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Electrical Surveying (part A)

Dr. Laurent Marescot

Course given at the University of Fribourg (2009)

Contact: laurent@tomoquest.com

www.tomoquest.com

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Introduction

Electrical surveying…

• Resistivity method

• Induced polarization method (IP)

• Self-potential (SP) method

Higher frequency methods (electromagnetic surveys):

• Electromagnetic induction methods

• Ground penetrating radar (GPR)

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Resistivity Method

The resistivity method is used in the study of horizontal

and vertical discontinuities in the electrical properties

(resistivity) of the subsurface

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Application

• Exploration of bulk mineral deposit (sand, gravel)

• Exploration of underground water supplies

• Engineering/construction site investigation

• Waste sites and pollutant investigations

• Cavity, karst detection

• Glaciology, permafrost

• Geology

• Archaeological investigations

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Structure of the Lecture

• Resistivity of Rocks

• Equations in Resistivity Surveying

• Survey Strategies and Interpretation

• Conclusions

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1. Resistivity of Rocks

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Resistivity and Units

Resistivity is the physical property which determines the aptitude of

this material to be opposed to the passage of the electrical current

resistivity in ohm.m (m)

=1/ conductivity in Siemens per meter (S/m)

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Electrolytic Conductibility

The current is carried by ions. The electrical resistivity of

rocks bearing water is controlled mainly by the water which

they contain.

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Electrolytic Conductibility

The resistivity of a rock will depend :

• on quality of the electrolyte, i.e., on the resistivity of the natural pore

water and consequently the quantity of dissolved salts in the electrolyte

1g/liter=1000 ppm

• on the mode of electrolyte distribution, porosity

• on the quantity of electrolyte contained in the unit of rock volume

(saturation)

• on the temperature

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Porosity

Total volume of communicating voids

Total volume of rocke

Total volume of voids

Total volume of rockt

Total porosity:

Effective porosity:

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Saturation

Volume of saturated voids

Total volume of voidswS

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Effect of Temperature

A rock totally frozen is infinitely resistant and it is

impossible to implement resitivity methods (use EM

methods)

18025.01

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tt

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Archie´s Law

nm

w Sa

• resistivity of the rock

• w resistivity of the fluid (water)

• porosity

• S saturation in water

• a factor which depends of the lithology

• m cementation factor (depends of the pores shape, of the compaction)

• n about 2 for majority of the formations with normal porosities containing water between 20 and 100 %.

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Formation factor F

m n

w

n

w

a S

F S

• For sand and sandstones: F≈ 0.62/2.15

• For well cemented rocks: F≈ 1/2

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Permeability

There is no direct relationship between resistivity and permeability.

This table shows also the problem in identifying rocks due to overlapping resistivity values (no contrast)

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Resistivity of Rocks and Minerals

Air, gas or oil: infinite or very high resistivity!

Liquid materials from landfills are generally conductive (<10 ohm.m)

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Effect of Clay

Clay has a high ionic exchange capacity, therefore the

resistivity of the pore fluid largely decreases.

Archie´s Law is not valid if clay is present!

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Summary…

The resistivity of a rock decreases if…

• The quantity of water increases (more water)

• The salinity increases (more ions)

• The quantity of clay increases

• The temperature increases (less water viscosity)

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2. Equations in Resistivity Surveying

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Current Flow in the Ground

r

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Potential from a single electrode

r

IV

2

r

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Two current electrodes

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Potential Difference

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21

112/2/2/

1 rrIrIrIVP

Vp1 is the sum of the

potential contribution

from the current

electrodes C1 and C2

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Potential field between

two current electrodes

A and B

Note the fast decrease near A and B (contact resistance)

A B

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Current Distribution

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Current Distribution

This has an influence on the depth of investigation!

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Current Distribution

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Heterogeneous Earth

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Effect of Topography

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3. Survey Strategies and Interpretation

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Two Potential Electrodes

111112

1111

2

11

2

11

2

NBANMBAMI

V

NBANMBAM

IVVV

NBAN

IV

MBAM

IV

MNa

NMMN

N

M

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Apparent Resistivity

In a heterogeneous medium, the measured resistivity is an

apparent resistivity, which is a function of the form of the

inhomogeneity and of the electrode spacing and surface

location.

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Geometric Factor

For a half-space and electrodes on the surface, a general

definition for the geometric factor K can be written:

11 1 1 1

2MNa

V

I AM MB AN NB

MNa

VK

I

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Device

• Current source: batteries in series

• Voltmeter and ammeter (resistivimeter)

• Electrodes: metallic stakes

current electrodes: stainless steel

potential electrodes: stainless steel or

nonpolarizing electrodes

Polarization occurs at the contact electrode/ground: this

creates an additional potential difference.

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To Decrease Contact Resistance…

• Add electrodes in parallel

• Increase the current intensity

• Increase the diameter of the current electrodes

• Put electrode deeper into the ground

• Add water (with salt) near the electrodes

About 90% of the contact resistance contribution comes from a portion of the ground around the electrode that is equal to 10 times the diameter of the electrode

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Origine of Noise

• Telluric currents

• Man-made currents

• Metallic conductors in the ground (short-circuits)

Solutions:

• Use of alternating current

• Stacking operations

• Rejection filters (16-20 Hz, 50-60 Hz)

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Survey Strategies

• Resistivity mapping, constant separation traversing (CST):

used to determine lateral variations of resistivity. The current and potential electrodes are maintained at a fixed separation and moved along profiles

• Vertical electrical sounding (VES):

used in the study of near-horizontal interfaces. The electrode spread is progressively expanded about a central point

• Electrical Resistivity Tomography (ERT):

is a mix between CST and VES. Also named electrical imaging

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Constant Separation Traversing (CST)

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Constant Separation Traversing (CST)

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Constant Separation Traversing (CST)

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Constant Separation Traversing (CST)

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• Demo during the lecture

Constant Separation Traversing (CST)

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Electrode Spreads

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Electrode spreads

I

Vaa

2

( 1)a

Vn n a

I

I

Vannna

)2)(1(

Wenner array

Schlumberger array

dipole-dipole array

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Penetration Depth

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Interpretation of CST

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Multiple “Twin Probes”

RM15 resistance meter with

multiplexer

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Sanctuary of Poseidon (island of Poros, Greece)

Papadopoulos et al., 2006. Archeological Prospection, 13, 75-90

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Ancient Royal Site of Rathcroghan, Ireland

Barton & Fenwick, 2005. Archeological Prospection, 12, 3-18

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Peristyle villa Gallo-romaine Yvonand (Vaud)

AB=4m

wall

fountain?

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Manually Dragged Systems

Dabas et al., 2000, Archeological Prospection, 7, 107-118

57Dabas et al., 2000, Archeological Prospection, 7, 107-118

Roman city, Wroxester (UK)

58Source: Geocarta, Paris

100 data points/seconde

1 data point each 20cm

Mobile Arrays with Vehicle

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Mobile Arrays

Source: Geocarta, Paris Vineyards investigations

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A B

M1 N1

M2 N2

M3 N3

Mobile Arrays

Current injection

Resistivity measurement

(three investigation depths)

Source: Geocarta, Paris

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Mapping example with mobile array (spacing 2m)

Surface: 155 hectares

30 ohm.m 160 ohm.m

Apparent resistivity

Source: Geocarta, Paris

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Mapping example with mobile array

(spacing 2m)

Surface: 140 hectares

Apparent resistivity

15 ohm.m 150 ohm.m

Source: Geocarta, Paris

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Profile spacing 6m Profile spacing 12m Profile spacing 24m

Apparent resistivity

10 ohm.m 90 ohm.mSource: Geocarta, Paris

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Ecartement 0.5m Ecartement 1m Ecartement 2m

10 ohm.m 60 ohm.m

Apparent resistivity

Source: Geocarta, Paris

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Inaccuracy in Location

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Electrostatic Mobile Arrays

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XVII and XVIII centuries structures (La Rochelle, France)

Panissod et al., 1998, Archeological Prospection, 5, 239-251

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Vertical Electrical Sounding (VES)

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Vertical Electrical Sounding (VES)

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Vertical Electrical Sounding (VES)

72• Demo during the lecture

Vertical Electrical Sounding (VES)

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One Layer and Two Layers

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Three layers and more…

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Interpretation

Field data Model Calculated data (response of model B)

Comparison between data A and C and modification

of model B

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Interpretation of VES

• Demo during the lecture

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Interpretation of VES

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Equivalence

constanth constant

h

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Parametric Sounding

A parametric sounding is a VES carried out on an outcrop

or near a borehole to precisely determine the resistivity of

a geological formation.

A precise determination of resistivity reduce the problem

of equivalence

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Suppression

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