Geology Reference
In-Depth Information
Figure 2.12
Detailed Bouguer anomaly profile over a subsurface cavity,
illustrating the definition of the 'half-width'. (See Examples 2.2 and 2.3).
2.5 Field Interpretation
Gravity results are usually interpreted by calculating the fields produced by
geological models and comparing these with the actual data. This requires
a computer, and until recently was rarely done in the field. Even now, an
appreciation of the effects associated with a few simple bodies can help
an observer when actually on traverse or otherwise temporarily severed
from his or her laptop to assess the validity and significance of the data
being collected. This can sometimes lead to a vital decision to infill with
additional stations being taken at a time when this can be done quickly and
economically.
Example 2.2
Interpreting the anomaly of Figure 2.12 as due to a roughly
spherical
air-
filled cavity in rock of density 2.5 Mg m
−
3
and working in metres:
ρ
=−
2.5 and the anomaly amplitude
=
g
=
0.05 mGal.
Anomaly half-width
=
2 m. Therefore, the depth to the sphere centre
=
h
=
2
×
4/3
=
2.7 m.
r
3
=
g
×
h
2
)
/
.
×
ρ
=
.
×
.
7
2
)
/
.
×
.
=
.
.
(
(0
028
)
(0
05
2
(0
028
2
5)
5
2
i.e.
r
=
1.7 m.
