Geology Reference
In-Depth Information
Figure 2.8 Field observer's Hammer Chart, for Zones B and C.
of the Earth, Moon and Sun, and follow linked 12- and 24-hour cycles
superimposed on a cycle related to the lunar month (Figure 2.10). Swings
are largest at new and full moons, when the Earth, Moon and Sun are in
line, when changes of more than 50 µGal may occur within an hour and
total changes may exceed 250 µGal. The assumption of linearity made in
correcting for drift may fail if tidal effects are not first removed.
Earth tides are predictable, at least at the 0.01 mGal level required for most
gravity surveys, and corrections can be calculated using widely available
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