Acquisition, processing and enhancement of multi-channel radiometric data collected with ultralight aircraft mounted detectors

摘要

An ultralight aircraft based airborne geophysical system was developed and udoperated by the Council for Geoscience during the period 1997-2004. The aim of udthis project was to collect geophysical data at lower cost and higher resolution udthan was possible using conventional airborne systems. This dissertation describes udthe development of the radiometric systems used in the ultralight airborne survey udproject. During the course of the study, a number of obstacles to the successful udcollection and processing of radiometric data with the ultralight-mounted systems udwere encountered. These are described and solutions proposed.udTo facilitate the development hardware systems and processing methods, a Monte udCarlo simulation method was developed to produce spectra containing realistic udsignal and noise components. This method was applied to the selection of detector udmaterials and the specification of detector sizes as well as being used to simulate udlarge spectral data sets for the development and testing of processing and udcalibration procedures.udRadiometric data follow a Poisson Distribution, with the signal to noise ratio udbeing dependent on the count rate recorded, which, in turn, depends on the size of udthe detector used. The ultralight aircraft were capable of carrying a detector one udeighth the size of that used in conventional systems. To allow for the use of the udsmaller detector, the noise adjusted singular value decomposition (NASVD) udprocessing technique was employed. While this technique is commonly applied in udnoise-reduction, the original application, namely the determination and mapping udof spectral components was also utilised.udiiiudDuring the course of the study no suitable calibration facilities were available udinside South Africa. This necessitated the development of a spectral stripping udmethod, utilising a technique generally applied to much higher resolution spectral uddata collected under laboratory conditions. Simulation studies and practical udapplication showed that this method performs well, in some cases outperforming udthe conventional stripping method. The method is also applicable to the study of udanthropogenic radionuclides, where suitable calibration facilities are generally udunavailable. An alternative to the conventional method of altitude correction was udalso applied to the radiometric data collected with the ultralight-mounted systems.udUsing simulated data, a spectrometer based on a bismuth germanate (BGO) uddetector was designed and constructed. This material is significantly denser than udthe more usual thallium activated sodium iodide used for detector fabrication and udhas a higher effective atomic number, giving it a greater photopeak efficiency. udHowever the poor light production of this scintillation material results in a poorer udenergy resolution than a conventional detector. Initial tests using small BGO uddetectors were promising and a larger detector was acquired and tested. udUnfortunately the poor energy resolution and high cost of BGO detectors led to udthe conclusion that they did not offer the advantages initially hoped for. udNevertheless a number of successful surveys were flown using the BGO detector.udUltralight-mounted systems were found to be ideal for small surveys where high udspatial resolution is required. The ultralight systems were successfully applied to udthe detection of radioactive pollution on a number of sites in the Witwatersrand udand related gold fields and one site where anthropogenic radionuclide udcontamination was present. In some cases, the data could be compared to data udcollected using a conventional airborne radiometric system. Here the ultralight-udmounted systems were found to perform satisfactorily, albeit with a poorer signal udto noise ratio except where adverse flying conditions necessitated flying at high udaltitude.udThe strengths, weaknesses and potential applications of ultralight-mounted udairborne radiometric systems are discussed.