Geoscience Reference
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(a)
200
(b)
100
200
Free air
Free air
P
ratt D = 80 km
Airy D = 30 km
Airy D = 20 km
100
0
Airy D = 30 km
0
−
100
−
200
−
100
−
200
−
300
Bouguer
Bouguer
−
400
−
300
With 100% compensation
With 75% compensation
0
20
0
2850 kg m
−
3
2850 kg m
−
3
20
40
40
60
60
3300 kg m
−
3
3300 kg m
−
3
80
80
0
200 km
0
200 km
(c)
Figure 5.8.
Gravity anomalies over a schematic mountain range.
(a) The mountain range is 100% compensated (Airy type). (b) The
mountain range is 75% compensated (Airy type). (c) The mountain
range is uncompensated. Dashed lines show the free-air and
Bouguer anomalies that would be measured over the mountain;
solid lines show isostatic anomalies calculated for the density
models (Pratt compensation depth
D
= 80 km; Airy
D
= 20 km and
30 km). Densities used to calculate the isostatic anomalies are
(fortuitously) those shown in the model. (After Bott (1982).)
400
Free air
300
Airy D = 30 km
200
100
Bouguer
0
−
100
With 0% compensation
0
2850 kg m
−
3
20
40
60
3300 kg m
−
3
80
0
200 km
The effect that isostatic compensation has on the gravity anomalies is illus-
trated in Fig. 5.8.Figure 5.8(a) shows a schematic wide mountain range that is
totally compensated. The Bouguer anomaly across this model is therefore very
large and negative, whereas the free-air anomaly is small and positive in the cen-
tre of the model and large and positive at the edge of the mountains. Also shown
in Fig. 5.8(a) are isostatic anomalies for three possible density models that have
been formulated in order to test whether the structure is in isostatic equilibrium.
All three isostatic anomalies are very close to zero and the anomaly calculated
for Airy-type compensation with
D
30 km is exactly zero. The fact that the
other two anomalies are almost zero indicates that the structure is in isostatic
equilibrium.
Figure 5.8(b) shows the same mountain range, but this time it is only 75% com-
pensated. Now the free-air anomaly is large and positive, whereas the Bouguer
=
