement array

Wenner-Schlumberger

Case Study 2


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Inversion Result

Case Study 2

DH3

Line3 CrossLine3

Line2Line1

DH1 DH5

DH2DH4

Forward Model

Case Study 2


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Forward Model

Case Study 2

Interpretation

In-situ measurement data

Logging

Forward model

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