Magnetic Behavior Of Rocks And Forward And Inverse Models Incorporating Demagnetization

A number of magnetic methods have been developed in order to gain geologic information about rocks.Three aspects of magnetism are used in the scope of this thesis: 1)rock magnetic methods to identify hematite when it occurs together with magnetite;2)paleomagnetic and magnetic anisotropy to gain an understanding of how paleomagnetic inclination responds to strain based on 3-D printed analogue rocks;and 3)incorporating remanent magnetization and demagnetization effects into forward and inverse models for magnetic anomalies.In the rock magnetism section,one understands the behavior of rocks for mineral identification,inclination flattening.The characterization of magnetic minerals in rocks often employs methods that measure induced magnetisation.When rocks,sediments,or soils contain two magnetic phases,in which one has a high saturation magnetisation(MS),e.g.,magnetite,and the other a low MS,e.g.,hematite,the induced magnetisation will be dominated by the stronger phase.An earlier study by Frank and Nowaczyk has shown that even when magnetite makes up < 10 wt % of the ferromagnetic content,it will mask hematite.This makes identification of phases with low MS difficult to identify.The first part of the thesis is interested determining which rock magnetic method is most sensitive in identifying hematite,when it is found together with magnetite.A systematic study of synthetic mixtures of single domain magnetite and hematite with a broad spectrum of particle size,was conducted,using hysteresis properties,acquisition of isothermal remanent magnetisation(IRM),and first-order reversal curve distributions(FORC).Hysteresis parameters and FORC distributions do not vary significantly from the pure magnetite sample for hematite concentrations ? 90 wt %.IRM is not saturated for hematite concentration of 30 wt % or higher.Principal component analysis(PCA)of the processed FORCs,detects the presence of hematite for concentrations 70 wt % at the very least.Our results illustrate the difficulty in identifying hematite when it is found together with magnetite.IRM acquisition is the most sensitive method for identifying hematite when it occurs together with magnetite.Paleogeographic reconstructions and construction of apparent polar wander paths are dependent on having reliable palaeomagnetic directions.The importance of inclination flattening in biasing the palaeomagnetic record has been debated for over 60 years.Correction for this effect often assumes that the paleomagnetic vector deforms as a passive line.In a novel approach of using 3-D printed analogue rocks,the second part of the thesis examines the question of how a paleomagnetic vector responds to deformation,specifically compaction.Maghemite nanoparticles are mixed in the printing resin with a concentration of 0.15 wt%,and five series of cylinders with 1 cm height and diameter,were printed with porosities between 0 to 20%.Samples were given an anhysteretic remanent magnetization 45° to the cylinder axis and subjected to incremental compaction.The magnetic fabric shows initial weak compaction in the printing plane that becomes larger with each step of compaction.The paleomagnetic inclination changes according to the strain which the sample undergoes,and the amount is less than predicted by a passive marker model of deformation.Our results show the need to examine closer methods for correcting inclination.Further I demonstrate the usefulness of 3D printing of analogue rocks.As printing technology develops,more realistic materials will be possible to produce.The final part of the thesis considers interpretation of data from magnetic surveys.In magnetic prospecting section,forward and inverse models incorporating demagnetization are carried on and only studied in static magnetic field.One emphasizes the importance of considering demagnetization via forward modeling.Taking the plate body as an example,systematically study is carried on the characteristics of magnetic anomaly affected by the magnetic susceptibility of the magnetic body,the axial ratio,the inclination of the plate body and geomagnetic inclination.Meanwhile,the influence of demagnetization in inversion is studied.The results of finite volume forward modeling and inversion of Maxwell equations based on static magnetic field show that the properties of high magnetization have a significant effect on the forward amplitude and shape of the magnetic anomalies.The value of magnetic susceptibility obtained via the inversion method of incorporating demagnetization is greater than that obtained by the traditional inversion method,which is much closer to the actual rock magnetic susceptibility.The results that are presented in this thesis demonstrate the versatility of magnetic methods to look at specific questions in the earth sciences.The topics have an effect on a broad range of scales ranging from mineral identification in rocks,sediments or soils to anomalies on the scale of meter to kilometers to global scales related to paleogeographic reconstructions.