(1) Gph 321- Principles of Electrical and Em (1)

8/20/2019 (1) Gph 321- Principles of Electrical and Em (1)

1/21


2/21

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


3/21

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


4/21

C./t

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


5/21

HOME*OR0 A$$IGNMENT$ IN PAGE :

12313141212324GRADING :

Midter5 e675 2 8 L79 2" 8

H.5e'.r Assi/5e/ts 1 8

%i/7; e675 4" 8

TEXT : R.9i/s./ & C.r7si? E6@;.r7ti./ Ge.@!si?s ./ *i;e! & $./sL.'rie, * ( 1# %


6/21

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.


7/21

#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


8/21

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.


9/21

*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.


10/21

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.


11/21

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.


12/21

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.


13/21

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.


14/21

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.


15/21


16/21

+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.


17/21

E;e?tri? ?ir?


18/21

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.


19/21

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.


20/21

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 .


21/21

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

Documents

(1) Gph 321- Principles of Electrical and Em (1)