Geoscience Reference
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materials within a few centimetres of the drillhole wall. The
denser the surrounding rock the smaller is the contributing
volume. This is not necessarily a major problem, however,
since it means that the log has high spatial resolution,
allowing centimetre-scale downhole variations to be iden-
ti
ed. This is also dependent on the logging speed, the rate
at which the tool is raised in the drillhole. Typical speeds
are 1 to 6 m/min with spatial resolution of
Since the 1990s, the reduction of radiometric data uses
entire recorded
-ray energy spectra, generally 256 energy
channels, whereas previously it was based primarily on the
counts recorded in just the K, U and Th energy windows.
Current developments in data reduction focus on
improved methods of noise reduction based on statistical
smoothing of the measured spectra. The various correc-
tions applied are known by the parameters that they com-
pensate, and we consider each one in the order that it is
A detailed description of these procedures applied to air-
borne surveying is provided by Minty et al.(
1997
).
The corrections described here are pertinent for
spectrometer measurements made above ground. Down-
hole measurements require less correction: for example,
those related to survey height, and to atmospheric and
cosmic effects, are not applicable. Total-count data are
not usually corrected at all.
γ
'
'
forma-
tional layers and contacts increasing with decreasing speed.
See
Section 4.7.5
for details of the interpretation of
thin
γ
-logs.
4.4
Reduction of radiometric data
The
-ray spectrum recorded in geophysical surveying
comprises radiation from a number of different sources
(see
Section 4.2.5
)
in varying proportions. Of these, radi-
ation originating from one or more of the three common
radioelements present in the ground, i.e. potassium (K),
uranium (U) and thorium (Th), is the signal of interest,
and all other sources of radiation form unwanted noise.
The survey data are reduced to remove various sources
of noise, both environmental and methodological (see
Section 2.4
)
, to reveal the radiometric response of geo-
logical signi
cance, which is used to calculate elemental
ground concentrations of the three radiometric elements.
Signi
cant non-geological factors affecting the measure-
ments include: the detector
γ
4.4.1
Instrument effects
The
first stage in the reduction process is to correct for the
effects of the measuring equipment. Thermal drift of the
spectrometer reduces its ability to accurately discriminate
the energy of the incident
γ
-rays, i.e. it degrades the spec-
trometer
s resolution. Stability of the system gain and loss
of sensitivity due to crystal or photomultiplier damage are
monitored by checking the resolution pre- and post-survey.
This is done by calibrating the instrument
'
'
s inability to perfectly count
γ
all the
-rays; the position, size, shape and chemical com-
position of the radioactive sources; survey height; nature of
the overburden and its thickness; vegetation; air tempera-
ture, pressure and humidity; the presence of moisture; and
the presence of atmospheric radon. The effects of some of
these can be partly corrected by making appropriate sec-
ondary measurements, for example air temperature and
pressure. Other variables require knowledge of the local
geology, some to the extent that would make a radiometric
survey super
s measured
energy response against a reference source of known
energy, either
137
Cs or
133
Ba as these elements emit
'
-rays
at prominent energy levels of 0.662 and 0.352 MeV respect-
ively. Modern spectrometers automatically monitor and
correct for drift during operation and contain heaters
which maintain the crystals at a constant temperature.
The
γ
finite time to record and
process the radiation detected during the integration
period. During the recording time it is unable to process
incoming
γ
-ray spectrometer takes a
uous (an example of the geophysical para-
dox, cf.
Section 1.3
). In the absence of the necessary data or
theoretical models from which to calculate their affect,
many of the required corrections are empirical. Conse-
quently, the reduction of radiometric survey data depends
upon complex instrument calibration procedures and the
adoption of some simplifying assumptions about the local
environment. Errors associated with each stage of the
reduction process are additive, so adherence to calibration
procedures and constant monitoring of ancillary param-
eters throughout the survey are essential in order to min-
imise uncertainty in the final result.
γ
-rays, so these are lost. This is known as the
instrument
s. It
only becomes important for count rates above about
1000 cps when the number of counts occurring during
the dead time, and therefore lost by the instrument,
becomes signi
cant. Modern instruments correct for this
automatically.
Other problems related to the instrument include
the fact that the NaI sensor is more efficient at detecting
lower-energy
'
s dead time and is typically less than 15
μ
γ
-rays, and to the possibility of accidental
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