Electrical Resisitivity and EM Methods

7/25/2019 Electrical Resisitivity and EM Methods

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ELECTRICAL AND ELECTROMAGNETICEXPLORATION

PRINCIPLES OF ELECTRICAL & EM Electrical properties of rocks

Mechanism of electrical conduction in materials Representative resistivity values Conductivity mechanism

FUNDAMENTALS OF CURRENT FLOW 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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D.C. RESISTIVITY METHOD Electrode configurations Electric sounding ! Electric profiling

field procedures pplications ! mbiguities "ualitative ! "uantitative #nterpretation Mise $ % la% Masse Method

ELECTROCHEMICAL METHODS self%potential method induced polari&ation method .

Cont.


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ELECTRICAL RESISTIVITY TECHNIQUES 45

'eophysical resistivity techni(ues are based on the responseof the earth to the flow of electrical current. #n these methods)an electrical current is passed through the ground and twopotential electrodes allow us to record the resultant potentialdifference between them) giving us a way to measure theelectrical impedance of the subsurface material. *he apparentresistivity is then a function of the measured impedance +ratioof potential to current, and the geometry of the electrode

array. -epending upon the survey geometry) the apparentresistivity data are plotted as %- soundings) %- profiles) orin /%- cross%sections in order to look for anomalous regions.


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#n the shallow subsurface) the presence of water controlsmuch of the conductivity variation. Measurement of resistivity

+inverse of conductivity, is) in general) a measure of watersaturation and connectivity of pore space. *his is becausewater has a low resistivity and electric current will follow thepath of least resistance. Inc!"#$n% #"t"t$on' $nc!"#$n%

#"($n$t) o* t+! n,!%on, -"t!' $nc!"#$n% oo#$t) o*oc/ 0-"t!1*$((!, 2o$,#3 "n, $nc!"#$n% n! o**"ct!# 0-"t!1*$((!,3 "(( t!n, to decrease !"#!,!#$#t$2$t). Inc!"#$n% co"ct$on o* #o$(# o oc/ n$t#-$(( !6!( -"t! "n, !**!ct$2!() $nc!"#! !#$#t$2$t). ir)with naturally high resistivity) results in the opposite responsecompared to water when filling voids. 0hereas the presenceof water will reduce resistivity) t+! !#!nc! o* "$ $n 2o$,##+o(, $nc!"#! ##*"c! !#$#t$2$t).


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Resistivity measurements are associated with varyingdepths depending on the separation of the current andpotential electrodes in the survey) and can be interpretedin terms of a lithologic and1or geohydrologic model of thesubsurface. -ata are termed apparent resistivitybecause the resistivity values measured are actually

averages over the total current path length but areplotted at one depth point for each potential electrodepair. *wo dimensional images of the subsurfaceapparent resistivity variation are called pseudosections.

-ata plotted in cross%section is a simplisticrepresentation of actual) comple2 current flow paths.Computer modeling can help interpret geoelectric data interms of more accurate earth models.


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'eophysical methods are divided into two types 3

ctive and Passive

P"##$2! !t+o,# +4atural 5ources,3 #ncorporatemeasurements of natural occurring fields orproperties of the earth. E2. 5P) Magnetotelluric +M*,)*elluric) 'ravity) Magnetic.

Act$2! M!t+o,# +#nduced 5ources, 3 signal isin6ected into the earth and then measure how the

earth respond to the signal. E2. -C. Resistivity)5eismic Refraction) #P) EM) Mise%%7%Masse) 'PR.


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DC Resistivity % *his is an active method that employsmeasurements of electrical potential associated withsubsurface electrical current flow generated by a -C) orslowly varying C) source. Factors that affect the measuredpotential) and thus can be mapped using this method includethe presence and (uality of pore fluids and clays. 8urdiscussions will focus solely on this method.

Induced Polarization (IP)% *his is an active method that is

commonly done in con6unction with -C Resistivity. #temploys measurements of the transient +short%term,variations in potential as the current is initially applied orremoved from the ground. #t has been observed that when a

current is applied to the ground) the ground behaves muchlike a capicitor) storing some of the applied current as acharge that is dissipated upon removal of the current. #n thisprocess) both capacity and electrochemical effects areresponsible. #P is commonly used to detect concentrations of

clay and electrically conductive metallic mineral grains.


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Self Potential (SP)%*his is a passive method that employsmeasurements of naturally occurring electrical potentialscommonly associated with the weathering of sulfide ore

bodies. Measurable electrical potentials have also beenobserved in association with ground%water flow and certainbiologic processes. *he only e(uipment needed forconducting an 5P survey is a high%impedance voltmeter and

some means of making good electrical contact to theground. Electromagnetic (EM) % *his is an active method that

employs measurements of a time%varying magnetic fieldgenerated by induction through current flow within the earth.

#n this techni(ue) a time%varying magnetic field is generatedat the surface of the earth that produces a time%varyingelectrical current in the earth through induction. receiver isdeployed 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 deposits) forlocating buried pipes and cables) for the detection of

une2ploded ordinance) and for near%surface geophysicalmapping.

. Magnetotelluric (MT) % *his is a passive method that

employs measurements of naturally occurring electricalcurrents) telluric currents) generated by magneticinduction of electrical currents in the ionosphere. *hismethod can be used to determine electrical properties ofmaterials at relatively great depths +down to and including

the mantle, inside the Earth. #n this techni(ue) a timevariation in electrical potential is measured at a basestation and at survey stations. -ifferences in the recordedsignal are used to estimate subsurface distribution of

electrical resistivity.


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Po#$t$on o* E(!ct$c"( M!t+o,# $n3+, Petroleum E2ploration.*he most prominent applications of electrical techni(ues in

petroleum e2pl. re in well logging. Resistivity and 5P arestandard 7ogging techni(ues.*he magnetotelluric method has found important applicationfor pet. E2ploration. #n structurally comple2 region +E9.

Rocky Mountains,.+/, Engineering ! 'roundwater.- C. Resistivity and EM have found broad use in civilEngineering and groundwater studies. 5aturated 1:nsaturated) 5altwater 1 freshwater

+;, Mineral E2ploration.Electrical methods interpretation difficult below


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O+7# L"-8hm>s 7aw describes the electrical properties of any

medium. O+7# L"-' V 8 IR) relates the voltage of acircuit to the product of the current and the resistance.*his relationship holds for earth materials as well assimple circuits. R!#$#t"nc!0 R3) however) is not amaterial constant. #nstead) resistivity is an intrinsicproperty of the medium describing the resistance ofthe medium to the flow of electric current.R!#$#t$2$t)9is defined as a unit change in resistancescaled by the ratio of a unit cross%sectional area and a

unit length of the material through which the current ispassing +Figure ,. R!#$#t$2$t)is measured in ohm%mor ohm%ft) and is the reciprocal of the conductivity ofthe material. *able displays some typical resistivities.


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4ote that) in *able ) the resistivity ranges of different earthmaterials overlap. *hus) resistivity measurements cannot be

directly related to the type of soil or rock in the subsurfacewithout direct sampling or some other geophysical orgeotechnical information. Porosity is the ma6or controlling factorfor changing resistivity because electricity flows in the nearsurface by the passage of ions through pore space in the

subsurface materials. *he porosity +amount of pore space,) thepermeability +connectivity of pores,) the water +or other fluid,content of the pores) and the presence of salts all becomecontributing factors to changing resistivity. ?ecause most

minerals are insulators and rock composition tends to increaseresistivity) it is easier to measure conductive anomalies thanresistive ones in the subsurface. However) air) with a theoreticalinfinite resistivity) will produce large resistive anomalies whenfilling subsurface voids.


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E(!ct$c c$c$t +"# t+!! "$n o!t$!#3

R!#$#t"nc! 0R3:resistance to movement of charge

C""c$t"nc! 0C3:ability to store charge

In,ct"nc! 0L3: ability to generate current fromchanging magnetic field arising from movingcharges in circuit

R!#$#t"nc! $# NOT " *n,"!nt"( c+""ct!$#t$c o*t+! !t"( $n t+! -$!.


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Mechanism of electrical conduction in Materials theconduction of electricity through materials can beaccomplished by three means 3

*he flow of electrons E2. #n Metal

*he flow of ions E2. 5alt 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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Metals 3

Conduction by the flow of electrons depends upon theavailability of free electrons. #f there is a large number offree electrons available) then the material is called ametal) the number of free electrons in a metal is roughly

e(ual to the number of atoms.*he number of conduction electrons is proportional to afactor

n ; < E=>T E ? @=n T ? n

@ 3 -ielectric constantA3 ?olt&man>s constant*3 bsolute *emperature.E ctivation Energy.


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Metals may be considered a special class of electron semiconductor for which E approaches &ero.

mong earth materials native gold and copper are truemetals. Most sulfide ore minerals are electron semiconductors with such a low activation energy.

b, *he flow of ions) is best e2emplified by conductionthrough water) especially water with appreciable salinity.5o that there is an abundance of free ions.

Most earth materials conduct electricity by the motion ofions contained in the water within the pore spaces .


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T+!! "! t+!! !6c!t$on#3

*he sulfide ores which are electron semi conductors.

Completely fro&en rock or completely dry rock.

Rock with negligible pore spaces + Massive lgneousrooks like gabbro . #t also include all rocks at depths

greater than a few kilometers) where pore spaceshave been closed by high pressure) thus studiesinvolving conductivity of the deep crust and mantlere(uire other mechanisms than ion flow throughconnate water.

c, Polari&ation of ions or sometimes electrons under the

influence of an electrical field) they move a shortdistance then stop. E2. Polari&ation of the dielectric in

a condenser polari&ation + electrical moment 1 unit


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Con,ct$2$t) !c+"n$# $n non1-"t!1!"$n% oc/#E2trinsic conductivity for low temperatures below B


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Resistivity +or conductivity,) which governs the amount of current thatpasses when a potential difference is created.Electrochemical activity or polari&ability) the response of certainminerals to electrolytes in the ground) the bases for 5P and #P.

-ielectric constant or permittivity. measure of the capacity of amaterial to store charge when an electric field is applied . #t

measure the polari&ability of a material in an electric field G 9

9 3 electrical susceptibility .Electrical methods utili&e direct current or 7ow fre(uency alternatingcurrent to investigate electrical properties of the subsurface.

Electromagnetic methods use alternating electromagnetic field of highfre(uencies.*wo properties are of primary concern in the pplication of electricalmethods.+, *he ability of Rocks to conduct an electrical current.+/, *he polari&ation which occurs when an electrical current ispassed through them +#P,.


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R!#$#t$2$t)For a uniform wire or cube) resistance is proportional

to length and inversely proportional to cross%sectionalarea. Resistivity is related to resistance but it notidentical to it. *he resistance R depends an length)

rea and properties of the material which we termresistivity +ohm.m, .Constant of proportionality is called Resistivity 3

R!#$#t$2$t) $# t+! *n,"!nt"( +)#$c"( o!t)o* t+! !t"( $n t+! -$!


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Resistivity is measured in ohm%m

Con,ct$2$t) is defined as 1I JKand is measured in 5iemens per meter +51m,)e(uivalent to ohm%m%.

E9. Copper has I . 9


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An$#oto)3 is a characteristic of stratified rocks which isgenerally more conducive in the bedding plane. *he

anisotropy might be find in a schist +micro anisotropic, or ina large scale as in layered se(uence of shale +macroanisotropic, .

.Coefficient of anisotropy It 1 Il

Il 3 7ongitudinal Resistivety .It 3 *ransverse Resistivity.

*he effective Resistivity depends on whether the current is

flowing parallel to the layering or perpendicular to it .

R@ 8 9@ +@


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*he total Resistance for the unit column + * ,

* N I h *ransverse unit resistance

*he transverse resistivity It is defined by .It *1H H is the total thickness

For current flowing hori&ontally) we have a parallelcircuit. *he reciprocal resistance is 5 1 R N hi 1 Ii7ongitudinal unit conductance7ongitudinal resistivity Il H 1 5

A %!o!(!ct$c n$t $# c+""ct!$!, ) t-oP""!t!#3, 7ayer Resistivity + Ii ,

/, 7ager *hickness+ ti ,


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Four electrical parameters can be derived for each layerfrom the respective resistivity and thickness. *here are 3

7ongitudinal conductance 57 h1I h.D

*ransverse resistance * h.I7ongitudinal resistivity Il h15*ransverse resistivity It *1h

nisotropy *ransverse resistivity It 1 7ongitudinal resistivity Il

*he sums of all 57 +N hi 1 Ii ,are called -ar Oarrouk functions.

*he sums of all * + N hi . Ii ,are called -ar Oarrouk variables.


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, Materials which lack pore spaces will show highresistivity such as

massive limestonemost igneous and metamorphic +granite)

basalt,?, Materials whose pore space lacks water will show

high resistivity such as 3 dry sand and gravel #ce .C, Materials whose connate water is clean +free

from salinity , will show high resistivity such as 3 clean sand or gravel ) even if watersaturated.

-, most other materials will show medium or low

resistivity) especially if clay is present such as 3

C("##$*$c"t$on o* M"t!$"(# "cco,$n% to R!#$#t$2$t$!# V"(!#


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*he presence of clay minerals tends to decrease theResistivity because 3

. *he clay minerals can combine with water ./. *he clay minerals can absorb cations in ane2changeable state on the surface.

;. *he clay minerals tend to ioni&e and contribute to thesupply of free ions.

F"cto# -+$c+ conto( t+! R!#$#t$2$t). 'eologic ge/. 5alinity.;. Free%ion content of the connate water.G. #nterconnection of the pore spaces

+Permeability,.=. *emperature.B. Porosity.

. Pressure


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I< bul roc resistivity

Iw pore%water resistivitya = empirical constant +


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Formation Factor:

Effects of Partial 5aturation3

Sw is the volumetric saturation.

n is the saturation coefficient (1.5 < n < 2.5).

rchie>s 7aw ignores the effect of pore geometry) but is a

reasonable appro2imation in many sedimentary rocksR!#$#t$2$t) #2!) $n#t!nt#:a,High tension battery pack +source of current,.b,Four metal stakes.

c,Milliammeter.d,oltmeter.e,Four reels of insulated cable.

C is preferred over -C as source of current. *he advantage

of using C is that unwanted potential can be avoided.


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F$!(, con#$,!"t$on# *o DC R!#$#t$2$t)

. 'ood electrode contact with the earth

% 0et electrode location.

% dd 4acl solution or bentonite

/. 5urveys should be conducted along a straight

line whenever possible .

;. *ry to stay away from cultural features whenever

possible . Power lines

Pipes 'round metal fences

Pumps


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Soc!# o* No$#!*here are a number of sources of noise that can effect our

measurements of voltage and current.

@1 E(!cto,! o("$"t$on. metallic electrode like a copper or steel rod in contact with

an electrolyte groundwater other than a saturated solution ofone of its own salt will generate a measurable contact

potential. For -C Resistivity) use nonpolari&ing electrodes.Copper and copper sulfate solutions are commonly used.1 T!(($c c!nt#.

4aturally e2isting current flow within the earth. ?yperiodically reversing the current from the current electrodes

or by employing a slowly varying C current) the affects oftelluric can be cancelled.

1 P!#!nc! o* n!") con,cto#. 0P$!#' *!nc!#3 ct as electrical shorts in the system and current will flow

along these structures rather than flowing through the earth.


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41 Lo- !#$#t$2$t) "t t+! n!" #*"c!.

#f the near surface has a low resistivity) it is difficult to getcurrent to flow more deeply within the earth.

51 N!"1 !(!cto,! G!o(o%) "n, Too%"+)Rugged topography will act to concentrate current flow invalleys and disperse current flow on hills.

1 E(!ct$c"( An$#oto).-ifferent resistivity if measured parallel to the bedding

plane compared to perpendicular to it .1 In#t!nt"( No$#! .

1 C(t"( F!"t! .


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C!nt F(o- $n Un$*o E"t+ -$t+ T-o E(!cto,!#Current in6ected by electrode at 5 and e2its by electrode at 5/3


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C!nt *(o- $# "(-")# !!n,$c(" to!$ot!nt$"( ($n!#.

0here ground is uniform) measured resistivity should

not change with electrode configuration and surfacelocation. 0here inhomogeneity present) resistivity varies with

electrode position. Computed value is called apparentresistivity I.

7ines of constant potential +e(uipotential, are nolonger spherical shells) but can be calculated frome2pression derived previously.

C!nt F(o- $n A Hoo%!n!o# E"t+


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C!nt F(o- $n A Hoo%!n!o# E"t+

@. Po$nt c!nt Soc! :#f we define a very thin shell of thickness dr we can define

the potential different dv,2 8 I 0 R 3 8 I 0 9 L = A 3 8 I 0 9 , = 3*o determine a t a point . 0e integrate the above e(.

over its distance - to to infinity 3

V 8 I 9 = DC3 current density per unit of cross sectional area 3


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. T-o c!nt !(!cto,!#*o determine the current flow in a homogeneous) isotropic

earth when we have two current electrodes. *he current

must flow from the positive +source , to the resistive+ sink ,.

*he effect of the source at C +, and the sink at C/ +%,V@ 8 $ 9 = @ J 0 1 $9 = 3

p iI 1 / S 1 T +d1/ 2 ,/ O/ U


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. T-o ot!nt$"( E(!cto,!#p i I 1 / r % iI 1 / r/p/ i I 1 / r; % iI 1 / rG

W p $p/ i I 1 / + 1r $ 1 r/ $ 1 r; 1 rG ,


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D!t+ o* C!nt P!n!t"t$onCurrent flow tends to occur close to the surface. Currentpenetration can be increased by increasing separation of

current electrodes. Proportion of current flowing beneathdepth & as a function of current electrode separation ?3


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E6"(!

#f target depth e(uals electrode separation) only ;


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ELECTRODE CONFIGURATIONS

*he value of the apparent resistivity depends on thegeometry of the electrode array used +A factor,

@1 W!nn! A"n%!!nt

4amed after wenner +YB, .*he four electrodes ) M ) 4 ) ? are e(ually spaced along astraight line. *he distance between ad6acent electrode iscalled Za[ spacing . 5o MM44? \ ? a.

"8 " V = I

*he wenner array is widely used in the western Hemisphere.*his array is sensitive to hori&ontal variations.

L!! P"t$t$on$n% A")


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1 L!!1 P"t$t$on$n% A").*his array is the same as the wenner array) e2cept that an

additional potential electrode 8 is placed at the centerof the array between the Potential electrodes M and 4.Measurements of the potential difference are madebetween 8 and M and between 8 and 4 .

*his array has been used e2tensively in the past .

"8 4 " V = I

1 Sc+(!%! A"n%!!nt.*his array is the most widely used in the electricalprospecting . Four electrodes are placed along a straight

line in the same order M4? ) but with ? ] = M4

=MN

MNAB

I

Va

22

22

*his array is less sensitive to lateralvariations and faster to use as only

the current electrodes are moved.


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41 D$o(! D$o(! A") .*he use of the dipole%dipole arrays has become common

since the Y=s ) Particularly in Russia. #n a dipole%dipole)

the distance between the current electrode and ?+current dipole, and the distance between the potentialelectrodes M and 4 +measuring dipole, are significantlysmaller than the distance r ) between the centers of thetwo dipoles.

9" 8 0 = " 3 2=$8r . if the separations a and b are e(ual and the distancebetween the centers is 0nJ@3 " then

9" 8 n 0nJ@3 0nJ3 . ". 2=$


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*his array is used for deep penetration ^ km.

Fo "#$c ,$o(! ,$o(! "")#


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Fo "#$c ,$o(!1 ,$o(! "")# .@. A$t+"(. R",$"(. P""((!(4. P!!n,$c("0hen the a&imuth angle 0 3formed by the line r and the

current dipole A 8 =) *he &imuthal array and parallelarray reduce to the e(uatorial rray.

0hen 8 O) the parallel and radial arrays reduce to thepolar or a2ial array .

#f M4 only is small is small with respect to R in the e(uatorialarray) the system is called ?ipole%-ipole +Ais the bipole andMNis the dipole ,) where Ais large and M4 is small.

#f ? and M4 are both small with respect to R ) the system is

dipole% dipole


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51 Po(!1D$o(! A") .

*he second current electrode is assumed to be a greatdistance from the measurement location + infiniteelectrode,

9" 8 " n 0nJ@3 2=$


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1 Po(!1Po(! .

#f one of the potential electrodes ) 4 is also at a greatdistance.

"8 " V = I


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REFRACTION OF ELECTRICAL RESISTIVITY

A. D$#tot$on o* C!nt *(o-

t the boundary between two media of different resistivitiesthe potential remains continuous and the current lines arerefracted according to the law of tangents.

@ t"n 8 t"n @

I* 9 @ ) *he current lines will be refracted away fromthe 4ormal. *he line of flow are moved downward becausethe lower resistivity below the interface results in an easier

path for the current within the deeper &one.


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. D$#tot$on o* Pot!nt$"(Consider a source of current # at the point 5 in the first

layers P of 5emi infinite e2tent. *he potential at any pointP would be that from 5 plus the amount reflected by thelayer P/ as if the reflected amount were coming from theimage 51

+P, i I 1 / T + 1 r, + A 1 r/ , U

A Reflection coefficient I/ $ I 1 I/ I


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#n the case where P lies in the second medium I/) *hentransmitting light coming from 5. 5ince only $ A istransmitted through the boundary.

*he Potential in the second medium is

V0P3 8 I 9= 0@ = @3 0> = @3

Continuity of the potential re(uires that the boundary where

r r/ ) +p, must be e(ual to / + P,.t the interface r r/ ) /

M!t+o, o* I"%!#


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M!t+o, o* I"%!#Potential at point close to a boundary can be found using_Method of #mages_ from optics.

In ot$c#:*wo media separated by semi transparent mirror of reflectionand transmission coefficients k and %k) with light source inmedium . #ntensity at a point in medium is due to source andits reflection) considered as image source in second medium)i.e source scaled by reflection coefficient k. #ntensity at point inmedium / is due only to source scaled by transmission

coefficient %k as light passed through boundary.

E(!ct$c"( R!*(!ct$on Co!**$c$!nt


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E(!ct$c"( R!*(!ct$on Co!**$c$!nt

Consider point current source and find e2pression for currentpotentials in medium and medium /3 :se potential from

point source) but G as shell is spherical3Potential at point P in medium 3

Potential at point " in medium/3

t point on boundary mid%waybetween source and its image3rr/r;r say. 5etting p ()

and canceling we get3

k is electrical reflection coefficient and used in interpretation


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T+! 2"(! o* t+! ,$$n% *"cto ' > "(-")# ($!#!t-!!n @

I* t+! #!con, (")! $# " ! $n#("to

0 9 8 3 t+!n > 8 J @I* t+! #!con, (")! $# " !*!ct con,cto

0 9 8 O 3 t+!n > 8 1 @W+!n 9@ 8 9 t+!n No !(!ct$c"( on,")

E6$#t# "n, > 8 O


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*wo categories of field techni(ues e2ist for conventional resistivityanalysis of the subsurface. *hese techni(ues are vertical electric

sounding +E5,) and Hori&ontal Electrical Profiling +HEP,.

V!t$c"( E(!ct$c"( Son,$n% 0VES3 .*he ob6ect of E5 is to deduce the variation of resistivity with

depth below a given point on the ground surface and tocorrelate it with the available geological information in order toinfer the depths and resistivities of the layers present.

In VES' -$t+ -!nn! con*$%"t$on) the array spacing Za[ isincreased by steps) keeping the midpoint fi2ed +a / ) B) L)

=G``., .In VES' -$t+ #c+(!%!) *he potential electrodes are moved

only occasionally) and current electrode are systematicallymoved outwards in steps

A 5 MN.


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1 Ho$ont"( E(!ct$c"( o*$($n% 0HEP3 .

*he ob6ect of HEP is to detect lateral variations in theresistivity of the ground) such as lithological changes)near% surface faults`` .

In t+! -!nn! oc!,!c o* HEP) the four electrodeswith a definite array spacing Za[ is moved as a whole insuitable steps) say


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M(t$(! Ho$ont"( Int!*"c!#For *hree layers resistivities in two interface case ) four

possible curve types e2ist.

Q t)! 9@ 9 9H T)! 9@ 9 9> T)! 9@ 9 9A T)! 9@ 9 9

In *o1 L")! %!o!(!ct$c #!ct$on#' T+!! "! o##$(!


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) %!("t$on# :9@ 9 9 94 HA T)!9@ 9 9 94 H> T)!

9@ 9 9 94 AA T)!9@ 9 9 94 A> T)!9@ 9 9 94 >H T)!9@ 9 9 94 >Q T)!

9@ 9 9 94 QH T)!9@ 9 9 94 QQ T)!

Q"nt$t"t$2! VES Int!!t"t$on: M"#t! C2!#


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Q"nt$t"t$2! VES Int!!t"t$on: M"#t! C2!#

7ayer resistivity values can be estimated by matching to a set of

master curves calculated assuming a layered Earth) in whichlayer thickness increases with depth. +seems to work well,. Fortwo layers) master curves can be represented on a single plot.

Master curves3 log%log plot with Ia 1 Ion vertical a2is and a1 h on hori&ontal +h isdepth to interface,


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Plot smoothed field data on log%log graph transparency. 8verlay transparency on master curves keeping a2es

parallel. 4ote electrode spacing on transparency at which +a 1 h,

to get interface depth. 4ote electrode spacing on transparency at which +Ia 1 I, to get resistivity of layer .

Read off value of k to calculate resistivity of layer / from3


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Q"nt$t"t$2! VES Int!!t"t$on: In2!#$on

Curve matching is also used for three layer models) butbook of many more curves.

Recently) computer%based methods have becomecommon3

forward modeling with layer thicknesses and

resistivities provided by user inversion methods where model parameters

iteratively estimated from data sub6ect to usersupplied constraints

E2ample +?arker) YY/,5tart with model of as many layers as data points and

resistivity e(ual to measured apparent resistivityvalue.


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Calculatedcurve doesnot matchdata) butcan beperturbed toimprove fit.

A($c"t$on# o* R!#$#t$2$t) T!c+n$!#


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)

@. !,oc/ D!t+ D!t!$n"t$on?oth E5 and C5* are useful in determining bedrock depth.?edrock usually more resistive than overburden. HEP profilingwith 0enner array at < m spacing and < m station intervalused to map bedrock highs.

. Loc"t$on o* P!"*o#tPermafrost represents significant difficulty to constructionpro6ects due to e2cavation problems and thawing afterconstruction.#ce has high resistivity of %/< ohm%m

. L"n,*$(( M"$n%Resistivity increasingly used to investigate landfills37eachates often conductive due to dissolved salts

7andfills can be resistive or conductive) depends on contents

L$$t"t$on# o* R!#$#t$2$t) Int!!t"t$on


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L$$t"t$on# o* R!#$#t$2$t) Int!!t"t$on

@1 P$nc$(! o* E$2"(!nc!.

#f we consider three%lager curves of A +I I/ I; , or "type +I I/ I;, we find the possible range of values forthe product */ I/ h/ *urns out to be much smaller. *his iscalled *%e(uivalence. H thickness) * 3 *ransverseresistance it implies that we can determine */ more reliably

than I/ and h/ separately. #f we can estimate either I/ orh/ independently we can narrow the ambiguity.E(uivalence3 several models produce the same results.

mbiguity in physics of - interpretation such that different

layered models basically yield the same response. -ifferent 5cenarios3 Conductive layers betweentwo resistors) where lateral conductance +Dh, is the same.Resistive layer between two conductors with sametransverse resistance +Ih,.


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1 P$nc$(! o* S!##$on.

*his states that a thin layer may sometimes not bedetectable on the field graph within the errors of fieldmeasurements. *he thin layer will then be averaged intoon overlying or underlying layer in the interpretation. *hin

layers of small resistivity contrast with respect tobackground will be missed. *hin layers of greaterresistivity contrast will be detectable) but e(uivalencelimits resolution of boundary depths) etc.

*he detectibility of a layer of given resistivity depends on itsrelative thickness which is defined as the ratio of*hickness1-epth.

Co"$#on o* W!nn! "n, Sc+(!%!


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Co"$#on o* W!nn! "n, Sc+(!%!

+, #n 5ch. M4 1= ?

0enner M4 1; ?+/, #n 5ch. 5ounding) M4 are moved only occasionally.#n 0enner 5oundings) M4 and ? are moved after eachmeasurement.

+;, *he manpower and time re(uired for making 5chlumbergersoundings are less than that re(uired for 0enner soundings.+G, 5tray currents that are measured with long spreads effectmeasurements with 0enner more easily than 5ch.+=, *he effect of lateral variations in resistivity are recogni&edand corrected more easily on 5chlumberger than 0enner.+B, 5ch. 5ounding is discontinuous resulting from enlarging

M4.

D$#",2"nt"%!# o* W!nn! A")


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D$#",2"nt"%!# o* W!nn! A")

. #nterpretations are limited to simple) hori&ontally layeredstructures

/. For large current electrodes spacing) very sensitive voltmetersare re(uired.

A,2"nt"%!# o* R!#$#t$2$t) M!t+o,#

. Fle2ible/. Relatively rapid. Field time increases with depth;. Minimal field e2penses other than personnel

G. E(uipment is light and portable=. "ualitative interpretation is straightforwardB. Respond to different material properties than do seismic andother methods) specifically to the water content and water salinity

D$#",2"nt"%!# o* R!#$#t$2$t) M!t+o,#


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. #nterpretations are ambiguous) conse(uently) independentgeophysical and geological controls are necessary to

discriminate between valid alternative interpretation of theresistivity data + Principles of 5uppression ! E(uivalence,

/. #nterpretation is limited to simple structural configurations.;. *opography and the effects of near surface resistivity

variations can mask the effects of deeper variations.G. *he depth of penetration of the method is limited by the

ma2imum electrical power that can be introduced into theground and by the practical difficulties of laying out long

length of cable. *he practical depth limit of most surveysis about Am.

=. ccuracy of depth determination is substantially lowerthan with seismic methods or with drilling.

L t ( $ + $t$ $ t+ , ** t $ t$ $t


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L"t!"( $n+oo%!n!$t$!# $n t+! %on, "**!ct !#$#t$2$t)!"#!!nt# $n ,$**!!nt -")#: T+! !**!ct ,!!n,# on

. *he si&e of inhomogeneity with respect to its depth

/. *he si&e of inhomogeneities with respect to the si&e of

electrode array

;. *he resistivity contrast between the inhomogeneity and

the surrounding media

G. *he type of electrode array used

=. *he geometric form of the inhomogeneityB. *he orientation of the electrode array with respect to the

strike of the inhomogeneity

M$#!1A1LA1M"##! M!t+o,


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*his is a charged%body potential method is a development of

HEP techni(ue but involves placing one current electrodewithin a conducting body and the other current electrode at asemi% infinite distance away on the surface .

*his method is useful in checking whether a particularconductive mineral% show forms an isolated mass or is part of a

larger electrically connected ore body.


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T+!! "! t-o "o"c+!# $n $nt!!t"t$on

.8ne uses the potential only and uses thema2imum values a being indicative of the conductivebody.

/.*he other converts the potential data toapparent resistivity and thus a high surface voltagemanifests itself in a high apparent resistivity

9" 8 4 X V=I :

0here 9 is the distance between C and P.

SELF1 POTENTIAL 0SP3


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SELF POTENTIAL 0SP3

5P is called also spontaneous polari&ation and is a

naturally occurring potential difference between points in theground. 5P depends on small potentials or voltages beingnaturally produced by some massive ores.

#t associate with sulphide and some other types of ores. #tworks strongly on pyrite) pyrrohotite) chalcopyrite) graphite.

5P is the cheapest of geophysical methods.

Con,$t$on# *o SP "no"($!#

% 5hallow ore body/% Continuous e2tension from a &one of o2idi&ing conditions to

one of reducing conditions) such as above and below watertable.


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either or bothpos1neg ions

4ote that it is not necessary that an individual ion travel theentire path. Charges can be e2changed.

*he implications of this for potential distribution would be


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*he implications of this for potential distribution would be

0hen we come to consider more specifically the


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T+"t $# -! #t +"2!

reverse process

neutral ion

positive ionnegative ion

neutral ion

mechanism) we see that it must be consistent with electron flow in the ore body ion flow in surrounding rock

no transfer of ions across ore boundary) althoughelectrons are free to cross

0hen we consider the possible ion species) the criteria


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would be common enough reversible couple under normal ground conditions mobile enough5ato and Mooney proposed ferric1ferrous couples to satisfy

these criteria.

made continuous by reactionsinvolving ferrous and ferric hydro2idewith presence of HF

made continuous by O2 H2O2 reactionwith O2 supplied from atmosphere


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Poo#!, !(!ctoc+!$c"( !c+"n$#


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*o #!(*1ot!nt$"(#

T+$# oo#!, !c+"n$# +"2! t-o %!o(o%$c $($c"t$on#:

.*he ore body must be an electronic conductor with high

conductivity.*his would seem to eliminate sphalerite +&inc sulfide, whichhas low conductivity.

/.*he ore body must be electrically continuous between a

region of o2idi&ing conditions and a region of reducingconditions. 0hile water table contact would not be the only

possibility have) it would seem to be a favorable one.

In#t!nt"t$on "n, F$!(, Poc!,!


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5ince we wish to detect currents) a natural approach is tomeasure current. However) the process of measurement

alters the current. *herefore) we arrive at it thoughmeasuring potentials.

P$nc$(!' "n, occ"#$on"( "ct$c!:

More usual practice

#nstruments

E(uipment3


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% ot!nt$o!t! o +$%+ $!,"nc! 2o(t!t!

1 non1o("$$n% !(!cto,!#1 -$! "n, !!(

Non1o("$$n% !(!cto,!# -!! ,!#c$!, $n

conn!ct$on -$t+ !#$#t$2$t) !6(o"t$on "(t+o%+t+!) "! not #"(() !$!, t+!!. H!!' t+!) "!!##!nt$"(. T+! #! o* #$(! !t"( !(!cto,!# -o(,%!n!"t! +%! cont"ct o coo#$on ot!nt$"(#

-+$c+ -o(, "#/ t+! ,!#$!, !**!ct. non1o("$$n% !(!cto,!# con#$#t o* " !t"( $n cont"ct-$t+ " #"t"t!, #o(t$on o* " #"(t o* t+! !t"( .Cont"ct -$t+ t+! !"t+ c"n ! ",! t+o%+ "

oo# c!"$c ot.


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T+! $n#t!nt -+$c+ !"#!# ot!nt$"( ,$**!!nc!!t-!!n t+! !(!cto,!# #t +"2! t+! *o((o-$n%

c+""ct!$#t$c#:c""(! o* !"#$n% J.@ $(($2o(t'c""(! o* !"#$n% to @ $(($2o(t# 0@ 2o(t3$nt $!,"nc! %!"t! t+"n @ !%"o+#'

!*!"() o!.T+! +$%+ $nt $!,"nc! $# !$!, $n o,! to "2o$,

,"-$n% c!nt t+o%+ t+! !(!cto,!#' -+o#!!#$#t"nc! $# #"(() (!## t+"n @ /$(o+#. In 2!),) con,$t$on# 0,) oc/' $c!' #no-' *o!n #o$(3' t+!

!(!cto,! !#$#t"nc! ") !6c!!, @ /$(o+#' $n-+$c+ c"#! t+! $n#t!nt $nt $!,"nc! #+o(,"(#o ! $nc!"#!,.

SP "! o,c!, ) " n! o* !c+"n$## :


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. Mineral potential +ores that conduct electronically , suchas most sulphide ores )4ot sphalerite +&inc sulphide,

magnetite) graphite. Potential anomaly over sulfide orgraphite body is negative *he ore body being a goodconductor. Curries current from o2idi&ing electrolytesabove water $ table to reducing one below it .

.. D$**#$on ot!nt$"(

0here#a ) #c Mobilities of the anions +ve, and cations+ %ve ,

R universal 'as constant + L.;GA% mol% ,* 3 absolute temperature + A,4 3 is ionic valenceF3 Farady>s constant YBGL C mol% ,

C ) C/ 5olution concentrations .

RT( Ia Ic)n F(Ia+Ic)

Ln 0C@ = C 3E,

. N!n#t Pot!nt$"(EN 0 RT = F 3 L 0 C@ = C 3


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EN 8 1 0 RT = nF 3 Ln 0 C@ = C 30here #a #c in the diffusion potential E(uation .

G. St!"$n% ot!nt$"(#due to subsurface water flow are thesource of many 5P anomalies. *he potential E per unit ofpressure drop P +*he streaming potentials couplingcocfficent, is given by 3

E> 8

9 Electrical Resistivity of the pore Fluld.E/ Electro%kinetic potential as a result from an electrolyte

flowing through aporous media.

< -ielectric constant of the pole fluid. iscosity of the pore fluidP pressure difference

CE electro filteration coupling coefficient.

CE P 4


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Int!!t"t$on

U#"(()' $nt!!t"t$on con#$#t# o* (oo/$n% *o"no"($!#.

T+! o,! o* "%n$t,! o* "no"($!# $#1 2 no"( 2"$"t$on15 2 o##$() o* $nt!!#t' !#!c$"(() $*

o#!2!, o2! " *"$() ("%! "!"o2! 5 2 ,!*$n$t! "no"()41@ 2 2!) ("%! "no"($!#

A($c"t$on#'roundwater applications rely principally upon potential


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pp y p p y p pdifferences produced by pressure gradients in thegroundwater. pplications have included detection of leaks in

dams and reservoirs location of faults) voids) and rubble&ones which affect groundwater flow delineation of water flowpatterns around landslides) wells) drainage structures) andsprings) studies of regional groundwater flow

8ther groundwater applications rely upon potentialdifferences produced by gradients in chemical concentration)pplications have included outline ha&ardous wastecontaminant plumes

*hermal applications rely upon potential differences

produced by temperature gradients.pplications have included geothermal prospecting map

burn &ones for coal mine fires monitor high%temperatureareas of in%situ coal gasification processes and oil field steam

and fire floods.

In,c!, Po("$"t$on 0 IP3


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IP depends on a small amount of electric charge being

stored in an ore when a current is passed *hrough it ) tobe released and measured when the current is switchedoff .

*he main application is in the search for ,$##!$n"t!,!t"(($c o!#and to a lesser e2tent) ground water andgeothermal e2ploration .

Measurements of #P using / current electrodes and / non%

polari&able potential electrodes. 0hen the current isswitched off ) the voltage between the potential electrodestakes a finite to decay to &ero because the groundtemporarily stores charge 0 !co! Po("$!,,

Fo #)#t!# o* IP .


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*ime domainFre(uency domain < HOPhase domain5pectral #P


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@ 3 No"( IP Membrane Polari&ation Most Pronounced with clays -ecreases with very high +


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'rain +electrode, polari&ation. +, :nrestricted electrolytic flowin an open channel.

+?, Polari&ation of an electronically conductive grain) blocking achannel

@. T$! ,o"$n !"#!!nt#.


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8ne measure of the #P effects is the ratio p 1 o which isknown chargeability which e2pressed in terms of millivolts

per volt or percent.p 3 overvoltageo 3 observed voltage

M8 V = Vo 0 2 =2 o Z3

pparent chargeability

Ma + 1 < , p +t, dt 1

Electrical Resisitivity and EM Methods

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Transcript of Electrical Resisitivity and EM Methods