Geology Reference
In-Depth Information
15 m grid at points whose elevations had been deter-
mined to ±3 mm, with base readings at 40 min intervals.
The soil thickness had been determined so that its effects
could be computed and 'stripped' from the observations
to remove gravity variations caused by undulating
bedrock topography. The resulting Bouguer anomaly
map is shown in Fig. 6.30. In the NE part of the site there
are two minima near the proposed perimeter of the cool-
ing tower, and subsequent drilling confirmed that they
originated from buried cavities. Remedial work entailed
the injection of grouting material into the cavities. A
check on the effectiveness of the grouting was provided
by a repeat gravity survey which, by an excess mass
calculation (Section 6.10.3), showed that the change in
the gravity field before and after grouting was caused by
the replacement of voids by grouting material. Casten
and Gram (1989) have described microgravity surveys
performed underground to locate cavities which might
pose a threat to the safety of mine workings.
Microgravity surveys also find application in archaeo-
logical investigations, where they may be used in the de-
tection of buried buildings, tombs and other artefacts.
The technique has also been used to study the temporal
movement of groundwater through a region.
An important recent development in gravity survey-
ing is the design of portable instruments capable of mea-
suring absolute gravity with high precision. Although
the cost of such instruments is high it is possible that they
will be used in the future to investigate large-scale mass
movements in the Earth's interior and small cyclic gra-
vity variations associated with neotectonic phenomena
such as earthquakes and postglacial uplift.
Gravitational studies, both of the type described in
this chapter and satellite observations, are important in
geodesy, the study of the shape of the Earth. Gravity sur-
veying also has military significance, since the trajectory
of a missile is affected by gravity variation along its flight
path.
N
17
17
14
17
0m 0
Fig. 6.30
Bouguer anomalies, uncorrected for topographic
effects, over the cooling tower area. Contour interval 0.3 gu.
(After Arzi 1975.)
method (see Section 5.4).The seismic control allowed a
mean density of the highly variable valley-fill deposits to
be determined. On the basis of the geophysical results,
two boreholes (Fig. 6.28) were sunk in the deepest parts
of the valley fill and located groundwater ponded in the
bedrock depressions.
In engineering and geotechnical applications, gravity
surveying is sometimes used in the location of subsurface
voids.Void detection has been made possible by the de-
velopment of microgravimetric techniques which can
detect gravity changes as small as a microgal. Arzi (1975)
described a microgravity survey of the proposed site of
a cooling tower serving a nuclear power plant, where it
was suspected that solution cavities might be present in
the dolomitic bedrock. Measurements were made on a
Problems
base, and (c) half-way up a vertical cliff 100 m
high?
3.
The table shows data collected along a
north-south gravity profile. Distances are mea-
sured from the south end of the profile, whose
latitude is 51°12
¢
24
≤
N. The calibration constant
1.
Compare and contrast the LaCoste-Romberg
and Worden-type gravimeters. State also the ad-
vantages and disadvantages of the two types of
instrument.
2.
What are the magnitudes of the terrain cor-
rection at gravity stations (a) at the top, (b) at the
