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GPH 321ELECTRICAL AND ELECTROMAGNETIC
EXPLORATION(3 Credits, Prereq: GEO 234 + P! 221 + MTH 2"3#
PRINCIPLE$ O% ELECTRICAL & EM ( 2 'ees# Electrical properties of rocks
Mechanism of electrical conduction in materials Representative resistivity values Conductivity mechanism
%)NDAMENTAL$ O% C)RRENT %LO* ( 2 'ees# Fundamentals of the current flow in the earth. Potential distribution in a Homogeneous Medium pparent and true resistivity Potential and current distribution across boundary
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DC RE$I$TIIT- METHOD ( 4 'ees # Electrode configurations Electric sounding ! Electric profiling
field procedures pplications ! mbiguities "ualitative ! "uantitative #nterpretation Mise $ % la% Masse Method
ELECTROCHEMICAL METHOD$ ( 2 'ees # self%potential method induced polari&ation method .
C./t
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MIDTERM EXAM
%INAL EXAM
E'EC(R)M*+E(#C ME(H),- / weeks 0 Classification of electromagnetic systems Principles of electromagnetics
Magnetotelluric Methods 1ertical loop 1'EM0 -lingram ! (uram -ystems 1ery 'ow Fre2uency 1'F0 udio Fre2uency Magnetics FM*0
(ime%,omain systems (,EM 0 irborne Method *round Penetrating Radar
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HOME*OR0 A$$IGNMENT$ IN PAGE :
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TEXT : R.9i/s./ & C.r7si? E6@;.r7ti./ Ge.@!si?s ./ *i;e! & $./sL.'rie, * ( 1# %
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ELECTRICAL RE$I$TIIT- TECHNI)E$ 4
*eophysical resistivity techni2ues are based on the response
of the earth to the flow of electrical current. #n these methods3
an electrical current is passed through the ground and two
potential electrodes allow us to record the resultant potential
difference between them3 giving us a way to measure the
electrical impedance of the subsurface material. (he apparent
resistivity is then a function of the measured impedance ratio
of potential to current0 and the geometry of the electrode
array. ,epending upon the survey geometry3 the apparentresistivity data are plotted as 4%, soundings3 4%, profiles3 or
in 5%, cross%sections in order to look for anomalous regions.
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#n the shallow subsurface3 the presence of water controls
much of the conductivity variation. Measurement of resistivity
inverse of conductivity0 is3 in general3 a measure of watersaturation and connectivity of pore space. (his is because
water has a low resistivity and electric current will follow the
path of least resistance. I/?re7si/ s7t
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Resistivity measurements are associated with varying
depths depending on the separation of the current and
potential electrodes in the survey3 and can be interpreted
in terms of a lithologic and7or geohydrologic model of the
subsurface. ,ata are termed apparent resistivity
because the resistivity values measured are actually
averages over the total current path length but areplotted at one depth point for each potential electrode
pair. (wo dimensional images of the subsurface
apparent resistivity variation are called pseudosections.
,ata plotted in cross%section is a simplisticrepresentation of actual3 comple8 current flow paths.
Computer modeling can help interpret geoelectric data in
terms of more accurate earth models.
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*eophysical methods are divided into two types 9
ctive and Passive
P7ssie 5et.ds +atural -ources09 #ncorporate
measurements of natural occurring fields or
properties of the earth. E8. -P3 Magnetotelluric M(03
(elluric3 *ravity3 Magnetic.
A?tie Met.ds #nduced -ources0 9 signal is
in:ected into the earth and then measure how the
earth respond to the signal. E8. ,C. Resistivity3-eismic Refraction3 #P3 EM3 Mise%%'%Masse3 *PR.
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DC Res!st!+!t% % (his is an active method that employs
measurements of electrical potential associated with
subsurface electrical current flow generated by a ,C3 or
slowly varying C3 source. Factors that affect the measured
potential3 and thus can be mapped using this method include
the presence and 2uality of pore fluids and clays. )ur
discussions will focus solely on this method.Indued o'r!/t!on 0I % (his is an active method that is
commonly done in con:unction with ,C Resistivity. #temploys measurements of the transient short%term0
variations in potential as the current is initially applied or
removed from the ground. #t has been observed that when a
current is applied to the ground3 the ground behaves muchlike a capicitor3 storing some of the applied current as a
charge that is dissipated upon removal of the current. #n this
process3 both capacity and electrochemical effects are
responsible. #P is commonly used to detect concentrations of
clay and electrically conductive metallic mineral grains.
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Se') otent!' 0S % (his is a passive method that employs
measurements of naturally occurring electrical potentials
commonly associated with the weathering of sulfide ore
bodies. Measurable electrical potentials have also beenobserved in association with ground%water flow and certain
biologic processes. (he only e2uipment needed for
conducting an -P survey is a high%impedance voltmeter and
some means of making good electrical contact to theground. E'etro,net! 0EM % (his is an active method that
employs measurements of a time%varying magnetic field
generated by induction through current flow within the earth.
#n this techni2ue3 a time%varying magnetic field is generatedat the surface of the earth that produces a time%varying
electrical current in the earth through induction. receiver is
deployed that compares the magnetic field produced by the
current%flow in the earth to that generated at the source.
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EM is used for locating conductive base%metal deposits3 for
locating buried pipes and cables3 for the detection of
une8ploded ordinance3 and for near%surface geophysicalmapping.
. Mnetote''ur! 0MT % (his is a passive method that
employs measurements of naturally occurring electricalcurrents3 telluric currents3 generated by magnetic
induction of electrical currents in the ionosphere. (his
method can be used to determine electrical properties of
materials at relatively great depths down to and including
the mantle0 inside the Earth. #n this techni2ue3 a timevariation in electrical potential is measured at a base
station and at survey stations. ,ifferences in the recorded
signal are used to estimate subsurface distribution of
electrical resistivity.
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P.siti./ .B E;e?tri?7; Met.ds i/9
40 Petroleum E8ploration.
(he most prominent applications of electrical techni2ues in
petroleum e8pl. re in well logging. Resistivity and -P arestandard 'ogging techni2ues.
(he magnetotelluric method has found important application
for pet. E8ploration. #n structurally comple8 region E;.
Rocky Mountains0.50 Engineering ! *roundwater.
, C. Resistivity and EM have found broad use in civil
Engineering and groundwater studies. -aturated 7
>> to 4?>>
ft. Electrical
e8ploration methods are the dominant geophysical tools in
Mineral E8pl.
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O5s L7'
)hm@s 'aw describes the electrical properties of any
medium. O5s L7', F I R3 relates the voltage of acircuit to the product of the current and the resistance.
(his relationship holds for earth materials as well as
simple circuits. Resist7/?e( R#3 however3 is not a
material constant. #nstead3 resistivity is an intrinsic
property of the medium describing the resistance of
the medium to the flow of electric current.
Resistiit! is defined as a unit change in resistance
scaled by the ratio of a unit cross%sectional area and a
unit length of the material through which the current ispassing Figure 40. Resistiit! is measured in ohm%m
or ohm%ft3 and is the reciprocal of the conductivity of
the material. (able 4 displays some typical resistivities.
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+ote that3 in (able 43 the resistivity ranges of different earth
materials overlap. (hus3 resistivity measurements cannot be
directly related to the type of soil or rock in the subsurfacewithout direct sampling or some other geophysical or
geotechnical information. Porosity is the ma:or controlling factor
for changing resistivity because electricity flows in the near
surface by the passage of ions through pore space in the
subsurface materials. (he porosity amount of pore space03 thepermeability connectivity of pores03 the water or other fluid0
content of the pores3 and the presence of salts all become
contributing factors to changing resistivity. Aecause most
minerals are insulators and rock composition tends to increaseresistivity3 it is easier to measure conductive anomalies than
resistive ones in the subsurface. However3 air3 with a theoretical
infinite resistivity3 will produce large resistive anomalies when
filling subsurface voids.
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E;e?tri? ?ir?
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Mechanism of electrical conduction in Materials the
conduction of electricity through materials can be
accomplished by three means 9
(he flow of electrons E8. #n Metal
(he flow of ions E8. -alt water .
Polari&ation in which ions or electrons move only a
short distance under the influence of an electric fieldand then stop.
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4 Metals 9
Conduction by the flow of electrons depends upon theavailability of free electrons. #f there is a large number of
free electrons available3 then the material is called a
metal3 the number of free electrons in a metal is roughly
e2ual to the number of atoms.
(he number of conduction electrons is proportional to a
factor
/ EJ0T E K 1J/ T K /
B 9 ,ielectric constant9 Aolt&man@s constant
(9 bsolute (emperature.
E ctivation Energy.
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Metals may be considered a special class of electron semi
conductor for which E approaches &ero.
mong earth materials native gold and copper are true
metals. Most sulfide ore minerals are electron semi
conductors with such a low activation energy.
b0 (he flow of ions3 is best e8emplified by conduction
through water3 especially water with appreciable salinity.
-o that there is an abundance of free ions.
Most earth materials conduct electricity by the motion of
ions contained in the water within the pore spaces .
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Tere 7re tree e6?e@ti./s 9
(he sulfide ores which are electron semi conductors.
Completely fro&en rock or completely dry rock.
Rock with negligible pore spaces Massive lgneous
rooks like gabbro . #t also include all rocks at depths
greater than a few kilometers3 where pore spaceshave been closed by high pressure3 thus studies
involving conductivity of the deep crust and mantle
re2uire other mechanisms than ion flow through
connate water.
c0 Polari&ation of ions or sometimes electrons under the
influence of an electrical field3 they move a short
distance then stop. E8. Polari&ation of the dielectric in
a condenser polari&ation electrical moment 7 unit