Geoscience Reference
In-Depth Information
reported over zones of hydrothermal alteration (see Irvine
A common occurrence is preferentially deeper
weathering of mineral deposits that are in contact with
the base of weathering. Weathering into the deposit
reduces density and causes a reduction in the overall grav-
ity response of the mineralisation. Similarly, kimberlite
pipes extending to the near-surface are sometimes prefer-
entially weathered deeper than the surrounding area to
produce a
process known as Compton scattering (see
Section 4.2.3
).
Gamma rays lose most of their energy during collision with
electrons, and the density of electrons in a substance is
closely correlated with the substance
s bulk density. The
scattered rays are detected by the probe
'
s sensor, located
some distance from the source, and their energy is related
to the electron density of the wall rocks (
Fig. 3.37a
). The
method is very sensitive to conditions in the drillhole.
Corrections need to be applied for the diameter of the
drillhole and for the density of drillhole fluids, all of which
affect the measured intensity. The gamma rays have very
little penetration, and the resulting logs represent the bulk
density of
'
gravity response over these features.
Surveys designed to detect gravity variations due to the
rocks hosting mineralisation include those targeting
sulphides in mafic and ultramafic intrusions and also kim-
berlite pipes. Sulphide-bearing intrusions are invariably
denser than their host rocks. Kimberlites have highly vari-
able density but generally it is lower than their host rocks.
Any diamonds they may contain will be too scarce to
in
'
low
'
the rocks within a few centimetres of
the
drillhole.
The density log can be a very useful lithological indica-
tor, especially for high-density ore minerals or low-density
coal seams (
Fig. 3.37b
). In soft-rock environments it can be
useful for hole-to-hole correlation as demonstrated by
Mach (
1997
), who describes the use of density logs for
correlation within coal measures in the Czech Republic.
cantly.
These complex controls on density mean that,
depending on the area being explored, geologically similar
exploration targets may be associated with either positive
or negative gravity anomalies.
uence the overall density of the intrusion signi
3.8.8
Analysis of density data
It is often necessary to estimate a representative density
of some geological entity, for example to correlate litho-
types with variations in gravity or for modelling. As with
other petrophysical data, rather than trying to identify a
speci
c value a better approach is to identify a likely
density range. This should use measurements made on
a large number of samples chosen to represent observed
variations in the formation
3.8.7
Measuring density
Density of rock specimens can be measured using Archi-
medes
s weight in air and when
immersed in water are compared. This gives the specific
gravity of the specimen, i.e. its density relative to water, but
given that water attains its maximum density of 1 g/cm
3
at
4 °C, and that it changes very little with temperature,
speci
'
principle. The specimen
'
s geological characteristics.
The data can then be analysed as a frequency histogram
may be available.
Ideally, the density measurements will show a normal
distribution, and the arithmetic mean and standard devi-
ation can be determined. Skew toward lower values may
occur owing to the greater porosity, and probably also
changes in mineralogy, of weathered materials taken from
the near-surface. Multimodal distributions may result from
inherently heterogeneous lithotypes such as gneisses,
layered intrusions and banded iron formation, when the
scale of variation is larger than the size of a sample. The
number of samples required to characterise the actual
distribution correctly depends on the homogeneity of
the formation. The more homogenous the rock, the fewer
the samples required. As with all geological sampling, the
danger exists of taking an unrepresentatively large number
'
c gravity is taken as the density of the rock specimen
(
ρ
specimen
) and is given by:
Weight
in air
Weight
in air
ρ
specimen
¼
ð
3
:
27
Þ
Weight
in water
In order to obtain accurate measurements the specimen
should be fully saturated, and should be as large as possible
to increase the likelihood of using representative samples
of what may be a heterogeneous lithotype. Lipton (
2001
)
describes appropriate laboratory procedures.
3.8.7.1
Density logging
Density can also be measured in situ with continuous
downhole density (gamma
gamma) logging. The method
is based on measuring gamma rays, emitted as a focused
beam from a radioactive source on the logging tool, that
interact with the electrons in atoms of the wall rocks in a
-
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