Chapter one (1)

Introduction

Magnetic susceptibility is the property of a material which determines how much magnetization will be

present due to an external magnetic field. Because everything in a rock or mineral contributes to the

magnetic susceptibility, it is a fertile source of information on the composition of the sample. Magnetic

susceptibility is extremely effective in geological mapping, macroscopically similar, but magnetically

different rocks can be distinguished and delineated.

Some processes of alteration of original rocks are accompanied by changes in rock's magnetic

mineralogy, thus phase changes or destruction of original magnetic minerals, formation of new

magnetic minerals. As these changes are usually reflected in rock's magnetic susceptibility, they are

easily detectable by susceptibility measurement.

Magnetic susceptibility mapping depends on geochemical or mineralogical composition of the rocks

and on later metamorphic processes and alterations. The aim of the survey was to map the site based on

mineralogical composition of the rocks or soil.

1.1 Literature review

The physical background for the existence of magnetic behavior in minerals is the magnetic moment

produced by electrons orbiting their nucleus and spinning around their axis. The major rock-forming

magnetic minerals are the following iron oxides: the titanomagnetite series, xFe2TiO4 (1 x)Fe3O4,

where Fe3O4 is magnetite, the most magnetic mineral; the ilmenohematite series, yFeTiO3 (1

y)Fe2O3, where -Fe2O3 (in its rhombohedral structure) is hematite; maghemite, -Fe2O3 (in which

some iron atoms are missing in the hematite structure); and limonite (hydrous iron oxides). They also

include sulfidesnamely, the pyrrhotite series, yFeS (1 - y)Fe1 - xS. The magnetic character of soils

is dominated by the presence of ferrimagnetic minerals such as magnetite and maghemite, and to a

lesser degree by pyrrhotite (Ward, 1990).

Minerals are classified as either diamagnetic or paramagnetic. Diamagnetic materials have negative and

very low susceptibilities. Most paramagnetic materials have positive susceptibilities but also with very

low values, so they are not of interest in geophysical surveys. Some paramagnetic materials are

ferromagnetic which have alignments of magnetic moments in small areas called magnetic domains.

These materials are not naturally occuring on earth, so again they are not of interest in exploration.

Ferrimagnetic minerals, on the other hand, are common and naturally occuring. These minerals (e.g.

magnetite, pyrrhotite) have a net magnetic moment and thus relatively high susceptibilities. In general,

igneous and metamorphic rocks have higher susceptibilities than sedimentary rocks. Except for rare

monomineralic rocks, rocks consist in general of all three kinds - i.e. diamagnetic, paramagnetic and

ferromagnetic minerals.

Table 1. Magnetic susceptibilities for several iron oxides and soil constituents. Data from Thompson &

Oldfield(1986) and Maher(1988).

Material Chemical formula Magnetic status Magnetic susceptibility

(10-8 m3 kg-1)

Water H2O Diamagnetic -0.9

Quartz SiO2 Diamagnetic -0.6

Pyrite FeS2 Paramagnetic 30

Ferrihydrite 5Fe2O3 9H2O Paramagnetic 40

Lepidocrocite -FeOOH Paramagnetic 70

Ilmenite FeTiO3 Superparamagnetic 200

Hematite -Fe2O3 Antiferromagnetic 60

Goethite -FeOOH Antiferromagnetic 70

Pyrrhotite Fe7S8 / Fe8S9 / Fe9S10 Ferrimagnetic ~5,000

Maghemite -Fe2O3 Ferrimagnetic 40,000

Magnetite Fe3O4 Ferrimagnetic 50,000

Table 1: Magnetic susceptibilities for several iron oxides and soil constituents. Data from Thompson &

Oldfield(1986) and Maher(1988)

Iron minerals within the soil can be altered through biological decay and burning which can enhance

the magnetic susceptibility of the soil. Field equipment can be used to measure the magnetic

susceptibility of the soil allowing zones to be mapped.

Le Borgne (1955, 1960) has suggested that the enhanced susceptibilities of soil is due to in situ

conversion of iron oxides from an antiferromgnnetic form such as haematite or goethite to

ferrimagnetic form maghaemite. He also proposed two possible mechanisms. In the first, reduction

occurs as a result of the decay of organic matter in the soil in anaerobic conditions achieved during wet

periods and reoxidation to maghaemite in aerobic conditions during dry periods. In the second the

burning of organic matter produces the temperature increase and reducing atmosphere necessary for the

reduction to magnetite in a thin layer of soil underlying the fire and reoxidation occurs during the

cooling down of the fires when air enters the system.

If rocks containing oil undergo elevated temperature, some Fe oxides and hydroxides can transform

in the reduction environment due to the presence of the organic substance into ferrimagnetic

minerals, which results in the increase of magnetic susceptibility. Particularly obvious is this process

in the oil deposits which underwent natural combustion. These are characterized by the existence of

intense magnetic anomalies and high susceptibility (Cisowski and Fuller 1987).

The natural variation in the soil properties gives rise to anomalies, similar to anomalies that are the

result of buried objects. The discrimination between these geological or soil anomalies and buried

object anomalies is extremely complicated. The location of buried features depends on the fact that the

magnetic susceptibilities of soil derived from sedimentary rocks is normally higher than that of the

parent rock. In strongly magnetic soils, magnetic and electromagnetic sensors often detect anomalies

that have a geologic origin. For most iron or steel objects, the susceptibility, k, falls between 10 and 200

in SI units. However, predicting the response of a magnetic susceptibility survey over metal is

complicated for several reasons. Remanent magnetisation is likely to be strong, and pointing in

different directions in the various components of a buried object. For example, a buried pipe will often

show up as a linear set of anomalies with variable character because each segment will have it's own

magnetic signature. It is worth mentioning that stainless steel is not magnetic, and many potential

targets may not even be ferrous (for example, aircraft frame parts are often some alloy with no

magnetic properties). (Breiner, 1973)

Magnetic susceptibility is an effective technique often used as a form of reconnaissance survey. Natural

magnetite and pyrrhotite (i.e. mixture of hexagonal and monoclinic phases), or other ferrimagnetic

minerals, tend to accumulate in ore deposits (including the non-iron ones) or in their

environs. Even though they do not often represent the economic minerals, their magnetic properties

can be important in the search for ore deposits, because these minerals often accompany the economic

metallizations in various ways. Thanks to their high susceptibility and remanent magnetization, these

minerals can be surveyed not only by geoelectrical methods, like the mineralizations of many other

minerals, but also by magnetometric methods. The measured magnetic anomalies can directly indicate

those ore deposits in which the distribution of ferrimagnetic minerals conforms to the distribution of

the economic mineralization and, of course, those ore deposits in which these minerals represent the

economic mineralization. However, in most cases these ferrimagnetic minerals create only haloes in

footwall rocks and the susceptibilities can indicate the deposit only indirectly. In both cases, it is useful

to measure the magnetic susceptibility in situ, in order to get an idea of the spatial relationship between

the susceptibility and the economic mineralization, which can be utilized in magnetic survey of the

deposits of a similar type. If a ferrimagnetic mineral creates an economic mineralization, the magnetic

susceptibility can be used in the fast control of the searched or exploited ore. This case often takes

place in metamorphosed oxidic Fe-ores, magnetite skarns and also in bodies of metamorphosed siderite

ores, originally metasomatic in origin.

In addition to magnetite and pyrrhotite, other rare magnetic minerals can be interesting from the

point of view of economic geology, such as: cassiterite, franklinite, cobaltite, Ni-sulphides, etc. The

magnetic minerals, hematite, pyrrhotite and magnetite, often occur in sulphide ore deposits. Various

alterations may give rise to transformation and even destruction and subsequent disappearance of

ferrimagnetic minerals from a deposit (Hrouda et al 2009).

Magnetic susceptibility is useful for mapping anthropogenic heavy metal pollution in soil based on the

idea that topsoil has more magnetic content than subsoil samples due to the settling of anthropogenic

magnetic particles.

Moreso, Magnetic susceptibility is enhanced where magnetite spheres produced in the combustion of

petroleum products are present as pollutants in dust particles. Therefore, magnetic susceptibility can be

used as a tracer of industrial pollution (e.g., Petrovsky et al. 2000).

1.2 Theory

Magnetic susceptibility is a parameter of considerable diagnostic and interpretational use in the study

of rocks. This is true whether an investigation is being conducted in the laboratory or magnetic fields

over a terrain are being studied to deduce the structure and lithologic character of buried rock bodies.

Susceptibility for a rock type can vary widely, depending on magnetic mineralogy, grain size and

shape, and the relative magnitude of remanent magnetization present, in addition to the induced

magnetization from the Earths weak field.

The physical background for the existence of magnetic behavior in minerals is the magnetic moment

produced by electrons orbiting their nucleus and spinning around their axis. Magnetism is controlled by

the inherent forces or energies created by electrons which make up atoms. Electrons spin around their

axis, and also around the atoms nucleus in their own orbits. Spins within spins, analogous to the orbit

of the Earth round the Sun whilst spinning on its axis. The way in which different electrons motions

are aligned determines the total magnetic energy or moment of the atom. Different atoms have different

numbers of electrons and types of motion. Atoms make up molecules and molecules make up materials,

so that the overall type of magnetic behaviour of a rock mineral is defined by the configuration and

interactions of all the electron motions in all its atoms.

In many types of material the overall magnetic moment is zero because the orbital and spin components

even out. When a mineral with zero magnetic moment is placed in a magnetic field the electron

motions will rearrange so that the net magnetic moment is in the direction opposite to the applied field.

These types of minerals are called diamagnetic. In contrast, when minerals with a small net magnetic

moment get subjected to a magnetic field the electrons will attempt to line up in the direction of the

magnetic field. These types of minerals are called paramagnetic. In some minerals, the interaction

between electron spin and orbital movement in adjacent atoms causes these minerals to behave as

active magnets. These types of minerals are called ferromagnetic when all magnetic moments line up in

the same direction, or ferrimagnetic, when one third of the magnetic moments line up in the opposite

direction. A special group of minerals are those in which the electron interaction leads to magnetic

moments being aligned in opposite directions. These minerals with a net magnetic moment of zero are

called antiferromagnetic. (Thompson and Oldfield, 1986).

When there is no external magnetic field, individual magnetic zones ("magnetic domains") within

rocks, soils or other materials will generally be oriented randomly. The net effect would be a zero

magnetic field. However, when the material is in the presence of an external magnetic field such as

Earths field, the individual magnetic domains become more or less aligned, resulting in a net non-zero

field. This is a secondary field distinct from, but caused by, the Earths field.

The strength of this so-called "induced" magnetisation is called the "dipole moment per unit volume",

m. It is related to the external magnetic field's strength, H, by

m = KH.

Magnetic susceptibility is therefore the property of a material which determines how much

magnetization will be present due to an external magnetic field.

K = m/H

It is effectively the ratio of the magnetisation effect to the applied magnetic field. Susceptibility K is a

dimensionless number related to the number of individual magnetic dipoles in the medium that can be

aligned with the main field.

Rocks and minerals may retain magnetization after the removal of an externally applied field, thereby

becoming permanent weak magnets. This property is known as remanent magnetization and is

manifested in different forms, depending on the magnetic properties of the rocks and minerals and their

geologic origin and history.

The concept of hysteresis is therefore fundamental when describing and comparing the magnetic

properties of rocks. Hysteresis is the variation of magnetization with applied field and illustrates the

ability of a material to retain its magnetization, even after an applied field is removed.

Generally there are three magnetic effects that impact the (electro)magnetic characteristics of the

subsurface, and thus electromagnetic sensors:

(1) remanent magnetization,

(2) induced magnetization, and

(3) viscous remanent magnetization.

Remanent magnetization Remanent magnetization exists in the absence of an applied field. The

remanent magnetization must be added to any magnetization effects resulting from an applied magnetic

field. Remanent magnetization occurs within ferromagnetic and ferromagnetic minerals that have a

natural alignment of the magnetic moments. This type of magnetization directly affects magnetic

sensors

Induced magnetization Induced magnetization results from a magnetic field being applied to a

magnetically susceptible object. In the low-intensity field region, the net magnetic moment (i.e., the

magnetization, m) is proportional to the strength of the applied field (H). Therefore, the low-field

magnetic susceptibility, defined as the ratio of the magnetization over the field strength, is a material-

specific property. The magnetic susceptibility is either expressed per unit volume (volume-specific

susceptibility, ) or per unit mass (mass-specific susceptibility, ). Induced magnetization can be

measured by applying a magnetic field to a sample (in the laboratory or in the field). By measuring the

difference between this primary magnetic field and the secondary magnetic field one can determine the

material specific magnetic susceptibility. The magnetic induction of a sample, measured by a magnetic

or electromagnetic sensor, is the sum of all the different entities of induced magnetization, weighed for

volume, distance to the sensor, and magnitude of the susceptibility.

Viscous remanent magnetisation Viscous remanent magnetization refers to the effect that the

secondary magneticfield gets delayed relative to the primary magnetic field (Thompson &

Oldfield,1986).

Chapter Two (2)

2.1 Project Site

The experimental site can be located close to the Great Hall of the Kwame Nkrumah University

of Science and Technology in Kumasi Ghana. It features a tropical wet and dry climate, with

relatively constant temperatures throughout the course of the year. The estimated area of the

site is approximately (250*200)m2. The topography of the land is characterized by high and

low lands and generally slopes down at the west. The project site is covered by grass and with a

canopy created by various tree species.

Illustration 1: Project site marked X

2.2 Data Acquisition

In taking the measurements, focus was placed on induced magnetization (magnetic

susceptibility) using the MS2D field loop sensor. The instrument; Bartington MS2 Susceptibility

System consists of a meter that can is attached to a sensor (loop sensor). The meter expresses magnetic

susceptibility in either cgs (centimetre, gram, second) or SI (standard international) units. SI was used

throughout the field measurements. The MS2D search loop sensor is a field sensor, 185 mm in

diameter, designed to make surface measurements. The sensor contains a coil that generates an AC

magnetic field and allows for the bulk susceptibility of a circular area approximately 200mm

Diameter to be quickly measured.

Two sensitivity positions are provided with the instrument: range 1 or 0.1. Accuracy on the

order of 1.0 x 105 SI can be obtained with a 1-s measurement cycle (1 range) and an accuracy

of 0.1 x 105 SI with the 10-s cycle (0.1 range). The Zero (Z) and Measure (M) functions are

selected on a three-position toggle switch. Measurements are displayed on a four-digit LCD

panel and were taken on the 0.1 range which takes about 10s.

Illustration 2: MS2 meter Illustration 3: MS2D sensor (loop)

Field Procedure

Magnetic objects worn by the surveyor can affect the data and was therefore removed before

collecting data. Also, as a precaution the battery was checked to make sure there was enough

power to give accurate and reliable measurements.

1. The MS2 Meter is carried in a shoulder bag for ease of use on the field

2. After proper assembling of the instrument, a zero operation is performed whilst holding the

probe at least 1m from all objects (in the air) by pressing the Z button.

3. The sensor is placed on the surface (ground) and measurement is triggered with the M button

which takes about 10s with a beep which indicates end. The reading is recorded and the GPS

point taken.

4. Measurements were taken at irregular intervals in a grid-like manner.

Chapter three (3)

3.1 Results

The results gathered from the field was entered into spreadsheet application for further processing of

the data. Below is the results obtained;

Magnetic Susceptibility x10-5 Northing Westing

32.8 657797 737981

27.6 657789 737981

50.7 657775 737980

20.2 657757 737981

19.3 657736 737984

707.9 657713 737980

44.1 657668 737981

15.2 657626 737981

16 657576 737980

6 657560 737982

6 657531 737978

18.6 657530 737942

36.6 657537 737939

33.6 657575 737930

19.3 657607 737912

5.7 657629 737914

19.7 657658 737906

62.3 657686 737911

25.2 657708 737914

12.3 657744 737913

10.3 657762 737909

66.3 657778 737883

28.8 657751 737847

78.6 657727 737827

74.6 657706 737843

38.3 657674 737851

11 657645 737856

44 657607 737869

38.8 657579 737868

24.8 657558 737881

16.4 657531 737886

15.1 657507 737885

25.5 657498 737821

44.5 657528 737819

15.5 657559 737820

50.6 657582 737816

67.1 657602 737810

32 657618 737806

18 657646 737796

26.1 657695 737793

16.1 657794 737928

53.9 657770 737936

20.7 657746 737942

15.4 657721 737947

38.8 657694 737949

80.6 657658 737953

116.3 657655 737970

64.2 657673 737985

92.7 657626 737957

21.3 657596 73795