Effects Of Near Surface Media On Exploration Seismic Wave Fields

The complexity of near surface and geological structure underground can intensify thediversity of seismic wave field, affect the quality of seismic records strongly, and bring somany difficulties for processing and interpretation of seismic data which mainly display inthe following aspects:(1) the pressure pulse of point-source produce surface wave andother secondary wave through the interaction between near surface’s media and freesurface, and these can make the seismic wave fields more complex;(2) low velocity andstrong absorbtion of near surface can cause changes of seismic wave in propagationdirection, wave-form, phase, frequency spectrum, energy and so on;(3) rolling surface caninduce transformation between surface wave and body wave, and generate scattered wavesof strong energy. These factors seriously affect the resolution and signal-to-noise ratio ofseismic data, so it is necessary to study the propagation law of seismic wave in nearsurface media.Based on the propogation theory of seismic wave in elastic or viscoelastic media,Firstly, research on the analytical solutions of wave fields excited by point source underfree boundary condition, and as a comparision, numerical simulation on homogenoushalf-space with free boundary by taking use of high-order finite difference method arepresented in this paper, and then to be analyzed to sum up several conclusions. Sencondly,the first item formula of direct wave, reflected P wave, reflected SV wave, transmitted Pwave and transmitted SV wave when spherical wave incident to the elastic interface arederived clearly, and numerical simulations are carried in the VSP observation system. P-Pwave reflection coefficients of four AVO models (elastic media and linear viscoelasticmedia) in accordance with spherical wave theory are provided by using the semi-analyticalmethod, and then to be compared respectively with that obtained from accurate Zeoppritzequation in light of plane wave theory in order to discuss the difference and applicabilitybetween them, and then the effective reflection coefficients(ERCs) based on Fresnelprinciple of the four elastic AVO models are presented, and to be compared with planewave respectively. Lastly, the near surface’s effects on receiving wave fields are discussedin two major aspects. On the one hand, accurate calculation methods for SH wave and P-SV wave propogating in low-velocity layered media are given in this paper. Recurrencemethod and transfer matrix method can be used to simulate SH wave, however it is moresuitable to adopt transfer matrix method for P-SV wave. And numerical simulations areconducted on established models which are corresponded with some features of actual nearsurface media, and quantitative analysis can be obtained by analyzing the effects of nearsurface meida embodied in seismic reflected wave. On the other hand, the wave energy’sdistribution subjected to free surface is raised by means of considering the polarizationcharacteristics and coupling effect between the vertical and horizontal wave field.This article innovation snot lies in that it takes two-demensional Lamb’s problem as anentry point, and then the initial-boundary value problem for wave equation is established toderive the seismic wave fields’ analytical solutions that excited by any direction p pressurepulse of point-source with the application of integral transform method, and the first itemformula of elastic sphere wave is researched theoretically.In conclusion, the comparative analysis between theory results and numericalsimulation results of this paper is designed to find out the impact mechanism andinterrelationship of these factors, and it is of high importance in theoretical value andpractical significance.