Geoscience Reference
In-Depth Information
gradients usually occur on the down-dip side of the source
and, similarly, for a plunging source the gradients are less
in the down-plunge direction. This general statement
assumes uniform and isotropic physical properties within
the source.
Another way to infer dip is to compare the geophysical
response in processing products emphasising responses
from different depths, or to compare responses from geo-
physical methods which have different depths of penetra-
tion. Dipping bodies can show apparent lateral offsets in the
various responses, which can be a good indicator of dip.
the observed data and, in an iterative process, the model is
adjusted and its response recomputed until there is a
satisfactory correspondence. This process is known as
modelling. We describe here the fundamental aspects of
geophysical models and modelling techniques common to
the analysis of all types of geophysical data, whilst details
speci
c to particular geophysical methods are given in our
descriptions of the various methods.
Most geophysical methods respond to changes (con-
trasts) in the physical property distribution of the subsur-
face, so only relative changes in the physical properties are
modelled. For example, a gravity target may be modelled
as, say, 0.2 g/cm
3
denser than its host rocks, with the actual
densities of the target and host unresolved by the model. In
some cases, for example with magnetic and electrical data,
the physical property of the source may be several orders of
magnitude different from that of the surrounding rock, so
that the observed relative response is, for practical pur-
poses, entirely due to the absolute properties of the anom-
alous body.
Models that are different in terms of the distribution of
their absolute physical properties may be the same in terms
of relative physical property contrasts, so that they produce
the same response, i.e. they are geophysically equivalent.
Generally the relative model is simpler. The physics under-
lying the different geophysical methods may make it impos-
sible to differentiate the physical property variation within
the source, so many different models may be equivalent.
Equivalent models are method-specific and discussed in
the chapters on each geophysical method. Equivalence is
extremely useful for reducing a complex physical property
distribution to a mathematically simpler model.
2.10.2.4
Using the interpretation
As the interpretation evolves, it can be used to determine
the geology of the area, and also for identifying conceptual
exploration targets, e.g. dilational zones where faults
change their orientation, intersecting faults, etc. Again,
we emphasise that geophysical data contain responses
from a range of depths, so the data should be interpreted
with this in mind. The pseudo-geological map is not a
geological map. Not only may there be differences in the
units mapped, care must be also taken when interpreting
relative timing of events based on cross-cutting relation-
ships; apparently younger features may actually be shal-
lower or simply have a much stronger geophysical
response obscuring that of others in the vicinity. Deter-
mining which linear offsets another can be very dif
cult to
establish, because of smoothing associated with gridding
and directional bias associated with data presented as
shaded-relief (see
Section 2.8.2.3
).
2.11
Data interpretation - quantitative
analysis
When a pseudo-geological map of the desired extent and
detail has been created, the interpreter may select areas for
more detailed analysis. This is most likely done to accur-
ately establish the depth and geometry of the sources of the
anomalies
2.11.1
Geophysical models of the subsurface
The geological environment can be very complex, but by
necessity it must be represented by a manageable number
of model parameters. This can result in a hugely simpli-
fied representation of the subsurface. Greater geological
complexity can be represented with more complex
models, but these require more effort to de
ne and
require a greater degree of competence to make effective
use of the additional complexities. Moreover, there is no
advantage in using a geophysical model whose complex-
ity is greater than the resolving capabilities of the data
being modelled, which may be quite limited, especially
for deeper regions.
information required for constructing cross-
sections to complement the qualitative interpretation, and
in particular for drill-testing the anomaly sources.
Fundamental to quantitative interpretation is the repre-
sentation of the local geology in terms of a numerical
model, whose purpose is to allow the geophysical response
of geology to be computed. The model is de
ned by a series
of parameters, which de
ne the geometry and physical
properties of regions within the subsurface. The geophys-
ical response of the model is computed and compared with
-
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