rjmf radiometric image, page-34

The radiometric image was taken from the following document prepared by JH Lally - worth having a look:

http://www.nt.gov.au/dpifm/Minerals_Energy/Geoscience/Content/File/2002_005/RJ_RECORD_2002_5_COVERPAGE.pdf

These figures are part of the following report:

Sep 06 - Lally JH and Bajwah ZU, 2006 - Report name: Uranium deposits in the NT - Ref: NTGS Report 20 - Format: CD

Here is an extract that discusses Radiametric Survey from page 8.

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AIRBORNE RADIOMETRIC SURVEY

Airborne radiometric surveys are the most common technique used in uranium exploration and have successfully discovered many significant deposits in the Northern Territory and worldwide. The technique is based on the measurement of natural emissions of gamma radiation from the radioactive decay of uranium, thorium and potassium by an airborne sensor containing a sodium iodide crystal. Early scintillometers measured the total amount (‘total count’) of gamma radiation emitted and were unable to distinguish between different element sources. Development of the gamma-ray spectrometer in the early 1970s allowed emissions from uranium, thorium and potassium to be separated, which provided a more useful exploration tool. Survey data is now generally displayed in a three-colour (red, green, blue) composite image in which red =potassium, green = thorium and blue = uranium. These images can be used not only for uranium anomaly detection, but also for lithology and regolith mapping.

In the ARUF, airborne radiometric surveys detected significant anomalies at Ranger 1, Koongarra and Nabarlek. The largest anomaly, at Ranger 1, was 15 times background intensity and extended over an area 7000 m by 1500 m (Rowntree and Mosher 1976). By contrast, the Koongarra anomaly was detected on one flight line and had an extent of 100 m. Airborne surveys over the Jabiluka area failed to detect any anomalism, due to transported overburden at Jabiluka 1 and overlying Kombolgie Subgroup sandstone at Jabiluka 2. Sandstone-hosted deposits in the Amadeus Basin were also discovered by airborne radiometric surveys.

The main limitation of airborne radiometric surveys is that a radioactive source must be at or very close to the surface to produce an anomaly. The experience at Jabiluka showed that even a large deposit is rendered ‘blind’ by a thin cover of overburden (less than 3 m over Jabiluka 1). These surveys are still used as a very cost-effective means of rapidly assessing large areas for priority targets from surface anomalies.

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Key points:

* Development of the gamma-ray spectrometer in the early 1970s allowed emissions from uranium, thorium and potassium to be separated, which provided a more useful exploration tool.

* The main limitation of airborne radiometric surveys is that a radioactive source must be at or very close to the surface to produce an anomaly.

Conclusion: Radiometric Surveys are a useful tool to help identify prospective anomalies but they may not pick up all the anomalies as the Jabiluka 1 example above highlights.