Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 68

The interpretation may commence by delineating the
large-scale features, which probably extend across large
areas of the survey, and then move on to progressively
smaller features. On the other hand, sometimes the data
in the vicinity of geological features of interest, or geologic-
ally well-known areas, are analysed in detail first, in order
to resolve the

must be accounted for in the interpretation. Some
examples of this effect are shown in Figs. 3.74 and 3.76 .
As in field geological mapping, an early and crucial deci-
sion is to determine what constitutes a

.In
both cases this is based on characteristics that can be easily
and reliably recognised. Whether working with geological
or geophysical data, it is often necessary for different litho-
types to be grouped together into a single mappable unit,
especially if individual units comprising a layered sequence
are thin, small, or discontinuous, or the inter-relationships
between them are too complex to illustrate at the scale of the
mapping. Sometimes a single distinctive

' mappable unit '

of the area. The
interpretation is then extended into the surroundings, pos-
sibly with the intention of identifying repeated occurrences
of the same signature. Sometimes only shallow responses
or only the deeper features may be all that is of interest, so
the interpretation will focus on the appropriate responses,
albeit with a

' geophysical signature '

horizon
may be recognised and mapped, conveniently revealing the
overall structure and structural style. In geological mapping
this might be a particularly well-exposed horizon, or one
with a distinctive mineralogy or fossil assemblage. In geo-
physical terms, it might be a unit with anomalously high or
low density, magnetism, conductivity or radioactivity etc.
The outcropmap provides important reference information
during this part of the interpretation. Nevertheless, the inter-
preter must be aware of apparent contradictions between the
geology and the geophysics caused by the fundamental differ-
ences between geological and geophysical responses (see
Section 1.1 ) and due to the depth information in the geophys-
ical signal. A common example of the latter is where a relatively
thin sequence of non-magnetic sediment overlies magnetic
bedrock. The outcrop map will show the sediment and the
magnetic dataset will show responses from the bedrock.
As the interpretation progresses, it is important to check
continually for geological credibility. In an area with a
layered succession, each unit must be assigned a place
within a local stratigraphy, although this will be a pseudo-
lithostratigraphy since it will reflect physical property vari-
ations rather than explicitly lithotype. A rigorous adherence
to stratigraphic rules and the implied outcrop patterns is as
necessary for a pseudo-geological map based on geophys-
ical data as it is for one based on geological data. Of course
physical properties may vary within a unit just as lithotypes
may vary laterally, potentially confusing the mapper.

' marker '

for the implications on the interpret-
ation of the other responses in the data.

' feeling '

2.10.2.1 Framework of linear features
The first step in making a pseudo-geological map is the

creation of a structural framework from the linear and curvi-
linear features identi ed in the data. These may be linear and
curvilinear anomalies, lineations interpreted from the align-
ment or truncation of other features, zones of greater struc-
tural complexity or areas of pronounced gradients, and
usually coincide with contacts and/or faults/shear zones.
Edge-enhancement forms of the data, i.e. some gradient-based
transform, are most useful for this aspect of the interpretation.
When identifying linear features on the basis of align-
ment and truncation of responses, it is important to
remember that the geophysical response, especially when
presented in its fundamental (untransformed) form, will
often extend beyond the surface projection of the source,
especially if the source is at depth ( Fig. 2.4 ). This means an
interpreted linear can legitimately the end of an

anomaly. However, this also means it is easy to mistakenly
interpret non-existent features, so caution is required.

' cut off '

2.10.2.2 Classification of regions
The next step is to map the units onto the

framework of linear features. This involves de ning poly-
gons in the GIS. It is essential when analysing images not
to assign too much signi cance to actual colours since
these depend on the colour map and the stretch function
(see Section 2.8.2 ) used for the display. Instead, consistent
textural and tonal characteristics are the basis for mapping
lithological subdivisions. The same unit will change char-
acter if it extends under cover owing to the increasing
separation between source and sensor (anomalies will
decrease in amplitude and increase in wavelength). This

' lithological '

2.10.2.3 Estimating dip and plunge
During the mapping process it can be very useful to have
some idea of the orientation of the sources of the geophys-
ical responses. For example, a fold may be recognised in
the geophysical data from its outcrop pattern, but this does
indicate whether it is a synform or an antiform. Orienta-
tion information can be most accurately determined using
modelling (see Section 2.11.2 ). However, much quicker