GRAVITY METHOD - Field Geophysics - page 42

Figure 2.2 A simple astatic gravity meter. The tension in the zero-length
spring is proportional to its length and, with the geometry shown, it provides
a moment that will support a mass M in some selected field g , regardless of
the value of the angle α . Measurements are made by rotating the dial, which
raises or lowers the measuring spring to provide a force M . δ g to return the
mass to a standard position.
seems likely to remain the case for the foreseeable future. Gravity surveys
are complicated by the fact that such meters measure gravity differences,
not absolute field strengths.
2.2.1 Astatic spring systems
Astatic systems use zero-length main springs, in which tension is propor-
tional to actual length. With the geometry shown in Figure 2.2 and for one
particular value of gravity field, the main spring will support the balance
arm in any position. In stronger fields, a much weaker auxiliary spring can
be used to support the increase in weight, which will be equal to the product
of the total mass and the increase in gravity field. To use an expression
common in other geophysical methods, the zero-length spring backs-off a
constant weight so that the measuring spring can respond to small changes
in gravity field. All modern commercial gravity meters use this principle,
although with additional complications. For example, LaCoste meters have