Gravity and Magnetic Maps

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
First-time Visitors: Please visit Site Map and Disclaimer. Use "Back" to return here.

 

Gravity and Magnetic Maps Compared

 

Gravity Maps

Magnetic Maps

Nature of Field Simple Field

Radial field

Latitudinal variations only

Gravity varies by about 0.5% from pole to equator

Complex Field

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 Corrections

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.

Complex Corrections

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.

Apparatus Mechanical

Basically very sensitive spring balance

Delicate

Temperature Sensitive

Prone to drift as springs age

Difficult to use in moving vehicles due to delicacy and sensitivity to non-gravitational accelerations.

Electronic

Magnetic fields can be measured electronically and the measuring device can be interfaced directly with other electronic devices

Easily adapted to moving vehicles

Measurements Discrete

Apparatus must be set up and calibrated before use

Data tend to be widely spaced and resolution coarse

Labor-Intensive

Continuous

Adaptability to moving vehicles allows continuous data recording from aircraft

Data closely-spaced and with fine resolution.

Signature Simple

Gravity effects located vertically above source

Same source always produces same effect

 

Complex

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 Simple

Gravity anomalies relate to density, which relates closely to lithology

Complex

Magnetic anomalies relate to magnetite content, which does not relate closely or uniformly to lithology

Summary


Return to Course Syllabus
Return to Techniques Manual Index
Return to Professor Dutch's Home Page

Created 13 May 1999, Last Update 31 January 2012
Not an official UW Green Bay site