A Comparison of Resistivity and Electromagnetics as Geophysical

Techniques

Felix Onovughe Oghenekohwo (felix@aims.ac.za)African Institute for Mathematical Sciences (AIMS)

Supervised by: Prof. George Smith

University of Cape Town, South Africa

22 May 2008

Submitted in partial fulfilment of a postgraduate diploma at AIMS

Abstract

A very precise understanding of what the resistivity and electromagnetic methods used in geophysicalexploration, is what this essay sets out to explain. More important is the fact that both methods maybe used to complement each other for a better understanding of the subsurface of the Earth in termsof structures, having a form of electrical property, which may be present.

We start by taking a quick look at what the concept of geophysics and the techniques which it makesuse of entails. A clear distinction is made in the major properties explored by each geophysical method.A more focussed analysis of the electrical methods used in exploration is then discussed, paving way forthe main discourse, which is the resistivity and electromagnetic methods.

In order to appreciate the correlation to Physics, a brief introduction of some basic concepts in Physicson which both methods (resistivity and electromagnetics) depend is discussed and the various methodsand systems then follow. Emphasis was laid on the possible modes of the resistivity survey as well assome of the systems used in electromagnetics.

Since no method is entirely perfect in exploration geophysics, we conclude by looking at the advantagesof one method over the other, their similarities and differences, their respective applications and majorlimitations faced during a survey using both methods.

Declaration

I, the undersigned, hereby declare that the work contained in this essay is my original work, and thatany work done by others or by myself previously has been acknowledged and referenced accordingly.

Felix Onovughe Oghenekohwo, 22 May 2008

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Contents

Abstract i

1 Introduction to Geophysical Techniques and Exploration 1

1.1 Introduction to Geophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Geophysical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2.1 Seismic Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.2 Electromagnetic Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.3 Electrical Resistivity Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.4 Magnetic Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.5 Gravity Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.6 Ground-penetrating Radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.7 Self-potential Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.8 Induced Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 General Uses of Geophysical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 An Overview of the Basic Electrical and Electromagnetic Methods 6

2.1 Natural Field Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.1 Self-potential Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.2 AFMAG Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.3 Telluric/Magnetotelluric Field Method . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Artificial Field Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Equipotential-line Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.2 Resistivity Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.3 Induced Polarisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.4 Electromagnetic Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Basis for Comparing Resistivity and Electromagnetic Method 11

3.1 Basic principle of resistivity of materials and current flow . . . . . . . . . . . . . . . . . 11

3.2 Conduction of electric currents in materials . . . . . . . . . . . . . . . . . . . . . . . . 12

3.3 Potential distribution in a homogeneous Earth . . . . . . . . . . . . . . . . . . . . . . . 13

3.4 Field procedure: configuration and method . . . . . . . . . . . . . . . . . . . . . . . . . 15

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3.5 Resistivity data interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.6 Electromagnetic method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.7 Continuous-wave electromagnetic systems . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.7.1 Fixed-source systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.7.2 Moving-source systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.8 Pulse-transient electromagnetic systems . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.9 Electromagnetic data interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.10 Comparing resistivity and electromagnetic method . . . . . . . . . . . . . . . . . . . . . 23

4 Limitations and Applications of the Resistivity and Electromagnetic Method 24

4.1 Limitations of both methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.2 Applications of both methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

References 27

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1. Introduction to Geophysical Techniques andExploration

1.1 Introduction to Geophysics

Applying the principles of Physics to the study of the earth is what is known as Solid Earth Geophysics,as Geophysics is a three-fold application of the principles of Physics to the study of the Earth, Moonand Planets [Rey97]. This study will focus on its relation to the Earth’s interior. This entails theinvestigation of the interior of the earth by taking measurements at or near the Earth’s surface. Thisaspect of geophysics is in fact called “Applied geophysics”. The other aspect, which is Pure geophysics,involves the study of the whole or substantial parts of the planet. The aim of Applied geophysics iscentred on the economy, how resources can be exploited for the use of the economy.

A detailed analysis of the measurements made in Solid Earth Physics, can reveal how the physicalproperties of the Earth’s interior vary vertically and laterally. The study also investigates specific featuresbelieved to exist within the Earth’s crust. Among such features are salt domes, oil reservoirs, ore bodies,geological faults and so on. The essence of studying the existence of such features has a very significanteffect on practical problems such as mineral exploration, oil prospecting, locating underground waterreservoirs, mapping of archaeological remains, locating buried pipes etc.

Several methods are involved in taking measurements. These methods may be applied to a wide rangeof investigations from studies of the entire Earth to the exploration of a localised region of the upperEarth’s crust for engineering and other purposes. Measurements within geographically restricted areasare used to determine the distribution of physical properties at depths that reflect the local subsurfacegeology.

Although the methods are prone to certain ambiguities or uncertainties of interpretation, they provide arelatively rapid and cost effective measure of deriving really distributed information on subsurface geology.In the exploration for subsurface resources, the methods are capable of detecting and delineating localfeatures of potential interest that could not be discovered by any realistic drilling programme [KB84].

1.2 Geophysical Techniques

Geophysical techniques are the various physical methods which are used in geophysical exploration.They are also referred to as geophysical survey methods. Geophysical exploration actually involvesthe applications of the principles of geophysics to geological exploration. Geophysical methods can beclassified in various ways.

In terms of their source of energy, they can be classified as active methods or passive methods. Activemethods are those which require the input into the earth of artificially generated energy or artificialsignal and the Earth’s response to the signal is measured. Some methods involve the generation of localelectrical or electromagnetic fields that may be applied analogously to natural fields. Examples of activegeophysical methods are seismic, electrical resistivity, electromagnetic methods, ground-probing radar,and induced polarization.

The passive methods are those which make use of the naturally occurring fields, thereby measuring the

1

Section 1.2. Geophysical Techniques Page 2

Earth’s response to the signal. Also known as the natural field methods, the passive methods utilisethe gravitational, magnetic, electric and electromagnetic fields of the Earth. Generally, natural fieldmethods can provide information on Earth properties to significantly greater depths and are logisticallysimpler to carry out than artificial source methods. Examples of passive methods are gravity, magnetic,radiometric decay method, self potential methods, and telluric methods.

Another classification of geophysical methods depends on their operational procedure, hence we havethe ground, airborne or borehole methods. Most of the ground methods can also be used in theair, under water or in boreholes as well. Examples of airborne geophysical methods include Magneticmethods, Electromagnetic methods, Airborne AFMAG (Audio Frequency Magnetics) and Radioactivity.The resistivity method and the seismic method are two examples of ground geophysical methods.

Of all the geophysical surveying methods, there exists an operative physical property to which themethod is sensitive. The type of physical property to which a method responds clearly determines itsrange of applications. For instance, seismic or electrical methods are suitable for the location of a buriedwater table because saturated rock may be distinguished from dry rock by its higher seismic velocityor higher electrical conductivity. Similarly, the magnetic method is very suitable for locating buriedmagnetic ore bodies because of their high magnetic susceptibility [KB84].

The mode of operation is another factor which determines the type of method employed in a geophysicalexploration programme. For example, due to a high speed of operation, reconnaissance surveys are oftencarried out from the air which makes the electrical or seismic methods to be non-applicable since theserequire physical contact with the ground for a direct input of energy [KB84].

The operative physical property and observed parameter of some of the geophysical methods are sum-marised in Table 1.1 which has been taken from Kearey and Brooks (see [KB84]).

Table 1.1: Geophysical methods, their measured parameters and operative physical properties

Method Observed Parameter Operative Physical Property

Gravity Spatial Variations in gravitational Densityfield strength

Magnetic Spatial variations in geomagnetic Magnetic susceptibilityfield strength

Seismic Travel times of reflected Density and elasticity of airand refracted seismic waves

Electrical Earth’s resistance Electrical conductivityResistivity

Induced Polarization voltages Electrical capacitancePolarization or ground resistance

Spontaneous Electrical potentials Electrical conductivitypotential

Electromagnetic Response to electromagnetic field Electrical conductivityand inductance

Ground-penetrating Radar

Travel times of reflected radar pulses Dielectric constant

Section 1.2. Geophysical Techniques Page 3

1.2.1 Seismic Method

The seismic method measures the response of seismic (sound) waves that act as input into the earthand then refract or reflect off subsurface soil and rock boundaries. The seismic source is usually asledgehammer-blow to a metal plate on the ground, a larger weight drop, or an explosion. The earthresponse is measured by sensors called geophones, which measure ground motion. Two basic methodsof seismic exploration are used namely, refraction and reflection.

Seismic Refraction: This method measures head waves that are refracted along geologic formationsbelow the Earth’s surface. Refractions generally occur along the top of the water table and the uppermostbedrock formation. The impulse source generates a seismic wave that travels through the subsurface.When the wave-front reaches a layer of higher velocity (e.g. bedrock), a portion of the energy is refracted,or bent, and travels along the refractor as a head wave at a velocity determined by the composition ofthe bedrock. A plot of the arrival time of the first seismic wave to the series of geophones, which arealready lined on the surface, gives information about the depth and location of the geologic horizons.This information is plotted in a cross section that shows the depth to the water table and to the firstbedrock layer. This method is used in geotechnical engineering to determine overburden thickness anddepth of bedrock for design and cost estimates for road cuts, investigating pipeline routes and otherengineering projects.

Seismic Reflection: This is very similar to the refractive technique. It considers the reflected waveinstead of the refracted wave. In this case, the receiving geophone is placed closed to the source toexclude refracted waves. The reflection method measures the time required for a sound impulse totravel from the source, bounce off a geologic boundary, and return to the surface at a geophone. It alsogives information about the depths to the reflecting layers and their dips. The reflection from a geologichorizon is similar to an echo off a cliff face.

1.2.2 Electromagnetic Method

The electromagnetic method, otherwise known as the EM method measures the response of an inducedalternating current into the ground. It can further be divided into Frequency domain electromagneticmethod (FDEM), Very low frequency (VLF) method which measures perturbations in the magnetic fieldof radio waves, Time-domain EM (TDEM), Airborne EM survey, Telluric and Magneto-telluric methods.Of the above, the most common are the TDEM and FDEM methods. The main principle of the EMmethod is that a current is induced into the ground by a transmitting coil. A receiving coil is placed ashort distance away to measure the induced earth current. The size of the induced current depends onthe geologic material beneath the transmitter and receiver. The property of phase of the received e.m.fis also important in this kind of survey, particularly for the TDEM.

1.2.3 Electrical Resistivity Method

Resistivity surveys measure variations in the electrical resistivity of the ground by applying small electriccurrents across arrays of ground electrodes. Also known as resistivity imaging, it involves the passage ofa direct current into the ground via electrodes and the measurement of the potential difference betweensome sections of the subsurface. This gives a measure of the electrical impedance of the subsurfacematerial. It also helps in understanding the horizontal and vertical discontinuities in the electricalproperties of the ground. Resistivity sounding involves gradually increasing the spacing between the

Section 1.2. Geophysical Techniques Page 4

current/potential electrodes in order to increase the depth of investigation. The data collected areconverted to apparent resistivity readings that can then be modelled in order to provide informationon the thickness of individual resistivity units within the subsurface. The limitations of the resistivitytechnique include the more difficult interpretation in the presence of complex geology and the existenceof natural currents and potentials.

1.2.4 Magnetic Method

Magnetic techniques measure disturbances in the Earth’s natural magnetic field. These disturbancesare caused by ferromagnetic materials, either magnetic rocks (usually bedrock) or man made objectscontaining iron or steel. The method makes use of a moving magnetometer over an area which measuresthe Earth’s magnetic field. No contact with the ground is required, so large areas can be covered withrelative speed. The result of the survey is a magnetic map and profiles.

The method is used extensively in environmental applications to locate ferrous underground storagetanks, drums and other objects. It is also used in oil and gas exploration. It is also used in archaeologicalapplications to locate ceramics and fibre pits and in various applications where the bedrock needs tobe mapped. The primary limitation of this method is its inability to locate non-ferrous objects such asplastic or unreinforced concrete.

1.2.5 Gravity Method

Gravity techniques measure minute variations in the Earth’s gravity field. Based on these variations,subsurface density and thereby composition can be inferred. In gravity survey, the measurements, whichare made by means of a gravity meter, are influenced by factors such as elevation, terrain, instrumentaldrift, Earth tides and the centrifugal force of the Earth’s rotation. The gravity map and profile obtainedfrom the survey is used to create a model of the subsurface.

This technique finds useful applications in groundwater investigations to map bedrock, and also ingeotechnical investigations to map bedrock and detect voids. It is also used in the exploration for oiland gas, regional and detailed geological studies.

1.2.6 Ground-penetrating Radar

Ground penetrating radar (GPR) is a high resolution geophysical method for investigating undergroundstructure. It uses a system which comprises of an antenna unit, control console, display monitor andgraphic printer. The antenna unit is in direct ground contact with the remaining equipment which arevehicle-mounted. This method is very useful for its accuracy in detecting hidden voids. Clear informationis provided about the presence of subsurface voids, often at a fraction of the cost of traditional intrusivemethods. These voids if not discovered could be an hindrance to construction work and could causeextensive damages to roads. The method is also used for mapping of subsurface soil, rock interfaces,and buried archaeological structures.

Section 1.3. General Uses of Geophysical Methods Page 5

1.2.7 Self-potential Method

This method makes use of small currents which are naturally produced beneath the Earth’s surface.The Self-potential or Spontaneous-potential (SP) geophysical method measures the potential differenceproduced by the currents, between any two points on the ground surface. This method is passive,non-intrusive and does not require the application of an electrical current, unlike the resistivity imagingmethod. The method is mainly used for exploration of massive sulphide ore bodies, finding leaks incanal embankments, defining zones and plumes of contaminants, etc. More details will be presented inthe next chapter.

1.2.8 Induced Polarization

This is actually a time based survey method. It involves measurement of the magnitude of the polar-isation voltage (Vp) that results from the injection of pulsed current into the ground. The current isapplied, with the polarisation voltage being measured over a series of time intervals after each currentcut-off using non-polarising electrodes. The measured value of Vp is divided by the steady voltage to givethe apparent chargeability of the ground. This gives qualitative information on the subsurface geology.The method is primarily used in mineral exploration surveys. More details will be presented in the nextchapter.

1.3 General Uses of Geophysical Methods

Geophysical methods find a very wide range of application. They are often used in combination. i.e.for efficiency, we may use two or more methods to be able to get accurate results. Thus the search formetalliferous mineral deposits often utilises air borne magnetic and electromagnetic surveying. Otheruses of geophysical methods include

• Exploration for underground water supplies which includes electrical resistivity, seismic, gravityand ground-penetrating radar surveying.

• Exploration for fossil fuels such as oil, gas and coal which includes simultaneous seismic, gravity,magnetic and electromagnetic methods.

• Exploration for metals used in atomic energy plant.

• Routine reconnaissance of continental shelf areas which involves simultaneous seismic, gravity,magnetic and electromagnetic methods.

• Investigation of engineering and construction sites which involves the simultaneous use of electricalresistivity, seismic, gravity and magnetic surveying.

• Archaeological investigations which includes simultaneous electrical resistivity, electromagnetic,magnetic, seismic and ground-penetrating radar surveys.

2. An Overview of the Basic Electrical andElectromagnetic Methods

In geophysical exploration, the electrical and electromagnetic methods are used almost exclusively inprospecting for ore deposits and in site investigation for engineering purposes. Although the depth ofpenetration of some electrical and electromagnetic methods is usually too shallow, the telluric prospect-ing technique and the newly discovered sea bed logging technique which is an example of a CSEM(Controlled Source Electromagnetic Method), has been used to penetrate depths where oil and gas arenormally found.

Apart from oil and gas prospecting and site investigation, the electrical and electromagnetic methodsalso have other range of applications, and case histories. The case histories will not be discussed as itis beyond the scope of this work, however we shall highlight some of the applications.

The electrical and electromagnetic methods may be broadly grouped into those which involve themeasurement of the natural electrical and electromagnetic fields in the Earth and those in which wemeasure the effects of artificially applied fields. We shall classify these as Natural field methods andArtificial field methods.

2.1 Natural Field Methods

2.1.1 Self-potential Method

As implied in the previous chapter, this process involves placing of two non-polarising electrodes somedistance apart, on the ground. It has been observed that due to local electrochemical action in theground, small electrical currents are generated and these will flow between the two electrodes, creatinga potential difference. The self (spontaneous) potential method ranks as the cheapest of the surfacegeophysical methods in terms of equipment required, and among the simplest to operate in the field[Rey97].

Small potentials are produced by the flow of electrolytic fluids (streaming potentials) through porousmaterials and by two electrolytic solutions of differing concentrations being in direct contact (diffusionpotentials). Larger potentials are produced by conductive mineralised ore bodies that are partiallyimmersed below the water. The potentials are normally of the order of a few millivolts, rising up toa several hundred millivolts in the presence of metallic sulphide ore-bodies and graphite deposits. Theresult of this survey is a profile which will, if there is no interference from irregularities of the surface,show a noticeable dip over the orebody.

The self-potential method has geothermal applications, where streaming potentials may be measuredas a result of the mobility of water bodies which have differing temperatures and salinity. The methodcan also be used in locating massive sulphide ore bodies, detecting leaks in Earth dam embankmentsand also in hydrogeology for ground-water borehole testing and detecting sites of leakages associatedwith man-made and natural dams.

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Section 2.1. Natural Field Methods Page 7

2.1.2 AFMAG Method

AFMAG is an acronym for Audio-Frequency MAGnetic fields. It is classified as an electromagneticmethod. This method is well suitable for airborne use, although it could be used on ground. In betweenthe Earth’s surface and the ionosphere, thunderstorm activities causes the production of pulses of naturalelectromagnetic waves. These propagating waves are the AFMAGS, and their direction of propagationis normally in a strictly horizontal plane but in a vague direction in azimuth. The frequency range ofthese waves is between 1 and 1000Hz. In the presence of a conducting body, the waves dip down fromthe horizontal and assume a more definite direction in azimuth [Dun70].

The basic principle of this method is to measure the tilt of the plane of polarization of the audio-frequencyfields. The azimuth is first found to a reasonable accuracy or degree. The tilt is then calculated alongthe azimuth, as an angle of dip to a very high degree of accuracy. These dips in the wave propagationdirection are registered as unequal induced voltages in the detecting coils. They are then recorded on aprofile, where analysis of the profile can reveal the size and location of the conducting orebody. Beingan example of an electromagnetic method, a few details is still presented in the next chapter.

2.1.3 Telluric/Magnetotelluric Field Method

Due to the variations in the Earth’s magnetic field and the various ionospheric electrical activity takingplace in the atmosphere, magnetotelluric fields are generated which produces telluric currents thatfluctuate and oscillate. They are in fact natural Earth currents and they flow everywhere along the surfaceof the Earth in large sheets, which extend well into the Earth’s crust. The resistivity of the formationscarrying the currents will determine the distribution of the current density within the sheets. This factorhelps to locate possible salt domes. The telluric method used to be the only electromagnetic/electricalmethod which is capable of penetrating depths where oil may be located until CSEM was discoveredvery recently. A major difficulty of the telluric method was having to distinguish between the potentialsset up by the Earth currents and the polarization caused by electrochemical action at the electrodes.

The magnetotelluric method is a new application of the telluric method where the magnetic fields in-duced by the variations in Earth currents is being measured along with the variation in voltage betweenthe electrodes at the surface [Dob60]. In other words, both the electric and magnetic fields are mea-sured in this method, hence providing more information on subsurface structure [KB84]. From thesemeasurements, appropriate estimates can be made of the absolute resistivity and thickness of rocks toa very great depth. The depth z of penetration of a magnetotelluric field depends on the frequency ofthe field and the density of the subsurface bodies. It has been shown that depth penetration increaseswith a decrease in frequency as shown from the equation below.

z = k

(

ρ

f

)1

2

(2.1)

where k is a constant, f is the frequency of the field, and ρ is the density of the subsurface.

Magnetotelluric methods yield conductivity information from much greater depths than artificial sourceinduction methods. It has been applied in the search for petroleum and deep zones of mineralization inthe upper crust [Low97].

Section 2.2. Artificial Field Methods Page 8

2.2 Artificial Field Methods

2.2.1 Equipotential-line Method

This method involves the application of a voltage into the ground via two current electrodes whichare fixed during the survey. If the conductivity of the ground is uniform, the current produced by theinduced voltage will flow uniformly and follow a regular pattern similar to the lines of force around abar magnet. If however, a body having a different conductivity to its surroundings is interposed in thecurrent flow, the lines of flow are distorted, either away from the body if its conductivity is lower ortowards it if it is higher [Dun70]. Lines having equal potentials (equipotential lines) will also be distortedin the presence of a body of contrasting conductivity. If a probe (search electrodes) is placed on any ofthese equipotential lines, no current will flow between them. This property is used to locate the lines.The equipotential lines must always be perpendicular to the lines of current flow, since no componentof the current at any point can flow along the equipotential line at that point [Dob60].

The equipotential-line method helps in locating highly conductive orebodies, and it ranks as the simplestartificial field method in terms of field procedure and the equipments involved. Interpretation of thedata obtained from its survey is mainly qualitative as it only reveals the nature of the geologic bodywithout much information about its geometry. A major limitation of this method is that, although ithelps to reveal ore bodies which may be concealed by overburden, it does not give information aboutthe depth of those bodies.

2.2.2 Resistivity Method

This is one of the most widely used electrical method. The method exploits the large contrast in resistivitybetween orebodies and their host rocks, especially for minerals that occur as good conductors [Low97].Its principle is very simple, as it involves a measurement of potential difference across electrodes, aftera direct current or a low frequency alternating current has been injected into the Earth by means ofcurrent electrodes. What is actually measured is the apparent resistivity since the resistivity values areaverages over the total current path length. In most cases the analysis and interpretation also involvethe use of a computer. The resistivity of the subsurface is then a function of the magnitude of thecurrent, the recorded potential difference, and the geometry of the electrode array.

The interpretation of the resistivity data actually entails a plotting of the data as 1-D sounding orprofiling curves, or in 2-D cross-section in order to observe regions where there is an anomaly. Thespacing of the electrodes can take different methods, which in most cases involves the placement ofthe potential electrodes between the current electrodes. In all cases, irrespective of the configuration ormethod used, the same ground will always give the same physical properties.

The resistivity method is useful for simultaneously detecting lateral and vertical changes in subsurfaceelectrical properties. It is also a useful technique in environmental applications. For example, due to thegood electrical conductivity of groundwater, the resistivity of a sedimentary rock is much lower when itis waterlogged than in the dry state [Low97]. Details such as the basic principles of the method, modesof deployment, and types of configuration used, will be discussed in the next chapter.

Section 2.2. Artificial Field Methods Page 9

2.2.3 Induced Polarisation

If an electric current is introduced into the Earth via ground electrodes, it sets up a potential difference.When the current is suddenly cut off, a small current continues to flow for a very short period of time.This effect is associated with electrochemical actions taking place in the Earth, and it arises at thesurfaces of buried metallic sulphide orebodies. It should be noted that induced polarisation involves theuse of a variable low frequency A.C source, unlike in resistivity method, where a D.C source is used.

The operational procedure of the Induced Polarization (IP) method is similar to the resistivity method,as it also employs the same electrode configurations. However, the most effective ones are the double-dipole and Schlumberger electrode configurations. The measurements are fraught with certain errorsor anomalies (noise) which may be due to telluric currents, and electromagnetic coupling betweenmeasuring equipments like the wires, where current can be induced on another wire as a result of theshorter distance of separation between the two wires (Ampere’s law).

Induced polarization may be time-domain, where controlled current signals are introduced into theground through the two current electrodes, and the overvoltage between the signals is measured acrossthe two potential electrodes. It could also be frequency domain, where the alternating current fed intothe ground depends on frequency. It makes use of the principle that, when an alternating current ispassed into the ground, the apparent resistivity of rocks in which polarisation can be induced is higherwith low-frequency current than with higher-frequency current. This is because the capacitance of theground inhibits the passage of direct currents but transmits alternating currents with increasing efficiencyas the frequency rises [KB84].

The IP method is extensively used in the detection of disseminated metals. This is as a result of itsefficiency in locating low-grade ore deposits. Under certain conditions, the IP method could also be usedfor buried waste characterisation, fault and fracture delineation, ionic contaminant plume delineation,and the mapping of sand and gravel deposits.

2.2.4 Electromagnetic Method

The electromagnetic techniques have the widest range of different instrumental systems. The mainprinciple used is the response of the ground to the propagation of the electromagnetic fields which arecomposed of alternating electric intensity and magnetic force. It has been observed that a conductorpresent in the subsurface will respond to an incoming primary electromagnetic field from a transmitter,the field will set up eddy currents on the conductor , which will in turn generate a secondary electro-magnetic field. It is the resultant of this secondary field and the primary field above the subsurface, thatis detected by the receiver on the surface or on air depending on the system being used. The observeddifferences between the transmitted and received electromagnetic fields provide information about thenature and electrical properties of the conducting body. This method does not necessarily require thephysical contact of the receiver and transmitter with the ground unlike the resistivity method. Just asresistivity is measured in the resistivity method, apparent conductivity of the ground is what is measuredby EM systems.

Electromagnetic methods in common use today fall into two classes: those using a stationary sourceand those using a mobile source. They can also be classified as either time domain (TDEM) whose mea-surements are as a function of time, or frequency domain (FDEM) which uses one or more frequencies.However, for geophysical applications, frequencies of the primary alternating field are usually less than afew thousand Hertz. TDEM, which could be either airborne, ground-based or borehole-based, has similar

Section 2.2. Artificial Field Methods Page 10

applications to FDEM. The advantages of TDEM over FDEM lies in its ability to determine depths andmap much deeper features. However, TDEM surveys are not as rapid as FDEM and thus large areascan not be mapped as economically. Another mode of classification involves the property which is beingmeasured. For instance, some systems measure the dip/tilt angle, while others measure the phase anglebetween primary and secondary waves. The AFMAG method which has been discussed above makesuse of the tilt angle method; the other technique using this method is the very low frequency (VLF),although this makes use of artificially generated electromagnetic field.

A great advantage of the electromagnetic methods is that they can be successfully applied even whenconductive ground connections indispensable for the electrical methods cannot be made owing to highlyresistive (or insulating) surface formations [Par62]. The electromagnetic methods also have a wide rangeof application. More emphasis on the method and its comparison with the resistivity method, will bepresented in the next chapter

3. Basis for Comparing Resistivity andElectromagnetic Method

The electrical resistivity method involves the measurement of the apparent resistivity of soils and rock asa function of porosity, permeability, ionic content of the pore fluids, and clay mineralization. The methodis mainly used in hydrogeologic and environmental investigations. As briefly explained in the previouschapters, during resistivity surveys, current is injected into the Earth via a pair of current electrodes,and the potential difference is measured between a pair of potential electrodes. The observed data isused to compute the apparent resistivity, which also depends on the type of configuration used. Moresophisticated software has been created to interprete the variation of resistivity with depth by using aforward and inverse modelling method. Two main techniques used in electrical resistivity survey are thevertical electrical sounding technique and the resistivity profiling method.

On the other hand, the electromagnetic method which has a very wide range of applications, can begrouped as either frequency domain methods or time domain methods. Electromagnetic Induction, EMutility location detection methods, very low frequency (VLF), are all frequency domain methods. Thesemethods could either be airborne, ground based, marine based or borehole methods. In any case, asdiscussed previously, the basic principle of these methods is the response of a detector to surroundingelectromagnetic waves, which may have been generated artificially or naturally and which are respondedto by subsurface bodies having some magnetic and electrical properties.

In this chapter, we want to compare the resistivity method as a singular electrical method, with thevariety of the electromagnetic methods. To do this, we shall attempt to revisit the basic principles ofPhysics on which both methods depend. We shall also look at some fundamental concepts in bothmethods. The field work, data collection procedure and interpretation methods will also form a basisfor comparison.

3.1 Basic principle of resistivity of materials and current flow

The resistivity of a material is defined as the resistance in Ohms between the opposite faces of a unitcube of the material [KB84]. Consider a conducting body (e.g. a uniform cylinder) of resistance δR,length δL and cross-sectional area δA (Figure 3.1)[KB84].

The resistivity ρ of the body is given by

ρ =δRδA

δL(3.1)

where R is in Ohms (Ω), A, the area in metre-squared(m2) and L is the length(m). The S.I. unit ofresistivity is the Ohm-metre (Ωm), while its inverse (σ = 1

ρ) gives the conductivity of the material, so

that

σ =δL

δRδA(3.2)

The S.I. unit of conductivity is the Siemens(S) per metre : 1Sm−1 = 1Ω−1m−1.

According to Ohm’s law, when a current I flows through a conducting body, it sets up a potential

11

Section 3.2. Conduction of electric currents in materials Page 12

δV

δA δR

δLI

Figure 3.1: The parameters used in defining resistivity

difference V between the ends of the body and they are related by

I =V

R(3.3)

where R is the resistance of the body. By comparing equations (3.1), (3.2) and (3.3), we find that

I =AV

ρL(3.4)

Since VL

is the potential gradient or electric field strength E and IA

is the current density J , we canwrite equation (3.4) as

E = ρJ (3.5)

Provided the dimension of the conductor is known and the electric field introduced is also known, theresistivity of the conductor can be deduced. Resistivity is one of the most variable physical propertiesranging from a smaller value of 1.6 × 10−8Ωm for native silver to 1016Ωm for pure sulphur.

3.2 Conduction of electric currents in materials

Depending on the nature of a material, conduction of electric currents could either be by an electrolyticprocess or by electronic process. For example, metals and crystals conduct electricity through thepassage of electrons (electronic process), whereas in liquids, conduction is mainly by the electrolyticprocess. In metals, electrons are loosely bound and they are in the conduction band, so that when anelectric field is applied, they move with a much smaller drift velocity in the direction of the field. Thisdrift velocity Vd is given by

Vd =E

ρne(3.6)

where E is the applied electric field, ρ is the resistivity of the material, n is the number of conductionelectrons, and e = 1.602 × 10−19Coulombs is the charge on an electron.

The implication of equation (3.6) is that a material with a low resistivity will give rise to frequentcollisions of the electrons, thereby increasing their drift velocity. For an Insulator, which has no free

Section 3.3. Potential distribution in a homogeneous Earth Page 13

electrons, the atoms of the material acquires an electric polarisation and therefore behaves like an electricdipole placed in an electric field. Conduction is extremely low in insulators [Ogh07].

However, what is found mainly in the subsurface are rock-forming minerals and it has been shown thatthese minerals, being insulators, conduct electric current also by the electrolytic process and the carriersof this current are the ions present in pore waters within the minerals.

In order to effectively determine the resistivity of a rock or rock-forming mineral, porosity ϕ (amountof groundwater in the rock) is a major factor which must be considered. Other factors which mustbe considered include the level of water saturation (S), resistivity of the saturated water (ρw) and thetype and quantity of the dissolved minerals and salts in the water. An empirical formula to actuallydetermine the resistivity of a rock based on the aforementioned parameters was given by Archie(1942)and it is referred to as Archie’s law. It states that resistivity ρ is given by

ρ = aρwϕ−mS−n (3.7)

where 0 < ϕ < 1; 0 < S < 1; 0.5 < a < 2.5; n ≃ 2 while m varies between 1.3 and 2.5. ρw variesaccording to the type and quantity of the dissolved minerals and salts in the water.

In terms of resistivity, Igneous rocks such as granite, diorite and gabbro have the highest resistivitieswhile Sedimentary rocks such as shale and sandstone have a lower resistivity compared to Igneous rocks;this is due to the high fluid content in them. Metamorphic rocks on the other hand have intermediatebut overlapping resistivities. Table 3.1 shows some common rock types and their resistivity values.

Table 3.1: Common rock types and their resistivity ranges

Rock type Approximate resistivity range(Ωm)

Clay 100− 101

Alluvium 101− 103

Shale 101− 104

Sandstone 100− 109

Quartzite 101− 108

Schist 101− 104

Gabbro 103− 106

Granite 102− 106

3.3 Potential distribution in a homogeneous Earth

Let us consider a homogeneous Earth of resistivity ρ. A single current electrode placed on the surface ofthe Earth will produce a radial flow of current I away from the electrode so that the current distributionis uniform over hemispherical shells of increasing radius r centred on the source (see Figure 3.2).

As shown in Figure 3.2, lines of equal potential (equipotential lines) intersect the lines of equal currentsat right angles. Since the potential decreases in the direction of current flow, the potential gradient dV

dr

is negative.

Section 3.3. Potential distribution in a homogeneous Earth Page 14

I

ρr

dVequipotentialsurface

currentflowline

Figure 3.2: Current flow from a single current electrode

According to Ohm’s law and from equation (3.1),

dV

dr= −

Iρ

2πr2(3.8)

On integration, we get for the potential V (r) at a distance r from a point current source,

V (r) =ρI

2πr(3.9)

Since there must be two current electrodes for a resistivity survey, the contribution of the other electrode(the negative current electrode) is given by

V (r′) = −

ρI

2πr′(3.10)

where r′ is the distance from the negative current electrode.

The total potential V at any point is therefore a superposition of potentials due to both current electrodesi.e.

V = V (r) + V (r′)

=ρI

2π

(

1

r−

1

r′

)

(3.11)

The above equation (3.11) gives the total potential due to one potential electrode at a point within thesubsurface. If we now consider the other potential electrode, the same principle also applies in findingthe potential at a point due to it. In effect, one cannot easily determine absolute potentials, so whatwe need to measure is the potential difference. To simplify this, consider Figure 3.3.

In Figure 3.3, A and B are the current electrodes; M and N are the potential electrodes. The potentialVM at the electrode M, from equation (3.11), is given by

VM =ρI

2π

(

1

rA−

1

rB

)

(3.12)

while the potential VN at electrode N is also given by

VN =ρI

2π

(

1

RA−

1

RB

)

(3.13)

Section 3.4. Field procedure: configuration and method Page 15

electrodecurrent

electrodepotential

Ground

I

rBrA

RA RB

∆V

A BM N

+I −I

Figure 3.3: The generalised form of the electrode configuration used in resistivity measurements

We require the potential difference between both electrodes, so we get

∆V = VM − VN

=ρI

2π

[(

1

rA−

1

rB

)

−

(

1

RA−

1

RB

)]

(3.14)

Therefore

ρ =2πI−1∆V

[(

1

rA−

1

rB

)

−

(

1

RA−

1

RB

)] (3.15)

What is actually measured in equation (3.15) is the apparent resistivity ρa, and as it implies, it dependson the mode of spacing of the electrodes. The equation also implies that when the ground is uniform,the resistivity should be constant and will not depend on the surface location or electrode spacing.Generally, the potential electrodes are placed between the two current electrodes.

The distance penetrated by the current increases with increase in the separation between the currentelectrodes. the depth of investigation is generally 20% to 40% of the outer electrode spacing, dependingon the Earth resistivity structure. In equation (3.15), the denominator on the right hand side is usuallycalled the geometric factor, as it is different for various electrode spreads or configurations.

3.4 Field procedure: configuration and method

Resistivity surveys can take different forms of arrangement of the current and potential electrodes. Inmost cases, there are always two current electrodes and two potential electrodes. Normally the potentialelectrodes are placed between the current electrodes. We shall consider three different kinds of electrodearrangements. These are the Schlumberger configuration, Wenner configuration and the Double-dipoleconfiguration. A major difference between all three configurations is the spacing between the currentand potential electrodes. Suppose the potential electrodes are M and N, while the current electrodesare A and B, Figure 3.4 shows the various electrode configurations.

Section 3.4. Field procedure: configuration and method Page 16

Wenner Array Schlumberger Array

Double−dipole Array

∆V

∆V∆V

I

II

rBrB rArA

RARA RBRB

A

AA

B

BB

M

MM

N

NN

a a

aaaa

L

L

Figure 3.4: The various electrode configurations: Wenner, Schlumberger, Double-dipole

In the Wenner arrangement, each potential electrode is separated from the adjacent current electrode bya distance a which is one-third the separation of the current electrodes. For this arrangement, equation(3.15) becomes

ρa = 2πa∆V

I(3.16)

In the Schlumberger arrangement, the spacing between the potential electrodes a is fixed, and is lessthan the separation between the current electrodes L which is progressively increased during survey. Bymaking reference to Figure 3.4, the apparent resistivity according to equation (3.15) becomes

ρa =π

4

∆V

I

(L2− a2)

a(3.17)

However, for most cases, L2 >> a2, hence

ρa =πL2∆V

4Ia(3.18)

The Double-dipole configuration is not very frequently used as it requires more electric current for itsoperation than the Wenner and Schlumberger. Also from Figure 3.4, the apparent resistivity in this caseaccording to equation (3.15) is given as

ρa = π∆V

I

(

L(L2− a2)

a2

)

(3.19)

Section 3.5. Resistivity data interpretation Page 17

There are two main types of procedures used during a resistivity survey, depending on the objective ofthe survey. These procedures are the Vertical electrical sounding (VES) and the Constant separationtraversing (CST). In most surveys, both procedures are employed and could be used with either theSchlumberger or the Wenner configuration.

Vertical electrical sounding (VES) is used for the purpose of determining the vertical variation of resis-tivity. The current and potential electrodes are maintained at the same relative spacing and the wholespread is progressively expanded about a fixed central point [KB84]. Since the aim of investigation isthe depth, the Schlumberger configuration is commonly used for VES investigations, as increase in thecurrent electrode separation creates more penetration and hence reaches more depth.

Constant separation traversing (CST), also known as horizontal or electrical profiling method is usedfor determining the horizontal or lateral variation of resistivity. The current and potential electrodesare maintained at a fixed separation and progressively moved along a profile [KB84]. The Wennerconfiguration is well adapted to this method.

Some of the equipments required in a resistivity survey include a power source to produce the current,tape rule for measurement of length, electrodes (current and potential), cables, crocodile clip for fixingcables to electrodes, hammer for fixing the electrodes in the ground, water to enhance conduction andthe Terameter which is the main equipment in the survey as it sends the signal, receives incoming signals,and calculates the resistance . A typical resistivity survey requires at least three people for speed andefficiency of the work. Suppose we are conducting a VES using the Schlumberger configuration, twopeople will be needed to measure the distances, lay the cables and electrodes, and also be in chargeof increasing the current electrode distance. The other person will remain with the terameter takingmeasurements, and also increasing the potential electrode distance when the voltages are too small tobe measured.

3.5 Resistivity data interpretation

Both the data obtained from a vertical electrical sounding and constant separation traversing techniquecan be interpreted using several methods. In both methods, what is actually recorded is the resistance,which is used to compute the apparent resistivity depending on the configuration employed. A typicaldata shows the various current and potential electrode distances, resistance and apparent resistivity forseveral stations.

A plot of the apparent resistivity against electrode spacing (on a bi-logarithmic paper) can be used toindicate vertical variations in resistivity. The curve obtained is then used to obtain the parameters of thegeoelectrical section i.e. the thickness, depth and resistivities of the layers present therein. It is possibleto obtain a similarity in the result for different sections. The interpretation is done by comparing thefield curve with some theoretical curves, otherwise known as Master Curves. A set of auxiliary curvesare also used with the Master curves during interpretation. These could either be two-layer curves orthree-layer curves.

The process of comparing the field curve with theoretical field curves is known as Partial Curve Matching.This process involves finding a near perfect match of the field curve with one of the theoretical fieldcurves. The first step is to obtain ρ1 and thickness h1, from the perfect or near perfect match and thecorresponding k value shown on the Master curve, such that ρ2−ρ1

ρ2+ρ1= k. This will give the resistivity of

the second layer. The process is repeated for the subsequent layers thus obtaining ρ2, h2, ρ3 e.t.c.

Section 3.6. Electromagnetic method Page 18

Partial curve matching interpretation depends on the shape of the curve, which also depends on thenumber of individual layers. For a three layer Earth model, there are four types of resistivity curveswhich depend on the relative magnitudes of the resistivities. Hence, we have

A-type(ρ1 < ρ2 < ρ3); K-type(ρ2 > ρ3 > ρ1); H-type(ρ1 > ρ2 < ρ3); Q-type(ρ1 > ρ2 > ρ3)

Now, when there are four layers with different resistivities on a field curve, the curves are representedby different letters. Hence, we have

HK-type(ρ1 > ρ2 < ρ3 > ρ4); HA-type(ρ1 > ρ2 < ρ3 < ρ4); AK-type(ρ1 < ρ2 < ρ3 > ρ4)

KH-type(ρ1 < ρ2 > ρ3 < ρ4); QQ-type(ρ1 > ρ2 > ρ3 > ρ4)

Another method for interpreting a resistivity data, is the Computer aided iteration technique. In thiscase, trial values of the layer parameters are guessed, checked with a computed apparent resistivitycurve, and adjusted to make the field and computed curves agree. The process will be much faster ifthe initial guess is guided by a semiquantitative comparison with two- and three-layer curves. Computerprograms have been written by several commercial software companies for the use of this method toobtain the layer parameters automatically by iteration, starting with an initial estimate obtained by anapproximate method [Ogh07].

The observed values of the resistivity and thickness obtained from the interpretation will give an ideaof the kind of rock present in the subsurface, and hence a model of the surface can be prepared. Themodelled results are displayed as scaled resistivity-depth pseudosections with different colours. Normally,Blues represent areas of low resistivity while reds are relatively higher.

Apart from the errors which may occur in the field measurements, certain errors occur during interpre-tation. The problem of equivalent models occurs since the thickness and resistivity cannot be derivedindependently, hence two different profiles may yield the same model, prompting the geophysicist tochoose the model which best agrees with the known geological structures of the ground. Another pos-sible interpretation error is the problem of suppression. This occurs for the A-type and Q-type curves.Sometimes, the middle intermediate layer may not be evident on the field curve. Also the effect of afault cannot be seen on an apparent resistivity curve and this may cause one to interprete a one-layercurve as a two-layer curve.

3.6 Electromagnetic method

Among all the geophysical methods, the electromagnetic techniques have the broadest range of differentinstrumental systems of any, matched by the remarkable range of applications to which these methodsare being applied [Rey97]. As briefly discussed in the previous chapter, the electromagnetic methodsbasically work on the principle of electromagnetic induction. An alternating magnetic field in a coil orcable induces electric currents in a good conductor. This current, which are called eddy-currents, inturn produces secondary magnetic fields that are superposed on the primary field and can be measuredat the ground surface [Low97].

Due to the principle of induction, the EM methods could either be ground-based (where either or bothtransmitter and receiver are on ground), airborne (either or both transmitter and receiver are in air),seaborne (either or both transmitter and receiver are on sea), or borehole-based in which both thetransmitter and receiver are placed in a hole dug in the Earth.

Section 3.6. Electromagnetic method Page 19

Generally, the electromagnetic methods/systems can be classified as Time-domain which makes mea-surements as a function of time or Frequency-domain which makes use of one or more frequencies.Both “domains” may either involve a passive method (where a natural electromagnetic field/groundsignals is required) or an active method (where an artificial field is required). When the transmitter isclose to the receiver, it is called a “near-field” method and when they are very far apart, it is calleda “far-field” method. Figure 3.5 shows a diagrammatic representation of the classification of the EMsystems/method being discussed.

active(artificialtransmitter)

passive

(natural field/ground signals)

near field(e.g. ground conductivity

meter)

far field

(e.g. VLF method)

TDEM

FDEM

Electromagnetic

Methods

Figure 3.5: Classification of EM methods

All the above mentioned classifications can also be grouped into

• methods in which transmitter is stationary and receiver is mobile

• methods in which transmitter as well as receiver is mobile

In order to clearly understand the various kinds of electromagnetic methods and systems commonly usedin geophysical exploration, we shall highlight some basic concepts and definitions.

Electromagnetic Waves: An electromagnetic wave consist of an electric field component (E) orthogonalto a magnetic field component (B), in a plane perpendicular to the direction of travel. An electromag-netic field can be generated by passing an alternating current through either a small coil comprisingof many turns of wire or a large loop of wire [Rey97]. In the EM method, it is the magnetic fieldcomponent of the EM wave which induces eddy current on a conductor in the subsurface according toFaraday’s Law of EM induction. The secondary field generated by these eddy currents (Ampere’s Law)are then received alongside the primary field travelling through the air, by the receiver. The resultanteffect of both fields give useful information on the nature of the subsurface.

Polarisation: If we consider the primary EM field P as a vector and the secondary EM field S as anothervector, their resultant R will also be a vector making an angle α with P. In most surveys, it is the angleθ this resultant makes with the horizontal, that is measured. Since R varies continuously in magnitudeand direction, as a vector, its tip describes an ellipse known as the “ellipse of polarisation” which alsomakes an angle θ (called the tilt angle) with the horizontal [Rey97].

Depth of penetration (Skin depth): This involves the extent in depth to which an electromagnetic ra-diation may penetrate the Earth surface. The depth of penetration largely depends on the frequency ofthe wave and the conductivity of the media present through which the EM radiation is to travel.

Section 3.7. Continuous-wave electromagnetic systems Page 20

The Skin depth is defined (Sheriff 1991) as the depth at which the amplitude of a plane wave hasdecreased to 1

eor 37% relative to its initial amplitude A0 [Rey97]. It is not the maximum depth of

penetration of the magnetic field. Mathematically, the skin depth δ (in metres) is given by

δ =

√

2

ωσµ(3.20)

where ω = 2πf and f is the frequency in Hz, σ is the conductivity of the media in S/m., and µ is themagnetic permeability (usually ≈ 1).

Equation (3.20) shows that the depth of penetration decreases with a decrease in resistivity and anincrease in frequency.

3.7 Continuous-wave electromagnetic systems

Continuous-wave EM systems may be regarded as the frequency dependent systems. They come indiverse forms depending on the relative positions of the transmitter and receiver. In most of thesesystems, it is the resultant effect of the primary and secondary wave which is being measured, however,some methods may still be distinguished. One of such methods is the Tilt-angle method.

The Tilt-angle method method involves the measurement of the angle of tilt or dip angle of the resultantof the applied primary field and the induced secondary field arising from a buried conductive body, such asa buried massive sulphide orebody [Rey97]. This method is used in both ground and airborne surveys,mainly for mineral exploration. The tilt angle is actually the angle the resultant field vector makeswith the primary field vector. During a survey, the tilt angle may change from a positive value to anegative value in the presence of a conductive body. The point at which the tilt angle is zero is calledthe “crossover point”, and it is the point directly above the body, where the resultant, primary andsecondary fields are all horizontal and parallel to each other. There are a number of EM techniqueswhich simply measure spatial variations in this tilt-angle [KB84]. Two principal techniques which makeuse of this method are the VLF and AFMAG (see section 2.1.2).

While AFMAG requires a natural field, VLF requires an artificially generated field from transmittersoperating in the low frequency band of 15 − 25kHz. The VLF receiver first determines the direction ofthe field from the transmitter, and then traverses are performed over the survey area at right angles tothis direction. The property that a conductor will lie below the positions of zero tilt, as measured by thereceiver, is used in the interpretation of the VLF data. Although the advantage of this method is thatit is very easy to operate and there is no need to install a transmitter, however for a particular surveyarea, there may be no suitable transmitter providing the required field necessary to cause a noticeablechange in the tilt angles, hence making it hard to detect a conducting body. Another disadvantage isthat the depth of penetration is somewhat less than that attainable by tilt-angle methods using a localtransmitter [KB84].

Other examples of electromagnetic methods which will not be discussed in this section are the Ground-penetrating radar (GPR) and Telluric/Magneto-telluric methods, as they have been briefly discussed insections 1.2.6 and 2.1.3 respectively.

We shall classify the continuous wave systems as either fixed-source systems or moving-source systems.

Section 3.7. Continuous-wave electromagnetic systems Page 21

3.7.1 Fixed-source systems

When the source is fixed and the receiver is mobile, depending on what is being observed, there are twomethods/systems under this category and these are

• the Sundberg method in which the primary magnetic field from a large loop of wire or longgrounded cable, is inclined towards the ground causing an interference between it and the secondarymagnetic field due to a conducting body. The receiver then measures the resultant field and thegradient of the secondary magnetic field which is displayed as a profile to reveal the position ofthe conducting body. A major factor which affects measurement is topography where the sourceand receiver may be at different elevations due to the rough nature of the surface.

• the Turam method in which two receiver coils are used. The coils are placed at a fixed distance(c) apart and are moved perpendicular to the source wire from one station to another wherethe phase (α) and amplitude (v) of the vertical component of the secondary field is measuredby each of the receivers. The ratio of the amplitudes at each successive pair of stations (e.g.v1/v2, v2/v3, v3/v4, . . .) and the horizontal gradient of the phases ((α2−α1)/c, (α3−α2)/c, (α4−

α3)/c, . . .) are plotted at the location of the midpoint between the coils along the profile [Rey97].If p is the amplitude of the primary wave which reduces as the receiver moves away from thesource, its ratio also changes along the profile and when combined with that of the secondary, weobtain the reduced ratios (RR) (v1p2/v2p1, v2p3/v3p2, v3p4/v4p3, . . .). The reduced ratios andhorizontal gradient of phase are often used to plot a Turam profile. The problem of topographyalso affects this method.

3.7.2 Moving-source systems

These are systems in which both the source (a coil) and receiver (coil) are mobile during survey. Thesource is connected to the receiver by means of a cable and the distance between them is fixed. Theyalso vary in terms of their frequency dependence. During survey, the point of reference is the midpoint ofthe coil separation and what is actually measured is quadrature component only or both the quadratureand in-phase components of the secondary electromagnetic field.

The ground conductivity meter (GCM) is an example of a moving-source system which measuresboth the quadrature and in-phase components. The quadrature component is measured in terms of theapparent conductivity σa of the ground, while the in-phase components is measured in parts per thou-sand. The true conductivity of subsurface bodies will contribute to the measured apparent conductivityvalue.

The GCM is designed to ensure that with the selected frequency (f), a given inter-coil separation (s), adesignated response of the primary magnetic field (Hp) for a given transmitter, the only unknowns arethe secondary field (Hs), which is measured by the instrument, and the true ground conductivity (σ).In other words

σa = (2/πfµ0s2)(Hs/Hp)q (3.21)

where µ0 is the permeability of free space and the subscript q denotes the quadrature phase.

If the ground is entirely homogeneous and isotropic, the instrument should give a measure of the trueconductivity of the ground, i.e. σa = σ. Examples of meters used for EM induction surveys are the

Section 3.8. Pulse-transient electromagnetic systems Page 22

Geonics EM-31, EM-34, and EM-38 which are all frequency dependent and can explore specific depths.There is also the GSSI GEM300 which is a multi-frequency EM system. In order to achieve a greaterdepth of penetration, one needs to use an EM conductivity meter with the lowest frequency and greatestinter-coil separation.

3.8 Pulse-transient electromagnetic systems

These may also be referred to as the time-domain EM systems. The systems work by generating anelectromagnetic field which induces a series of currents in the Earth at increasing depths over time.These currents create a magnetic field which is measured by the receiver in order to deduce subsurfaceproperties and features at great depth. In other cases, it is the decaying voltage observed while thecurrent is turned off, that is measured and recorded as a function of time. The magnitude and rateof decay of the eddy currents depend on the conductivity of the medium and on the geometry of theconductive layers. Currents will decay very rapidly in media with high resistivity. A conductive layer ata depth may “trap” currents in that layer, while currents elsewhere decay more rapidly.

Pulse-transient EM systems have similar applications to Continuous-wave EM systems. Their majoradvantage lies in their ability to determine depths and map much deeper features. However, TDEMsurveys are not as rapid as FDEM and thus large areas can not be surveyed or mapped as economically.

TEM measurements are affected by errors such as topography, static cultural noise and dynamic culturalnoise. The effect of topography stems from the relative positions of the transmitter and receiver. Thepresence of pipes, cables and metal fences around the survey area causes a static cultural noise, whiledynamic cultural noise is caused by geomagnetic signals, lightning discharges producing natural EMtransients, A.C. power lines and VLF transmitters which are higher frequency sources of noise.

The analysis and interpretation of a TEM survey depends largely on the kind of system which is usedto obtain the data. In most cases, it is the plot of the transient voltage with time that is made. Otherplots of observed data which may be made include response profile of decay voltage at all stationsat a selected decay time and apparent resistivity versus time decay curve. The interpretation of thedata involves actually observing the location and shape of a target from the observed profiles which isquantitative. After determining the location, the quality of such target is then determined by checkinghow the field intensity changes from one station to another using the transient voltage-time curves.Computer modelling of observed data has also been used to interprete TEM data, however it cannot beapplied to three-dimensional models, although efforts are on to produce a software which can meet theneed.

3.9 Electromagnetic data interpretation

The interpretation and analysis of electromagnetic data, either from a TDEM survey or from a FDEMsurvey, can be done in a different number of ways, according to the manner in which they have beenacquired. However most analysis entails

• Profiling and depth sounding which involves the representation of observed data as a plot (profilesor contour maps) on which anomalous areas can be identified. This only gives a qualitativeinformation and is not often sufficient when the nature and shape of the anomaly is required.

Section 3.10. Comparing resistivity and electromagnetic method Page 23

• Computer analysis which yields quantitative information. With the aid of software packages, EMdata may be entered into a computer program which inverts it to produce a layered Earth modelof the changing conductivity with depth. The estimated thicknesses and conductivities are thencompared with observed values to have a fair idea of the nature of the underlying materials. Inmost cases, the computer iterates the given data until a reasonable error is reached and then amodel of the layer will be presented based on the observed values.

• Resolution which involves the act of determining the specific meter required for a particular caseand also distinguishing between two close but not identical values of the conductivity as measuredby two separate EM equipments. Two EM equipments can locate the same target body but withdifferent resolutions or settings in terms of intercoil separations.

3.10 Comparing resistivity and electromagnetic method

Having discussed the details of both the resistivity method and electromagnetic methods, it is clear thatthey both have a few similarities as well as a lot of differences.

Some of the differences are hereby itemized

• While the EM methods can be carried out effectively in air, sea and ground, the resistivity methodcan only be done on ground.

• The resistivity method is not as complex and complicated as most EM surveys, particularly theairborne surveys where aircrafts and a sizeable number of personnel will be required.

• As a result of the property of electromagnetic induction, EM methods can be used on frozen groundwhere it will be difficult for resistivity method, as the electrodes may not easily be implanted intothe ground.

• While softwares for interpreting a three-layer Earth model using resistivity method are available,it is not easy to interprete a three-layer model in Time-domain EM survey using the computer.

Apart from the similarities in some of the applications of both methods, other similarities include

• In most cases, what is actually measured in resistivity or EM survey are the electrical propertiesof the ground such as resistivity, inductance and conductivity.

• As in some EM equipments, increasing the transmitter-receiver separation increases depth pene-tration. Likewise in resistivity survey, greater depth is attained by increasing the current electrodeseparation.

• Just as resistivity data are affected by self-potentials present in the ground, secondary currentsalso affect the primary EM field needed in an EM survey.

• The interpretation of EM data and resistivity data follow the same procedure, where profiles areobtained, and if need be, a computer is used to provide more information from the observed data.

4. Limitations and Applications of the Resistivityand Electromagnetic Method

4.1 Limitations of both methods

Some of the limitations of the resistivity method include

• It may be unsuitable for examining highly industrialised and urbanised areas where cultural featuressuch as buildings, fences and power lines may interfere with the collection of accurate data.

• The target depth, size and resistivity contrast may also pose limitations.

• The interpretation is more difficult in the presence of complex geology and existence of naturalcurrents and potentials.

• Interpretation is limited to simple structural configurations. Any deviations from these simplesituations may be impossible to interprete [KB84].

• The depth of penetration is limited by the maximum electrical power that can be introduced intothe ground and by the practical difficulties of laying out long lengths of cable. The practical depthlimit for most surveys is about 1Km [KB84].

Some of the limitations of the electromagnetic methods are hereby itemized

• According to Parasnis 1962, one of the troublesome effects in the electromagnetic methods is thatthe secondary currents in superficial layers of good conductivity, e.g. clays, graphitic shales etc.may screen the deeper conductors partially or wholly from the primary field. The latter which arethe real objects of exploration will then produce weak or no distortions (anomalies) in the primaryfield and may therefore be undetectable.

• The frequency dependence of depth penetration also places constraints on the EM method. Pene-tration is not very great, being limited by the frequency range that can be generated and detected[KB84].

• The quantitative interpretation of electromagnetic anomalies is complex [KB84].

4.2 Applications of both methods

The general applications of the resistivity method (which may either be a Vertical electrical sounding orConstant separation traversing method) include

• environmental audits and site assessment prior to construction

• water resource management and groundwater resource studies such as mapping and plume delin-eation

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Section 4.3. Conclusion Page 25

• public/private remedial investigations and feasibility studies, landfill closures and geological map-ping

while some specific applications of the resistivity method include

• determination of the electrical properties of a surrounding area

• determination of the depth of water table and the overburden depth

• determination of extent of saltwater intrusions

• delineation of salt water from fresh water and design of corrosion control

• location and monitoring extent of groundwater pollution

• location of fractures and faults, and detection of solution features and voids

Applications of EM methods (either TDEM or FDEM) are very wide, however a few will be highlighted.

• location and exploration of orebodies or metalliferous mineral deposits, and oil reservoirs (CSEM)

• groundwater investigations (FDEM for near-surface mapping investigations and TDEM for deeperinvestigations)

• EM methods are mainly used in the follow-up ground surveys (such as resistivity survey) whichprovide more precise information on the target area

• detection of underground cavities (TDEM) and location of frozen ground (FDEM)

• contaminated-land mapping, landfill investigations (FDEM), contaminant plume mapping (TDEM),mapping subsurface voids (TDEM)

4.3 Conclusion

The resistivity method and electromagnetic methods are two very important and useful techniques ingeophysical exploration. They both require a field introduced into the ground and rely on the responseof the ground to the applied field for an understanding of what lies beneath the Earth’s surface. For anysurvey to be more meaningful, one might have to “double-check” the output of the survey with anothermethod, and that is why some geophysicists employ the resistivity and electromagnetic methods, as theyhave similar applications and a similar style of interpreting the data. However, it is not always necessaryto use both methods on a survey area for the same purpose.

Some of the factors which may influence the choice of any of the methods are the availability ofpersonnel, the cost of operation, presence of a naturally occurring field generated by a transmitter asin VLF, the area of the location to be studied etc. It is also pertinent to note that both the resistivitytechnique and electromagnetic method are two of the best methods used in geotechnical survey andgeological investigations. There are lots of examples of cases in history where both methods have beenapplied.

Acknowledgements

All thanks be to God who has sustained me all through my time at AIMS.

I wish to acknowledge the efforts of my supervisor Prof. George Smith for his patience in reading throughmy work and suggesting very important additions and omissions, despite his busy schedule . I also wantto thank Louise Soltau of the CSIR in Stellenbosch for her effort in making me understand more ofresistivity and electromagnetics.

I am also full of gratitude to my parents and family for their love, support and encouragement from adistance.

The staff of AIMS led by the director, Prof. Fritz Hahne, are also worthy of acknowledgement. Thelast nine months would not have been what it was but for your wonderful roles and impact in my lifetoo numerous to be stated. Special gratitude goes to Jan, Igsaan, Emmanuel, Laure, Matteo and othertutors. You’ve all been wonderful.

At this point, I wish to acknowledge my friends (AIMS Students 2007 set), especially my Nigeriancolleagues, for their show of love and support as a family in the last few months of living together. Youguys have been great.

To others who have contributed in their own way to the success of this work and my stay at AIMS, Isay a big thank you.

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References

[Dob60] Milton B. Dobrin. Introduction to Geophysical Prospecting. McGraw-Hill Book Company, Inc.,1960.

[Dun70] F.W. Dunning. Geophysical Exploration. Her Majesty’s Stationery Office., 1970.

[KB84] Philip Kearey and Michael Brooks. An Introduction to Geophysical Exploration. Blackwell,London, 1984.

[Low97] William Lowrie. Fundamentals of Geophysics. Cambridge University Press. London, 1997.

[Ogh07] F. O. Oghenekohwo. An Exploitation of the Possible Applications of a Multi-layer Earth Model

Using Electrical Resistivity Sounding Technique. BSc, University of Ibadan, 2007.

[Par62] D.S. Parasnis. Principles of Applied Geophysics. Methuen and Co. Ltd. London, 1962.

[Rey97] John M. Reynolds. An Introduction to Applied and Environmental Geophysics. John Wileyand Sons Ltd., 1997.

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