Geophysics ( Electric Method )

8/2/2019 Geophysics ( Electric Method )

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GEOELCTERICALMETHODS

BY

DR. ESSAM A. MORSY

CAIRO UNIVERSITY


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Geophysical Surveys:

Can be divided

into

two classes

Active

Versus

Passive


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Passive geophysical surveys

Involving measurementsofnaturally occurringfields or propertiesof the earth.

The naturally occurringfields are the:

gravitational field,

magnetic field, and

EM field.


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Active geophysical Surveys.

A signal is injected intothe earth and

we then measure how

the earth responds tothis signal.

DC resistivity andseismic refraction.


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Electrical Methods Overview

The electrical

and

electromagnetic methodsrepresent

the largest class

of all geophysical methodsDue to the wide field applications.


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INTRODUCTION

Several authors pointed out thatgeoelectrical resistivity method can beused in:

Searching for water bearing formations,Stratigraphic correlation in oil fields,

Prospecting for conductive bodies,

Determining the depths of high or lowconductive-anomalous layers,


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Determining the depths andapproximate shape of resistivity-

anomalous ore bodies,

Civil engineering and environmental

investigations,

Cavity detection, and

Identifying the depth of sand andgravel deposits.


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What are the electricalProperties of rocks depend on:-

The electrical properties of mostrocks in the upper part of the Earthscrust are dependent primarily upon

The amount of water in the rock,

The salinity of the water, and

The distribution of the water in therock.


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Saturatedrocks have lowerresistivities

than unsaturated and dry rocks.The higher theporosityof thesaturated rock, the lower its

resistivity, and

the higher thesalinityof thesaturating fluids, the lower the

resistivity.

The presence ofclays andconductive minerals also reduces

the resistivity of the rock.


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Properties are of primary concern inthe application of electrical methods

Two properties are of primary concernin the application of electricalmethods:

1. The ability of rocks to conduct anelectric current, and

2. The polarization which occurs when anelectrical current is passed throughthem (induced polarization).


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The resistivity method is used in thestudy ofhorizontal and vertical

discontinuities in the electricalproperties of the ground.

It utilizes direct currents or low

frequency alternating currents toinvestigate the electrical properties(resistivity) of the subsurface.

A resistivity contrastbetween thetarget and the background geologymust exist.


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Common applications ofresistivity surveying

Groundwater exploration


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Mineral exploration,

detection of cavities


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Oil exploration


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Waste site exploration


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Electrical methods employa variety of measurements

within the Earth.

DC Resistivity

Induced Polarization (IP)Self Potential (SP)

Electromagnetic (EM)

Magnetotelluric (MT)


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BASICSCurrent Flow and Ohm's Law

In 1827, Georg Ohmdefined an empiricalrelationship between

the current flowingthrough a wire and

the voltage potentialrequired to drive that

current.


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It's Resistivity, NOT Resistance

In the case of a wire,resistivity is definedas the resistance inthe wire, times the

cross-sectional area

of the wire, divided bythe length of the wire.The units associated

with resistivity arethus, ohm - m

(ohm-meters).


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Resistivity is one of the most variablephysical properties.


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There is

a large range of resistivities,not only between varying

rocks and minerals

butalso

within rocks of the same type.


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ResistivityIs the reciprocal ofconductivity


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RESISTIVITY UNITS

It is reasonable to expect that R is directly

proportional to the length of the resistor andinversely proportional to the cross-sectionalarea of the resistor. The conductivity is definedas

Dimensionally, the units of resistivity must be in

ohm-m, and conductivity then is:


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Dependencies of Earthresistivities:

1. Water , r 2. Salinity , r 4. Porosity , r (water filled)5. Clays , r 6. Metallic minerals , r


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Rock types and resistivity

Igneous rocks highest resistivities

Sedimentary rocks tend to bethe most conductive due to theirhigh fluid content

Metamorphic rocks haveintermediate but overlappingresistivities


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Most rock-forming minerals are insulators:

1081016m

However, measurement in situ: sedimentary rocks: 51000 m

metamorphic/crystalline rocks: 100105m

Reason: Rocks are usually porous and poresare filled with fluids, mainly water. As theresult, rocks are electrolytic conductors.Electrical current is carried through a rockmainly by the passage of ions in pore waters.

Most rocks conduct electricity byelectrolytic rather than ohmic processes.


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Schematic current flow insoil sample

An increase in the number of ions in soilwater (groundwater contamination) linearlydecreases the soil resistivity.


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The approximate resistivity

values of common rock types


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Age of the rock and Resistivity

Age of the rock is also important forthe resistivity.

For example:

Young volcanic rock (Quaternary)

10200 m

Old volcanic rock (Precambrian)1002000 m


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Current Densities andEquipotentials

How does thecurrent flowthrough the Earth?


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At any point in the medium, thecurrent densityis defined as theamount of current passing through a

unit area of an equipotential surface.


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A First Estimate of Resistivity

Vis voltage,Iis current,ris resistivity,and ris the distance

this expression is nothingmore than Ohm's law with theresistance, R equal to rover2pr.


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If the Earth had a constant resistivity,

C t Fl F T Cl l


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Current Flow From Two CloselySpaced Electrodes

A Practical Way of Measuring


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A Practical Way of MeasuringResistivity

r1, r2, r3, and r4, as shown in the figure.


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El d fi i


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Electrode configurations

and general case

General CaseThe general case is considered, where thecurrent sink is a finite distance from the

source.


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Principle of

measurementand potentialfield for forgeoelectric

DC surveys


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ELECTRODE ARRAYS

There are three commonlyused electrode arraysincluding

1- Wenner array,

2- Dipole-Dipole array, and3- Schlumberger array


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Fig. (I-2): Electrode arrays in common use(a) Schlumberger, (b) Wenner, and (c) Dipole- dipole configurations.

I

V

A M N B

2 L

2 l

I V

a a

L

( c ) Dipole - Dipole

( a ) Schlumberger

A B M N

I

V

A M N B

aaa

( b ) Wenner


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Comparison Of Electrode Arrays


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Comparison Of Electrode Arrays

R l ti b t l t d


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Relation between electrodespacing and depth penetration

Depth of penetration

is

equal toone fifth

of

electrode spacing

Factors affecting selection of


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Factors affecting selection ofresistivity electrode arrays.

Sensitivity to surface

inhomogeneties

Resolution of

Steeply dippingstructures

Resolution of

Horizontalstructures

Signal

Response

ProfilingSoundingArrayCBCAAenner

CABABchlumberger

BDDBEipole-Dipole


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Generally

The Schlumberger and Wennerarrays are the most common arrays

used for different engineering,environmental, and groundwaterapplications. There are two

essential differences between thesetwo types of electrode arrays:


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In the Schlumberger array, the distance between thepotential electrodes MNis small and is always keptequal to, or smaller than, one-fifth the distance

between the current electrodes AB; that is,AB5MN.In the Wenner array, on the other hand, AB is alwaysequal to 3MN,

The potential electrodes in a Schlumberger soundingare moved only occasionally. In a Wennersounding,however, they and the four electrode are moved aftereach measurement.

K i h l i f h f l d


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Knowing the locations of the four electrodes,and by measuring the amount of current input

into the

ground, iand the voltage difference betweenthe two potential electrodes, DV, we can

compute the

resistivity of the medium, ra, using the

following equation.

In this particular case,


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In this particular case,regardless of the location of the

four electrodes, ra will be exactly

equal to theresistivity of the medium. Theresistivity computed using the

equation given above is referredto as the

apparent resistivity.

Depth of Current Penetration Versus Current


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Depth of Current Penetration Versus CurrentElectrode Spacing

Current Flow in Layered Media


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Current Flow in Layered Media


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Variation in Apparent Resistivity: Layered VersusHomogeneous

Media

Current Flow in Layered Media Versus Current


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Current Flow in Layered Media Versus CurrentElectrode Spacing

Consider the earthmodel shownbelow: a highresistivity layerover a lowerresistivity layer.

Plotting the Resistivity Data


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Plotting the Resistivity Data

Thus, if we were to compute and plotapparent resistivity for a variety of currentelectrode spacings while holding thepotential electrodes fixed, we wouldgenerate a plot similar to that shownbelow.

A Second Example of Current


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A Second Example of CurrentFlow in Layered Media

As another example of

current flow in layeredmedia and howapparent resistivitycan vary with varying

electrode spacing*,consider the earthmodel shown below.In this case, a low

resistivity layeroverlies a

higher resistivityhalfspace.


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Survey Types Overview:


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Survey Types Overview:Soundings and Profiles

Resistivity Soundings

Resistivity Profiling

VES


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VES


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VES

The relation between electrodespacing and depth penetration



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Electrode Spacings and Apparent


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gResistivity Plots


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Flow Diagram showing the basic Steps of Acquisition,


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Processing and interpretation of VES data.



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resistivity profiles are resistivity surveys inwhich the electrode spacing is fixed

for all readings. Apparent resistivity is

computed for different electrode centerpoints as the entire

electrode spread is moved. Usually, thecenter point is moved along the line of the

electrodes, although

this does not have to be the case.

Profiling is accomplished by fixing


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Profiling is accomplished by fixingthe electrode spacing and movingthe entire array.

The distance between stations D, isdictated by the lateral resolutiondesired


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Geo-electric Layering


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Geo electric Layering

Often the earth can be

simplified as consistingof a series ofhorizontal beds thatare infinite in extent.

Goal of the resistivitysurvey is then todetermine thicknessand resistivity of thelayers.

VES Data Plotting Convention


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VES Data Plotting Convention

Plot apparent resistivity as a functionof the log of some measure ofelectrode separation.

Wennera spacing

SchlumbergerAB/2

Dipole-Dipolen spacing

Future? Towed Array


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Future? Towed Array


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Measurement System

TransmitterPower SupplyCan be DCAC more

common

AmmeterMetal electrodes

Receiver

Volt meterMetal electrodes

Field Considerations for DC


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Resistivity

Good electrode contact with the earth.

Wet electrode location

Add NaCl solution or bentonite.Surveys should be conducted along astraight line whenever possible.

Stay away from cultural featureswhenever possible (power lines, pipes,grounded metal fences,pumps, etc)

Geophysics ( Electric Method )

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