Font Size: a A A

Study On Processing, Forward Modeling And Inversion Algorithms Of Satellite Magnetic Anomaly Data In Spherical Coordinate System

Posted on:2015-01-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:J S DuFull Text:PDF
GTID:1220330431470446Subject:Geophysics
Abstract/Summary:PDF Full Text Request
The lithospheric magnetic field mainly reflects the magnetised physical properties (susceptibility, magnetization or magnetic moment) distribution of the rocks inner the lithosphere. Due to the differences in the rocks’ magnetised environment and its changes with time, the magnetic carriers and the tectonic evolution they experienced, the magnetic field carries amount of geology and geodynamic information such as the material composition, the structure, the pressure and thermal state, the deep rock metamorphism and the tectonic movement and evolution processes.Based on the accumulation of the ground magnetic survey, ship-borne magnetic survey and aeromagnetic survey, as well as the fast development of the satellite magnetic survey techniques, massive regional and even global magnetic field observations of the Earth, the Moon, the Mars and other planets have been obtained, and the accuracy and resolution of lithospheric magnetic field models based on the abundant observed data become better and better.However, for regional and global scales, the observation locations of the magnetic data are on a curved surface and the lithospheric magnetic distribution also shows as an approximately spherical or ellipsoidal shell. Therefore, it needs us to take the expression, processing, forward, inversion and interpretation of the magnetic survey data in the spherical coordinate system. On one hand, this can avoid new errors introduced by the traditional plane coordinate projection. On the other hand, the in situ spatial correspondence relationship between the observed magnetic field and the source location could be more reasonably maintained. Specially, the satellite magnetic survey has salient features as global coverage, uniformity of observed data processing and consistency of the accuracy and resolution of the regional data, relative to the ground, marine and aeromagnetic surveys. Moreover, the satellite magnetic data is mainly dominated by medium and long-wavelength components of the lithospheric magnetic field, which has suppressed the disturbances caused by human life, the local environment and shallow geological bodies. This leads a better study of the regional and global tectonics, the magnetic distribution frame of the deep crust and the uppermost mantle and also the thermal state of the lithosphere.Therefore, in this paper, the three dimensional (3D) forward, inversion and the quantitative interpretation of the magnetic data in the spherical coordinate system were systematically carried out. The influence factors and effectiveness of the methods were analyzed and investigated by the corresponding synthetic model tests. As a following step, based on the lithospheric magnetic field model data (e.g., GRIMM_L120v0.0model), the three-dimensional lithospheric anomalous susceptibility model of China and its adjacent area is constructed preliminarily. And then, such works could supply basis of the regional magnetization distribution in the lithosphere for the China’s SinoProbe project and also the scientific problems such as regional tectonics and lithospheric dynamics of China and its adjacent areas, referenced methods of the lithospheric magnetic field interpretation of the Moon, the Mars and the other planets, as well as knowledge accumulation for the achievement of the scientific objectives of the lithosphere in the SWARM satellite mission and also for China’s satellite magnetic survey program in the future.Research contents and major achievements in this paper are as following:(1) Signal enhancements of the lithospheric magnetic field in spherical coordinate systemBecause of the superposition of potential field and the rocks’skewed magnetization, the correlation between the lithospheric magnetic anomaly and the horizontal locations of the sources is not apparent. In this paper, transformed signals (such as gradient fields, analytical signal, modulus and normalized magnetised source strength) were employed to improve the correlation between the magnetic anomalies and the horizontal locations of the geological sources. Besides, these transformed signals at a low altitude are always related to the contacts such as the geological unit’s boundaries, the fault zones and magmatic dikes. Then, utilizing the lithospheric magnetic field model data, the enhanced lithospheric regional magnetic anomaly maps of China and its adjacent areas were obtained. Meanwile, the spherical harmonic expressions of the magnetic potential, vector and its gradient fields with non-singularity in the polar region were derived, wich makes the transformation of the magnetic field in the polar regiones possible. These kinds of transformed and strengthened magnetic signals can provide the assistant information to evaluate the reliability horizontally of the latter inversed3D model.(2) Forward modeling algorithems for magnetic potential, vector and its gradient tensor fields of the spherical prism in spherical coordinate systemForward modelling is the foundation of the inversion and interpretation. In this paper, the three kinds of forward modelling methods for magnetic potential, vector and its gradient tensor fields of the spherical prim based on the Taylor series expansion, triple and double Gauss-Legendre numerical integration and the approximations by traditional elements (e.g., magnetic dipole source and rectangular prism) were given. Then the accuracy, the influencing factors and the computing effectiveness of different methods were comparatively analyzed.(3) Inversion algorithems for3D anomalous susceptibility imaging in the spherical coordinate systemThe classical3D inversion methods in the gravity and magnetic exploration were extended to the spherical coordinate system from the Cartesian coordinate system. The3D inversion method of magnetic anomaly in the spherical coordinate system was systematic discussed, especially for the key issues such as the model discritization, the forward methods, the model objective functions, as well as the radial weighting functions. By series of synthetic model tests, the impact factors on the recovered models were investigated. Then, the particularity of the regional inversion was discussed and the self-constrained inversion method by the maximum depths of the lithospheric magnetization layer was presented. Futhermore, the newly constructed3D inversion algorithem can adapt to different data types (e.g., single component, multi components,△T magnetic anomaly and magnetic anomaly modulus), take the continental topography and the seafloor depth variations into consideration, and also employ different kinds of norms or measures to compute the structural complexity of the model and also the mis-fit of the data.(4) The3D distribution of the regional anomalous magnetic susceptibility in the lithosphere beneath China and its adjacent areasUsing the lithospheric magnetic model of GRIMM_L120v0.0, the3D distribution of the regional anomalous magnetic susceptibility in the lithosphere of China and its adjacent areas was inversed after some necessary data corrections. The overall distributions perform as that the lithosphere of areas such as the North China Craton, the Tarim Basin, Junggar Basin, Songliao Basin, Qaidam Basin, Sichuan Basin and China’s southeast coast all with high anomalous magnetic susceptibility, but in the areas of South China fold belt, the Yellow Sea and Subei Basin, Qinghai-Tibet Plateau, North-South Seismic Belt, Tianshan and Qinling-Dabie orogenic belts were all with low anomalous magnetic susceptibility. This kind of the recovered susceptibility distribution was in accordance with the distribution of the transformed lithospheric magnetic field, which indicates the high reliability of the recovered magnetization model in horizontal direction.(5) Analysis of the affecting factors of the magnetic distribution in the lithosphereAccording to the analysis of this thesis, the regional magnetic distribution of the lithosphere mainly reflects the large-scale magnetic property of the tectonic units, and rarely reflects the distributions of metamorphic rocks, igneous rocks, and mineral deposits that sporadically found near the surface; Also, there is a low positive correlation between the density, P-wave and S-wave velocity of lithosphere and the magnetic distribution, and exists no common anticorrelation between magnetic distribution and the thickness of lower crust and total crust in China. In Qinghai-Tibet plateau region, for example, it shows a low radial integrated anomalous magnetic susceptibility yet with a thick lower crust and total crust; Moreover, the regional magnetic distribution is negatively associated with the surface heat flow, the S-wave velocity in the uppermost mantle and the temperature of lithosphere; And there is a difference between the bottom interfaces of magnetization layer and crust in continental China; And also the bottom depth variation and temperature distribution are of anticorrelation, but it does not appears to be a isothermal surface; On the contrary, there are some factors showing good agreements with the magnetic distribution, such as the lithospheric deformation and the effective elastic thickness of lithosphere (Te), which show that high magnetic areas are correspond to high Te values and weak deformations; Another factor which shows good compliance is that earthquakes mainly happen in low magnetic areas and the sharp changing zones which means two possible mechanisms of the earthquakes, for examples, the Tibet, Tianshan-Baikal and Pamir belong to the former one, but the Longmenshan orogen and the middle of the North China Craton the latter one. Similarly, most of volcanoes are located in the sharp changing zones of the magnetization variations. Besides, the magnetization distribution provides a believable interpretation of the location of the North-South Earthquake Zone and its three-section feature; At last, there is a positive correlation between the magnetic distribution and the geological age of the structure units of which the last (or the newest) thermaltectonic event shows a great influence on the magnetization distribution. In short, there is an obvious relationship between the thermal structure and the magnetization distributions in the continental lithosphere.(6) Some insights on the regional tectonics and lithospheric dynamics in continental China and its adjacent areas from the regional magnetization distributionThe regional high magnetised units of lithosphere are correspond to ancient, stable and high mechanical strength’s blocks, and the inversed magnetization variations reveal the possible ancient blocks existing of Precambrian basement in China Mainland including the Erie Basin, the Junggar Basin, the Tarim Basin, the Qaidam Basin, the Turpan Basin, the Alxa Block, the Sichuan Basin, the Chuandian Fragment, the Songliao Basin, the North China Craton (NCC), the Central Dabie Organic Belt (diamictite), Cathysia, the Yunkai Block, and two high magnetized sources in the northeastern part of Qiangtang block. However, the low magnetization indicates a heated lithosphere, such as the Tibet, NCC and the Yangtze craton except the Sichuan Basin. Besides, these regions with a large area of low magnetization may be caused by the moving up of the isotherms resulting from the convections in the upper mantle. For the low magnetization beneath the Gangetic plain, it is possible that, in the subduction of the Indian Plate towards the Tibet, the mantle convections exist not only in front of the subducting slab but also behind the the subducting slab.The lithosphere of the Tarim Basin is dominated by a high magnetization gradually weaked and narrowed in the direction of northeast and is surrounded by the low magnetization of the Tianshan, Pamir and the Tibet. The Northeast China Region is dominated by a large area of high magnetization, which agrees well with the seldom earthquakes and weak surface deformation field. The3D anomalous susceptibility model shows a ring-shaped high magnetized block with the most strongly magnetized body from northeast to southwest in the Xing-An block. Providing the Songliao microcontinent exists, we can deduce that the thickness of this microcontinent is thicker in the Xing-An block than that in the Song-Nen block, the minimum thickness appears in the center of the Songliao basin and the geometric trend is from northeast to southwest. Compared with the Tarim Craton and the India Craton, the NCC contains a relatively lower magnetization which means demagnetization process happened, namely destruction and deformation occurred in the NCC. The eight high magnetised units in the North China Craton might be the Micro-blocks by Archean continental nucleus with continental crust accretion. Inside of the NCC, there is the high-low regional magnetization distribution with the directions of the tectonic trend. Specially, the east of the Tanlu fault shows widely low magnetised area and high-low magnetised belts in the west, which means the destruction of the NCC mainly happened in the east part and particularly in the east of Taihang Mountain. In the Yellow Sea and the south region of40°N of the Korea peninsula, the lithospheric magnetization appears to be low value similarly with the feature of the lower Yangtze area. On contrary, in the north region of40°N of the Korea peninsula, the lithosphere is dominated by high magnetization. This distribution indicates that relative to the Sino-Korea block, the lithosphere in the Yellow Sea and the south region of40°N of the Korea peninsula is more nearly the characteristics of the lower Yangtze block.In the Tibetan Plateau, the lithosphere is widely, wholly and weakly magnetized with highly consistency with the frequent earthquakes distribution, both of which indicates a widespread hot lithosphere. Combined with the field trends of the residual gravity anomalies and the feature of north-south magnetised distribution in the central and south Tibetan Plateau, it is proved that the collision of the India Block and Tibetan Plateau has inhomogeneity in time and also in space. There exists a low magnetized zone, which becomes wider gradually from southeast to south, between the Sichuan Basin and the Eastern Himalayan syntaxis with extremely high lithospheric magnetization. Therefore, we deduce that this zonic magnetization distribution with high temperature could be the reflection of the crustal low channel caused by the collision of the India Block against the Tibetan Plateau. In addition, the magnetization and also the temperature distribution show that this zone is obstructed by the highly magnetized Indo-China Block and thus break into two branches in west-south and east-south directions. Moreover, the inversed3D model suggests that, the rigid Sichuan basin tilts from northwest to southeast. This structural feature is also revealed by the distributions of the Te and the earthquakes. Especially, the inversed3D model also shows that, the western part of the Sichuan basin has a higher magnetization in upper lithosphere than that in the lower, which indicates that the western part possibly is heated or is rotated downward, and the whole Sichuan basin has a counterclockwise rotation.In Indian Peninsula, the magnetization distribution is highly agreed with the geotectonic frame. The3D anomalous susceptibility model shows the oblique contact between the adjacent oceanic lithosphere and the Indian continental lithosphere. The Indian continent extends west to the ocean around14°N, and the oceanic lithosphere beside the continent both obliquely underly the Indian continent, which makes the Indian continent looks like an inverted triangle. Moreover, the result of the3D inversion of the regional magnetization distribution of lithosphere depicts that the85°E ridge is a boundary of magnetization, that is, highly magnetised in the east part and lowly magnetised in the west part, and both parts are obliquely located in two sides from the85°E ridge.
Keywords/Search Tags:Spherical coordinate system, Lithosphere, Magnetic anomaly, Data processing, Forward modelling, Three dimensional (3D) inversion, North China Craton, TibetanPlateau, Magnetization structure, Susceptibility imaging, China and its adjacent area
PDF Full Text Request
Related items