Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 23

data usually assumes that the location of each reading is
adequate for the survey objectives. If this is not the case the
results will be erroneous, or at best lack resolution. With
the advent of satellite-based positioning, errors in location
have been greatly reduced, although elevation is often of
more importance than lateral position. Obtaining suffi- -
ciently accurate heights from satellite-based data requires
specialised equipment and processing of the location data.
Another potential source of positioning-related noise is the
incorrect and/or inconsistent use of map projections and
height datum. It goes without saying that any treatment of
data that combines information defined in different pos-
itional reference frames will result in errors unless the
various data are transformed to a single reference frame.
The problem is particularly common when satellite-based
positioning information, which is based on a global coord-
inate system, is combined with data positioned on a
ground-based local coordinate system.
Most geophysical surveys also record the time of the
measurements to allow temporal corrections to be applied
to some types of data, and to enable the integration of
different datasets. Minimising errors in time is, therefore,
crucial. Satellite navigation systems provide a convenient
and highly accurate time signal for this purpose.
Data processing and display techniques may inadvert-
ently create noise. A common example is short-wavelength
variations, or ripples, appearing in the processed data
through an effect known as ringing (see online Appendix 2 ),
which is caused by inappropriate design of the processing
algorithm. Sometimes geophysical surveys can be designed
to measure responses that predominantly originate from a
particular depth. By varying the depth of penetration and
the location of the surface measurements a

geophysical method that should be used and also the type
of survey, for example airborne or downhole. The two
most common objectives of mineral geophysical surveys
are to map the local geology and/or to measure responses
originating from the mineralised environment. Mapping
surveys provide essential geological context, and in areas of
poor outcrop the data may comprise the only useful form
of

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map available. Surveys designed to target the
mineralised environment may be intended to detect or to
define the geological/geophysical features of potential sig-
nificance. Detection simply involves ascertaining whether
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geological

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has to produce a detectable geophysical response so that its
presence can be detected. Surveys designed to characterise
the source of a response are required when information
about the nature of the source is required, usually to design
a drilling programme to sample it.
Putting aside the inevitable in uence of costs and
budgets (see Section 1.2.3 ), the decision over whether to
use geophysics in an exploration or mining programme,
and if so which method to use, depends on the geophysical
detectability of the features of interest. If signi cant phys-
ical property contrasts do not occur in the survey area then
the chances of a successful outcome are much reduced.
Clearly, some understanding of the likely physical proper-
ties of the geological environment being surveyed is para-
mount. This might be based on petrophysical data from
the survey area; more probably, especially in the early
stages of exploration, it will be based on data from other
areas or from published data compilations. Successful out-
comes are more likely when there is a good understanding
of petrophysics, one reason that we emphasise this subject
throughout this text.

something ' is there or not, and obviously the ' something

of
data can be constructed. However, the measured responses
are not entirely due to features at a particular depth, and
determining the depth that has the predominant control on
the response is fraught with dif culty. This is a further
source of error as some data processing techniques require
the depth of particular features to be known in order to
apply them. Furthermore, the shape of the measured
response pro le/cross-section usually does not mimic the
true shape of features in the subsurface. In all cases, the
result is a potentially misleading display.

' cross-section '

2.5.1 Geological mapping

Mapping the local geology seeks to identify geological
settings conducive to the formation of orebodies. In poorly
explored areas the primary intention may be simply to
create a geological ' base map ' , which will form a basis for

assessing the area s prospectivity. In better known areas

particular deposit types may be sought and the features of
primary interest will depend on the exploration model
being applied. A common example is seeking to map major
faults which may have acted as conduits for mineralising
fluids. Alternatively, if the mineralisation being sought is
strata-bound then mapping the prospective lithotypes or
lithological contacts may be the aim of the survey.

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2.5 Survey objectives

A geophysical survey should be undertaken with a clear
objective in mind since this critically affects both the