Study On The Method Of VSP Deformable Layer Velocity Tomography

Seismic tomography is a method of knowing the internal stratigraphic physical parameter distribution map through the inversion by using transformation of physical quantity such as time, amplitude and waveform when the seismic waves going through the medium of strata rocks. It is more and more widely used in the oil and gas field exploration and development for this technology can clearly show the structure property of the material underground in the way of images. VSP method is a seismic survey technology in wells. Compared with surface seismic technology, VSP has a higher signal/noise ratio and resolution, and its waves'motive and dynamic characteristics are more conspicuous.Based on the innovation fund project of CNPC"VSP Multiwave Image with Kirchhoff Migration", this thesis mainly studies on the method of VSP deformable layer velocity tomography by using the VSP downgoing waves. The main contributions are as follows:1. VSP travel-time field forwardThe seismic wave field numerical simulation forward can be divided into forward modeling and raytracing forward modeling. In comparison, the ray-tracing method has the advantages of small amount calculation, intuitive image, simple operation and strong adaptation. Based on the comments and analysis of the raytracing methods, this thesis does the raytracing by solving the raytracing equations with Runge-Kutta method, which can provide traveltime for VSP velocity tomography.2. VSP underground velocity field inversion by DLTThe velocity distribution can be inversed from the difference between the computed traveltime and the observed traveltime of the VSP transmitted wave.After reading the relative references and comparing the tomography algorithms, I will use the conjugate gradient method for solving inverse equations.Firstly, I do the inversion based on the grid-based velocity-depth model, from which we can derive the slowness of the grids'crunodes. Secondly, the deformable layer tomography method is applied for the VSP traveltime tomography, and we can get the location of the interface if the layer's velocity is known. Trial calculation of the models shows that this algorithm is of high accuracy.