Sources of Noise

Thus far we have shown how variations in the gravitational acceleration can be measured and how these changes might relate to subsurface variations in density. We've also shown that the spatial variations in gravitational acceleration expected from geologic structures can be quite small.

Because these variations are so small, we must now consider other factors that can give rise to variations in gravitational acceleration that are as large, if not larger, than the expected geologic signal. These complicating factors can be subdivided into two catagories: those that give rise to temporal variations and those that give rise to spatial variations in the gravitational acceleration.

• Temporal Based Variations - These are changes in the observed acceleration that are time dependent. In other words, these factors cause variations in acceleration that would be observed even if we didn't move our gravimeter.
  • Instrument Drift - Changes in the observed acceleration caused by changes in the response of the gravimeter over time.
  • Tidal Affects - Changes in the observed acceleration caused by the gravitational attraction of the sun and moon.
  
  
• Spatial Based Variations - These are changes in the observed acceleration that are space dependent. That is, these change the gravitational acceleration from place to place, just like the geologic affects, but they are not related to geology.
  • Latitude Variations - Changes in the observed acceleration caused by the ellipsoidal shape and the rotation of the earth.
  • Elevation Variations - Changes in the observed acceleration caused by differences in the elevations of the observation points.
  • Slab Effects - Changes in the observed acceleration caused by the extra mass underlying observation points at higher elevations.
  • Topographic Effects - Changes in the observed acceleration related to topography near the observation point.

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