|Nature of Field||
Latitudinal variations only
Gravity varies by about 0.5% from pole to equator
Approximately dipole field with both vertical and horizontal components
Dipole is not aligned with earth's axis
Dipole is off-center with respect to earth's center
Significant non-dipole components to field
Magnetism varies by about 50% over earth's surface
Details of field change measurably in a human lifetime.
|Corrections to Data||
Simple latitude correction accounts for earth's rotation and equatorial bulge.
Correction necessary for altitude (free-air correction).
Additional altitude correction can be made for excess mass above sea level (Bouguer correction).
Bouguer correction overcorrects in mountains because of greater crustal thickness. Correction for variations in crustal thickness is called isostatic correction.
Field does not vary with time.
Complexity and great variability of earth's magnetic field requires detailed models of magnetic field for accurate correction.
Correction models must be adjusted to time of data because of secular changes in the earth's field.
Data sets processed at different times may have mismatches at boundaries.
Basically very sensitive spring balance
Prone to drift as springs age
Difficult to use in moving vehicles due to delicacy and sensitivity to non-gravitational accelerations.
Magnetic fields can be measured electronically and the measuring device can be interfaced directly with other electronic devices
Easily adapted to moving vehicles
Apparatus must be set up and calibrated before use
Data tend to be widely spaced and resolution coarse
Adaptability to moving vehicles allows continuous data recording from aircraft
Data closely-spaced and with fine resolution.
Gravity effects located vertically above source
Same source always produces same effect
Dipole nature of magnetic fields results in complex patterns even around simple sources
Same source will have different signature at different locations on earth. A rock body at the magnetic poles will look different from the same rock body at the magnetic equator.
|Correlation to Lithology||
Gravity anomalies relate to density, which relates closely to lithology
Magnetic anomalies relate to magnetite content, which does not relate closely or uniformly to lithology
Created 13 May 1999, Last Update 31 January 2012
Not an official UW Green Bay site