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Graduation date: 1994

Bibliography: v. 1, leaves 213-226 / 2 v. : ill. (some col.) ; 31 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Geophysics, 1986

[Truncated abstract] Magnetic gradiometers are powerful tools for mineral exploration. The magnetic field contains valuable information about the mineral content of the surveyed terrain. The magnetic gradient specifies the amount of spatial variation in the direction and magnitude of the magnetic field. Surveys that measure the magnetic gradient provide vastly more information about geological targets than the magnetic field alone. This technology could have enormous benefits in terms of new discoveries and lower exploration costs. The magnetic gradient is normally calculated by subtracting the outputs of two total field magnetometers which are separated by a baseline. In 1997, a direct string magnetic gradiometer (DSMG) was developed that directly measures magnetic gradients using only a single string as its sensing element. This thesis describes research conducted to improve the sensitivity and performance of the DSMG. The main advantage of the DSMG is that only gradients can induce second harmonic vibrations in the string. Thus, the DSMG is insensitive to uniform magnetic fields that we are not interested in, such as the global magnetic field of the Earth. By using inductive electronics to measure second harmonic string vibrations, we can select to measure the magnetic gradient of nearby targets. Recent work has shown that a magnetic gradiometer with a noise floor of 0.01 nT/m/ v Hz should be sufficiently sensitive for geophysical exploration. In order to reach this goal, this thesis presents an investigation of all noise sources affecting the DSMG. ... Gas damping is negligible in high vacuum and no vibration isolation is required. This means that longer strings with low resonant frequencies can be used. Using theoretical modelling, I show that a space borne DSMG should be able to match the white noise level of SQuID based magnetic gradiometers and have a lower 1/f noise corner. Deployment in space could be the most viable application of the DSMG because of the ease of operation and enhancement of sensitivity. If the thermal noise level is reduced then other sources of noise will start to become more important. When rotated in the Earth's magnetic field, the DSMG detects a pseudo magnetic gradient despite the field being almost uniform. A possible cause is magnetically susceptible parts which are magnetically aligning with the Earth's field. I have conducted a thorough investigation of magnetic susceptible parts in the DSMG and reported the results in this thesis. In the DSMG, a pair of inductive pickup coils are used to measure the string's displacement with a root mean square accuracy of 1011 m/ v Hz. This is adequate at present but the inductive electronics may not be sensitive enough after other improvements in the DSMG are implemented. Here, I present a new capacitive displacement readout with a high sensitivity of 1013 m/ v Hz. The thesis also presents some magnetic gradient measurements in the lab and the results of a ground survey in the field. These trial measurements are used to characterise the DSMG and demonstrate its effectiveness for airborne surveying.

Thesis (MSc)--Stellenbosch University, 2005. / Some digitised pages may appear illegible due to the condition of the original hard copy. / ENGLISH ABSTRACT: Airborne gamma-ray spectrometry is used for exploration and environmental monitoring over large areas. Such datasets of 7-ray are acquired using large 16l NaI(T1) detector crystals mounted in an aircraft. The response of these detectors to naturally occurring radioactive isotopes is a function of detector volume, efficiency and resolution. This response is modelled using the Monte Carlo method. A photon-electron transport computer code, NIPET, is written to calculate the intrinsic efficiency and photofraction of these large prismatic crystals. The intrinsic efficiency indicates the number of incident photons that interact in some way with the detector resulting in a deposition of energy in the crystal. The photofraction, on the other hand, indicates how many of these photons that impinge on the detector are actually recorded with their full initial energy. This study presents explicity calculated photofraction values for 16l NaI(Tl) crystals. Comparison of these new values, calculated with this computer program, with that of published values for small 3" x 3" crystals shows good agreement. However, for large crystals, the calculated values differ from those in the literature by almost 15%. The study also finds that for the gamma-ray energy range between 0.2 - 3.0 MeV, these detectors have an intrinsic efficiency above 90%, whereas the photofraction values range from 80% for low energy photons to above 40% for high energy photons in the case of large crystals. / AFRIKAANSE OPSOMMING: Lug gamma-straal spektroskopie is nuttig vir eksplorasie en omgewings ondersoeke oor groot areas. Metings word tipies gedoen met 16l NaI(T1) detektors wat monteer is in ’n vliegtuig. Die respons van hierdie detektors ten opsigte van natuurlike radioaktiewe isotope is ’n funksie van detektorvolume, doeltreffendheid en oplosvermoë. Hierdie respons word gemodeleer deur gebruik te maak van ’n Monte Carlo tegniek. ’n Rekenaarprogram, NIPET, is geskryf om die intrinsieke doeltreffendheid en fotofraksie vir hierdie groot prismatiese kristalle te bereken. Die intrinsieke doeltreffendheid dui die breukdeel van die invallende fotone aan wat ’n interaksie ondergaan binne die detektor kristal en sodoende energie deponeer. Die fotofraksie, daarenteen, dui die breukdeel invallende fotone aan wat absorbeer word deur die kristal met hul voile invallende energie. In hierdie tesis word eksplisiete fotofraksiewaardes vir 16l NaI(T1) kristale bereken. Goeie ooreenstemming word gevind tussen fotofraksie waardes bereken met hierdie program en die waardes gepubliseer in die literatuur vir klein 3" x 3" kristalle. Vir groter kristalle is daar egter swakker ooreenstemming. Berekende fotofraksie waardes verskil met 15% van die waardes in die literatuur. Daar word ook bevind dat vir gamma-energie tussen 0.2 - 3.0 MeV, die intrinsieke doeltreffendheid vir hierdie tipe detektors bo 90% is en dat fotofraksiewaardes wissel tussen 80% vir lae energie tot bo 40% vir hoë foton energie.