Geology Reference
In-Depth Information
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Fig. 6.32
Bouguer anomaly observations pertaining to Question 8.Values in gu.
which can be represented on the map as con-
tours drawn in a different colour.
(b) Construct gravity profiles along line A-A
ยข
illustrating the observed, regional and residual
anomalies.
(c) Perform a direct interpretation of the residual
anomaly, obtaining as much information as pos-
sible on the depth, thickness and shape of the
source.
(d) The bedrock constitutes part of a Precam-
brian shield. Speculate on the nature of the
anomalous body, giving reasons for your ideas.
8.
Contour the gravity data on the map shown in
Fig. 6.32 using an interval of 10 gu. Draw a repre-
sentative profile.
(a) Use limiting depth calculations based on the
half-width and gradient-amplitude methods to
determine the depth to the centre of mass of the
anomalous body. Comment on any difference
between the depth estimates provided by the two
methods.
(b) Determine the mass deficiency present using
the formula for the gravity anomaly of a point
mass. If the anomaly is caused by a salt dome of
density 2.22 Mg m
-
3
within sediments of density
2.60 Mg m
-
3
, calculate the volume and mass of
salt present and the depth to the top of the salt
dome. Compute the actual gravity anomaly of
the salt and comment on any differences with the
observed anomaly.
(c) What is the lowest possible density contrast
of the anomalous body?
(d) Determine the mass deficiency present using
a method based on Gauss' Theorem. Comment
on the accuracy of the value obtained and
compare it with the answer to (b). Calculate the
actual mass present assuming the same densities
as in (a).
9.
The map in Fig. 6.33 shows Bouguer anom-
alies over a gabbro intrusion in a schist terrain. In
the eastern part of the map, horizontally bedded
Mesozoic sediments unconformably overlie the
schists. A seismic refraction line has been estab-
lished over the sediments in the location shown.
Time-distance data and typical velocities and
densities are given below.
Interpret the geophysical results using the fol-
lowing scheme:
(a) Use the refraction data to determine the
thickness and possible nature of the Mesozoic
rocks beneath the seismic line.
(b) Use this interpretation to calculate the gra-
vity anomaly of the Mesozoic rocks at this loca-
tion. Correct the observed gravity anomaly for
the effect of the Mesozoic rocks.
(c) Determine the maximum gravity anomaly of
