Impact of the pre-processings on the fractal properties of the airborne gamma ray measurements: a case study from Hoggar
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ORIGINAL PAPER
Impact of the pre-processings on the fractal properties of the airborne gamma ray measurements: a case study from Hoggar (Algeria) S. Gaci & N. Zaourar & L. Briqueu & M. Djeddi
Received: 7 March 2011 / Accepted: 18 September 2011 / Published online: 4 October 2011 # Saudi Society for Geosciences 2011
Abstract The analysis of gamma ray measurements provides valuable information about the basement’s lithospherical model and the radioactive elements contained in the subsurface. To be exploited, the raw data need preprocessing operations which consist of corrections of the Compton effect, altimetry and the aircraft background, and 2D interpolations. These transformations may modify the data’s stochastic component which reflects heterogeneities of the investigated medium. In this study, a fractal analysis is carried out on airborne gamma ray data measured in the Hoggar’s area (Algeria). First, it is shown that the analyzed data exhibit fractal properties. Due to the dependence of the latter on the spatial location of the measures, the data can be considered as paths of multifractional Brownian motions. Another result worth noting is that the fractal nature of the raw data is not modified by the pre-processing operations. The fractal analysis can be then performed on the raw spectrometric measurements without submitting them to any pre-processing; thus, the stochastic component of the measurements is kept intact. S. Gaci (*) : N. Zaourar : M. Djeddi Geophysics Department—FSTGAT, University of Sciences and Technology Houari Boumediene (USTHB), BP 32 El Alia, 16111( Algiers, Algeria e-mail: [email protected] N. Zaourar e-mail: [email protected] M. Djeddi e-mail: [email protected] L. Briqueu Laboratoire Géosciences, Université Montpellier 2, 34095 Montpellier, France e-mail: [email protected]
Keywords Hoggar . Airborne spectrometric data . Fractal . Hölder exponent
Introduction The remote sensing of environmental radioactivity is achieved mainly through the detection of gamma radiation. Gamma rays are the most penetrating radiation from natural and artificial sources. Gamma ray surveys are conducted from aircraft, on ground (from field vehicles, on foot, in boreholes, and in laboratories) and on the sea bottom. The obtained maps of terrestrial radiation and radioelement concentrations put in evidence the regional trends of the radionuclide distribution and allow to assess the environment in terms of radioactivity. Gamma ray spectrometry is widely used for geochemical mapping, structural geology, and mineral prospecting (Ruffell and Wilson 1998; Brown et al. 2000; Aydin et al. 2006; Sulekha et al. 2009). This study does not deal with the conventional approach of gamma ray measurements, but aims at suggesting a fractalbased analysis which enriches the results of the latter. Geophysical signals are composed of deterministic and stochastic components. The former presents a low-frequency content due to the regional geological trends’ effect, while the latter is a high-frequency component
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