Research On Forward Modeling Of Borehole Seismic In Complex Media Using Finite Element Methods On Irregular Meshes

In modern geophysical exploration,the target body is becoming more and more complex,the precision and resolution required are getting higher and higher.Because of the influence of the low velocity zone under the surface,the recording accuracy of ordinary surface seismic is lower than that of borehole seismic.In addition,the wave frequency in wells is higher,and better vertical resolution can be achieved.In order to understand the wave field information about the complex geological conditions,an accurate forward method is needed.For complex geological bodies,irregular meshing is very suitable.It can adjust the size of the meshing according to the geological parameters of the model to make it more in line with geological conditions.Among the methods suitable for irregular mesh generation,the simplest one is the finite element method.In order to solve the problem of forward modeling under complex geological conditions and undulating surface conditions,the Delaunay triangulation technique is used to divide the underground complex structure and the geological model with the undulating interface into Delaunay triangles,the finite element method is used to solve the elastic wave wave equation in this region.It is concluded that the irregular triangular mesh can better simulate the undulating stratum and the undulating interface under the same grid condition,and the obtained wave field is more refined.In the process of solving the finite element method,the element-byelement method is used to replace the whole assembly matrix,which effectively reduces the memory occupancy of the finite element method.Because the element-by-element method is easy to realize parallel processing,the program is accelerated by Openmp to improve the computing speed.When suppressing dispersion,the combination of lumped mass matrix and dispersed mass matrix is used to suppress dispersion effectively,but the mass matrix is not diagonal matrix.Therefore,the preconjugate gradient iteration is used to accelerate the solution of linear equations.At the same time,the Sarma absorbing boundary is improved,and the quadratic incremental piecewise function is used instead of the primary function to reduce the artificial reflection of the boundary.In the forward part of the model,from simple to complex,it includes undulating stratum interface,undulating surface,the isotropic medium,the anisotropic medium,etc.At the same time,VSP record and cross-well record are forward,and the wave field information in the record is explained in detail.