Geology Reference
In-Depth Information
0
10 km
268
Sea
49
423
46
640
45
60
48
330
141
78
402490
650
698
642
481
Seismic Line
635
51
497
N
295
Sea
45
Fig. 6.33
Map of geophysical
observations pertaining to
Question 9. Bouguer anomaly
values in gu.
Mesozoic
sediment
Schist
Gabbro
Bouguer anomaly (gu)
the gabbro. Assuming the gabbro to have the
form of a vertical cylinder, determine the depth
to its base.
The gravity anomaly
D
g
of a vertical cylinder of
density contrast
D
r
, radius
r
, length
L
, depth to
top
z
U
and depth to base
z
L
is given by
Seismic data
Dist. (m)
Time (s)
530
0.349
600
0.391
670
0.441
1130
0.739
1200
0.787
(
)
g
=
2
GLz
-
2
+
r
2
+
z
2
+
r
2
D
pr
D
1270
0.831
L
U
1800
1.160
1870
1.177
where
G
is the gravitational constant.
State any assumptions and possible causes of
error in your interpretation.
1940
1.192
2730
1.377
2800
1.393
2870
1.409
Typical densities and seismic velocities
3530
1.563
3600
1.582
r
(Mg m
-
3
)
Veloc. (km s
-
1
)
3670
1.599
Jur./Cret.
2.15
1.20-1.80
Trias
2.35
2.40-3.00
10.
Over a typical ocean spreading centre, the
free-air gravity anomaly is approximately zero
and the Bouguer anomaly is large and negative.
Why?
Schist
2.75
3.60-4.90
Gabbro
2.95
Jur.
=
Jurassic; Cret.
=
Cretaceous.
Bott, M.H.P. (1973) Inverse methods in the interpretation of
magnetic and gravity anomalies. In: Alder, B., Fernbach, S.
& Bolt, B.A. (eds),
Methods in Computational Physics
,
13
, 133-
62.
Dehlinger, P. (1978)
Marine Gravity
. Elsevier, Amsterdam.
Gibson, R.I. & Millegan, P.S. (eds) (1998)
Geologic Applications of
Further reading
Baranov, W. (1975)
Potential Fields and Their Transformations in
Applied Geophysics
. Gebrüder Borntraeger, Berlin.
Blakely, R.J. (1995)
Potential Theory in Gravity and Magnetic Applica-
tions
. Cambridge University Press, Cambridge.