Geoscience Reference
In-Depth Information
computed response of a model, i.e. a (simpli
ed) numerical
representation of the subsurface geology. This is a form of
quantitative analysis. Obtaining a match between the
model
contact. They may be located at different depths. Shallow
and deep responses need to be recognised, a subject we
describe later for each geophysical method.
s geophysical response and actual observations
places constraints on the geology of the subsurface, e.g.
the depth of the source of an anomaly, the dip of a
stratigraphic contact etc. In most cases, a match is not a
de
nitive result, since more than one model can be made
to fit the data. In other words, there is no unique arrange-
ment of the geological elements that explain the observed
geophysical data, so the interpretation is ambiguous. This
fundamental and important aspect of geophysics is known
as non-uniqueness, and is discussed in more detail
'
There is a strong possibility of responses originating in
the near-surface cover, regolith, permafrost etc., and that
bedrock responses may be distorted and/or attenuated
by the effects of the cover.
Distortions caused by topography may be present. We
stress the importance of integrating topographic data
into the interpretation of all types of geophysical data;
because for most geophysical methods, the shape of the
land surface affects the geophysical measurements to
produce terrain-induced responses. Nevertheless, it is
important to be aware that a correlation between geo-
physical responses and topography is not necessarily
indicative of artefacts in the dataset, since both the
topography and the geophysical responses are controlled
by the local geology. More likely to be artefacts in the
geophysical data are responses not directly correspond-
ing with topographic features, but occurring adjacent to
them. For example, a
in
Before discussing the general aspects of interpretation,
we consider a number of issues affecting interpretation and
describe the important matter of accounting for long-
wavelength variations, i.e. the regional response.
2.9.1
Interpretation fundamentals
surrounding a hill or a
linear anomaly adjacent to a ridge should be treated with
scepticism. Even then the responses may be re
ecting
the local geology: a hill due to an intrusion may have a
contact aureole, the cliff may be indicative of a fault etc.
'
moat
'
Aside from the dif
culties in reconciling the geological and
geophysical perspectives of the geological environment (see
Section 1.1
)
, an interpretation must consider the following
aspects of geophysical data and geophysical responses:
A map or image of the survey stations, or the survey
lines and the terrain clearance for airborne surveys,
should always be available for the interpretation. Inte-
grating these with the survey data and its various trans-
forms can reveal correlations with responses, possibly
indicating artefacts in the data caused by the station
distribution itself, or by variations in survey height.
Features occurring in areas of sparse data or oriented
parallel to the survey lines should be carefully analysed
to ensure they are not survey artefacts.
Petrophysics is the link between the geophysical
response and the geology. It is a complex subject.
Changes in lithology, mineralogy, alteration, fabrics,
weathering and structure can cause great variability in
physical properties, so the available data may not be
representative of the rocks in the ground. Also, some
physical properties are commonly anisotropic, notably
electrical conductivity, magnetic susceptibility and seis-
mic velocity, often being different when measured paral-
lel and perpendicular to planar fabrics such as bedding
or metamorphic foliations. This will affect the geophys-
ical response.
The survey line direction and spacing have effects on the
resolution of particular geological features.
Geophysical responses may have a complicated form
which is not indicative of their source geometry and
may be offset from their source location or extend
beyond the edges of their source.
The scale of resolvable features needs to be understood:
in particular, the diminishing resolution of features with
depth and the concept of footprints, i.e. the volume of
the subsurface contributing to an individual measure-
ment (see
Section 2.6.2
).
Artefacts caused by data processing and display may be
present. Levelling errors may introduce artefacts
trending parallel to survey lines. Gridding may introduce
beading and bulls-eyes (see
Section 2.7.2.3
)
. Apparent
'
Since a geophysical map contains responses from fea-
tures at a range of depths, the interpreter needs to think
in terms of three dimensions. For example, the fact that
one anomaly apparently cuts across another does not
mean the sources of
are not necessarily indicative
of relative ages as gridding may result in datasets
where higher-amplitude anomalies appear to cross-cut
cross-cutting relationships
'
the anomalies are actually in
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