Geoscience Reference
In-Depth Information
When magnetic data needs to be modelled, a represen-
tative value, or more realistically a range of values, is
required. When interpreting a distribution of magnetic
susceptibility measurements to obtain the representative
susceptibility of a geological
compass. Drillcore samples should ideally be oriented in
three dimensions: with the uphole and downhole ends
labelled, and the upper and lower surfaces of the inclined
drillhole also shown. Otherwise, the axis of the drillhole/
core can be used as a partial reference; this constrains the
magnetism
formation,
the following
'
should be considered:
s direction to lying within a cone centred on
the core (drillhole) axis.
As for susceptibility measurements, care is required in
selecting samples to ensure that they are representative of
the rocks being investigated. They need to be free of the
effects of weathering and lightning strikes, which can result
in unrepresentatively high Königsberger ratios. Over the
time scales required to erode outcrops, it is virtually certain
that every point on the top of a hill will have been struck,
so these are generally not good locations from which to
take samples. Drillcore samples can be magnetically over-
printed by the
Where multiple populations are present, it is necessary
to determine whether these represent the heterogeneity
of the geology being sampled, or are a sampling artefact.
As noted above, susceptibility readings made with a
hand-held instrument will be affected by the surface
roughness and volume of the sample.
It is possible that one or more populations are of local
significance and can be ignored: for example chilled
margins of intrusions and areas of alteration close to
faults and joints etc., which may be either magnetite
constructive or destructive. Also, weathering will tend
to skew the distribution towards lower susceptibilities or
may even result in an identi
able weathered population.
fields caused by the steel (magnetic) drill
rods, and strong external
fields such as pencil magnets can
magnetically contaminate the samples.
The NRM (see
Section 3.2.3.4
) may be the sum of a
number of component remanent magnetisms acquired at
different times and generally in different directions. It is
possible to isolate the different remanent components
using laboratory palaeomagnetic cleaning techniques; see
for mineral exploration purposes; an exception is the
removal of young, and sometimes very strong, remanent
magnetism associated with lightning strikes in order to
establish the likely remanent magnetism in the subsurface.
A fundamental aspect of all geological
field mapping is
that outcrop is often a biased sample, being overly rep-
resentative of those units that are more resistant to
erosion. This may also lead to bias in susceptibility
measurements.
The inherent complexity of magnetic mineralogy and limi-
tations of sampling mean that it is very difficult to assign
an accurate
susceptibility value to a geological
unit. Selecting a range that encompasses the majority of
the data is often the best approach. The aim should be to
identify likely susceptibility ranges for individual units and
where significant inter-unit contrasts occur.
'
average
'
3.9.8.5
Analysis of remanent magnetism data
Variations in the various components throughout a rock
unit produce scatter in the strength and direction of the
NRM, so the estimation of the
3.9.8.4
Measuring remanent magnetism
Being a vector quantity, both the strength and direction of
remanent magnetism need to be measured. Measurements
are made in the laboratory on geographically oriented rock
samples. The methodology is described by Clark and
detailed descriptions of rock sampling and sample prepar-
ation techniques. For accurate measurements a specialised
drill is used for taking a mini-core 16 mm in diameter and
10
'
average
'
remanence of a
rock unit is not straightforward.
Like susceptibilities, a number of measurements of rem-
anent magnetism should be made on each rock unit.
Strengths can be plotted as histograms and multimodal
distributions are common. Directions can be plotted on a
stereographic projection, as used for working with other
geological orientation data. However, both the upper and
lower hemispheres must be used when plotting magnetic
directions. By convention the orientations of fold axes etc.
are plotted in the down-plunge direction: that is, the lower
hemisphere is used to project onto the stereo net. An up-
plunge measurement of, say, 45° upwards towards the
north would be converted to 45° downwards to the south
before plotting. The convention does not apply to magnetic
30 mm in length. For most mineral exploration appli-
cations, oriented bulk samples are usually suitable. When
using a magnetic compass to determine orientation, care
should be taken to ensure that the magnetism of the
sample does not de
ect the compass. For strongly magnet-
ised rocks, such as BIF, it may be necessary to use a sun
-
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