Physics for Geologists, Second edition

36 Gravity
The benefits of space flights are not limited to the mapping of the oceans.
They have also led to the Global Positioning System (GPS). This depends
upon very precise timing of radio waves from at least four of the twelve
navigation satellites whose position in space is accurately known. This has
revolutionized navigation and mapping, for it is now possible to determine
one's position with a hand-held unit, in fine weather or foul, to within
10-20 m. It will also determine the altitude relative to the geoid used as the
standard. This may be several tens of metres different from the elevations
printed on maps, but they are just as consistent.
While many of these developments came from military applications, their
scientific applications have been significant. The military applications, of
course, relate to the precise positions of ships, aircraft and launching sites;
but when launching a missile from an ocean aimed at a target the size of
a tennis court, it is also necessary to know the gravity patterns between
launch and target. If the vector g is not 'vertical' in the global sense or varies
slightly in value, the trajectory will vary slightly. We touched on this in the
discussion of Coriolis forces. As mentioned earlier, this technology has had
its commercial application in the development of equipment for air-borne
gravity surveys. Falcon, as the system is called, is an application involving
accelerations and vibrations that would have rendered the old gravimeters
useless. Fifty years ago, a gravimeter on land could be seriously affected by
vibrations in the ground from the wind in the trees.
Miscellaneous gravity-driven geological processes
The effects of gravity are not, of course, confined to the global scale. Gravity
is with us everywhere, and has long been put to work. We shall postpone
the mechanical processes of compaction and down-slope sliding because they
will be discussed in context in some detail in later chapters.
Ground-water flow to rivers, streams and bores
Between 1668 and 1670 (not a misprint!) Pierre Perrault studied rainfall in
the catchment area of the Seine river in France, and the rate of flow of the river
in Paris. He found that the volume of rainfall was about six times the vol-
ume of river flow (Perrault 1674, 1967). This was later confirmed by Edme
Mariotte (1717: 338-9). From Mariotte's work grew the understanding that
river flow was due more to ground-water flow than to run-off. Otherwise
rivers would not have maintained flow during droughts, as many do.
The physics of the process is both simple and instructive. It has long been
known that the free water table follows the topography in general, with
reduced amplitude, and that the ground-water flow follows the downward
slope of the topography. So a cross section of part of a river valley in an
ideal porous and permeable reservoir would be as in Figure 3.5.
Copyright 2002 by Richard E. Chapman