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Introduction to Magnetic Exploration
 Often cheap relative to other geophysical techniques.
 Can be measured with ground-based or airborne
equipment.
 Not usually very useful when looking at sedimentary
structures. (We’ll see why later!)
 Measuring a potential field (like gravity!)
 Interpretation is more difficult than for gravity data
because magnetization is a vector
 Changes in the field, not the absolute value of the field is
important.
Magnetic Force: Coulomb’s Law
r
m = Magnetic
p1
^
r
F
Pole Strength
Scalar
1
F= m
p2
Permeability
p1p2
r2
Gravitational Force:
F = G m1m2
r2
Vector
1
F= m
p1p2 ^
r
2
r
Magnetic Induction (B)
Magnetic Induction is the force exerted on a magnetic pole (po)
when placed in an existing magnetic field.
 Also called
 Magnetic flux density
 Magnetic field density
 Magnetic Induction B has units of N/(amp-m) = Tesla (T)
Earth’s magnetic field measured in air is very small.
We use nanoTeslas (nT) to measure anomalies in Earth’s magnetic field.
B=
F
=
po
1
m
p’ ^
r
r2
Relationship of Induction & Field Strength
B= mH
where
H = magnetic field strength or intensity
m0 = mag. permeability of a vacuum ≈ mag. permeability of air
= 4πx10-7 N/(amp2)
In air or a vacuum, the solution is very simple:
B = m0 H
Relationship of Induction & Field Strength
B= mH
where H = magnetic field strength or intensity
(units amp/m)
m0 = mag. permeability of a vacuum ≈ mag. permeability of air
= 4πx10-7 N/(amp2)
m = mrm0
mr = m/m0
In other materials:
B = m0 H + m0 I
B = mr m0 H
where
M = (mr-1)H
In materials other than air, the magnetic field strength B
is increased by M, the intensity of magnetization, which
is induced by the field H. ➜ Induced Magnetism
Magnetic Susceptibility ()
M = (mr-1)H = (mr-1)B/m0
 = mr -1
M =  H =  B/m0
Magnetic Susceptibility, , is dimensionless and describes the
proportional relationship between field strength and the
intensity of magnetization for a given material.
(but 1 cgs emu = 4𝜋 SI emu)
http://www.epa.gov/
Maxwell’s Equations
Gauss’s Law for Magnetism
By the divergence theorem this gives:
➜
➜
➜
No NET sources or sinks of magnetic flux
No magnetic ”point charges” or monopoles
A positive magnetic charge is always accompanied by a
negative one
Unlike Gauss’s Law for Gravity !
Magnetic Dipole
 Unlike gravity, there is no net magnetic
field – all positives and negatives are
balanced.
 No net sources or sinks of magnetic
flux.
 No monopoles exist in reality, but it
can be useful mathematically, to
consider a hypothetical one.
 We can visualize the magnetic field as
“field lines” running from + to -,
indicating the orientation of H.
http://solarscience.msfc.nasa.gov/magmore.shtml
S
N
Another Maxwell Equation – Ampere’s
Law
J = current density
D = electric displacement - D = 𝜀0 E + P
𝜀0 = Permittivity of free space
P = Polarization density
For static magnetic fields in the air, J = 0 and ∂D/∂𝒕 = 0
(no current, no changing electric fields) ➔ ∇ X H = 0
By Stokes Theorem:
Because the line integral around a closed loop = 0,
the magnetic field intensity is a conservative field.
Magnetic Potential
Because the magnetic field intensity H is a conservative field,
we can express it as the gradient of a magnetic potential U:
H= -∇U
Note that this is an identical formulation to gravity:
g= ∇U
(where U is gravitational potential)
Classifications of Magnetic Susceptibility
(how do materials behave in a magnetic field?)
Diamagnetic:
Electrons are paired, but orbits induce
a small net magnetic moment
Very small negative 
Moment is near zero.
Examples: quartz & feldspar
Paramagnetic:
Unpaired electrons partially align
Positive  typically very low
Moment is typically small.
Examples: pyroxene & olivine
Magnetic Domains
Ferromagnetic and Ferrimagnetic materials generally made
up of domains with uniform magnetic direction
Within domains the magnetic moments of atoms are aligned
The domains form when cooled below the Curie Temperature
Magnetic domains (bands) visible in
Microcystalline grains of NdFeB
Magnetism can be either permanent
or induced.
Permanent magnetism remains
when the field is removed.
For surveys, magnetism induced by
the Earth’s field is generally the most
important
Classifications of Magnetic Susceptibility
Ferromagnetic:
Domains within the material are aligned.
Very high 
Common examples are pure iron, nickel
Do not occur naturally on Earth.
Anti-Ferromagnetic:
Domains within the material are both anti
-parallel and parallel, with both orientations
having the same strength.
 is very low
Moment = 0
Common example is hematite.
Ferrimagnetic:
Domains within the material are both antiparallel and parallel, with one direction
being stronger than the other.
High 
Common examples are magnetite*
and ilmenite.
Magnetic
Susceptibility of
Naturally
Occurring
Materials
Induced vs. Remnant Magnetism
I = H = B/mo
 A magnetic field can be temporarily induced in
a material by another magnetic field. The
intensity of this induced field will depend on the
magnetic susceptibility of the material.
 This induced magnetism disappears when the field
is removed!
 In other cases, the magnetic field may be
permanent. This is called “remnant magnetism.”
 Remnant magnetism does not disappear with the
inducing field is removed!
Remnant Magnetism
 Depositional remnant magnetism –
The alignment of magnetite grains
during or after deposition of
sediments.
Curie Temperature
~580oC for
magnetite
Alignment of magnetic grains during
phase change or growth.
 Thermoremnant magnetism –
Alignment of magnetic minerals
during cooling and crystallization.
Magnetization
 Chemical remnant magnetism –
Temperature
Induced vs. Remnant Magnetism
 For exploration and environmental surveys, we generally
assume induced magnetism.
 But, remnant magnetism has important applications as well!
http://www.ngdc.noaa.gov
http://www.whoi.edu