Geoscience Reference
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the fact that some of these may be in the cover material
rather than the bedrock should borne in mind. The ana-
lytic signal (see
Section 3.7.4.2
)
is also very effective for
resolving contacts. It has the distinct advantage of being
independent of the magnetic inclination and is particularly
useful at low magnetic latitudes where the nature of mag-
netic sources can be dif
cult to resolve from TMI images,
but the polarity of the physical property contrast across the
contact is lost. The merits of these and other methods for
locating contacts using magnetic data are discussed by
Pilkington and Keating (
2004
).
A vast number of enhancement filters have been pro-
posed for potential field data in addition to the options
available regarding display (see
Section 2.8
). These ought
to be selected to provide complementary information
useful in terms of the geological information being sought.
For example, when the aim is to map the geology, a
product that emphasises contacts and one emphasising
linear features would complement each other. Where the
geology changes signi
cantly, for example strongly mag-
netised basalt at the surface in some places, it may be
necessary to create different enhancement products applic-
able to particular areas. Some
filters are more susceptible to
noise than others reducing their effectiveness on lower
quality datasets. In aeromagnetic data, noise commonly
manifests itself as corrugations due to mis-levelling (see
Section 3.6.4
)
, and in gravity data as a speckled appearance
related to incomplete sampling of the variations due to
non-ideal station spacing (see
Section 2.6.1.1
).
The removal of long wavelengths with high-pass filters
(see
Section 3.7.3
) or using upward continuation (see
Section 3.7.3.2
)
, or by simply removing a regional gradient
(see
Section 2.9.2
)
, improves the resolution of short-
wavelength shallow features. When the intent is to resolve
speci
pseudocolour drape useful for displaying multiple aspects
of the data (see
Fig. 2.36
)
; shaded relief (see
Section 2.8.2.3
)
applied as a directional enhancement; shaded-relief grey-
scale of
first vertical derivative with the gravity or TMI data
as a pseudocolour overlain; or contours of one particular
parameter overlaid on an image of another parameter.
Ternary images (see
Section 2.8.2.4
) can be created by
integrating different forms of gravity and magnetic data.
Stacked profiles are useful for displaying detail on survey
line data that may otherwise be distorted by the gridding
process (see
Fig. 2.30
).
As with all types of geophysical data, topographic data in
the form of a digital elevation model (DEM) ought to be
included in the interpretation to identify correspondences
between the terrain and possible anomalies, and also to
identify effects due to changes in ground clearance (see
3.10.3
Interpretation pitfalls
Gravity and magnetic data are subject to a variety of
spurious responses from a number of sources. Their
responses distort genuine target anomalies and can mas-
querade as target anomalies. They are usually fairly easy to
recognise in the data, particularly after removal of the
regional response.
3.10.3.1
Cultural effects
Cultural features can produce anomalous responses in
gravity and magnetic data. Features such as dams, reser-
voirs, buildings, open pits and mine dumps influence the
gravity terrain correction and are a source of error. The
iron content of buildings, roads, railway lines, pipelines
and powerlines produces responses in magnetic data.
Because they are at, or near, the surface, the responses
are usually localised. Aerial photographs can be used to
con
c target anomalies from the response of the local
geology this is particularly important. A little judicious
experimentation with the various parameters is usually
required to resolve anomalous responses in the local geol-
ogy. An example of regional removal to facilitate interpret-
ation of a gravity map is presented in
Section 3.11.1
.In
many cases it is also worthwhile to produce a version of the
data containing the highest frequencies (shortest wave-
lengths) to identify density or magnetisation contrasts in
the cover material; second horizontal derivatives computed
on the line data can be very effective in this regard.
Various transformations of the data can be integrated
into single images using imaging techniques (see
Section
2.8
). Examples include grey-scale shaded relief with a
rm the presence of cultural features.
3.10.3.2
Overburden effects
The near-surface environment can also be a source of
noise. For the case of gravity, variations in the thickness
of the regolith or cover produce responses that superim-
pose geological noise on the responses of underlying
sources, which can be mistaken as bedrock responses. In
lateritic terrain, the near-surface may be highly magnetic,
giving rise to large-amplitude, short-wavelength anomalies,
which when gridded create a mottled appearance (Dentith
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