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Petro-Log™ · Uranium · technical note
Total count and spectral
gamma for uranium
Gamma-ray logging is the main borehole method in uranium exploration. Total count logging is faster and cheaper; spectral logging separates potassium, uranium and thorium for more confident interpretation. Which to use depends on cost, programme scale and data quality, and either needs laboratory checks for disequilibrium before grade estimation.
The question
Gamma-ray logging detects variations in the natural radioactivity of the rock, from the trace elements uranium and thorium and the major rock-forming element potassium. Because their concentrations vary between rock types, gamma logs map lithology and correlate stratigraphy between holes, and in uranium deposits they also help estimate in-situ grade and the grade thickness of ore intersections.
Two systems are used, and they differ in cost, speed and what they can tell apart. Choosing between them means balancing cost, the scale of the programme and the data quality the project needs.
Total count gamma suits large, cost-sensitive programmes; spectral gamma resolves potassium, uranium and thorium for confident interpretation; and either should be checked for disequilibrium in the laboratory before logs are used for grade.
- Total countCounts per second, converted to equivalent uranium (eU)
- SpectralFull energy spectrum, processed to K, U and Th plus total count
- RangeAbout 10 to 3,000 keV in 255 channels
- CautionUranium-series disequilibrium, checked in the laboratory
The two methods
Total count gamma is the simpler system and the standard depth and lithology correlator. It cannot tell the radioelements apart: every natural gamma ray is counted. Its native output is counts per second; calibrating against sources of known uranium concentration gives a factor that converts it to an equivalent uranium (eU) log. eU logs are a staple of uranium exploration because they are fast to acquire and process and need no specialist processing when many holes must be evaluated. Their weakness is the assumption that every peak is uranium, ignoring changes in potassium and thorium.
Spectral gamma, or gamma-ray spectrometry, records a complete energy spectrum at each measurement, from about 10 keV to 3,000 keV in 255 channels roughly 12 keV wide. The spectrum is processed after logging into potassium, uranium and thorium concentrations, plus a total count log. It is slower to log and process, and needs careful calibration at a test facility, but it gives specific radioelement concentrations the total count cannot.
Figures
The PDF includes two published example logs: a spectral gamma log of an organic-rich shale in which uranium rises to 30 ppm while thorium and potassium stay constant, and a set of total gamma, potassium, thorium and uranium curves beside a lithology log.
What to check
Both systems depend on the uranium decay series being in equilibrium. Uranium's decay series is the most complex, and any disequilibrium undermines the reliability of either log's output.
That should be checked by laboratory analysis before the downhole results are accepted, especially if the logs will be used for grade or grade-thickness estimation.
- Total count for large programmes and fast correlation
- Spectral gamma where potassium or thorium could mislead an eU log
- Calibration: known-concentration sources for eU, a test facility for spectral
- Laboratory disequilibrium checks before grade estimation
A scintillation crystal flashes when a gamma ray deposits its energy, and each flash is counted. A total count tool reports the sum; a spectral tool also measures each flash's energy, so the characteristic energies of potassium, uranium-series and thorium-series decays can be separated into three concentrations.