magnetic+interpretation_09

8/7/2019 Magnetic+interpretation_09

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Interpretation of the magneticInterpretation of the magnetic

anomaliesanomalies

Saman Tavakoli

October 2009


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FundamentalsFundamentals

The force between the poles is defined by coulomb as :

F = p1p2/r2 or

is the magnetic permeability of free space (i.e. vacuum)

orWb/(Am). in the SI system and 1 in the cgs

Is the relative permeability of a medium

The magnetic field B due to a pole of strength p1 at a distance r from the pole is

defined as the force per unit positive pole at that point

2

210

4 r

ppF

rTQ

Q!

0Q

7

0104

v! TQ

rQ

2NA


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In actuality, the magnetic field can be either represented by:

H, the magnetic field intensity (A/m, mks-SI; orested, cgs)

- or

B, the magnetic induction (Teslas, SI; gauss, cgs)

1 A/m = 4 x 10-3 oersteds1 Tesla = 104 gauss

1 Gauss = 100,000 nT

B and H are useful when considering a magnetic field inside the magnetic object, but

magnetic field of the earth is measured in none-magnetic media such as air or water,

therefore the equations in free space for H and B are :

At the Earth's surface the total intensity varies from 24,000 nanotesla (nT) to 66,000

nT


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Magnetic susceptibility :Magnetic susceptibility : The degree to which a body becomes

magnetized is determined by its magnetic susceptibility and is the

fundamental parameter in magnetic prospecting.

K for sedimentary rocks :

K for granite and gneiss rocks :

K for intrusive basic rocks :

410

K

431010

"" K

310

"K


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Minerals which are mainly responsible for the magnetic properties of the Rocks :

And Pyrothite87

SFe

Ilmenorutile

TiO2FeOFe2O3 ternary diagram


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3Main classes of the magnetic behavior based on the magnetic susceptibility :3Main classes of the magnetic behavior based on the magnetic susceptibility :

Diamagnetic minerals : the magnetic susceptibility is low and negative to an external magneticfield, normally is masked since is overlain by the stronger paramagnetic or ferromagneticproperties. is independent of the temperature : quartz ,feldspars, salt

Diamagnetic minerals are not able to retain the magnetic properties when the external field isremoved.

Paramagnetic minerals : have low and possitive susceptibilities, minerals containing transitionalelements (Fe, Cu, Zn, Ni, Mn, Cr, Ti ) are paramagnetic. susceptibility varies inversely with thetemperature: pyroxine , olivine and amphibole with susceptibility of

Paramagnetic minerals are not able to retain the magnetic properties when the external field is

removed.

Ferromagnetic minerals : have high possitive susceptibilities and strong magnetic properties : iron, nickel.

They are able to retain their magnetic properties after the external field has been removed.

610

451010


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FundamentalsFundamentals The origin of the Earth's magnetic field is not completely understood, but is thought to be

associated with electrical currents produced by the coupling of convective effects and rotationin the spinning liquid metallic outer core of iron and nickel. This mechanism is termed thedynamo effect.

Elements of the earth magnetic field :Elements of the earth magnetic field :

At any point of the earths surface, the magnetic field can be defined by three vectors and 2angles :

Vectors :1. Horizontal field(H)

2. Vertical field(Z)

3. Total field(T)

Angles :1. Declination (D) : Or magnetic variation is the angle between magnetic meridian and

the geographic meridian

2. Inclination (I) : The angles at which the magnetic vector dips below the horizontal

RelationshipH = T cosIZ = T sinI = H tanIX = H cosDY = H sinD

X and y are North and East geomagnetic fieldsX and y are North and East geomagnetic fieldsrespectivelyrespectively

Magnetic meridianMagnetic meridian


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The purpose of the magnetic surveying is to identify and describe regions

of the earths crust that have unusual (anomalous) magnetizations.

Magnetic surveying consists of :Magnetic surveying consists of :

1. Measuring the magnetic field at predetermind points

2. Correcting the measurement for known changes

3. Comparing the resultant value of the field with expected value at each

measurement station

The expected value of the field at any place is that ofInternational GeomagneticReference Field(IGRF). The difference between the observed and expected

values is magnetic anomaly


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Measurement methods :Measurement methods :

1. Land : An operator uses a portable magnetometer to measure the magnetic field

at the center of the earth, over a suspected geological structure,slow, but gives

very good detail of the anomalous source since is close to the source

2. Sea : A magnetometer in a waterproof fish is towed behind the ship at the end of

the long cable, since the ship are consisting of thousands ton of steel, they cause

big magnetic disturbance, so the cable must be 100-300m long. The depth of

measurement depends on the speed of ship and length of cable. For 10km/h, 20-

30 meter depth investigation is expected.

3. Air : The most effiecient method of the surveying magnetic anomalies. The

magnetometer must be removed as far as possible from the magnetic

environment of the aircraft


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Magnetic gradiometer :Magnetic gradiometer :

A magnetic gradiometer is an installation comprising two magnetic sensors

oriented either vertically above each other or horizontally beside each other.

Gradients are calculated by subtracting the value of measurements made atone sensor from those made at the next sensor, and dividing by the

distance between sensors.

Why using that?Why using that?1. Removing diurnal effects. Diurnal and other temporal changes

in the magnetic field are eliminated by synchronously taken readings.

2. Resolving complex anomalies related to overlapping sources.

Magnetic gradiometers


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Land

Air

Sea


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a. Constant flight elevation above the sea level

b. A helicpter can keep the distance from magnetic

source as small as possible by flying at constant

height above the surface

c. The usual way of survey along parallel flight lines


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Corrections for the magnetic data :Corrections for the magnetic data :

In comparison to the gravity data, magnetic data require very few corrections

1. Diurnal correction: One effect that must be compensated is variation in intesity of

the geomagnetic field at the earths surface during the day. ( The best solution forits correction is to instal a constantly rocording magnetometer at a fixed base

station within the survey area.)

2. Terrain correction : Topography may give rise to magnetic anomalies. There is no

general rule to do the terrain correction. Normaly, if there is a good correlation

between topography and magnetic anomalies, the anomaly is not very significant!

3. Temperature correction : Older instruments were temperature dependent but in

modern instruments this is negligible. These changes are generally incorporated

into the diurnal correction.

4. Normal Correction : Removal of the International Geomagnetic Reference Field

(IGRF)


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anomalyanomaly Advantages and disadvantages :Advantages and disadvantages : Disadvantages :Disadvantages :

1. No unique solution is available for an anomaly, similar to gravity interpretation

2. High remanent magnetization lead to difficulties when interpreting magnetic anomalies3. Magnetic susceptibilies can vary within a certain rock type

4. The characteristic of anomalies are dependent of direction of magnetization.

Advantages :Advantages :

1. Cost effective method given the area surveyed.(Both ground and aeromagnetic surveys)

2. The magnetic anomaly have normaly high precision3. Although the earths magnetic fields orientation affect the anomaly, in a certain survey area

even large, is constant, helping geophysicist for interpretation.

4. Since the susceptibility values are small, they can represent specific rock types easier than thegravity anomalies.


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anomalyanomaly Half-Maximum technique : Considering a slender vertical srtraightforward and bottom far away from observer

For a sphere and a cylinder, total width of an anomaly curve at ZA (magnetic field intensity) max/2

equals the depth to center of sphere and cylinder

766.0

max2/1x

Z !


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Interpretation of theInterpretation of the

magnetic anomalymagnetic anomaly Peters slope method to depth problem :Peters slope method to depth problem :1. Identifying maximum slope on the anomaly curve

2. Creating a line with a one-half slope of maximum slope value

3. Moving the current line until two points of tangency are identified

4. Distance between points of tangency is d

5. The relationship between d and depth to the top

of the prism,z is :

This method assumes :This method assumes :1. Length of prism is great compared to width

2. Depth to prism top is the same as the prism width

3. And prism has infinite length and oriented parallel to

the magnetic meridian

6.1

dZ !

Z=0.3km

1

2,3

4


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anomalyanomaly

If the magnetic field of theanomalous body at a givenlocation along the profile line isvertical then is a maximum(positive or negative dependingon whether the magnetic fieldreinforces or opposes the Earthsfield) at that location.

If the vertical component of themagnetic field of the anomalous bodyopposes the vertical component ofthe Earths magnetic field along the

profile line, then D Z at that locationhas a negative value.


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anomalyanomaly a. Magnetic anomaly of a monopole vertical dike :a. Magnetic anomaly of a monopole vertical dike :

We assume the dike is infinite in the depth

So that the dike is a monopol model

The magnetometer mesearues the sum of the

Indcing field and anomalous field

The magnetic anomaly is possitive everywhere here,

increasing from zero far from the dike to maximum

value directly over it

Inducing magnetic fieldInducing magnetic field

Anomalous field of magnetized bodyAnomalous field of magnetized body

Component of anomalous field parallel to inducing fieldComponent of anomalous field parallel to inducing field


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anomalyanomaly b. Magnetic anomaly of a dipole vertical dike :b. Magnetic anomaly of a dipole vertical dike :

The dike here is finite, as what we see in the real world

The distribution of south poles produce possitive magnetic anomaly

Distribution of the North poles produce negative magnetic anomaly

Close to the dike, the dipole has a component that reinforces

the inducing field, caused a positive central anomaly.

Why the North poless negative anomaly is weaker than the

Possitive anomaly of the South pole ?

Why The negative anomaly of North poles becomes stronger

than the South poles anomaly Right of R and Left of L ?


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anomalyanomaly Effect of the block width on anomaly shape :

A narrow but extended in depth block gives a sharp possitive anomaly(a)

For a body in which its width is considerably large compared to its depth,

South and North poles distribution is equal, so the possitive anomaly isalmost zero, near the edge of block, the effect of South

pole is larger than that of North(d)

The observed dip in the middle of the anomaly

is due to the depth of the block, if it wasinfinitely deep, the effect of North pole would

be missing and the anomaly would look

flat-topped!


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anomalyanomalyExample of upward continuationExample of upward continuation (Blue line)(Blue line)

Initial anomaly

1500 m upward continuation




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anomalyanomalyExample of downward continuationExample of downward continuation (Blue line)(Blue line)

Initial anomaly

100m downward continuation



1500m downward continuation,

Look at the sharp anomalies,Is this anomaly reliable ?


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Useful linksUseful links

The geomagnetic calculator : http://williams.best.vwh.net/magvar.htm

magnetic+interpretation_09

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