Background Image

The background image for these pages is created from satellite-derived free air gravity anomalies (gray-scale) and color contours of the Earth's geomagnetic inclination, declination and field strength.

Satellite-derived free air gravity anomalies

Ocean surfaces represent a gravitational equipotential surface. Free air gravity anomalies are then derived by calculating the rate of change in elevation of this surface. The area of the image is the central Atlantic Ocean with the Lesser Antilles Islands and Venezuela located to the west, and Africa to the East. The Mid-Atlantic Ridge (short north-south segments) and related transform faults and fracture zones (generally east-west) are well defined through the center of the area.

Geomagnetic Parameters

The Earth's gravity field is monopolar so it always points down towards the Earth's center. But the magnetic field is dipolar, which means that the direction of the field changes with geographic position. At the geomagnetic poles the field is vertical or perpendicular to the Earth's surface (see diagram below). At the geomagnetic equator it is horizontal or parallel to the Earth's surface, and oriented north-south. Geomagnetic inclination and declination describe the magnetic field vector with respect to geographic location. Geomagnetic field strength generally is strongest near its poles and weakest over its equator.


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Tasa Graphic Arts  has a lot of cool geo-slides.


Below is an "unwashed" version of the background image.

  • Geomagnetic inclination (10 red contours) increases from -20 in the south to 50 in the north.
  • Geomagnetic declination (5 blue contours) decreases to -15 through the center of the area and increases westward and eastward to -10 and -5 respectively.
  • Geomagnetic field strength (5,000 nT magenta contours) increases from 30,000 nT in the south to 45,000 nT in the north.


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