Geoscience Reference
In-Depth Information
3.10
Interpretation of gravity and
magnetic data
a)
10
0
Field of common
lithotypes
Interpreting spatial variations in the Earth
'
s gravity and
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-1
magnetic
task in mineral
exploration. Importantly, gravity and magnetics are often
interpreted together and, since the principles of interpret-
ation are fundamentally the same for both data types, we
considered them in combination. In so doing, we consider
the relationships between anomalies and their source pos-
itions and geometries, in particular the in
uence of source
depth. This is fundamental to making geologically mean-
ingful interpretations of potential
field data. We present a
number of case studies demonstrating the applications and
interpretations of gravity and magnetic data.
Features of interest identified during the qualitative
analysis of spatial variations in the gravity and/or total
magnetic intensity (TMI) data can be modelled (see
Section
2.11
) in order to estimate the location, depth and geometry
of the anomalous sources. Both forward and inverse model-
ling techniques are routinely used. Simple automated tech-
niques of estimating the location and depth of the source
offer the advantage of being able to analyse large volumes of
data rapidly, the results being used as the basis for more
sophisticated analysis of targeted features. Parametric
models (see
Section 2.11.1.1
)
provide additional informa-
tion for a particular source shape, such as the depth, width
and dip. Property distributions of greater complexity (see
Section 2.11.1.2
)
can be resolved using inversion techniques,
but require considerably more time and effort to apply.
Removal of the regional response is often required in
order to resolve the target anomaly and can be done with
any of the techniques described in
Sections 2.9.2
and
3.7.3
;
but obtaining satisfactory resolution of the local anomaly
is not always easy. Non-uniqueness or ambiguity (see
Section 2.11.4
) is always prevalent, being reduced with
greater geological control. These two factors represent the
greatest sources of uncertainty in the modelling of gravity
and magnetic data.
fields is a common
'
geophysical
'
Granulite
10
-2
10
-3
10
-4
Amphibolite
10
-5
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
Density (g/cm
3
)
b)
10
0
10
-1
10
-2
10
-3
10
-4
Mafic & ultramafic rocks
Other rock types
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-5
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
Density (g/cm
3
)
Figure 3.63
Correlation between density and magnetic susceptibility.
(a) Correlation fields for most lithotypes; and some common
geological processes, schematically indicated, that move the property
values from one field to the other. Redrawn, with permission, from
Henkel (
1991
). (b) Data for various lithotypes from a granitoid-
greenstone terrain in the Abitibi Subprovince. Data from Ontario
Geological Survey (
2001
).
greenstone
terrain in the Abitibi Subprovince, Ontario, Canada (see
Section 3.11.3
). Data plot in both the paramagnetic and
magnetite
fields. The metamorphic grade in the region is
greenschist facies so ma
c rocks tend to have low suscepti-
bilities and, therefore, occupy the paramagnetic
eld,
although the data falling on the serpentinisation trend
suggest this process has occurred.
Figure 3.63b
shows data from a granitoid
-
3.10.1
Gravity and magnetic anomalies and
their sources
The fundamental features of geophysical responses
These are con
rmed here by showing variations in total
magnetic intensity and gravity across sources with simple
geometry embedded in a
'
background
'
volume.
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