Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 232

￿ Many complex, secondary near-surface processes can remove, transport and deposit K, U and Th. These processes may
be sufficient to create ore-grade U concentrations in surficial materials.
￿ Both 238 U and 232 Th decay via series, and it is

-rays from daughter elements in the series that are detected. Daughter
elements in the U decay series that are mobile in the geological environment may cause disequilibrium, whereby the
level of the measured radiation does not correlate with the abundance of U present.

γ

￿ Radioactive decay is a random process, and measurements involve counting the

-ray photons, using a process known as
scintillation, over a finite integration time. The longer the integration time the more accurate is the measurement.

γ

￿ Gamma-ray spectrometers record the -ray count at different energies allowing the source of the radiation to be

determined, i.e. K-, U- or Th-rich source.

γ

￿ Gamma-ray detectors have an acquisition footprint that depends on the source-detector separation and whether the
detector is in continuous motion.

￿ The reduction of radiometric data is based mostly on complex calibration procedures. Radiation originating from radon
gas in the atmosphere is particularly difficult to compensate for.

￿ Concentrations of K, U and Th are often correlated in most rock types, so ratios of the concentrations of each element
are powerful tools for detecting anomalous regions.

￿ Radiometric data are best interpreted in conjunction with multispectral remote sensing data and digital terrain data.
These ancillary data are especially useful when mapping cover material and regolith, and when working in rugged
terrains.

￿ Radiometric data are conventionally displayed as ternary images with K displayed in red, Th in green and U in blue.

￿ Mineralisation itself, associated alteration zones and lithotypes favourable for mineralisation may all have anomalous
radiometric compositions, detected either in a particular radioelement or in a particular ratio of the elements.

Review questions

.....................................................................................................
1. What is meant by the terms half-life and Compton scattering, and why are these important for radiometric surveys?
2. Compare and contrast individual channel values and channel ratios. How can they be used in the interpretation of
radiometric survey data?
3. Describe how the radiometric element concentration of igneous rocks varies as rock chemistry varies from ultramafic to
ultrafelsic.
4. Describe the nature and distribution of K, U and Th in overburden material.
5. Give some examples of mineral deposit types which may be directly detected by radiometric surveys.
6. What is disequilibrium and why is it important?
7. What is the difference between a scintillometer and a spectrometer? Explain stripping ratios and how they are
determined.
8. Calculate the statistical measurement error for the following count levels measured over one second: 10, 40, 100, 400
and 1000. How does the error change with count level and how can it be reduced for a given detector?
9. Describe the circle-of-investigation and how this can be used to select the most appropriate survey height.
10. What is the source of atmospheric radon gas and how are its effects removed during reduction of radiometric survey
data?