Geology Reference
In-Depth Information
6.3 Units of gravity
The mean value of gravity at the Earth's surface is about
9.8 m s -2 . Variations in gravity caused by density varia-
tions in the subsurface are of the order of 100 m ms -2 .This
unit of the micrometre per second per second is referred
to as the gravity unit (gu). In gravity surveys on land an
accuracy of ±0.1 gu is readily attainable, corresponding
to about one hundred millionth of the normal gravita-
tional field.At sea the accuracy obtainable is considerably
less, about ±10 gu.The c.g.s. unit of gravity is the milligal
(1 mgal = 10 -3 gal = 10 -3 cm s -2 ), equivalent to 10 gu.
s
s +
δ
s
m
m
mg
m ( g +
δ
g )
Fig. 6.1 Principle of stable gravimeter operation.
6.4 Measurement of gravity
Since gravity is an acceleration, its measurement should
simply involve determinations of length and time.
However, such apparently simple measurements are not
easily achievable at the precision and accuracy required
in gravity surveying.
The measurement of an absolute value of gravity is
difficult and requires complex apparatus and a lengthy
period of observation. Such measurement is classically
made using large pendulums or falling body techniques
(see e.g. Nettleton 1976, Whitcomb 1987), which can
be made with a precision of 0.01 gu. Instruments for
measuring absolute gravity in the field were originally
bulky, expensive and slow to read (see e.g. Sakuma
1986).A new generation of absolute reading instruments
(Brown et al . 1999) is now under development which
does not suffer from these drawbacks and may well be in
more general use in years to come.
The measurement of relative values of gravity, that is,
the differences of gravity between locations, is simpler
and is the standard procedure in gravity surveying. Ab-
solute gravity values at survey stations may be obtained
by reference to the International Gravity Standardiza-
tion Network (IGSN) of 1971 (Morelli et al. 1971), a
network of stations at which the absolute values of grav-
ity have been determined by reference to sites of absolute
gravity measurements (see Section 6.7). By using a rela-
tive reading instrument to determine the difference
in gravity between an IGSN station and a field location
the absolute value of gravity at that location can be
determined.
Previous generations of relative reading instruments
were based on small pendulums or the oscillation of
torsion fibres and, although portable, took considerable
time to read. Modern instruments capable of rapid
gravity measurements are known as gravity meters or
gravimeters .
Gravimeters are basically spring balances carrying a
constant mass.Variations in the weight of the mass caused
by variations in gravity cause the length of the spring to
vary and give a measure of the change in gravity. In Fig.
6.1 a spring of initial length s has been stretched by an
amount d s as a result of an increase in gravity d g increas-
ing the weight of the suspended mass m . The extension
of the spring is proportional to the extending force
(Hooke's Law), thus
mg ks
=
d
d
and
m
k
s
=
g
d
d
(6.4)
where k is the elastic spring constant.
d s must be measured to a precision of 1 : 10 8 in instru-
ments suitable for gravity surveying on land. Although a
large mass and a weak spring would increase the ratio
m / k and, hence, the sensitivity of the instrument, in
practice this would make the system liable to collapse.
Consequently, some form of optical, mechanical or
electronic amplification of the extension is required in
practice.
The necessity for the spring to serve a dual function,
namely to support the mass and to act as the measur-
ing device, severely restricted the sensitivity of early
gravimeters, known as stable or static gravimeters. This
 
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