Geology Reference
In-Depth Information
battery-powered thermostatting system. The range of
the instrument is 50 000 gu.
The other unstable instrument in common use is the
Worden-type gravimeter. The necessary instability is
provided by a similar mechanical arrangement, but in
this case the beam is supported by two springs. The first
of these springs acts as the measuring device, while the
second alters the level of the 2000 gu reading range of the
instrument. In certain specialized forms of this instru-
ment the second spring is also calibrated, so that the
overall reading range is similar to that of the LaCoste
and Romberg gravimeter.Thermal effects are normally
minimized by the use of quartz components and a
bimetallic beam which compensates automatically for
temperature changes. Consequently, no thermostatting
is required and it is simply necessary to house the instru-
ment in an evacuated flask. The restricted range of
normal forms of the instrument, however, makes it un-
suitable for intercontinental gravity ties or surveys in
areas where gravity variation is extreme.
Gravimeters for general surveying use are capable of
registering changes in gravity with an accuracy of 0.1 gu.
A new generation of more efficient zero-length springs
has been developed. Microprocessor-controlled instru-
ments are now available which are, within limits, self-
levelling, and which allow observations to be made
rapidly. Also available for more specialized surveys (Sec-
tion 6.12) are gravimeters capable of detecting gravity
changes as small as 1 microgal (10
-8
ms
-2
).
A shortcoming of gravimeters is the phenomenon of
drift
.This refers to a gradual change in reading with time,
observable when the instrument is left at a fixed location.
Drift results from the imperfect elasticity of the springs,
which undergo anelastic creep with time. Drift can also
result from temperature variations which, unless coun-
teracted in some way, cause expansion or contraction of
the measuring system and thus give rise to variations in
measurements that are unrelated to changes in gravity.
Drift is monitored by repeated meter readings at a fixed
location throughout the day.
Gravity can be measured at discrete locations at sea
using a remote-controlled land gravimeter, housed in
a waterproof container, which is lowered over the side
of the ship and, by remote operation, levelled and read
on the sea bed. Measurements of comparable quality to
readings on land can be obtained in this way, and the
method has been used with success in relatively shallow
waters.The disadvantage of the method is that the meter
has to be lowered to the sea bed for each reading so that
the rate of surveying is very slow. Moreover, in strong
Adjusting screw
Beam
θ
mg
θ
'
Hinge
m
(
g +
δ
g
)
Fig. 6.2
Principle of the LaCoste and Romberg gravimeter.
problem is overcome in modern meters (unstable or
astatic) which employ an additional force that acts in the
same sense as the extension (or contraction) of the spring
and consequently amplifies the movement directly.
An example of an unstable instrument is the LaCoste
and Romberg gravimeter.The meter consists of a hinged
beam, carrying a mass, supported by a spring attached
immediately above the hinge (Fig. 6.2). The magnitude
of the moment exerted by the spring on the beam is de-
pendent upon the extension of the spring and the sine of
the angle
q
. If gravity increases, the beam is depressed and
the spring further extended. Although the restoring
force of the spring is increased, the angle
q
is decreased to
q
¢. By suitable design of the spring and beam geometry
the magnitude of the increase of restoring moment with
increasing gravity can be made as small as desired. With
ordinary springs the working range of such an instru-
ment would be very small. However, by making use
of a 'zero-length' spring which is pretensioned during
manufacture so that the restoring force is proportional to
the physical length of the spring rather than its exten-
sion, instruments can be fashioned with a very sensitive
response over a wide range. The instrument is read by
restoring the beam to the horizontal by altering the ver-
tical location of the spring attachment with a micro-
meter screw. Thermal effects are removed by a
