Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 24

We reiterate the important point made in Section 1.1
that the contrasts in physical properties that control geo-
physical responses do not necessarily have a one to one
correlation with contrasts in lithotype. This is because it is
the rock-forming minerals that are the basis for assigning
lithological names, and these in turn are controlled by rock
chemistry. In contrast, the physical properties of relevance
to geophysics are not entirely, or sometimes even slightly,
controlled by the rock-forming minerals. For this reason, a
'

the responses of unwanted features, such as the sur-
rounding rock formations and mineralisation of no eco-
nomic signi cance. The strategy can be applied at
regional scale to select areas for detailed work, and at
prospect scale to detect a target anomaly. It is also
applicable when a deposit is being mined, with adjacent
orebodies being the target. Also, surveys designed to
detect faults or dykes ahead of coal mining are basically
aimed at identifying anomalous parts of the geological
environment.
Anomalous responses associated with the mineralised
environment may be caused by the mineralisation itself,
although not necessarily the actual ore minerals. Deposits
comprising massive or disseminated metal sulphides and
oxides are commonly targeted in this manner. Also
targeted are alteration zones caused by mineralising

map created using geophysical measurements
should be referred to as a pseudo-geological map.
Compilation of the pseudo-geological map involves iden-
tifying the near-surface and deeper responses and classify-
ing the remaining responses according to their possible
sources (see Section 2.11 ) . The integrated analysis of mul-
tiple data types, i.e. magnetics, radiometrics, conductivity
etc., helps to produce a more reliable and accurate model of
the subsurface geology.
Surveys designed for geological mapping should provide
a uniform coverage of geophysical data across the area of
interest. With large areas to cover this will probably require
an airborne survey and survey speci cations typical of
reconnaissance objectives (see Section 2.6.3 ) . As explor-
ation focuses on areas considered to be most prospective,
more detailed surveys may be undertaken and ground
surveys may be used. The most common types of geophys-
ical surveys for geological mapping are airborne magnetic
and radiometric surveys. Airborne gravity surveys are
becoming more common as surveying technology impro-
ves. These geophysical methods produce responses that
distinguish a wide range of lithotypes and are favoured
in most geological environments. The more complex, diffi-
cult to interpret and expensive electrical and electromag-
netic methods tend to be used less, although airborne
electromagnetics is increasingly being used in a mapping
role.

geological '

fluids, which have the advantage of usually being much
larger in area than the target mineralisation, so the geo-
physical response from the alteration covers a corres-
pondingly larger area, which may help to facilitate its
detection. Porphyry style copper deposits are an example
of deposits with extensive, geophysically distinctive, alter-
ation haloes. Another form of anomaly targeting seeks
to locate speci c lithotypes in which mineralisation
occurs, e.g. potentially diamondiferous kimberlitic and
lamproitic intrusions, or ultrama c intrusions that might
contain platinum group elements, or palaeochannels
hosting placer deposits.
A survey designed to detect the responses from the
mineralised environment requires a survey strategy
based on the probability of making a measurement in
the right place, i.e. within the bounds of its geophysical
response (see Section 2.6.4 ). This ensures that the anom-
alous responses are both recorded and recognised as
signi

cant (see Section 2.5.3 ). Needless to say, some
knowledge of the physical properties of the targets is
required to ensure that responses are anticipated in the
chosen form of geophysical data. The depth and volume
of the source, and the magnitude of the physical prop-
erty contrast with its host, are also important since the
amplitude of the anomaly depends on this (see Section
2.5 ) . In this context an appreciation of noise levels is
required, especially geological noise. For example it may
becomparativelyeasytoidentifythemagneticresponse
of a kimberlite intruding a weakly magnetised sediment-
ary sequence, but the response may be unrecognisable in,
say, a terrain comprising a variably magnetised succes-
sion of basalts.

2.5.2 Anomaly detection

The exploration strategy can involve surveys intended
to detect localised responses distinctly different from
their surroundings, i.e. anomalies. This approach is
sometimes referred to as

' searching for bumps ' , .e.

looking for localised anomalously ' low ' or anomalously

' values in various presentations of the survey data.

This is a simple and effective form of targeting and is
a valid approach when exploration targets give rise to
distinct anomalies that are easily distinguishable from

high '