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between aeromagnetic and airborne electromagnetic
anomalies was very successfully used as a target selection
criterion for nickel sulphide mineralisation in the Can-
adian Shield in the 1950s (Dowsett,
1967
).
a)
b)
When the principal object is anomaly detection rather than
geological mapping, context can be the only means of
ranking the anomalies as exploration targets. However,
discriminating a target anomaly from unwanted target-
like anomalies is a fundamental challenge for the inter-
preter. Often geophysical data are unable to differentiate
between responses from mineralised environments and
geologically similar, but unmineralised, settings.
d)
c)
e)
f)
2.10.1.4
Human perceptions of spatial data
The interpretation process relies on the human mind pro-
cessing information collected by the human eye to perceive
patterns in spatial data, and then interpreting these pat-
terns in terms of
h)
g)
the geology. Despite the expression
'
, this is far from an infallible process.
This aspect of being human is poorly understood and
almost completely unexplored from a geophysical context,
so only a few general observations are possible. Import-
antly, we emphasise and demonstrate the real possibility of
making erroneous interpretations of patterns in a dataset.
The reader will be familiar with optical illusions, where
the eye/brain system misinterprets one or more character-
istics of an object in a picture. The most relevant examples
in the context of geophysical image analysis are linear and
circular features, since these are fundamental components
of the geological environment.
Figures 2.41a
and
b
dem-
onstrate how obliquely intersecting lines produce illusion-
ary offsets and misjudgement of parallelism. The former is
a commonly occurring line-geometry in geophysical inter-
pretations, with the potential result being incorrect inter-
pretations of faulting. The next two examples,
Figs. 2.41c
ceive non-existent lines. The potential exists in geophysical
data for the interpretation of spurious linear features.
Figure 2.41e
shows the strong human tendency to see
circles, even when there are none present, or even when
there are no curved lines in the area.
Figure 2.41f
shows
how abrupt changes in image intensity create the illusion
of deviations in straight lines. Although all three arcs in
Fig. 2.41g
have the same radius, they are perceived to be
different, which may cause problems when interpreting
data containing curvilinear features. Finally, the two cen-
tral squares in
Fig. 2.41h
are the same shade of grey, but
seeing is believing
'
Figure 2.41
Examples of some optical illusions that can be misleading
in the interpretation of geophysical data. See text for explanation.
the different shades of the surrounding areas disguise this
fact. The same illusion occurs with coloured regions.
Another common form of optical illusion is the inver-
sion of apparent topography associated with the illumin-
ation direction in a shaded relief display (see
Section
specifically rely on the identification of positive features,
for example the analytic signal transformation applied to
potential
field data (see
Section 3.7.4.2
)
. The possibility of
making erroneous interpretations is obvious.
2.10.1.5
A cautionary note
Figure 2.42a
shows the locations of sources of geophysical
responses, perhaps magnetic sources derived from Euler
deconvolution (see
Section 3.10.4.1
) or anomalous radio-
metric responses (see
Section 4.5.1
)
. From the various
patterns that can be seen in
Fig. 2.42a
, in particular linear
and circular alignments, and variations in their clustering,
a plausible geological interpretation has been made. Linear
alignments were identified first and then regions with
similar internal distributions of sources were delineated.
The interpretation recognised a faulted and folded layered
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