Geology Reference
In-Depth Information
set at a known elevation and a mobile field set, can pro-
vide elevations to an accuracy of some 25 mm.
(Fig. 6.10) and drift is assumed to be linear between
consecutive base readings. The drift correction at time
t
is
d
, which is subtracted from the observed value.
After drift correction the difference in gravity be-
tween an observation point and the base is found by
multiplication of the difference in meter reading by the
calibration factor of the gravimeter. Knowing this differ-
ence in gravity, the absolute gravity at the observation
point
g
obs
can be computed from the known value of
gravity at the base. Alternatively, readings can be related
to an arbitrary datum, but this practice is not desirable
as the results from different surveys cannot then be tied
together.
6.8 Gravity reduction
Before the results of a gravity survey can be interpreted
it is necessary to correct for all variations in the Earth's
gravitational field which do not result from the differ-
ences of density in the underlying rocks. This process
is known as
gravity reduction
(LaFehr 1991) or
reduction
to the geoid
, as sea-level is usually the most convenient
datum level.
6.8.1 Drift correction
Correction for instrumental drift is based on repeated
readings at a base station at recorded times through-
out the day. The meter reading is plotted against time
6.8.2 Latitude correction
Gravity varies with latitude because of the non-spherical
shape of the Earth and because the angular velocity of a
point on the Earth's surface decreases from a maximum
at the equator to zero at the poles (Fig. 6.11(a)).The cen-
tripetal acceleration generated by this rotation has a
negative radial component that consequently causes
gravity to decrease from pole to equator.The true shape
of the Earth is an oblate spheroid or polar flattened ellip-
soid (Fig. 6.11(b)) whose difference in equatorial and
polar radii is some 21 km. Consequently, points near the
equator are farther from the centre of mass of the Earth
than those near the poles, causing gravity to increase
from the equator to the poles. The amplitude of this ef-
fect is reduced by the differing subsurface mass distribu-
tions resulting from the equatorial bulge, the mass
underlying equatorial regions being greater than that
underlying polar regions.
The net effect of these various factors is that gravity at
d
t
Time
Fig. 6.10
A gravimeter drift curve constructed from repeated
readings at a fixed location.The drift correction to be subtracted
for a reading taken at time
t
is
d
.
(a)
(b)
Fig. 6.11
(a) The variation in angular
velocity with latitude around the Earth
represented by vectors whose lengths are
proportional to angular velocity. (b) An
exaggerated representation of the shape
of the Earth.The true shape of this
oblate ellipsoid of revolution results in a
difference in equatorial and polar radii
of some 21 km.
