Geoscience Reference
In-Depth Information
2.4.2 Relative Gravimetry
Relative gravimetry is a technique used to determine the gravity difference between
two points, and then to obtain the gravity value of each point in a pointwise manner
through at least one point of known gravity value.
The static method of relative gravimetry is to use a kind of force (such as the
spring force) to work against the force of gravity that is acting on the object and
balance the gravitational pull. By changing gravity, the location of the equilibrium
position (location of the spring) is also changed. As long as the change of the
equilibrium position (the amplitude of the spring) is determined, the variation in
gravity can be calculated (according to Hooke's Law). The gravity difference
between the two locations is thus obtained (Lu 1996).
Currently, the most frequently used gravimeter is called the spring gravimeter, in
which the spring force is used to balance gravity. Examples are the quartz spring
gravimeter ZSM series manufactured by Beijing Geological Instrument Factory and
the (LCR) metal spring gravimeter by LaCoste and Romberg in the USA. Both of
these spring gravimeters incorporate a spring mass system, optical system, mechan-
ical device for measurement, instrument panel, and insulated case. The range of
gravity difference measured by ZSM is 80
10
5
m/s
2
to 120
10
5
m/s
2
and the
10
5
m/s
2
The
LCR gravimeter can also be classified as Model G and Model D. The range of direct
measurement of the Model G is up to 7,000
10
5
m/s
2
and 0.3
precision of measurement is between 0.1
10
5
m/s
2
It can be utilized for
relative gravity measurement on a worldwide scale and its measurement precision
amounts to
10
8
m/s
2
The range of direct measurement of Model D is only
20
10
5
m/s
2
It is widely used in regional gravity surveys and its precision of
measurement is slightly higher than that of Model G.
200
2.4.3 Airborne Gravimetry
Airborne gravimetry is a method employed to determine the near-Earth gravita-
tional acceleration using an integrated airborne gravity remote sensing system,
which consists of an aircraft as carrier, airborne gravimeter, GPS, altimeter, and
attitude determination devices, etc. (Fig.
2.21
). It can operate in areas where
terrestrial gravity measurement is hard to conduct such as deserts, ice sheets,
marshlands, and primeval forests. It can acquire information on the gravity field
at a fast pace, with high precision, on a large scale, and with even distribution.
Compared to the classical technique of terrestrial gravity measurement, it is entirely
different in terms of measuring instrument, motion carrier, measuring technique,
methods of data collection, as well as theory of data reduction, etc. Airborne gravity
measurement has fully demonstrated the integrated application of modern technol-
ogies in the field of geodetic survey. It is of vital significance to geodesy, geophys-
ics, oceanography, resources exploration, and space science.
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