Geology Reference
In-Depth Information
from the gravity station and can be readily comput-
erized. Such an approach is likely to be increasingly
adopted as digital elevation models for large regions be-
come available (Cogbill 1990). Correction for inner
zones, however, must still be performed manually as any
reasonable digitization scheme for a complete survey
area and its environs must employ a sampling interval
that is too large to provide an accurate representation of
the terrain close to the station.
Terrain effects are low in areas of subdued topography,
rarely exceeding 10 gu in flat-lying areas. In areas of
rugged topography terrain effects are considerably
greater, being at a maximum in steep-sided valleys, at the
base or top of cliffs and at the summits of mountains.
Where terrain effects are considerably less than the
desired accuracy of a survey, the terrain correction may
be ignored. Sprenke (1989) provides a means of assessing
the distance to which terrain corrections are necessary.
However, the usual necessity for this correction accounts
for the bulk of time spent on gravity reduction and is thus
a major contributor to the cost of a gravity survey.
effects are predictable and can be computed by a small
computer program.
6.8.5 Eötvös correction
The Eötvös correction (EC) is applied to gravity mea-
surements taken on a moving vehicle such as a ship or an
aircraft. Depending on the direction of travel, vehicular
motion will generate a centripetal acceleration which
either reinforces or opposes gravity. The correction
required is
2
EC
=
75 03
.
V
sin
af
cos
+
0 04154
.
V
gu
where
V
is the speed of the vehicle in knots,
a
the head-
ing and
f
the latitude of the observation. In mid-
latitudes the Eötvös correction is about +75 gu for
each knot of E to W motion so that speed and heading
must be accurately known.
6.8.6 Free-air and Bouguer anomalies
The
free-air anomaly
(FAA) and
Bouguer anomaly
(BA)
may now be defined
6.8.4 Tidal correction
Gravity measured at a fixed location varies with time be-
cause of periodic variation in the gravitational effects of
the Sun and Moon associated with their orbital motions,
and correction must be made for this variation in a high-
precision survey. In spite of its much smaller mass, the
gravitational attraction of the Moon is larger than that
of the Sun because of its proximity. Also, these gravita-
tional effects cause the shape of the solid Earth to vary in
much the same way that the celestial attractions cause
tides in the sea. These
solid Earth tides
are considerably
smaller than oceanic tides and lag farther behind the
lunar motion.They cause the elevation of an observation
point to be altered by a few centimetres and thus vary its
distance from the centre of mass of the Earth. The peri-
odic gravity variations caused by the combined effects of
Sun and Moon are known as
tidal variations
. They have
a maximum amplitude of some 3 gu and a minimum
period of about 12 h.
If a gravimeter with a relatively high drift rate is used,
base ties are made at an interval much smaller than the
minimum Earth tide period and the tidal variations are
automatically removed during the drift correction. If a
meter with a low drift rate is employed, base ties are nor-
mally made only at the start and end of the day so that the
tidal variation has undergone a full cycle. In such a case, a
separate tidal correction may need to be made.The tidal
FAA
ob
=-+
g
g
f
FAC
(
±
EC
)
(6.12)
BA
ob
=-+ ±+ ±
g
g
f
FAC
BC
TC
EC
(
)
(6.13)
The Bouguer anomaly forms the basis for the inter-
pretation of gravity data on land. In marine surveys
Bouguer anomalies are conventionally computed for in-
shore and shallow water areas as the Bouguer correction
removes the local gravitational effects associated with
local changes in water depth. Moreover, the computa-
tion of the Bouguer anomaly in such areas allows direct
comparison of gravity anomalies offshore and onshore
and permits the combination of land and marine data
into gravity contour maps.These may be used, for exam-
ple, in tracing geological features across coastlines. The
Bouguer anomaly is not appropriate for deeper water
surveys, however, as in such areas the application of a
Bouguer correction is an artificial device that leads to
very large positive Bouguer anomaly values without sig-
nificantly enhancing local gravity features of geological
origin. Consequently, the free-air anomaly is frequently
used for interpretation in such areas. Moreover, the FAA
provides a broad assessment of the degree of isostatic
compensation of an area (e.g. Bott 1982).
Gravity anomalies are conventionally displayed on
