| The joint inversion of gravity and magnetic data can obtain more accurate underground density and magnetic structure which provides significant reference for the study of geological structure and resource exploration.The cross-gradient function utilizes the similarity of underground density and magnetic structure to establish constraints to achieve the joint inversion of gravity and magnetic data.This method is appropriate for solving geological problems in areas such as oceans,deserts,and ice-covered regions,as it does not require information on physical properties of rocks.In recent years,with the increasing demand for detection resolution and depth and the continuous integration of multi-dimensional gravity and magnetic data(satellite,airborne,ship and ground surveys),large-scale geo-scientific explorations based on a wide range of gravity and magnetic data have become a hot research topic.Tthe influence of the Earth curvature must be considered for the inversion of large-scale gravity and magnetic data,and this problem can be considered by performing inversions in a spherical coordinate system.Therefore,there is a call for the development of a fast joint inversion method with high-resolution of gravity and magnetic data in a spherical coordinate system as an effective and rapid technique for solving large-scale geologic problems.In this thesis,to develop a fast joint inversion method with high-resolution of gravity and magnetic data in spherical coordinate system,we focus on the improvement of joint inversion resolution and the calculating efficiency of the inversion in spherical coordinate system.The joint inversion of gravity and magnetic data with cross-gradient establishes constraints through the gradient similarity of the X density and magnetic structures,prompting one of the physical structures to align as closely as possible to the other.It has two main drawbacks:(1)the cross-gradient function emphasizes the constraints of the other structure in the joint inversion,which makes the joint inversion results inevitably lose the original structural features in the respective inversion results;(2)the cross-gradient constraints are constructed from the regularized inversion results and characterized by a smooth structure.Therefore,the cross-gradient constraint term is not sufficient to significantly improve the resolution and convergence of the joint inversion results.To address the above problems,we proposed a component power type self-mutually constrained joint inversion method to improve the resolution of the joint inversion and to make the density-magnetic structure of homologous geological bodies have good consistency.The proposed method combines the self-mutual constraint and the powers of physical gradient components constraint.The self-mutual constraint is constructed by a fusion of density and magnetic structures,so that the two physical structures in the joint inversion are constrained to each other without losing the original resolution characteristics of their respective inversion results.The power of physical gradient component constraint term improves the resolution and convergence of the joint inversion results by enhancing the constraint strength of the constraint term.For inversions of gravity and magnetic data in spherical coordinate system,two kinds of subdivision mesh are usually selected:(1)the latitude-longitude mesh,which use the tesseroid as the subdivision unit,has the advantage of simple mesh structure and convenient generation,but it also has the disadvantages of insufficient forward modeling accuracy of tesseroid cell and inconsistent mesh resolution.(2)the triangular prism subdivided mesh based on the recursion of the icosahedron,has a more consistent mesh resolution,and there exists the analytical formula for the gravity and magnetic field forward calculation of the triangular prism cell.However,This subdivision pattern is inflexible for mesh construction non-global inversion,and no efficient computational method for the inversion under this mesh has been established yet,which is facing serious computational efficiency problems.In this paper,we firstly propose a triangular prism mesh for rectangular projection in a local range(<20 °),which is more flexible in mesh generation and does not have computational accuracy issues.Aiming at the computational efficiency of triangular prism mesh inversion,we proposed an equivalent fast computation strategy for the kernel function of triangular prism mesh based on the structural characteristics of triangular prism mesh.This method requires only a small number of elements in the kernel function to be calculated and the kernel function matrix can be obtained quickly without loss of accuracy,which significantly improves the computational efficiency of the inversion under the triangular prism mesh.The East Antarctic is generally considered to be a combination of several different terranes during the global orogeny.Princess Elizabeth Land,an important part of the East Antarctic,is usually considered to be associated with the Indian Shield,but its geological properties are still unknown due to the lack of geological studies in its hinterland,which is covered by an ice cap.It is significant to study the crust-mantle structure and geologic subdivision of Princess Elisabeth Land based on airborne gravity and magnetic data.In this paper,the proposed method is applied to the airborne gravity and magnetic data of Princess Elisabeth Land in East Antarctic to establish a high-resolution crustal density and magnetic susceptibility model.Then,the following understandings are derived based on the inversion results:(1)The American Highlands crust is characterized by a distinct density boundary line.Although the American Highland is a whole in terms of topographic morphology,the characteristics of crustal density and magnetism suggest that it may be two crusts with different lithologies,or it may be due to the fact that metamorphism during the Pan-African period covered the southern part of the highland more extensively,resulting in differences of crustal density and magnetism in the northern and southern parts of the highland..(2)The crust in the subglacial rift area of the Princess Elisabeth Land is characterized by high density and high magnetic susceptibility.It is inferred that the mantle source material intruded upward along the main rift during the rift formation period.In that period,the basaltic magma continuously reformed the crust on both sides,which increased the density and magnetization of the crust,and at the same time formed a high-density,high-magnetization intrusion at the location of the main rift.(3)In the northern part of the subglacial rift of the Princess Elisabeth Land,there is a triangular uplift with the highest density and low magnetic susceptibility crust.The physical characteristics of the rocks are completely different from other parts of the study area.It is suggested that it is likely to be a separate terrane which is rich in high-density,low-magnetism minerals(e.g.,pyroxene,olivine,etc.)that was incorporated into the East Antarctic Craton during the Gondwana period.(4)The crust of the study area is divided into four terranes based on the different density and magnetic characteristics,and a new direction for the extension of the Luker suture to the east of South Prince Charles Mountain has been provided. |
