The Application Of Gravity Methods In The Investigation Of Crustal Structure

Gravity is one of the oldest geophysical methods. The theory of gravity methods has been fully developed. With the advancement of the technology, however, gravity methods are still playing an important role in the investigation of the crustal structure.In this thesis, the application of gravity methods for the investigation of deep structure is carefully analyzed and studied. The discussion is concentrated on such important aspects of gravity methods as: the processing and inverse methods of gravity data, the theory and application of the isostasy, and the application of the satellitic gravity data for the deep structure interpretation.For the large number of processing methods, their geophysical significances and the preconditions are analyzed. The conclusion of the analysis is that the processing methods with clear geophysical significance, reasonable precondition and mathematical stability are the most useful in the crustal structure study. Base on the discussion about potential-field inverse problem and its ambiguity, it is presented that the interactive modeling interpretation of the gravity data and integrative interpretation of multiple geophysical data are the important approaches to overcome the ambiguity of potential-field inverse problem. An analytical expression for the gravity field of tetrahedron with linearly varying density, which can be used in three-dimensional interactive modeling interpretation, is presented in this paper.To construct a seismic-gravity simultaneous inversion algorithm, a parameterization method for linearly varying density distribution is presented. This parameterization method is similar with that used in some three-dimensional seismic tomography algorithm. The analytical formal of gravity effect calculation for linearly varying density model is then deduced and used in the seismic-gravity integrative interpretation of the Dabie mountain area, The integrative interpretation shows that it is very helpful for understanding the geological significance of the high -velocity body, which was detected by the three-dimensional seismic tomography.Based upon the coincident parameterization of velocity and density, a new seismic-gravity simultaneous inversion algorithm for density distribution is presented in this thesis. The algorithm follows the seismic tomography, using the velocity-density relationship to establish the connection between the seismic travel-time residuals and the density disturbance. By solving the travel-time equations and potential-field equations simultaneously, I construct a seismic-gravity join inversion algorithm. The test with synthetic data shows that the solution of gravity inversion is improved with the constraints of seismic data, under the condition that the relationship between velocity and density is known. In the crustal structure investigation of northwestern Xinjiang area, Bouguer gravity data, along with aeromagnetic data and geothermal data, were processed and analyzed. The results reveal mach information about the crustal structure of research area, such as the tectonic framework, the properties of basin's basement, the positions and deep structural significance of large-scale fracture belts, the magma activity and the depth of Moho discontinuity and the lower interface of earth's magnetic layer. The two-dimensional interactive modeling of gravity and magnetic data gives us the density and magnetic models along the geoscience transection of western Xinjiang. These two models exhibit the structure and properties of the deep crust, and are helpful for the geological-geophysical integrative interpretation of the geoscience transection. In the northwestern Xinjiang area, the isostatic gravity anomalies show that the Tianshan mountains and the southern margin of Jumgger basin is not in a status of isostatic equilibrium. The directions of vertical movement of the crust and that of isostatic adjustment are opposite. It means that the compressive stress is mach stronger than that of the isostattic adjustment and is the predominant t...