The Inversion Study On The Low-Energy Area Of Aviation Gamma-Ray Spectrum

This paper based on the "method-technology research and software development of the airborne gamma spectrometry" ,which was the 3rd subject of "the development of airborne gamma spectrometry survey system (major projects of National 863 Plan)".On the analysis of the development and current situation about the Aviation gamma spectrometry , there are three problems on the domestic and international: (1) in the traditional Aviation Gamma Spectrometry, only use the area of the natural gamma spectrometry, which is more than 1MeV, and getting the target information of peaks spectrum is still limited to the U, Th, K three nuclides, and the area of the energy less than 1MeV of the natural gamma spectrometry dose not be used , which cause losing a large information of the natural gamma spectrometry; (2) Determining the rock density, ash composition and so on, always use the information of the low-energy spectrum of Man-made gamma-ray spectrum, and using the information of natural gamma-ray spectrometry has not been reported; (3) the application of natural gamma spectrum in lithology, there is no basis for the choice of the energy window.The purpose of taking advantage of the information of this low-energy area of aviation gamma (<1MeV) spectrum, is for geological exploration, and the light of these studies focus on the variation between the low-energy area of natural gamma spectrometry and petrochemical parameter (medium (effective) atomic number Zeff, media densityρ). This paper argues consistency of characteristics between the aviation gamma spectrometry and the ground natural gamma spectrometry, and between the man-made gamma spectrometry and the natural gamma spectrometry, and design spectra test equipment and the MC simulation test device. The system study, which formed on the low energy area of gamma spectrum of concrete, marble, iron, C, Na, Mg, Al, P, Ca, Fe, SiO2 and other different materials, identified the relations between the low-energy area of gamma spectrum and petrophysical parameter. And then under the Mathematical Regression Analysis, analysis the part area of the low-energy spectrum which associated with medium physical parameters, further confirmed the highest level of energy range of natural gamma spectrum, which associated with medium physical parameters. And it is the basis for the low-energy area of aviation gamma spectrum used in inversion of petrophysical parameter.This paper has the following features and innovations:1. The paper, based on the theory of the interaction betweenγ-ray and matter, made a comprehensive analysis on the forming mechanism of the low-energy area of natural gamma spectrum, and argued consistency of characteristics between the aviation gamma spectrometry and the ground natural gamma spectrometry, and between the man-made gamma spectrometry and the natural gamma spectrometry;2. The low-energy area(<1MeV) of the aviation gamma spectrum has nothing to do with theγsource types, energy and spatial distribution,only relevant to the absorption materials. And the paper designed spectra test equipment and the MC simulation test device for the study on the low-energy spectrum;3.Qualitative determination the variation feature of the low-energy area of natural gamma spectrum with the Medium physical parameters: with the increase of the medium (effective) atomic number, the low energy peaks of the low energy spectrum moves high-energy; the counter of the low-energy spectrum reduced with the increase of media density; under 200keV of the low-energy spectrum is related with medium (effective) atomic number and media density, but the affection of the (effective) atomic number is the major role; the role of 200～600keV of the low-energy spectrum is media density, and the medium (effective) atomic number is little affected;4.Using mathematical regression analysis, quantitative determination that the energy region 80～160keV of the natural gamma spectrometry associated with the medium (effective) atomic number highest degree, and the 360～600keV of the natural gamma spectrum associate with media density the highest degree;5.The paper establish the inversion equation of the use of the information of the low-energy area of natural gamma spectrometry inverse medium physical parameters(effective atomic number and medium density).