The Study On Joint Inversion Of Gravity And Gravity Gradient Data And Its Application In Vinton Dome Area

With the development of gravity exploration technology,especially the wide application of gravity gradient measurement,gravity data processing and interpretation technology also need to be improved.Gravity gradient measurement is a relatively new technology,which has been widely used in geophysics for its advantages,such as,large amount of information,high accuracy and less disturbance,etc.Gravity gradient is defined as the second order derivative of gravity potential.Its high frequency signal is complementary to the low frequency signal of gravity data.Thus,the joint inversion of gravity and gravity gradient data is of great significance.The five components of the gravity gradient contain anomaly information from different geological bodies.And the joint inversion of different components of the gravity gradient can better reflect the distribution of the anomalous body.Therefore,the research on the joint inversion of gravity and gravity tensor data will achieve the purpose of complementing information and reduce the multiplicity of inversion results.Firstly,we build a theoretical model and calculate full tensor gravity gradient data using forward equations.Then,random noise is added artificially according to the characteristics of practically measured noise.And we compare the three horizontal components(Vxx,Vxy,Vyy)before and after adding noise.The Welch power spectral density map are used to analyze the three horizontal components(Vxx,Vxy,Vyy)after adding noise.According to the analysis results,we design and compare two kinds of low-pass filters,Butterworth filter and Gaussian filter.In order to select the physical quantities involved in joint inversion,we make a comparative analysis of gravity and gravity tensor data.We derive the relationship between gravity gradient components in frequency domain and analyze the correlation between each component and component combination based on singular value of tensor data.The results show that the lowfrequency energy of the gravity data Vz is higher than gravity gradient component Vzz.However,the high-frequency energy of the gravity data is lower than gravity gradient component Vzz.To conclude,gravity data Vz can better reflect the deep information,gravity gradient Vzz can better reflect the shallow anomaly and Vzz contains the most abundant information among all components.The joint inversion has greatly increased the amount of data.And the sensitivity matrix of each component in the inversion process also increases,which requires more computing time and storage space.In order to solve this problem,we design a new method for the fast calculation of sensitivity matrix based on previous studies,which greatly reduces memory requirements and the calculation time.The experimental data prove that the storage space is less,and the computation time is shortened.This paper conducts three-dimensional smooth constrained inversion of gravity data and gravity gradient tensor data.We add roughness in three directions of the model to achieve the maximum smooth,and add depth weighting function to offset the impact of the "skin effect".Selection of the depth weighting function directly affects the inversion results,so we expatiate on the selection of depth weighting function in this study,compare the two different depth weighting function,and choose the improved depth weighting function in our inversion method.In this paper,a model of two abnormal bodies is established to carry out gravity and tensor data inversion respectively.From the inversion results,we can see that in vertical direction Vz and Vzz can better reflect the deep information of geological bodies.But the deep inversion results are both divergent,with the tailing phenomenon.The lateral resolution of Vzz is higher than that of Vz.and the recognition ability of Vzz for many geological bodies is higher.The convergence of inversion results of gravity gradient tensor components Vxx and Vxz is better in X direction,which contributes to the identification ability of geological body in north-south direction.And the convergence of Vyy and Vyz is better in Y direction,which can help us to accurately identify the boundary of abnormal body in East-West direction.In addition,Vxy can help identify corner information of abnormal body.In general,the inversion results of gravity and gravity gradient components can basically reflect the geological features.The results of multi-component joint inversion is much better than those of single component inversion.The advantages are that the inverted density variation is closer to real value,noise immunity is enhanced,the horizontal and vertical resolution is improved,the convergence is better,and the geological boundary is more obvious.This paper also introduces the cross gradient function into the joint inversion of gravity gradient data,and applies this method to Vinton Dome rocky data.The results prove the superiority and practicability of this method.