Geological mapping using airborne gammaray spectrometry

V.C. Baranwal1, R.J. Watson1, M.A. Smethurst1,2 and J.S. Rønning1,3

1.Geological Survey of Norway, Postboks 6315 Sluppen, 7491 Trondheim2.Avalonia Geophysics, University of Exeter Cornwall Campus, Penryn, TR10 9EZ, UK

3.Norwegian University of Science and Technology (NTNU), Trondheim

Introduction

NGU is a government institution responsible for geological study of bedrock, soil, marine, mineral resources and groundwater

NGU had a 256 channel GR820 gamma ray spectrometer with sodium iodide detector packs (16.7 l downward and 4.2 l upward detector)

0.2 to 3 MeV signal is recorded in channel 1 to 255 and above 3 meV in channel 256 as cosmic radiation

The system is upgraded in 2010 to RSX-5 to operate in 1024 channels

We use it for geological mapping of natural radioelements K, U and Th

It can also be used for Cs falldown and other radioactive objects

We are also part of the National Nuclear Accident Preparedness Organization

Survey around Kongsberg, Norway

Kongsberg-Modum complex is in southern Norway and also known as silver mining district

The well known uranium rich Alum shale is very well exposed in the easternmost part of the area (Figure 1)

The Survey was performed by a helicopter at a nominal height of ~ 60 m in E-W direction with ~ 200 m line spacing

Survey always started and ended nearby lakes or water bodies to collect background radiation data

Figure 1. Geological map of study area (modified after Lutro and Nordgulen, 2008). Surveyed area is marked by a polygon.

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Processing of the radiometry data

Processed with the aid of Oasis Montaj software package and some in- house software

Quality control , splitting the survey in lines and K, U and Th windows

Random noise reduction using clustered NASVD or MNF method

Dead time, aircraft background and cosmic background correction

Stripping correction to remove radiation from nuclides of other elements

Airborne radon correction using upward detector method or spectral ratio method

Flight height and topographic correction

Microleveling and conversion to ground element concentration

Total counts window- Channels 34-220 (410-2810 keV)

Figure 2. Raw total counts window data

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Figure 3. Processed total counts window data

Total counts window- Channels 34-220 (410-2810 keV)

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Figure 4. Raw K window data

K- window- Channels 109-125 (1370-1570 keV)

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K- window- Channels 109-125 (1370-1570 keV)

Figure 5. Processed ground concentration of K in %

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Th- window- Channels 189-220 (2410-2810 keV)

Figure 6. Raw Th window data

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Th- window- Channels 189-220 (2410-2810 keV)

Figure 7. Processed ground concentration of Th in ppm

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U- window- Channels 132-148 (1660-1860 keV)

Figure 8. Raw U window data

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U- window upward - Channels 132-148 (1660-1860 keV)

Figure 9. U window counts in upward detector

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U- window- Channels 132-148 (1660-1860 keV)

Figure 10. Calculated radon counts in U window

November2008

June2009

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U- window- Channels 132-148 (1660-1860 keV)

Figure 11. U window counts after Radon correction

November2008

June2009

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Figure 12. Calculated radon counts in U window after calculating it separately for autumn and summer season

U- window- Channels 132-148 (1660-1860 keV)

November2008

June2009

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Figure 13. U window counts after Radon correction calculated separately for autumn and summer season

U- window- Channels 132-148 (1660-1860 keV)

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U- window- Channels 132-148 (1660-1860 keV)

Figure 14. Processed ground concentration of U in ppm (displayed by histogram equalization method)

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U- window- Channels 132-148 (1660-1860 keV)

Figure 15. Processed ground concentration of U in ppm (displayed in linear scale)

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Figure 16. Ternary image of the area using linear scale

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Conclusions

Maps of K, eU and eTH are obtained by processing of the data

There was different amount of background radon in the air in the autumn and summer season in the area

The K and eTH map largely show lineaments and lithology variation in the region

eU map clearly marks presence of alum shale in the region which matches well with existing geological maps of the region

ReferencesHeincke B.H., Watson, R.J. and Thomas M. 2009. Determination of altitude- dependence of standard spectra and stripping ratios for the GR820 Airborne Gamma Ray Spectrometer, NGU repot 2009.042, pp. 42.

IAEA. 2003. Guidelines for radioelement mapping using gamma ray spectrometry data, IAEA-TECDOC- 1363, Vienna, Austria. pp. 173.

Lutro, O. and Nordgulen, Ø. 2008. Oslofeltet, bedrock map M 1:250000. Geological Survey of Norway.

Mauring, E. and Kihle, O. 2000. Levelling aerogeophysical data using a moving differential median filter. Geophysics, 71, L5-L11.

Thank you for attention !!