Steven Dutch, Professor Emeritus, Natural and Applied Sciences,University
of Wisconsin - Green Bay
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Gravity Maps |
Magnetic Maps |
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| 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
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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 |
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Created 13 May 1999, Last Update
12 June 2020