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Philosophiae Doctor - PhD / This study presents the results of radiometric investigation of soil and beach sand in Zanzibar. The activity concentration of natural radionuclides (40K, and 232Th and 238U decay products) in beach sand and soil samples was measured in-situ using the NaI(Tl) and the MEDUSA gamma ray detectors and ex-situ using the low background HPGe detector system. The activity concentrations of 232Th, 238U and 40 K in beach sand are much lower than in soil samples, with one major exception at Kukuu. Two beach sand samples from Kukuu beach were found to have enhanced radioactivity levels due to the presence of heavy minerals. The spatial distributions maps for 40K, 238U and 232Th show large variation in soil samples for two relatively small islands. These strong variations are unexpected, that could have implications for agriculture, is one of the major outcomes of this study. The outdoor gamma dose rates obtained in beach sand and soil samples ranged from 3 to 2156 nGy h-1 and 50 to 294 nGy h-1, respectively. The highest absorbed dose rates in soil samples and beach sand are respectively 5 and 38 times higher than the average world level of 57 nGyh-1 for terrestrial doses (UNSCEAR, 2008). Apart from the Kukuu black sand samples that contain the high 238U and 232Th levels, the beach sands and soil in this study do not pose any radiological threat to the public using beaches for various activities. Based on elemental concentrations, the beach samples in the study area have been classified into four groups; silicate sand (rich in SiO2), carbonate sand (dominated with CaO), mixed sand (with high amount of CaO and SiO2) and heavy mineral sand (with high contents of Fe2O3 and TiO2. A strong correlation between SiO2, Fe2O3, TiO2, Zr, V, Ce, Nb, Hf, Y, La, and Nd show these elements are linked with high activity concentration of 232Th in the studied samples. Moreover, the high concentrations of Y, V, P2O5 and Fe2O3 in the beach sand samples may relate to high 238U activity concentration. This study set out to investigate how radiometric studies can quickly and easily provide an idea of the variation of soil type found over a large area as well as provide detailed information on a small scale such as the beach where heavy minerals were found. The in-situ laboratory measurements were supplemented by XRF and ICP-MS measurements of samples to investigate the distribution of heavy mineral sands.

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An ultralight aircraft based airborne geophysical system was developed and operated by the Council for Geoscience during the period 1997-2004. The aim of this project was to collect geophysical data at lower cost and higher resolution than was possible using conventional airborne systems. This dissertation describes the development of the radiometric systems used in the ultralight airborne survey project. During the course of the study, a number of obstacles to the successful collection and processing of radiometric data with the ultralight-mounted systems were encountered. These are described and solutions proposed. To facilitate the development hardware systems and processing methods, a Monte Carlo simulation method was developed to produce spectra containing realistic signal and noise components. This method was applied to the selection of detector materials and the specification of detector sizes as well as being used to simulate large spectral data sets for the development and testing of processing and calibration procedures. Radiometric data follow a Poisson Distribution, with the signal to noise ratio being dependent on the count rate recorded, which, in turn, depends on the size of the detector used. The ultralight aircraft were capable of carrying a detector one eighth the size of that used in conventional systems. To allow for the use of the smaller detector, the noise adjusted singular value decomposition (NASVD) processing technique was employed. While this technique is commonly applied in noise-reduction, the original application, namely the determination and mapping of spectral components was also utilised. iii During the course of the study no suitable calibration facilities were available inside South Africa. This necessitated the development of a spectral stripping method, utilising a technique generally applied to much higher resolution spectral data collected under laboratory conditions. Simulation studies and practical application showed that this method performs well, in some cases outperforming the conventional stripping method. The method is also applicable to the study of anthropogenic radionuclides, where suitable calibration facilities are generally unavailable. An alternative to the conventional method of altitude correction was also applied to the radiometric data collected with the ultralight-mounted systems. Using simulated data, a spectrometer based on a bismuth germanate (BGO) detector was designed and constructed. This material is significantly denser than the more usual thallium activated sodium iodide used for detector fabrication and has a higher effective atomic number, giving it a greater photopeak efficiency. However the poor light production of this scintillation material results in a poorer energy resolution than a conventional detector. Initial tests using small BGO detectors were promising and a larger detector was acquired and tested. Unfortunately the poor energy resolution and high cost of BGO detectors led to the conclusion that they did not offer the advantages initially hoped for. Nevertheless a number of successful surveys were flown using the BGO detector. Ultralight-mounted systems were found to be ideal for small surveys where high spatial resolution is required. The ultralight systems were successfully applied to the detection of radioactive pollution on a number of sites in the Witwatersrand and related gold fields and one site where anthropogenic radionuclide contamination was present. In some cases, the data could be compared to data collected using a conventional airborne radiometric system. Here the ultralight- mounted systems were found to perform satisfactorily, albeit with a poorer signal to noise ratio except where adverse flying conditions necessitated flying at high altitude. The strengths, weaknesses and potential applications of ultralight-mounted airborne radiometric systems are discussed.

SAR backscatter data contain both geometric and radiometric distortions due to underlying topography and the radar viewing geometry. Thus, applications using SAR backscatter data for deriving various scientific products (e.g. above ground biomass) require accurate absolute radiometric calibration. The calibration process involves estimation of the local radar scattering area through knowledge of the imaged terrain, which is often obtained through DEMs. High resolution UAVSAR data over a New Hampshire boreal forest test site was radiometrically calibrated using a low resolution SRTM DEM, and different calibration methods were tested and compared. Heteromorphic methods utilizing DEM integration are able to model scattering area better than homomorphic methods based on the local incidence or projection angle with a resultant backscatter calibration difference of less than 0.5 dB. Additionally, the impact of low DEM resolution on the calibration was investigated through a Fourier analysis of different topographic classes. Power spectra of high-resolution airborne lidar DEMs were used to characterize the topography of steep, moderate, and flat terrain. Thus, errors for a given low resolution DEM associated with a particular topographic class could be quantified through a comparison of its power spectrum with that from the lidar. These errors were validated by comparing DEM slope derived from SRTM and lidar DEMs. The impact of radiometric calibration on the biomass retrieval capabilities of UAVSAR data was investigated by fitting second-order polynomials to backscatter vs. biomass plots for the HH, HV, and VV polarizations. LVIS RH50 values were used to calculate biomass, and the process was repeated for both uncalibrated and area calibrated UAVSAR images. The calibration improved the $R^2$ values for the polynomial fits by 0.7-0.8 for all three polarizations but had little effect on the polynomial coefficients. The Fourier method for predicting DEM errors was used to predict biomass errors due to the calibration. It was revealed that the greatest errors occurred in the near range of the SAR image and on slopes facing towards the radar. / text

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