Study About Investigation Of Surface Layer Structure By Surface Wave Information In Petroleum Exploration

Rayleigh has proved in theory that there exists a kind of wave which travelsalong the free surface and which amplitude attenuates by the depth rapidly. Now, thiswave is named Rayleigh surface wave. In homogeneous halfspace Rayleigh surfacewave travels by a constant velocity. But in vertically heterogeneous media differentfrequencies travel by different velocities, which is called dispersion. Surface wavedispersion rule is related to the structure of surface layers. Rayleigh surface wave wasfound firstly in the nature seismic records and is used to study the earth structure.Now, by the development of computer and requirements in geotechnical exploration,surface wave method is applied to survey the surface layers structure broadly. Surfacewave is one of the main noise in the reflection petroleum exploration. In petroleumexploration, surface layers can be simplified as three layers, which are the lowvelocity layer, middle velocity layer and high velocity layer. In some area, there is nomiddle velocity layer. The surface layers structure of petroleum exploration is belongto normally dispersive profile, which stiffness is monotonically increasing with depth.By computation we know that the first modal displacements is bigger than these ofhigher models and the first modal dispersion cure exists in the whole petroleumsurface wave range of 5-20Hz and only the second, the third modal etc. higher modalscan exist. Therefore, the main energy of petroleum surface wave concentrates on thefirst modal. The length of survey array, offset range, receivers, seismic instrument,time sampling interval and record length don't effect the recording of surface wavefield. But the exploding depth and receiver array are not of benefit to surface wavefield. The effect of trace interval is notable in the thin surface layers area. There aremany methods to estimate the phase and group velocities dispersion curve, p-ωstacking firstly changes the common shot gather V(x,t) to x-ωdomain.Secondly, it stacks all the same phase wave and gets F(p,ω) wave field in p-ωdomain. Finally, the biggest value points of amplitude spectrum |F(p,ω)| represent the dispersion curve of surface wave. p-ωstacking technology can estimate thefirst modal dispersion curve of petroleum surface wave. The problem of inducing thesurface layers structure is a non linear one, which math model can be concluded as aproblem to estimate parameters of n_L layers. Every layer is defined by four mediaparameters, which are V_Pi, V_Si,ρ_i and h_i. Therefore, this is a four-dimensionalinverse problem. V_Pi andρ_i have little effect to dispersion curve and can bereplaced by approximate values. At the same time a thick layer is equal to a series ofthin layers when computing the dispersion curve. Therefore, we needn't considerlayer thick parameter during inversion. The four-dimensional problem is simplified asa one-dimensional one about V_Si. By smoothness strategy we build the thin layerssmooth model inversion of petroleum seismic surface wave. In this thesis, we use 24shot gathers acquired in XinJiang to analyse the surface wave and get a about 1800mtransverse wave section, which accords basically with the results of little refractionand micro logging. In a word, the method of investigation of surface layers structurebased on the surface wave information in shot gathers of petroleum exploration isuseful and has an application future.