Geology Reference
In-Depth Information
of the Worden gravimeter used on the survey
is 3.792 gu per dial unit. Before, during and
after the survey, readings (marked BS) were taken
at a base station where the value of gravity is
9 811 442.2 gu. This was done in order to moni-
tor instrumental drift and to allow the absolute
value of gravity to be determined at each obser-
vation point.
D
g
=
2
p
G
Dr
t
where the gravitational constant
G
is 6.67
¥
10
-
11
m
3
kg
-
1
s
-
2
.
(a) Scale this equation to provide
D
g
in gu when
D
r
is expressed in Mg m
-
3
and
t
in m.
(b) This equation is used to provide a prelimi-
nary estimate of the gravity anomaly of a body of
specified thickness. Using this equation, calcu-
late the gravity anomaly of (i) a granite 12 km
thick of density 2.67 Mg m
-
3
; and (ii) a sandstone
body 4 km thick of density 2.30 Mg m
-
3
, where
the density of the surrounding metamorphic
rocks is 2.80 Mg m
-
3
. Are the anomalies so calcu-
lated liable to be over- or underestimates?
6.
Show that the half-width of the gravity anom-
aly caused by a horizontal cylinder is equal to the
depth of the axis of the cylinder.
7.
Figure 6.31 is a Bouguer anomaly map, con-
toured at an interval of 50 gu, of a drift-covered
area.
(a) On the map, sketch in what you consider to
be the regional field and then remove it from the
observed field to isolate residual anomalies,
Station
Time
Dist. (m)
Elev. (m)
Reading
BS
0805
2934.2
1
0835
0
84.26
2946.3
2
0844
20
86.85
2941.0
3
0855
40
89.43
2935.7
4
0903
60
93.08
2930.4
1
0918
2946.5
BS
0940
2934.7
1
1009
2946.3
5
1024
80
100.37
2926.6
6
1033
100
100.91
2927.9
7
1044
120
103.22
2920.0
X
1053
140
107.35
2915.1
1
1111
2946.5
BS
1145
2935.2
1
1214
2946.2
9
1232
160
110.10
2911.5
10
1242
180
114.89
2907.2
11
1300
200
118.96
2904.0
1
1315
2946.3
BS
1350
2935.5
A
(a) Perform a gravity reduction of the survey data
and comment on the accuracy of each step.
Use a density of 2.70 Mg m
-
3
for the Bouguer
correction.
(b) Draw a series of sections illustrating the
variation in topography, observed gravity, free-
air anomaly and Bouguer anomaly along the
profile. Comment on the sections.
(c) What further information would be required
before a full interpretation could be made of the
Bouguer anomaly?
4.
Two survey vessels with shipborne gravity
meters are steaming at 6 knots in opposite direc-
tions along an east-west course. If the difference
in gravity read by the two meters is 635 gu as the
ships pass, what is the latitude?
5.
The gravity anomaly
D
g
of an infinite horizon-
tal slab of thickness
t
and density contrast
Dr
is
given by
-550
-700
-650
A'
0
km
10
Fig. 6.31
Bouguer anomaly map pertaining to Question 7.
Contour interval 50 gu.
Continued
