Geology Reference
In-Depth Information
mGal
-0.0075
-0.015
-0.0225
-0.030
24
48
72
96
120
d
(m)
5.0
Z
(m)
-10.0
4.5-m diameter air void
10-m diameter zone of weak chalk
Top at 0.5 m depth
Top at 3.25 m depth
Density = 0.0 Mg m
-3
-20.0
Density = 1.9 Mg m
-3
Chalk bedrock
Density = 2.1 Mg m
-3
-30.0
Figure 1.3
Ambiguity in potential field interpretation. The two very different
sources produce almost identical gravity anomalies.
modelled with the density of 2.1 Mg m
−
3
typical of more competent chalk.
The gravitational attraction of each sphere can be calculated assuming the
mass deficit is concentrated at its centre. The two anomalies are almost
identical, and a follow-on intrusive investigation of each, or a survey using a
corroborative geophysical method such as electrical resistivity tomography
(Section 6.5) would be required to resolve the ambiguity. Even non-identical
anomalies may, of course, differ by amounts so small that they cannot be
distinguished in field data.
Ambiguity worries interpreters more than it does the observers in the
field, but its existence does emphasise the importance of those observers
including in their field notes anything that might possibly contribute to a
better understanding of the data that they collect.
1.2.3 Two-dimensional sources
Rates of decrease in field strengths depend on source shapes as well as on
the inverse-square law. Infinitely long sources of constant cross-section are
