Magnetic surveying...

Equations in magnetic surveying 2. Geomagnetic field (refresher) 3. Magnetic properties of rocks (refresher) 4. Survey strategies and interpretation 5. Conclusions We will talk about magnetic properties at an atomic scale, paleomagnetics or the magnetic structure of the Earth. These notions were developed last year. We will focus on magnetics for environmental and engineering applications and emphasize links with gravimetry.

• Exploration of fossil fuels (oil, gas, coal)

• Exploration of ore deposits

• Regional and global tectonics

• Large scale geological structures, volcanology

Structure of the lecture

1 Equations in magnetic surveying

2 Geomagnetic field (refresher)

3 Magnetic properties of rocks (refresher)

4 Survey strategies and interpretation

We will talk about magnetic properties at an atomic scale, paleomagnetics or the magnetic structure of the Earth These notions were developed last year We will focus on magnetics for environmental and engineering

1 Equations in magnetic

surveying

• Like a lot of phenomena in Physics, understanding magnetismrequires an understanding of quantum theory, but perhaps morethan most

• We’ll need to get into this a bit, but there are some useful ideas wecan discuss without going too deeply

• Scientists first investigating magnetism noticed a lot of similaritiesbetween magnetic fields and electrical fields, and so presumed theywere due to the same physical mechanism

• In fact, Gauss proposed that Coulomb’s law for the forces betweenelectrical charges could be modified for magnetic force, except thatthe property eo, the electrical permittivity, is replaced by somethingcalled the magnetic permeability - mo

1 Equations in magnetic

surveying

2

2 14

1

r

Q

Q F

Thus, the electric force:

becomes the magnetic force

Where P1 and P2 are called magnetic poles The main

difference is that the magnetic field always looks like there are two poles of opposite sign in some proximity to each other, but as far as we know the concept of a magnetic pole

µ0 =4π*10-7 H/m

1 Equations in magnetic

surveying

The field of a magnetic pole

These ideas cannot be transferred directly to magnetism, because magnetic poles do not really exist Nevertheless, many magnetic properties can be described and magnetic problems solved in terms of fictitious poles For example,

we can define a magnetic field B as the force exerted by a pole of strength p on a unit pole at distance r

1 Equations in magnetic

surveying

The potential of a magnetic pole

In studying gravitation we also used the concept of a field todescribe the region around a mass in which its attraction could befelt by another test mass In order to move the test mass away fromthe attracting mass, work had to be done against the attractive forceand this was found to be equal to the gain of potential energy of thetest mass When the test mass was a unit mass, the attractive forcewas called the gravitational field and the gain in potential energywas called the change in potential

We can define the magnetic potential W at a distance r from a pole

of strength p

1 Equations in magnetic

surveying

midpoint of the pair of poles, in adirection that makes an angle to the axis,

is the sum of the potentials of thepositive and negative poles At the point

(r, θ) the distances from the respective poles are r + and r– and we get for themagnetic potential of the pair

1 Equations in magnetic

surveying

The magnetic dipole

The pair of opposite poles is considered to

form a dipole when their separation becomes

infinitesimally small compared to the distance

to the point of observation (i.e., d « r) In this

case, we get the approximate relations

When d « r, we can write and terms of order

(d/r)2 and higher can be neglected This leads to

the further simplifications

1 Equations in magnetic

surveying

The magnetic dipole

The quantity m(dp) is called the magnetic moment of the dipole.This definition derives from observations on bar magnets Thetorque exerted by a magnetic field to turn the magnet parallel to the

field direction is proportional to m This applies even when the

separation of the poles becomes very small, as in the case of thedipole

2 Geomagnetic field

2 Geomagnetic field

More complex than the gravity field:irregular variations with latitude,longitude and time

Inclination varies depending onthe hemisphere

Geocentric dipole is inclined atabout 11.4°

2 Geomagnetic field

The magnetic pole is moving in time

2 Geomagnetic fieldDeclination: Then and Now

2 Geomagnetic fieldDeclination: Then and Now

2 Geomagnetic field

US/UK world magnetic chart – Epoch 2000

Declination – Main

2 Geomagnetic field

US/UK world magnetic chart – Epoch 2000

Inclination – Main

2 Geomagnetic field

2 Geomagnetic field

Migration of the North Pole

2 Geomagnetic field

Origin of the geomagnetic field

Magnetic field of the Earth measured at the surface comes from

three sources:

 97-99% Main field generated by dynamo action in the outer core

 1-2% External field generated in space in the magnetosphere

 1-2% Crustal field from remnant magnetization above the Curie depth

 Main field varies significantly with time scale of years (secular variation)

 External field varies with time scales of minutes to days

 Crustal field on varies over geological time scales

 At any point the magnetic field is defined by the magnetic field elements

 Magnetic field is more complicated in spatial form than gravity field

2 Geomagnetic field

Origin of the geomagnetic field

Not a remanent origin (temperature too high)

Dynamo action produced by the circulation of charged particles incouples convective cells within the outer, fluid, part of the Earth'score The secular variation, and alignment of dipole

with rotation axis, suggest that the magneticfield originates in the relatively rapid fluidmotion in a part of the Earth with a highelectrical conductivity

2 Geomagnetic field

The external component of the magnetic field is generated in the atmosphere and magnetosphere.

 The solar wind (a stream of H and

He ions) is deflected by the Earths internal magnetic field to create the magnetosphere.

 The interactions between the solar wind and the Earth’s magnetic field are very complex.

 Temporal changes in the solar wind, due to sunspots, solar flares and coronal mass ejections can

 From 50-1500 km above the Earth’s surface

is the ionosphere, a region of plasma with

high electrical conductivity.

 Changing magnetic fields from the

magnetosphere can induce large electric

currents in the ionosphere.

 Changes in these currents produce large

External component of the Earth’s magnetic field

2 Geomagnetic field

External component of the Earth’s magnetic field

When the solar wind is in a steady state, the Earth’s magnetic field shows

a daily variation that is due to the Earth turning within the current systems of the magnetosphere and ionosphere The typical variation is

called the solar quiet day variation

(Sq) The amplitude is typical 10-20

nT and varies with latitude Clearly seen in time series above.

 When the solar wind is active, the Earth’s magnetic field is said to be disturbed Magnetic

storms occur when the current systems change over a period of several days and the field at the

Earth’s surface can change by 100’s of nanotesla These changes are largest beneath major ionospheric current systems A small substorm can be seen in the middle of the time series plotted above.

 Smaller magnetic field disturbances are classified as substorms and bays and have timescales of several hours.

 Solar activity is characterized by an 11 year cycle and we are currently in a minimum.

 Maximum solar activity results in high levels of activity in the Earth’s external magnetic field and frequent magnetic storms and strong auroral displays.

2 Geomagnetic field

Diurnal variations

field induced by the flow of charged particles within the ionized ionosphere towards the poles

2 Geomagnetic field

Interaction of the terrestrial magnetic field with particles from the solar wind sets up

the conditions for the aurora phenomena near the poles.

2 Geomagnetic field

2 Geomagnetic field

Top: A simulation of Earth's magnetic field structure Bottom: An image of what Earth's magnetic field might look like during a reversal, something humans may have to worry about.

3 Magnetic properties of rocks

Rock magnetism

The measured total magnetic field is the sum of the geomagnetic field and the remanent magnetic field

3 Magnetic properties of rocks

Rock magnetism

• All substances are magnetic at the atomic scale Each

and the orbital path of the electrons around the nucleus

• Two electrons can exist in the same state provided their spins are in opposite directions ( paired electrons ).

In this case their spins cancelled When unpaired

scale appears

• Paired and unpaired electrons are mainly at the origin

of the various magnetic rock properties

3 Magnetic properties of rocks

A simplified picture of the magnetic moments inside a

material is shown in Fig 1 The magnetic moment m of

each atom is associated with a current loop as illustrated and described in the previous section The net magnetic moment of a volume V of the material depends on the degree of alignment of the individual atomic magnetic moments It is the vector sum of all the atomic magnetic moments in the material The magnetic moment per unit volume of the material is called its magnetization, denoted

M:

3 Magnetic properties of rocks

Fig 1: Schematic representation of the magnetic moments

inside a material; each magnetic moment m is associated

with a current loop on an atomic scale.

𝐕

3 Magnetic properties of rocks

In a vacuum there is no magnetization (M=0); the vectors

B and H are parallel and proportional (B=µ0H) Inside a magnetizable material the magnetic B-field has two

sources One is the external system of real currents that

produce the magnetizing field H; the other is the set of

internal atomic currents that cause the atomic magnetic moments whose net alignment is expressed as the

magnetization M In a general, anisotropic magnetic material B, M and H are not parallel However, many

magnetic materials are not strongly anisotropic and the elementary atomic magnetic moments align in a statistical

3 Magnetic properties of rocks

Rock magnetism

B = 0(H+M) =(1+k) 0H

• Diamagnetic: k < 0

• Paramagnetic: k > 0

• Ferromagnetic (e.g iron), ferrimagnetic (e.g magnetite)

and antiferromagnetic (e.g hematite)

3 Magnetic properties of rocks

Remanent magnetization

The strength of the magnetization of ferro and ferrimagnetic material decreases with temperature and disappears at the Curie temperature (for most of the rocks about 500oC, i.e to a depth of 40 to 50 km).

Origin of remanent magnetization:

• Thermoremanent magnetization

• Detrital remanent magnetization

• Chemical remanent magnetization

• Viscous remanent magnetization

3 Magnetic properties of rocks

Classifications of magnetic materials

 Diamagnetic

• All electron shells are full, thus there is no net moment

• In the presence of an external field, the net moment opposes theexternal field, i.e., slightly negative susceptibility

 Paramagnetic

• Materials contain unpaired electrons in incomplete electron shells

• However magnetic moment of each atom is uncoupled from others

so they all behave independently

• Results in weakly magnetic materials, i.e small susceptibility

3 Magnetic properties of rocks

Classifications of magnetic materials

 Ferromagnetic

• Materials contain unpaired electrons in incomplete electron shells

• Magnetic moment of each atom is coupled to others in surrounding

‘domain” such they all become parallel

 Caused by overlapping electron orbits

 Gives rise to a spontaneous magnetization

even in absence of an external field

 Magnets are ferromagnetic

3 Magnetic properties of rocks

Classifications of magnetic materials

 Ferromagnetic

• A rock sample may contain thousands of tiny ferromagneticmineral grains The magnetization loop of a rock sample shows theeffects of magnetic hysteresis (Fig 2) In strong fields themagnetization reaches a saturation value (equal to Ms), at which theindividual magnetic moments are aligned with the applied field Ifthe magnetizing field is reduced to zero, a ferromagnetic materialretains part of the induced magnetization The residual magnetization

is called the remanence, or isothermal remanent magnetization(IRM); if the sample is magnetized to saturation, the remanence is asaturation IRM (Mrs)

3 Magnetic properties of rocks

Hysteresis

 When a ferromagnetic material is magnetized in one direction, itwill not relax back to zero magnetization when the imposedmagnetizing field is removed It must be driven back to zero by afield in the opposite direction

 If an alternating magnetic field is applied to the material, itsmagnetization will trace out a loop called a hysteresis loop

 The lack of retrace ability of the magnetization curve is theproperty called hysteresis and it is related to the existence ofmagnetic domains in the material Once the magnetic domains arereoriented, it takes some energy to turn them back again

3 Magnetic properties of rocks

Hysteresis

Fig 2 The magnetization loop of an

arbitrary ferromagnetic material.

3 Magnetic properties of rocks

Classifications of magnetic materials

 Anti-ferromagnetic

• Almost identical to ferromagnetic except that the

moments of neighboring sublattices are aligned

opposite to each other and cancel out

• Thus no net magnetization is measured

• Example: Hematite

 Ferrimagnetic

• Sublattices exhibit ferromagnetically but then

couple antiferromagnetically between each other

3 Magnetic properties of rocks

Magnetic properties of materials of interest

• Basement: tends to be igneous or metamorphic, thus greater magnetic properties.

• Soils and other weathered products: because magnetic minerals tend to weather rather rapidly compared to quartz, will get reduction of magnetic materials with weathering.

• Man-made objects: iron and steel

• Ore deposits: many economic ores are either magnetic, or associated with magnetic minerals.

3 Magnetic properties of rocks

Remanent magnetizations in rocks

• The induced magnetization is directly proportional to the susceptibility and concentration of magnetic minerals present in the rocks The orientation is, naturally, the same

as that of the external field (geomagentic field in our case) However, the measured magnetization is not always of this direction Responsible for this phenomena is the remanent magnetization The remanent magnetization is present even

if we remove the external magnetic field The most common types of remanent magnetization are described

3 Magnetic properties of rocks

Remanent magnetizations in rocks

magnetic material is cooled below the Curie temperature in the presence of external magnetic field (usually the Earth’s magnetic field) Its direction depends on the direction of the external field at the time and place where the rock cooled.

When magnetic particles slowly settles they are oriented into a direction of an external field Various clays exhibit this type of remanence.

3 Magnetic properties of rocks

Remanent magnetizations in rocks

grown of crystals or during an alteration of existing minerals The temperature must be low (below the Curie point) This type might be significant in sedimentary or metamorphic rocks.

left following the removal of an external field Its amplitude is low unless it was created within very large magnetic field like during the lightning strike.