Processing and interpretation of near-earth satellite magnetic data

A thesis submitted to the faculty of science, University of the Witwatersrand, Johannesburg, In fulfilment of the requirements for the Degree of Doctor of Philosophy Johannesburg 1990The processing of near-earth satellite magnetometer data towards extracting an improved lithosphere anomaly map is addressed. The two aspects of the data reduction route where data quality has been compromised most by contemporary methods is researched, namely;[Abbreviated Abstract. Open document to view full version]GR201

Ultra-high sensitivity moment magnetometry of geological samples using magnetic microscopy

Useful paleomagnetic information is expected to be recorded by samples with moments up to three orders of magnitude below the detection limit of standard superconducting rock magnetometers. Such samples are now detectable using recently developed magnetic microscopes, which map the magnetic fields above room-temperature samples with unprecedented spatial resolutions and field sensitivities. However, realizing this potential requires the development of techniques for retrieving sample moments from magnetic microscopy data. With this goal, we developed a technique for uniquely obtaining the net magnetic moment of geological samples from magnetic microscopy maps of unresolved or nearly unresolved magnetization. This technique is particularly powerful for analyzing small, weakly magnetized samples such as meteoritic chondrules and terrestrial silicate crystals like zircons. We validated this technique by applying it to field maps generated from synthetic sources and also to field maps measured using a superconducting quantum interference device (SQUID) microscope above geological samples with moments down to 10⁻¹⁵ Am². For the most magnetic rock samples, the net moments estimated from the SQUID microscope data are within error of independent moment measurements acquired using lower sensitivity standard rock magnetometers. In addition to its superior moment sensitivity, SQUID microscope net moment magnetometry also enables the identification and isolation of magnetic contamination and background sources, which is critical for improving accuracy in paleomagnetic studies of weakly magnetic samples.National Science Foundation (U.S.) (DMS-1521765)National Science Foundation (U.S.) (DMS-0934689)Peterson, Thomas F

Unmixing of Magnetic Hysteresis Loops Through a Modified Gamma‐Cauchy Exponential Model

Abstract Quantifying the contributions of distinct mineral populations in bulk magnetic experiments greatly enhances the analysis of environmental and rock magnetism studies. Here, we develop a new method of parametric unmixing of susceptibility components in hysteresis loops. Our approach is based on a modified Gamma‐Cauchy exponential model that accounts for variable skewness and kurtosis. The robustness of the model is tested with synthetic curves that examine the effects of noise, sampling, and proximity (similar coercivities) of susceptibility components. We provide a Python‐based script, the Hist‐unmix, which allows the user to adjust a forward model of up to three ferromagnetic components as well as a dia/paramagnetic contribution. Optimization of all the parameters is achieved through least squares fitting (Levenberg‐Marquardt method), with uncertainties of each inverted parameter calculated through a Monte Carlo error propagation approach. For each ferromagnetic component, it is possible to estimate the saturation magnetization (Ms), saturation remanent magnetization (Mrs) and the mean coercivity (Bc). Finally, Hist‐unmix was applied to a set of weakly magnetic carbonate rocks from Brazil, which typically show distorted hysteresis loops (wasp‐waisted and potbellied). For these samples, we resolved two components with distinct coercivities. These results are corroborated by previous experimental data, showing that the lower branch of magnetic hysteresis can be modeled by the presented approach and might offer important mineralogical information for rock magnetic and paleomagnetic studies

Late glacial palaeomagnetic secular variations from France

SIGLEAvailable from British Library Document Supply Centre- DSC:D67688/86 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

A lock-in model for the complex Matuyama-Brunhes boundary record of the loess/palaeosol sequence at Lingtai (Central Chinese Loess Plateau)

ISSN:0956-540XISSN:1365-246

Spectral analysis and inverse modeling of satellite data and aeromagnetic data

A series of Earth observation satellite missions has opened a new era for the study of Earth’s magnetic field. Due to the homogeneous global coverage and high accuracy of satellite data, magnetic models derived from those provide reliable estimates of the long-wavelength components of the crustal magnetic field. How such satellite magnetic models can contribute to our understanding of the characteristic of the crustal structures is the main topic of this thesis. First, a comparison of conventional filtering methods was made and a new method for regional spherical harmonic analysis is presented and a thorough discussion is provided by considering the case study of the Australian continent. Next, together with reduction to the pole of satellite data and long-wavelength corrected aeromagnetic compilations, correlated tectonic signatures over the neighboring continents in a Gondwana framework are shown. Finally, a positivity constraint was applied to the global magnetic susceptibility inversion and a globally inverted susceptibility model for a reconstructed Gondwana framework is presented and discussed

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2007The origin of symmetric alternating magnetic polarity stripes on the seafloor is investigated in two marine environments; along the ridge axis of the fast spreading East Pacific Rise (EPR) (9º 25’-9º 55’N) and at Kane Megamullion (KMM) (23º 40’N), near the intersection of the slow-spreading Mid Atlantic Ridge with Kane Transform Fault. Marine magnetic anomalies and magnetic properties of seafloor samples are combined to characterize the magnetic source layer in both locations. The EPR study suggests that along-axis variations in the observed axial magnetic anomaly result from changing source layer thickness alone, consistent with observed changes in seismic Layer 2a. The extrusive basalts of the upper crust therefore constitute the magnetic source layer along the ridge axis and long term crustal accretion patterns are reflected in the appearance of the axial anomaly. At KMM the C2r.2r/C2An.1n (~ 2.581 Ma) polarity reversal boundary cuts through lower crust (gabbro) and upper mantle (serpentinized peridotites) rocks exposed by a detachment fault on the seafloor, indicating that these lithologies can systematically record a magnetic signal. Both lithologies have stable remanent magnetization, capable of contributing to the magnetic source layer. The geometry of the polarity boundary changes from the northern to the central regions of KMM and is believed to be related to changing lithology. In the northern region, interpreted to be a gabbro pluton, the boundary dips away from the ridge axis and is consistent with a rotated conductively cooled isotherm. In the central region the gabbros have been removed and the polarity boundary, which resides in serpentinized peridotite, dips towards the ridge axis and is thought to represent an alteration front. The linear appearance of the polarity boundary across both regions indicates that the two lithologies acquired their magnetic remanence during approximately the same time interval. Seismic events caused by detachment faulting at Kane and Atlantis Transform Faults are investigated using hydroacoustic waves (T-phases) recorded by a hydrophone array. Observations and ray trace models of event propagation show bathymetric blockage along propagation paths, but suggest current models of T-phase excitation and propagation need to be improved to explain observed characteristics of T-phase data.My thesis was funded by several different sources: National Science Foundation grants OCE-9819261, OCE-0221832 and OCE-0118445, a Geological Society of America Graduate Student Research Grant and the Academic Programs Office at Woods Hole Oceanographic Institution

Origin of Magnetic Instability in Sediment Cores From the Central North Pacific

Previous paleomagnetic studies on deep-sea sediment cores from the central North Pacific have shown that the natural remanent magnetization (NRM) of these 'red clay' sediments was unstable below several meters depth in each core. It was also noted that the magnetic instability was related to the presence of a relatively large low-coercivity component of magnetization. The purpose of this investigation was to characterize the rock magnetic properties in three select cores from this region to determine the physical origin of the unstable magnetization. The principal findings of our investigation were as follows. (1) The ability to acquire a viscous remanent magnetization increased with depth in each core, particularly at about the level where unstable magnetization became evident. (2) The magnitude and stability of the observed NRM of the magnetically unstable section of each core can be explained by a viscous remanence acquired in the presence of the earth's magnetic field over a period of time ranging from only several weeks to several thousands of years. (3) The unstable magnetization, believed to be of viscous origin, was attributed to the presence of a magnetic mineral similar in structure and composition to maghemite. This mineral may have resulted from the low-temperature oxidation of very fine grained magnetite at about the time of deposition of these sediments. The extrapolated ages of the levels at which unstable magnetization becomes evident in the cores from this region suggest a close correspondence with the times of established upper Cenozoic climatic changes. Considerations of the alteration of the sedimentary regime resulting from the changes in climate can provide a satisfactory explanation for the observed change in magnetic properties