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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5596 |
| Date | 05 September 2008 |
| Creators | Coetzee, Hendrik |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
Page generated in 0.0024 seconds