Dissertations / Theses on the topic 'Aeromagnetic interpretations'

Nautanen Deformation Zone, is a prominent deformation zone in the Malmfälten area, which is of importance to understand for mineral exploration purposes. In spite of diverse geophysical data being available in Malmfälten and the good correlation between airborne measurements and geological observations, the area has not been fully investigated in detail using the aforementioned available data. A geological feature in connection with the Mertainen magnetite-breccia apatite iron ore deposit has been studied. Methods include the study of geological maps, the study of analytic signals of magnetic and gravity data, data processing, potential field- and 3D modelling and the interpretation of aforementioned models. Based on the observed and modelled data a fold structure has been detected in connection with Mertainen, and several mineralizations are believed to be structurally related to this fold. Furthermore, a potential mineralization structurally related with the fold has been detected, though it is quite likely that it isn't economically viable.
Nautanen Deformation Zone, är en framträdande deformationszon i Malmfälten området, vilken är av betydelse att förstå för mineral prospekterings ändåmål. Trotts att det finns ett stort utbud av geofysiska data i Malmfälten och att det finns en god korrelation mellan de flyggeofysiska mätningarna och geologiska observationer, så har området inte undersökts fullständigt med den tillgängliga datan. En geologisk struktur i koppling till apatit järn malms fyndigheten Mertainen has studerats. Bland metoder ingår studie av geologiska kartor, studie av de analytiska signlar hos magnetiska och gravimetriska data, data processering, potential fält- och 3D modellering samt tolkningen av ovannämnda modeller. Baserat på den observerade samt modellerade datan har en veck strucktur upptäckts i koppling till Mertainen, och flertalet mineraliseringar tros vara strukturellt relaterade till detta veck. Dessutom har en potentiell mineralisering strukturellt relaterad till vecket upptäckts, dock är det väldigt troligt att den inte är ekonomiskt brytbar.

A procedure to upward continue magnetic anomalies observed on an irregular surface onto a horizontal plane has been developed and applied to the aeromagnetic map of Great Britain. Pseudogravimetric transformation was then carried out on this reduced anomaly and both data sets have been used for analysis and interpretation of several prominent anomalies in Scotland along the Great Glen fault and over the Midland Valley. A prominent linear positive magnetic anomaly occurring along the Great Glen fault has been modelled as due to a locally magnetized outward dipping body almost symmetrical about its apex beneath the fault line, together with a magnetized crustal slab to the northwest of the fault. The outward dipping body has its top lying within the upper crust, a magnetization of greater than about 1.0 A/m, a half-width of about 40 km at its base and a thickness of the order of 7-18 km. The origin of the outward dipping magnetized body may possibly be explained by metamorphism produced by frictional heating resulting from the transcurrent fault movement. Alternatively the metamorphism may be associated with some other fault related process such as crustal fluid flow. Thermal modelling has been used to demonstrate this. The magnetization contrast across the fault may be the direct result of blocks of differing magnetization on opposite side, juxtaposed as a result of transcurrent movement. The modelling along a profile over the Clyde Plateau (Midland Valley of Scotland) using a well-constrained lava body reveals the presence of a long wavelength anomaly component due to a deeper crustal source. The basement anomaly is conspicuous on the pseudogravimetric map but not on the aeromagnetic map. A near circular magnetic anomaly near Bathgate in the Midland Valley can be explained by an unexposed intrusive body superimposed on the deep crustal source as above.

To simplify the complex total magnetic field intensity (T) on datasets obtained from locations close to the geomagnetic Equator (inclinations |α| ≤ 20°) such datasets are routinely reduced-to-equator (RTE), since they cannot be stably reduced-to-pole (RTP). RTE anomalies tend to have small amplitudes and exhibit azimuth-based anisotropy, unlike RTP anomalies. Anisotropy describes the dependence of the amplitude and shape of an RTE anomaly on the strike direction of its source. For example, an East-West striking contact/fault will generate a strong RTE anomaly response whereas a North-South striking equivalent will not. Where adjacent sources occur, anisotropy causes interference between anomalies, displacing anomalies relative to their sources. This makes using magnetic data to map structures in regions that are close to the geomagnetic equator difficult or potentially of limited value. This thesis develops a strategy to interpret RTE datasets and applies it to determine the basement structure in NE Nigeria where |α| ≤ 8°. This area has >50% of the basement concealed beneath Cretaceous and Quaternary sediments of the Benue Trough and Chad basin, respectively. The aim of the study is to structurally map the basement underlying the Benue and Chad rifted basins in NE Nigeria, by tracing and determining the depths of basement faults and associated structures. The first-order derivative-based "Tilt-Depth" method has been evaluated to determine its effectiveness when applied to RTE datasets to determine the location and depth of structures. The method was tested first using RTE and RTP equivalents of synthetic  datasets obtained from profiles across East-West striking, 2D contacts at various depths, inclinations of effective magnetisation (ϕ), and dips (d). RTP datasets were used throughout as reference models. Errors in "Tilt-Depth" method estimates were invariant to changes in depth, but sensitive to changes in ϕ and d of sources. At error limits of 0-20%, the method effectively estimates locations and depths of 2D contacts when dip is within the 75 ≤ d° ≤ 105 range, inclination of remanent magnetisation relative to induced magnetisation is within the 155 ≤ β° ≤ 205 range (magnetisations are collinear), and Koenigsberger ratio (Q) of remanent to induced magnetisation amplitudes ≤ 1. Relationships between Q, α , β and ϕ suggests that the simplification of remanence-laden anomalies due to magnetisations being collinear results from deviations of ϕ from α of ≤12° when Q≤1. Similar deviations occur between ϕ and α , for all β values, when Q≤0.2. Hence, remanent magnetisation is negligible for RTP or RTE datasets when a priori information suggests Q≤0.2. The "Tilt-Depth" method was further tested for anisotropy-induced anomaly interference effects using RTP or RTE of the Complex “Bishop” Model (CBM) and Tanzania grids. The CBM grid contains 2D contacts of various strikes and three-dimensional (3D) sources with non-2D contacts at various depths (all precisely known), and satisfy the d, ϕ and Q requirements above. The Tanzania grid presented a real dataset from a Karoo rift basin, where more randomly striking 2D contacts occur at unknown depths. For comparison, the second vertical derivative, analytic signal amplitude, local wavenumber, and the horizontal gradient magnitudes of Ѳ (HGM(Ѳ)) and  (HGM()) methods were also tested using these grids. Locations estimated from all these methods show that: (1) Sources of all shapes and strikes are correctly imaged on RTP grids; (2) North-South striking 2D contacts are not imaged at all on RTE datasets, but can be inferred from linear alignments of stacked short wavelength East-West striking anomalies; (3) 2D contacts with strikes ranging from N045 to N135° are correctly imaged on RTE datasets; (4) Anomalies from poorly isolated 2D contacts with N±020° strikes interfere to further complicate RTE datasets, making it difficult to correctly image these sources; and (5) RTE anomalies from 3D sources tend to smear in an East-West direction, extending such anomalies well past edges of their sources along this direction. These North-South striking non-2D edges are not imaged at all, whilst their East-West striking non 2D (Northern and Southern edges are correctly imaged. Depths estimated for 2D and non-2D contacts with strikes ranging from N045 toN135° from RTP and RTE of the CBM grids, using the local wavenumber, analytic signal amplitude and |Ѳ| = 27°- based “Tilt-Depth" methods show that: (1) "Tilt-Depth” and local wavenumber methods underestimate the actual depth of sources, while the analytic signal amplitude method provided both severely underestimated and overestimated depths. Thus, “Tilt-Depth” and local wavenumber estimates were easier to utilise and interpret; (2) "Tilt-Depth" and local wavenumber methods underestimate 2D contacts from RTP and RTE grids by up to 25 and 35% of their actual depths, respectively; (3) 'Tilt-Depth" and local wavenumber methods, respectively, underestimate depths of East-West striking non-2D edges of 3D sources by about 35 and 30% from the RTP grid; and (4) "Tiit-Depth" method consistently underestimates non-2D contacts from RTE grids by up to 40%. Using knowledge gained from the above tests, all the methods were applied to a NE Nigeria  (RTE) dataset, to delineate basement structures in the area. The dataset was a 1 km upward-continued grid with 1 km x 1 km cell size, and extended well beyond NE Nigeria into Niger, Chad and Cameroon Republics. While basement depths were estimated from the dataset using the "Tilt-Depth" and local wavenumber methods only, these methods and the second vertical derivative, analytic signal amplitude, local wavenumber, as well as the horizontal gradient magnitudes of Ѳ (HGM(Ѳ)) and  (HGM()) methods, were used to map source edge locations. A basement structure map of NE Nigeria was obtained using the above methods and found not to be dominated by North-South striking faults. Instead the basement is dissected mainly by near vertical, NE-SW trending faults against which NW-SE or E-W trending faults terminate. The relationship between these inferred faults, basement horsts, volcanic plugs, and basement depressions, and outcrop information suggests that rifting was episodic as the mainly NorthEast directed rift propagation direction was occasionally deflected by transcurrent faults to relieve differential stresses built up from wall rock and/or crustal resistance. Apparent stress relief features include the Yola basin, flood basalts, Lamurde Anticline and Kaltungo Inlier. A number of isolated depocenters, mainly half grabens, with sediment thickness exceeding 11km seem to occur in NE Nigeria. Outside these depocenters, basement occur at depths generally shallower than 0.5 km, except where intra-basinal horsts occur, at depths shallower than 2.5 km. These depths agree well with well information and seismic data interpretation, and show the SW Chad basin depocenter to be isolated from adjoining basins in Cameroon, Chad and Niger Republics.

L'analyse des donnees stratigraphiques et structurales de la serrania del interior confirme l'existence de 2 cycles sedimentaires cretace moyen-miocene. L'interpretation des profils sismique montre que la partie enfouie de l'edifice est structuree par des accidents suggerant la presence d'un decollement. L'interpretation gravimetrique et aeromagnetique confirme que la serrania est un exemple de chaine d'avant pays decollee et mise en place dans un contexte particulier associant une composante principale en coulissage dextre avec une collision oblique

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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

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.
Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Geophysics, 1989

During the summer and fall of 1982, personnel from the Geophysics Group in the School of Oceanography at Oregon State University conducted an aeromagnetic survey in the northern Oregon Cascades to assess geothermal potential and study the thermal evolution of the Cascade volcanic arc. Total field and low-pass filtered magnetic anomaly maps obtained from the survey data show high amplitude positive and negative anomalies associated with volcanic cones and shallow source bodies along the axis of the High Cascades. Spectral analysis of the aeromagnetic data yielded source depths and depths-to-the-bottom of the magnetic sources. The magnetic source bottom, in the northern Oregon Cascades, is interpreted as the depth to the Curie-point isotherm. The northern Oregon study area shows shallow Curie-point isotherm depths of 5 to 9 km below sea level (BSL) beneath the axis of the High Cascades from the southern boundary (44°N latitude) to near Mt. Wilson (45°N latitude). A smaller region of shallow Curie-point depths of 6 to 9 km BSL lies west of Mt. Wilson (45°N latitude, 122°W longitude). The shallow Curie-point isotherm suggests the emplacement of relatively recent intrusive bodies in the upper crust beneath the axis of the High Cascades and west of Mt. Wilson. A major northeast trending structure observed in magnetic and residual gravity anomalies near Mt. Wilson, is the northernmost. extent of shallow Curie-point depths and high geothermal gradients mapped in the northern Oregon Cascades. This northeast trending structure appears to mark a division between high intrusive activity in localized areas south of Mt. Wilson and intrusive activity confined beneath the major cones north of Mt. Wilson.
Graduation date: 1986

Figure 5.13 is folded and in plastic pocket inside back cover.
Bibliography: leaves 147-168.
xiv, 184, [14] leaves, [35] leaves of plates : ill. (chiefly col.), maps ; 30 cm.
Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Geophysics, 1998

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2015
Three new methods for determining the structural index and source distance for magnetic field data are presented. These methods require only the calculation of the first and second order analytic signal amplitudes of the total field and are applicable to both profile and gridded data. The three methods are first tested on synthetic data and then on two real datasets to test for applicability and repeatability. It was found that each method had different strengths and weaknesses and thus one method cannot be favoured over the others. Cooper (2014) describes how to calculate the distance to source over both profile and gridded data given a user defined structural index. Often however, particularly in the case of real data, the structural index is not known or varies over the surveyed area. These three new methods however do not require any user input since the structural index is calculated thus making them more applicable to regions of unknown geology. It was found that the first of the three new methods, the multi-distance inversion method, was best used as an edge-detection filter, since the use of higher order derivatives resulted in increased noise levels in the distance to source calculation. The third of these new methods, the unconstrained inversion method, discussed in Chapter 7, not only solves for the structural index but also determines the depth of the source. In that particular case, the structural index is used as a rejection filter, whereby, depth solutions associated with structural index values outside of the expected range are deemed to be invalid. Unlike the third new method, the first two methods require the distance to source to be calculated via the approach described by Cooper (2014) (which requires the user to define the structural index), the results of which are later rescaled by the calculated structural index to yield what is termed a rescaled distance to source. All three of the new methods are fully automatic and require no user control. The techniques were first tested on both profile and gridded theoretical data over sources with known structural index values. All of the methods were able to estimate the structural index of each of the particular sources and give depth estimates that varied from the true depth by less than 20 percent (with deeper sources being more inaccurate). Noise was also added to the theoretical data in an attempt to assess how the methods can be expected to perform with real data. It was found that when applied to noisy data, these methods performed equally well to slightly worse, than when the method developed by Cooper (2014) was used. As a real world case study these three new methods were tested on aeromagnetic data collected over the Kuruman Military Area, Northern Cape, South Africa. Regional deformations as well as later intrusive dykes and cross cutting faults were imaged by the chosen depth determination procedures. The dolerite dykes in the area were found to occur between 20 to 60 m deep. While the sand cover was estimated to be between 30 to 40 m thick. Overall, the techniques yield distance to source estimates that differ by less than 15 m, over sources, to the results obtained by using the source distance method (Cooper, 2014). To test for repeatability a second aeromagnetic dataset, collected over a dyke swarm within the Bushveld Complex, South Africa was considered. Again comparable (less than 15 m over sources) depth estimates were made between the unconstrained and constrained inversions. Since the distance to source estimates produced by these new unconstrained inversion methods are comparable to those produced by constrained inversion (Cooper, 2014) the project can be deemed successful.