Dissertations / Theses on the topic 'Magnetic prospecting Magnetic prospecting Magnetometers'

The eastern branch of the late Proterozoic Damara Orogenic Belt of central Namibia hosts various copper, gold, manganese and uranium deposits, but in the vicinity of Steinhausen, approximately 145 km northeast of Windhoek, the Damara Belt becomes increasingly covered by recent Kalahari cover sediments resulting in little known geology and subsequent lack of discovered economic mineral deposits. Airborne magnetic and radiometric data over the Steinhausen Study Area was enhanced through image processing and filtering to accentuate characteristics of subsurface geology that, by comparing these characteristics to known geology, aided in the interpretive mapping of lithology, structure and targets for follow-up exploration. As a result, some important observations regarding the regional lithology can be drawn. An arenaceous stratigraphic unit that includes a coarse grained, glassy quartzite below the Kuiseb Formation equates to either the eastern Damaran equivalent of the Nosib Group subjected to high grade metamorphism or, alternatively, the upper part of the pre-Damaran sequence, immediately underlying the Damara. The Kuiseb Formation within the study area is uncharacteristically varied as compared to the same formation further west along the Damaran Orogen and can be subdivided into 5 separate units based on geophysical signature. Structural features evident within the study area include the prominent Kudu and Okahandja Lineaments and straddle an area of inferred uplifted stratigraphy of possibly pre-Damara age. The Ekuja Dome (Kibaran age and host to the Omitiomire copper deposit) is also clearly discernible on the airborne magnetic data and is cross-cut by an east-northeast structural zone. Direct targets for follow-up exploration include the Rodenbeck intrusion, anomalous magnetic bodies and numerous radiometric anomalies present within the study area. Identified dome-like features are considered prospective for Omitiomire-style deposits and the Okatjuru Layered Complex is considered a possible source of copper, chromite, magnetite, ilmenite, nickel and the platinum group elements.

[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.

Starting from the dynamic equations of electromagnetics we derive mutual impedance formulas that include the effects of induced polarization (IP) and electromagnetic (EM) coupling. The mutual impedance formulas are given for four geometries: a fullspace, a cylinder in a fullspace, a halfspace, and a layer over a halfspace. IP effects are characterized by a Cole-Cole model, the properties of which are fully investigated. From the general mutual impedance formulas specific limiting forms are defined to characterize the IP and EM effects. Using these limiting forms a framework is developed to justify the addition or multiplication of the two effects. The additive and multiplicative models are compared in the cylinder and layer geometries with the conclusion that the additive model proves to be more accurate over a wider range of frequencies than the multiplicative model. The nature of the IP and EM effects is illustrated in all four geometries showing the effects of relevant parameters. In all cases it is shown that the real part of the mutual impedance contains important IP information that is less influenced by EM effects. Finally the effects of boundaries are illustrated by the cylinder and layer geometries and a theory is developed to incorporate EM effects and IP effects from multiple regions which utilizes frequency dependent real dilution factors. We also include a brief review of some EM "removal" schemes and dilution theory approximations.

A method is presented for the determination of interface topography of multi-layered crustal models from gravity or magnetic data. The technique is developed within the framework of discrete linear inverse theory and exploits the quasi-linearity of the problem to derive a simple cost-effective algorithm suitable for use on large gridded data sets.
Variation of auxiliary parameters allows a suite of acceptable models to be produced rapidly and appraised in the light of available geological and geophysical evidence. When independent knowledge concerning the behaviour of specified interfaces is available, the incorporation of such data in the form of linear equality constraints is outlined.
The proposed method is applied to Curie isotherm and Moho mapping in the Abitibi greenstone belt.

The aim of this thesis has been to investigate the benefits, pitfalls and possibilities of using geophysical methods in archaeological projects. This is exemplified by surveys carried out at archaeological sites in different geographical and chronological contexts. The thesis also aims at investigating the cause for the under-use of the methods in Swedish archaeology by looking at previously conducted surveys. The methods used during these surveys have been Ground-penetrating radar (GPR), magnetometer, slingram and a kappameter. The surveys in the mountain tundra region of Lapland show that magnetic susceptibility surveys is a valuable aid in discovering heaps of fire-cracked stones and when combined with magnetometry, also hearths. GPR and magnetometer surveys within the Migration Period ringfort Sandbyborg provided the spatial layout of the fort and indicated, along with results from recent excavations and metal detections, many similarities with the ringfort Eketorp II. The non-magnetic character of the sedimentary bedrock on Öland and Gotland is suitable for magnetometer surveys and the method is also highly appropriate for the detection of the remains of high-temperature crafts. GPR surveys at St. Mary’s Dominican convent in Sigtuna produced the spatial layout of the central cloister area. The investigations also show that the geology, pedology, land use and the character of commonly occurring prehistoric remains in Sweden, in certain circumstances and in certain areas, have restricted the possibility of successfully carrying out geophysical surveys. Care must therefore be taken to choose the right instrument for the survey and to tailor the sampling density of each geophysical survey, according to the character and size of the expected archaeological remains, in order to maximize their information return. To increase the use of geophysical methods in Sweden the educational opportunities, both for surveyors and professional archaeologists, need to improve.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Manuscript.

Processed.
Bibliography: leaves 115-128.
xi, 128 leaves : ill., maps (some col.), 1 folded ; 30 cm. + 2 microfiche, 5 maps (folded), appendix and errata in pocket inside back cover.
Title page, contents and abstract only. The complete thesis in print form is available from the University Library.
Thesis (Ph.D.)--Dept. of Geology and Geophysics, University of Adelaide, 1989

The need to construct optimal drape surfaces arises in airborne geophysical surveys where it is necessary to fly a safe distance above the ground and within the performance limits of the aircraft used, but as close as possible to the surface. The problem is formulated as an LP with constraints at every point of a grid covering the area concerned, yielding a very large problem. The objective of the LP is to create as "good" a surface as possible. This formulation is new, as previous methods did not aim to minimise an objective function. If the desired surface has only slope limitations, the resulting drape surface must be constrained in the first derivative. Such a drape surface is readily constructed using the Lifting Algoritlun. It is shown that the Litling Algorithm is both exact and has great speed advantages. Some numerical results confinning exacmcss and speed are presented, as is the algorithm's analogy to a flow network method. An enhanced lifting method with a better order of complexity is also proposed and tested numerically. In most practical situations a drape surface is required which has both first and second derivatives constrained. If only a cut through such a surface is considered, the problem can be solved with relative ease by exploiting its nctwork~Jike structure. This method fonns the basis of one of the preferred heuristics developed later. It was not possible to generalise this method to a full two~dimensional drape surface. A commercially available LP package fares better in finding the optimal solution. Several heuristic methods were examined. first a general heuristic method based on a lifting approach was developed. This was followed by a method using repeated application of the method used for sections (the Alternating One-dimensional Dual Algorithm ["AODA"]). Three heuristics based on thimbles were also designed. Thimbles are caps whose first and second derivatives are acceptable and which are placed over local infeasibilities in the topography The work ends with a chapter comparing the efficiency of various heuristics and comparing the results obtained using a number of test datasets. It was fOLmd that heuristic methods provide acceptable drape surfaces and that the choice lies between speed and accuracy, with a previously designed smoothing method being the fastesl and the AODA the most accurate and quick enough.
Operations Research
D.Phil.

Tese de doutoramento em Engenharia Geológica, no ramo de Geofísica Aplicada, apresentada ao Departamento de Ciências da Terra da Faculdade de Ciências e Tecnologia da Universidade de Coimbra
A área em estudo enquadra-se no sector Santa Comba Dão-Oliveira do Hospital-Seia, na região Centro de Portugal, onde afloram rochas granitóides pertencentes ao batólito granítico das Beiras, que intruiram as sequências metassedimentares do Complexo xisto-grauváquico (CXG). O principal objectivo deste trabalho é conhecer em profundidade a geometria dos corpos graníticos aflorantes neste sector, e deste modo contribuir com dados relevantes para a avaliação do potencial geotérmico na região. Para tal realizaram-se campanhas de prospecção geofísica, por métodos gravimétricos e magnéticos, que permitem obter as cartas da anomalia de Bouguer e da anomalia magnética. Com recurso a programas informáticos de modelação de campos potenciais, considerando toda a informação geológica regional e as propriedades físicas das rochas (densidade e susceptibilidade magnética), as anomalias são modeladas em conjunto. Desta forma consegue-se caracterizar as estruturas geológicas em profundidade e nomeadamente definir uma geometria para os principais corpos graníticos aflorantes na região (plutão Tábua-Santa Comba Dão (TSCD), plutão da serra da Estrela (SE) e o plutonito de Avó (AV)). A modelação demonstra que a fracturação regional condiciona a geometria dos blocos graníticos e de toda a bacia do CXG, sendo observada a influência das principais direcções dos alinhamentos estruturais. De acordo com o modelo proposto a estimativa da profundidade média dos corpos graníticos é cerca de 5-6 km e a profundidade máxima é de 11.6 km. O corpo de TSCD não apresenta enraizamento bem definido e ocorre sobrejacente à unidade do CXG, localizando-se a Oeste da falha da Lousã (FLS). Os corpos graníticos de TSCD e da SE afloram em continuidade, sendo atribuída uma idade similar. Desta forma poderá inferir-se a presença de uma zona de alimentação comum aos plutões, localizada a Este da estrutura da FLS, onde o granito contacta directamente com a crosta média. O plutão TSCD estabelece um contacto brusco e verticalizado, possivelmente por falha com o plutonito de Avô.
The study area is located in the Santa Comba Dão-Oliveira do Hospital-Seia sector of the Central Portugal region. In this region, granitic rocks of the Beiras batholith are present, intruding the metasedimentary sequences of the Complexo xisto-grauváquico (CXG). A better understanding of the subsurface geometry of the granitic bodies in the region is the main goal of this work. This effort is also relevant for the assessment of the geothermal potential of the region. To achieve this, magnetic and gravimetric surveys where performed in order to obtain the Bouguer and magnetic anomalies. Considering and integrating all the available geological data and physical proprieties (density and magnetic susceptibility) the mentioned potential fields were simultaneously modeled. In this way it was possible to characterize the subsurface geometry of the granitic bodies in the region (Tábua-Santa Comba Dão (TSCD), Serra da Estrela (SE) and Avó (AV) plutons). The modeling results show that the regional tectonic setting controls the geometry of the granitic bodies as well as the structure of the CXG basin. The modeled average depth for the granitic bodies is 5-6 km, with a maximum depth of 11.6 km. The TSCD pluton is located west of the Lousã fault (FLS) and overlies the CXG unit. The model suggests that this pluton does not have a well defined root zone. Since the TSCD and SE plutons outcrop in continuity and they have similar ages, a common feeding zone located eastward of the FLS structure is the most likely scenario. In this area the granite is in direct contact with the middle crust. The limit between the TSCD and AV plutons is more abrupt and vertical, suggesting the presence of a fault contact.
FCT - SFRH/BD/48628/2008