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Airborne gamma-ray spectrometer surveying

Airborne gamma-ray spectrometer surveying (AGS) studies areamean content of natural radioactive elements — uranium (radium), thorium and potassium in the near surface layer of rocks and soil. Interpretation of AGS data is based on the law of distribution of radioactive elements in earth material and ore fields. The maps of uranium, thorium and potassium content as well as their various transformations (function of contents) allow to solve the following tasks:

  • to determine the elements of geological framework of the explored acreage;
  • to search for radioactive raw materials deposits;
  • to study the quality and intensity of superimposed processes (metasomatosis) and search for hydrothermal type deposits;
  • to study prospective oil and gas bearing territories;
  • to perform environmental and engineering remote monitoring.

We use high-end digital 1024 channel spectrometers RSX-4 (RSX-5) with polistsin detectors NaJ (Tl), with total capacity of 48 litres (usual capacity is 32 litres) to perform AGS surveying. Spectrometer energy resolution through 0.662 mega-electron-volt line is not worse than 9%.

Spectral registration range of gamma rays is 0.38 — 5.0 mega-electron volts. Frequency of registration is 1 second. Stabilization of spectrometer energy scale is performed by photopeaks of the natural radioactive elements (potassium and thorium). The spectrometer registers and processes the data supplied by detectors chip by chip. Besides that, the detectors which are used possess relatively high photoefficiency, and the recording unit has high failure tolerance and self-testing capability.

Depending on the tasks at hand, helicopters (such as Mi-8, Eurocopter AS350 B3) or planes (AN-2, AN-3, AN-26, Cessna 208B and others) can be used as carriers.

Besides standard activities aimed at solving geological tasks, the company was also the central coordinating body for performing airborne gamma spectrometry surveying to estimate the level of nuclear contamination of the territories which suffered from disasters that occurred within nuclear power generation industry. Over the last years the company fulfilled the government program for radiation mapping of the territories which suffered from Chernobyl disaster, “Mayak” manufacturing group disaster in Chelyabinsk region, proving-ground trials at Novaya Zemlya and Semipalatinsk. The scope of this estimation was over 4 million square km.

The method of AGS surveying and data processing comply with IAEA, 2003 requirements. A standard processing is done through four channels:

ChannelEnergy window (MeV)
Potassium (40K)1.370 — 1.570
Uranium (214U)1.660 — 1.860
Thorium (208Th)2.410 — 2.810
Integrity channel0.400 — 2.810

Besides the standard method, an alternative processing based on calculating the area of photopeaks is used. The main advantage of the new technology is using photopeaks area calculation instead of solving Compton correction equation. In this case it is possible to almost completely get rid of space radiation influence with its inevitable “Poisson statistics” commensurable with our “useful signal” and of a set of Compton correction equations solution error (contribution of scattered radiation from any monochrome source with E0 energy to the left part of the spectrum). Moreover, in general normal cases, this method allows to calculate uranium concentration from 3 photopeaks (1.12+1.238, 1.76 and 2,20+2,119 MeV) and thorium concentration from 2 photopeaks (0.966+0.911 МэВ and 2.62 MeV), which in case of other conditions being equal, lets to significantly increase the quality of the result thanks to the increased statistical representativeness of the data.

Correction for a free radium in the atmosphere is determined through measuring area ratio of each photopeak of the main uranium emission lines (Bi-214) towards the area of a photopeak with the energy of 0.609MeV, which increases statistical representativeness of the correction that is being calculated.

The preliminary correction of the area of 0.609MeV photopeak is done by means of contributing a thorium line (Tl-208) with the energy of 0.583 MeV.

Map of uranium content
Map of uranium content:
A – before the correction “for radium” was introduced,
Б – after the correction “for radium” was introduced
(before the procedures for internal leveling)

As “for radium” correction value is characterized by the concentration of free radium in presurface atmosphere at the time of airborne surveying, this parameter can be used to build a corresponding map. If we take into account the super dynamic nature of this value chargeability, it makes sense to analyze only the spatial variations of the local content of radium concentration.

AGS researches can be done in combination with other methods: magnetometry, gravimetry, electric, aerosol, gas and thermal infra-red exploration.