Fundamental Relationships

Magnetic Force - proportional to product of the pole strengths divided by the square of the distance between them. Coulomb's law similar to Newton's Law for gravity

Magnetic field strength - Magnetic force on a unit magnetic pole. Units are nanotelsa.

Magnetic Moment (of a dipole) - magnetic pole strength times length of dipole.

Intensity of Magnetization - magnetic moment divided by volume.

Magnetic Susceptibility - material property, which determines the Intensity of Magnetization induced in an object by an external magnetic field

Diamagnetic - negative susceptibility (e.g., quartz and feldspar),

Paramagnetic - positive susceptibility but values are low (e.g., olivine and pyroxene)

Ferrimagnetic - large positive susceptibility (e.g., magnetite, ilmenite, pyrrhotite)
 

Hysteresis and Remanent Magnetization - When a material is subjected to an external magnetic field the induced magnetization may not return to zero when the external magnetic field is removed (non-linear). Thus, the object is permanently magnetized. Most common type is thermoremanent magnetization when ferrimagnetic minerals crystallize and cool during the solidification of igneous rocks.

Inclination - angle the total field vector makes with a horizontal plane

Magnetic equator - inclination of zero. Intensity is 30,000 nanotesla

Magnetic dip poles - inclination of 90°. Intensity is 70,000 nanotesla

Declination - angle the vertical plane containing total field vector makes with geographic north.

Dipolar aspects of the Earth's field - most of the Earth's magnetic field is similar to the field produced by a dipole at the center of the Earth with an angle of 11.5° relative to the rotational axis. The projection of this imaginary dipole to the surface is the geomagnetic pole.

Non-dipolar aspects of Earth's field - Currents within the Earth's core produce a major non-dipole aspect to the Earth's magnetic field (dip magnetic poles and geomagnetic poles are different). The non-dipole part of the Earth's field changes on time scales of years (Secular variation).

Flux Gate Magnetometer - Measures Total or Vertical Component of Earth's field. Two equal but opposite polarity electromagnets which are wrapped by a third wire. The earth's field strengthens one electromagnet, which generates an electrical current in the third wire.

Proton-Precession Magnetometer - External magnetic field causes protons to spin like a top. When the external field is turned off, the earth's magnetic field causes the spinning protons to wobble like a top. The precession of the wobble gives the intensity but not the direction of the Total Field Component of the Earth's magnetic field.

Magnetic Cleanliness - man-made metal objects must be kept away from instrument (e.g., belt buckles, cars, metal pipes)

Diurnal Corrections - Earth's field changes with time. As it is the Earth's field not the instrument that is changing it is common practice to have one continuous recording magnetometer at a base site to determine variations in time.

Elevation and Horizontal Position - these corrections are not as important for the Earth's magnetic field as for the gravity field primarily because we do not need the same level of precision to detect anomalies.

Rock Susceptibilities - Sedimentary < Metamorphic < Igneous (0.00005 to 0.012 cgs emu)

Monopole - Vertical component looks like a gravity profile for a sphere (max. over the monopole). Horizontal component is asymmetric (zero over the monpole). Total component field resembles vertical component at high latitudes and horizontal component at low latitudes.

Dipole - Superposition of two monopoles. At latitudes above 30°, Total field and vertical component look similar to a monopole except there is some asymmetry. At lower than 30° latitudes, vertical component is asymmetric (zero over the dipole). Total field component is zero over the dipole with flanking positive values.

Sphere - Similar to dipole. Magnetic anomalies are complex because the direction of induced magnetization and the total field component direction both vary with latitude.

Horizontal Sheet - At high latitudes, magnetization is the difference in the angles subtended by the top and bottom of the object. Thick objects of limited spatial extent (e.g., igneous dike) produce a larger anomaly than thin objects of large horizontal extent (e.g., igneous sill).

Complex bodies - are usually modeled using a polygon technique similar to gravity.

Half-Maximum Technique and Slope Methods - Deep objects produce low amplitude, broad magnetic anomalies. Shallow objects produce high amplitude, narrow magnetic anomalies (just like gravity). Thus, the shape of the anomaly can be used to infer the approximate depth to an object (assuming that the approximate shape of the object is known).