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