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 p1p1 p2p2 r Scalar F = 1 p 1 p 2  r 2 F Vector F = 1 p 1 p 2  r 2 r ^ r ^ Pole Strength = Magnetic Permeability  Gravitational Force: F = G m 1 m 2 r 2 Gravitational Force: F = G m 1 m 2 r 2

Magnetic Induction ( B )  Also called  Magnetic flux density  Magnetic field density  Magnetic Induction B has units of N/(amp-m) = Tesla (T) B = 1 p’  r 2 r ^ F popo = Magnetic Induction is the force exerted on a magnetic pole (p o ) when placed in an existing magnetic field. Earth’s magnetic field measured in air is very small. We use nanoTeslas (nT) to measure anomalies in Earth’s magnetic field.

Relationship of Induction & Field Strength B =   H In air or a vacuum, the solution is very simple: B =  H    mag. permeability of a vacuum ≈ mag. permeability of air = 4πx10 -7 N/(amp 2 ) where H = magnetic field strength or intensity

Relationship of Induction & Field Strength B =  H    mag. permeability of a vacuum ≈ mag. permeability of air = 4πx10 -7 N/(amp 2 )  r =    =  r   B =  r   H In other materials: B =   H +   I where M = (  r -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 where H = magnetic field strength or intensity (units amp/m)

Magnetic Susceptibility (  )  =  r -1 M =  H =  B/  0 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 = 4 SI emu) M = (  r -1)H = (  r -1)B/  0

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. S N

Another Maxwell Equation – Ampere’s Law J = current density D = electric displacement - D = 0 E + P 0 = Permittivity of free space P = Polarization density For static magnetic fields in the air, J = 0 and ∂ D /∂ = 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 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 Magnetic domains (bands) visible in Microcystalline grains of NdFeB

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 anti- parallel 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  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! I =  H =  B/  o

Remnant Magnetism  Depositional remnant magnetism – The alignment of magnetite grains during or after deposition of sediments.  Chemical remnant magnetism – Alignment of magnetic grains during phase change or growth.  Thermoremnant magnetism – Alignment of magnetic minerals during cooling and crystallization. Curie Temperature ~580 o C for magnetite Temperature Magnetization

Induced vs. Remnant Magnetism  For exploration and environmental surveys, we generally assume induced magnetism.  But, remnant magnetism has important applications as well!