Two-dimensional Joint Inversion Of Magnetotelluric And Seismic Data In Spherical Coordinates

Based on natural alternating electromagnetic field as the field sources,the magnetotelluric sounding method is a geophysical detection method which takes the differences in the electromagnetic properties of underground medium as the main material basis.It has been widely used in many areas due to its advantages of large exploitable depth,wide frequency band,and the sensitive to low resistance medium.With the development of science and technology,detection of research area expands gradually,continental level of magnetotelluric exploration has become possible.The geometric deformation effects of curvature of the earth could not be ignored anymore when the survey line is more than hundreds kilometers long or located at high latitudes because the survey line shape has obvious arc shape.The traditional cartesian coordinate system and the rectangular mesh division method is no longer suitable for large scale in the inversion of magnetotelluric data interpretation.In the paper,we make systematic studies on the two-dimensional magnetotelluric forward and inversion research in spherical coordinates,using fan-shaped grids when we divide the subsurface medium.We dericed the forward equation and used the finite difference method for the forward solution again.We used dataspace OCCAM methods to do inversion work.Then,this paper compared the inversion results in the spherical coordinates and orthogonal coordinates of small scale models that we construct to verify the accuracy of the newly developed forward modeling results.The theoretical synthesis data of the large-scale model is retrieved in the two coordinate systems,and the result of comparing the inversion results shows that some impact are made due to the Earth curvature while doing large-scale detection.The magnetotelluric forward and inversion results obtained in the spherical coordinate system are more consistent with the actual earth shape as well as can reflect the more real structure and location of the underground anomaly more accurately.Based on the fast marching method(FMM),we made some forward modeling of some seismic reflection and refraction data research.We used the subspace inversion method as well as the FOTOMO software package for calculation when doing the inversion work.When constructing the three-dimensional model,the physical property in the direction of strike does not change,and the forward and inverse results of the two-dimensional profile can be obtained by slicing the calculated results.In addition,this paper makes studies on the joint inversion of magnetotelluric and seismic data based on cross gradient algorithm in the two-dimensional spherical coordinates.In this process,we add cross gradient in the objective function and use the central difference format to discrete cross gradient formula.The resistivity and velocity models are mutually restricted and coupled in the inversion to realize the complementary advantages of the two methods.In the result,the multiple solutions of the inversion are reduced and more reliable inversion results are obtained.