Joint Inversion Algorithms Of Multiple Types Of Seismic Data And Gravity Data And Their Applications In The China Region

Using different geophysical data to acquire accurate and reliable geophysical models through geophysical inversion methods is of great significance to understand the interior structure of the earth and its evolution history.Due to the limited sampling of the model space and different sensitivities to the subsurface medium for different geophysical data,different geophysical models in the same area are usually not consistent.In order to reduce the non-uniqueness of the geophysical inversion methods and obtain a more accurate geophysical model,this thesis has developed new joint inversion algorithms to combine different types of seismic data and gravity data into common joint inversion systems,and then new methods have been applied to different areas in China.Based on the new methods,the advantages of different geophysical inversion methods are effectively combined to obtain unified velocity and density models,which provide a basis for the study of earthquake locations and the seismogenic structure.Specifically,the following four researches have been carried out:1)An improved joint inversion method which simultaneously uses body wave arrival times and surface wave dispersion curves was developed by considering topography variations and the updated unified velocity models for Chinese continent were constructed by using the new developed algorithm.This study collected the body wave travel-time data recorded by the permanent seismic stations in the Chinese continent during the 10-year period from 2008 to 2018 and surface wave group and phase velocity maps,and then constructed the seismological reference models for continental China by joint inversion.In order to evaluate the seismic velocity models,this study collected airgun data and surface wave dispersion travel-time data recorded by the portable seismic stations.The results show that the velocity structure in the shallow part beneath the Qinghai-Tibet Plateau has been corrected and the model accuracy in other regions has been improved.2)A new method for joint inversion of three different seismic datasets including the body wave travel times,surface wave dispersion curves and teleseismic P-wave receiver functions were developed,and a more refined Vs velocity structure in South China was obtained by using this method.The traditional joint inversion method which uses body wave travel-time and surface wave dispersion data can only obtain the largescale smooth component of the subsurface velocity structure.This study developed a new joint inversion algorithm by introducing P-wave receiver functions into the body wave and surface wave joint inversion system.The synthetic test results indicated that the new developed method can improve the velocity structure around the Moho.Then the algorithm was applied to South China and Vs model with finer velocity structure near the Moho was obtained,which revealed the relationships between the velocity jump across the Moho with the tectonic environments.3)A new method for joint inversion of multiple types of seismic data based on cross-gradient structural consistency constraint has been developed and applied to southwest China.The introduction of the P-wave receiver functions makes the Vs structure around the Moho well constrained,but the Vp model was not improved.In order to obtain a more accurate Vp model,this study developed a joint inversion algorithm which uses body wave travel-time,surface wave dispersion data and receiver functions based on cross gradient constraint and applied the method to southwest China.The results show that the Vp and Vs models obtained by new developed joint inversion method can be more consistent.4)The joint inversion algorithm for seismic data and gravity data was developed and the unified velocity and density structure in southwest China were obtained.Seismic data have good vertical resolutions,but lateral resolutions depend on the distribution of the earthquakes and seismic stations.Gravity data have poor depth resolutions,but are sensitive to lateral changes in density structure.Considering the complementarity sensitivities of the two kinds of data on the underground structure,this study developed a parallel joint inversion algorithm which simultaneously makes use of seismic and gravity data.Then this method was applied to southwest China and the unified velocity and density structure models in this region were obtained.