Study On Joint Inversion Method Of Gravity And Its Gradient And Seismic Data With Unstructured Mesh Division

Geophysical inversion method is an effective way to obtain the distribution of underground physical properties.Because geophysical inversion problems are often trapped in the uncertainty of inversion results,this paper selects a variety of geophysical data joint inversion methods to effectively reduce the multi-solution of inversion results and obtain higher resolution inversion results.With the exploration of mineral resources gradually shifting from plains to large relief areas such as mountains and hills,it is important to develop joint inversion methods suitable for relief areas to reveal structural evolution and mineral distribution.Splitting the underground space into unstructured meshes can better simulate undulating terrain and irregular model,and achieve more accurate inversion.Therefore,it is meaningful to develop the refined inversion method with unstructured mesh.Gravity data can better highlight the characteristics of deep geological bodies,and the gravity gradient data can better highlight the shallow anomaly characteristics.The traditional joint inversion of gravity and its gradient data is realized by data-constraint which cannot effectively improve the resolution of deep geological bodies.Therefore,this paper proposes a cooperate density-integrated inversion method of gravity and its gradient data.Firstly,the inversion results of gravity and its gradient data are obtained separately and the weight function is formed.Then,it is cross-added to the subsequent cross-gradient joint inversion of gravity and its gradient data to achieve structural constraints,and the results are used as the initial model after data fusion.It is brought into the regularization inversion to obtain the final high-resolution density inversion results.The calculation of cross-gradient function in unstructured meshes is different from that in structured meshes.In order to better simulate the nonlinear variation characteristics of underground physical parameters,this paper compares the cooperative density inversion results under different order finite difference functions.Finally,considering the accuracy and computational efficiency of the inversion results,the second-order finite difference function is determined to calculate the physical gradient changes.The cooperative density inversion method can better simulate the undulating terrain and model irregularity with unstructured mesh division.The proposed method is applied to the Lu-zong ore concentration area to obtain threedimensional density distribution,and delineate the distribution range of minerals which provides a reliable basis for the next exploration plan.Gravity and its gradient data have high horizontal resolution,and seismic data have high vertical resolution.Therefore,the joint inversion gravity and its gradient data with seismic data can obtain more accurate inversion results with unstructured mesh division.Taking advantages of the respective resolution of different data,the cross-gradient function is introduced to realize the cross-gradient joint inversion of three kinds of data in which the gravity and its gradient data adopt the cooperate density-integrated inversion method.For the calculation of cross-gradient function with triangular meshes division,we also select the second-order finite difference function by comparing the calculation efficiency and accuracy.Through the verification of theoretical model and actual data,the accuracy and anti-noise ability of the method are proved.At the same time,the results of joint inversion of gravity and its gradient and seismic profile density in the actual area of Luzong verify the speculated mineral distribution,which provides a strong basis for determining the mineral delineation of Luzong ore concentration area.When facing with the problems of fine unstructured mesh subdivision,large amount of calculation and time-consuming of seismic data,the calculation of frequency domain can solve this problem well.In this paper,the fast joint inversion method of gravity and its gradient and earthquake data is carried out.NUFFT is used to realize the fast forward modeling in frequency domain under unstructured meshes and the calculation of physical property gradient change in cross-gradient function.Through the tests of the theoretical model,the method proposed in this paper can greatly improve the inversion efficiency without losing the calculation accuracy.The proposed method is applied to the Luzong ore concentration area,and the same three-dimensional density results are obtained.The computational efficiency has been greatly improved.