Download - Lecture-12- Introduction & Theoritical Background -Electrical Method

7/30/2019 Lecture-12- Introduction & Theoritical Background -Electrical Method

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Subject 12: Introduction (Electrical Method)Lecturer: Dr. Bakhtiar Q. Aziz

Objective: The basic principle of electrical methods is explain to the students, the

electrical methods classify into two groups active and passive methods. Dc

resistivity, self potential, induced polarization, electromagnetic and Magnetotelluric

methods are discuss in detail to them.Scientific contents

1- Some basic principle and historical review.

1- Active and passive electrical method.

3-Dc resistivity, self potential, induced polarization, electromagnetic and Magnetotelluric

methods.

References

1. Applied and environmental geophysics, 1999, Sharma,V.,P.2. Introduction to geophysical prospecting, 1988, Durbin, M. B.3. www.Geophysics.net4. www.hager-richter.com/ resistivity.htm5. www.geovision.com/PDF/M_ Resistivity.pdf

Subject 13: Theoretical backgroundLecturer: Dr. Bakhtiar Q. Aziz

Objective: The resistivity depends on ohms law, so a review of electrical

parameters will explain to the students, such as current (I), Voltage (V) andresistance. This subject is providing good information on the electrical terms and

some application and limitation of the electrical method to the students.Scientific contents

1- Ohms law.

1- Active and passive electrical method.

3-Dc resistivity, self potential, induced polarization, electromagnetic and Magnetotelluricmethods.


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Electrical Method

Prepared

By

BAKHTIAR QADER AZIZ

2010-2011B.Sc


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Electrical methods divides into the following:

1- Passive Techniques.

2- Active Techniques.

1- DC Resistivity - This is an active method that employs measurements ofelectrical potential associated with subsurface electrical current flow generatedby a DC.

2-Induced Polarization (IP)- This is an active method that is commonly donein conjunction with DC Resistivity.

3- Self Potential (SP) - This is a passive method that employsmeasurements of naturally occurring electrical potentials commonly associatedwith the weathering of sulfide ore bodies.

4- Electromagnetic (EM) - This is an active method that employsmeasurements of a time-varying magnetic field generated by inductionthrough current flow within the earth.

5- Magnetotelluric (MT) - This is a passive method that employsmeasurements of naturally occurring electrical currents, or telluric currents,generated by magnetic induction of electrical currents in the ionosphere.


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It's Resistivity, not ResistanceThe problem with using resistance as a measurement is that it depends not only onthe material out of which the wire is made, but also the geometry of the wire. If wewere to increase the length of wire, for example, the measured resistance wouldincrease. Also, if we were to decrease the diameter of the wire, the measured

resistance would increase. We want to define a property that describes a material'sability to transmit electrical current that is independent of the geometrical factors.

The quantity that is used is called resistivity and is usually

indicated by the Greeksymbol rho*,* *.In the case of the wire, resistivity is defined as theresistance in the wire, multiplied by the cross-sectional

area of the wire, divided by the length of the wire.The units associated with resistivity are thus ohm.mResistivity is a fundamental parameter of the materialmaking up the wire that describes how easily the wirecan transmit an electrical current. High values of resistivity imply that the materialmaking up the wire is very resistant to the flow of electricity. Low values of resistivityimply that the material making up the wire transmits electrical current very easily.

*Unfortunately, the symbol rho is used throughout the geophysical literature torepresent both density and resistivity. Although one would suspect that this could leadto some confusion, it rarely does because the context in which rhois used will usuallydefine whether it is representing density or resistivity unambiguously.
http://g/GRAV/NOTES/reldens.htmlhttp://g/GRAV/NOTES/reldens.html


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Electrical Resistivity1- FundamentalsThe electrical resistivity method is used to map the subsurface electrical resistivitystructure, which is interpreted by the geophysicist to determine geologic structureand/or physical properties of the geologic materials. The electrical resistivity of a

geologic unit or target is measured in ohmmeters, and is a function of porosity,permeability, water saturation and the concentration of dissolved solids in pore fluidswithin the subsurface.The purpose of a DC electrical survey is to determine the subsurface resistivitydistribution of the ground, which can then be related to physical conditions of interestsuch as lithology, porosity, the degree of water saturation, and the presence or absence

of voids in the rock. The basic parameter of a DC electrical measurement is resistivity.Resistivity is not to be confused with resistance.

2- AdvantagesA principal advantage of the electrical resistivity method is that quantitative modeling is

possible using either computer software or published master curves. The resultingmodels can provide accurate estimates of depth, thickness and electrical resistivity ofsubsurface layers.The layered electrical resistivities can then be used to estimate the electrical resistivityof the saturating fluid, which is related to the total concentration of dissolved solids inthe fluid.


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3- LimitationsLimitations of using the electrical resistivity method in ground water pollutioninvestigations are largely due to site characteristics, rather than in any inherentlimitations of the method. Typically, sites are located in industrial areas that contain anabundance of broad-spectrum electrical noise.

In conducting an electrical resistivity survey, the voltages are relayed to the receiverover long wires that are grounded at each end. These wires act as an antenna receivingthe radiated electrical noise that in turn degrades the quality of the measured voltages.Electrical resistivity surveys require a fairly large area, far removed from power lines andgrounded metallic structures such as metal fences, pipelines and railroad tracks. Thisrequirement precludes using this technique at many ground water pollution sites.

However, the electrical resistivity method can often be used successfully off-site to mapthe stratigraphy of the area surrounding the site. A general rule ofthumb for electricalresistivity surveying is that grounded structures be at least half of the maximumelectrode spacing away from the axis of the electrode array. Electrode spacing andgeometry or arrays (Schlumberger, Wenner, and Dipole-dipole) are discussed in detail inthe section below entitled, Survey Design, Procedure, and Quality Assurance.

Another consideration in the electrical resistivity method is that the fieldwork tends to bemore labor intensive than some other geophysical techniques. A minimum of threecrewmembers is required for the fieldwork.


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4- InstrumentationElectrical resistivity instrumentation systems basically consist of a transmitter andreceiver. The transmitter supplies a low frequency (typically 0.125 to 1 cycles/second orHertz) current waveform that is applied across the current electrodes. Either batteriesor an external generator, depending on power requirements can supply power for the

transmitter. In most cases, the power requirements for most commonly used electrodearrays, such as Schlumberger (pronounced schlum-bur-zhay) and Wenner arrays areminimal and power supplied by a battery pack is sufficient. Other electrodeconfigurations, such as Dipole-dipole arrays, generally require more power, oftennecessitating the use of a power generator. The sophistication of receivers range fromsimple analog voltmeters to microcomputer-controlled systems that provide signal

enhancement, stacking, and digital data storage capabilities. Most systems have digitalstorage of data. Some systems may require the field parameters to be input via PC(personal computer) prior to collection of the data. The trend in manufacturers ofresistivity equipment is to have the entire system controlled form a PC orpreprogrammed software built into the instrument.

Fig (1) SYSCAL Jr Switch-72


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Electrical resistivity methods:

Resistivity measurements are made by passing an electrical current into the groundusing a pair of electrodes and measuring the resulting potential gradient within thesubsurface using a second electrode pair (normally located between the current

electrodes). Resistivity sounding involves gradually increasing the spacing betweenthe current/potential electrodes (or both) in order to increase the depth ofinvestigation. The data collected in this way are converted to apparent resistivityreadings that can then be modelled in order to provide information on the thicknessof individual resistivity units within the subsurface.The electrical resistivity method is one of the most useful techniques in groundwater

hydrology exploration because the resistivity of a rock is very sensitive to its watercontent. In turn, the resistivity of water is very sensitive to its ionic content. In general, it is able to map different stratigraphic units in a geologic section aslong as the units have a resistivity contrast. Often this is connected to rock porosityand fraction of water saturation of the pore spaces.

Applications:1. Water table depth.2. Groundwater quality3. Brine plumes.4. Seawater intrusion5. Well sitting.

6. Aquifer exploration7. General stratigraphic mapping

Advantages:1. Less costly thandrilling.2. Non disturbing.

Disadvantages:1. Cultural problems causeinterference, e.g., power lines,pipelines, buried casings,fences .2. Resolution.


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Possible applications of resistivity surveying

Groundwater exploration

Mineral exploration, detection

of cavities

V

I

A M N B

MA MB

NA NB


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Waste site exploration

Oil exploration

D i ti it T h i R i ti it t f th d ll d


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Dc resistivity Techniques :Resistivity measurements of the ground are normally madeby injecting current through two current electrodes and measuring the resulting voltagedifference at two potential electrodes. From the current (I) and voltage (V) values, anapparentresistivity (a) value is calculated,

Consist of the following:1- Energy source, (Battery).2- Resistivity meter.3- Two potential electrodes.4- Two current electrodes.

Current-1 Current-2Potential-1 Potential-2

Fig (1) SYSCAL Jr Switch-72 resistivity meter


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IV

IL

VA

Theory


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TheoryIn a homogeneous earth, current flows radially outward from the source to dfine ahemispherical surface. The current distribution is equal everywhere on this surfacewhich is also called an equipotential surface. Starting with Ohms law (V = IR) anddefining the resistance R in terms of the resistivity and the area of the shell(equipotential surface), the potential difference across the shell is

where Vis the voltage (or electrical potential), Iis the current, is the resistivity, and ris

the radius of the equipotential surface. Integrating the above equation and setting thepotential at infinity to zero, the electric potential at a distance Rfrom the source is givenby:

Resistivity has units ofohm mand is not to be confused with resistancewhich has unitsofohms. The resistivityof a material is defined as = RA Lwhere Ris the resistance ofthe material, A is the cross-sectional area through which current flows and L is thelength on the material.The potential has been derived due to a single current source. The goal in resistivitysurveying is to measure the potential different between two points due to the current

from two current electrodes. The potential at each electrode is determined due to thecurrent sources:


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The potential difference V= VP1 VP2 which simplifies to :

The above equation can then be solved for the resistivity . In a nonhomogeneous earth,the resistivity which is measured is not actually the true resistivity of the subsurface. Foran earth with more than one layer, the apparent resistivitymeasured will be an averageof the resistivities of the additional layers. The apparent resistivity data needs to beinterpreted in terms of a subsurface model in order to determine the actual resistivities

of the layers.


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Apparent ResistivityIf the resistivity in the ground is uniform, then the measured resistivity will be constantand independent of electrode spacing and surface location. If the resistivity in theground is inhomogeneous, then the measured resistivity will vary with relative andabsolute location of the electrodes. In this case, the measured resistivity is an

apparent resistivity, a, which depends on the shape and size of anomalous regions,layering and relative values of resistivities in these regions.The apparent resistivity is similar to the equivalent resistivity of a circuit with resistors inparallel and series. In order to figure out how many resistors there are in the circuit andtheir individual resistivity, you would need to interrupt the circuit at various locations andmeasure the voltage. With several measurements you may be able to isolate theparticular circuitry. Similarly, in the earth, by changing the relative spacing and locationof the potential electrodes you can unravel where the resistors are below the surface.

SurfaceSurface

C1 C2P1 P2 C1 C2P1 P2

The reading represent apparent resistivity The reading represent True resistivity

Calculation of Apparent Resistivity :The most common problem encountered in resistivity


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Calculation of Apparent Resistivity :The most common problem encountered in resistivitysounding work is high contact resistances at the current electrodes. Whilst this does notdirectly affect the measured value of resistance, high contact resistances (>2kOhms)will reduce the maximum current that can be applied with the output voltage availablefrom the meter (typically 300-400V). In order to overcome high resistances electrodescan be watered with a saturated salt solution or placed in hole filled with bentonite orclay slurry. A BM N

r2r1

r3 r4

A, B : Are current electrodes

A, B : Are potential electrodes

After introducing current , the potential calculate by:

x

Iv

2The total potential at M and N are VM and VNThe potential at M calculate by:

)11

(

222

2,

2

2121

2

2

1

1

21

rr

I

r

I

r

Iv

r

Iv

r

Iv

vvv

M

rr

rrM


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By the same way the potential at N calculate by:

)11

(222

2,2

4343

44

33

43

rr

I

r

I

r

Iv

r

I

vr

I

v

vvv

N

rr

rrN

Then the potential difference between M and N calculate by:

4321

4321

1111

12

)1111

(2

rrrr

I

v

rrrr

Iv

vvv

a

NM


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)(1111

:

4321

GFactorlGeometricacalledisrrrr

quantityThe

yresistivitapparentis

truea

a

GI

va ..2

In homogenous area

Apparent resistivity for Wenner spread:

The distances between electrodes are constant , equal to (a)

A BM Nr2r1

r3 r4a a a

arr

arr

BNMNAM

232

41


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aRoraI

v

aaaa

I

v

aa

a

.2.2

1

2

1

2

11

12

Apparent resistivity for Schlumberger spread:

The distances between potential electrodes are too small compared to thecurrent electrodes.

A BM Nrr r

rr

rrrr

rrr

32

41


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AMR

r

r

I

v

r

rI

v

sozeror

rrr

rrrI

v

rrr

I

v

rrrrrr

I

v

a

a

a

a

a

2

2

2

..

..1

..

....

)(2

12..2

22

1.2

1111

1.2


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