Factors that Affect the Gravitational Acceleration
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 categories: 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 Effects - Changes in the observed acceleration
caused by the gravitational attraction of the sun and moon (which move).
- 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 effects, 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.
