Geoscience Reference
In-Depth Information
CHAPTER
4
Radiometric method
4.1
Introduction
The radiometric method, or radiometrics, measures nat-
urally occurring radioactivity in the form of gamma-rays
(
data are acquired simultaneously with magnetics during
airborne surveying, although it would be unusual for a
solely radiometric survey to be conducted. Ground radio-
metric surveys are usually conducted with hand-held
instruments, but
-rays) (
Fig. 4.1
). Most of this radiation originates from
mineral species containing radioactive isotopes of potas-
sium (K), uranium (U) and thorium (Th). Radiometrics is a
passive geophysical method because it measures a natural
source of energy. Radiometric surveys for mineral explor-
ation are routinely made from the air, on the ground and
within drillholes. Airborne radiometrics is particularly
common in mineral exploration where the radiometric
γ
-ray detectors are sometimes mounted
on a moving vehicle for larger-scale surveys. Downhole
radiometric measurements are limited to
γ
-logging;
downhole surveying as defined in
Section 1.2.1
is not
possible in radiometrics. Gamma-logging is a common
component of multiparameter logging for rapidly and
economically measuring in situ physical properties,
correlating stratigraphy between drillholes and assisting
in the evaluation of uranium deposits. It is also used
extensively by the petroleum industry in well-logging.
Historically, the main use of radiometrics in mineral
prospecting was detection of anomalies caused by out-
cropping, highly radioactive, uranium deposits. With
improvements in sensor technology and data-
processing algorithms, downhole radiometric tech-
niques were developed for estimating the grade of
uranium ores intersected by drillholes. The advent of
multichannel detectors for airborne surveying, capable
of distinguishing radiation from different radioactive
elements, the increased sensitivity and resolution of
airborne surveying techniques, and the development
of new data reduction algorithms have focused air-
borne radiometrics more toward geological mapping
than purely anomaly detection. Newer applications
include detecting and mapping areas of hydrothermal
alteration and weakly radioactive mineral deposits,
γ
Measured
radioactivity
Response from
surface source
No response from
subsurface source
Location
Response in
intersecting
drillhole
No response in
non-intersecting
drillhole
γ
Radiation
K, U or Th
γ
Log
K, U or Th
Figure 4.1
Schematic illustration of the radiometric method,
which measures natural radioactivity from K, U and Th. The
emissions are absorbed by even minor amounts of rock, so surface
occurrences or drillhole intersections are required to obtain
recognisable responses.
Search WWH ::
Custom Search