A Study On The Relations Of The Surface Attenuation With The Surface Velocity And Seismic Signal Frequency

In spreading process, the energy decay of the seismic wave depends on the attenuation caused by the stratums'viscoelastic nature besides geometrical spreading. Because of inner friction and viscidity in the rocks, we can get strong high-frequency and weak low-frequency attenuation. Past research shows that geometrical attenuation is the main factor of geometrical spreading energy, and absorption is the main factor of narrowed signal bandwith, especially around the near surface.The coefficient of absorption (α) or quality factor (Q) is the important parameter of the seismic attenuation, which depicts viscoelastic nature of the medium and represents stratum's internal nature character. The faithful values of the quality factor not only improve seismic resolution through inverse Q deconvolution, but also can be used as the information of the lithologic interpretation with the velocity because it is sensitive to parameters such as saturation, porosity and permeability. The researches of the absorption regular of the consolidated layers have much more than that of the unconsolidated layers.Inversion of the surface quality factor is a very significant work. Q can be obtained by means of technical know-how collection or VSP datum. In the base of the available field VSP datum, the work develops two methods to estimate Q, builds Q estimation flow chart, and at last acquires Q values in the near surface. It proposes the statistical method in time domain in which the coefficient of absorption first is calculated. It utilizes the VSP common detector gathers received on the surface, process the actual amplitude through analysis of the near surface structure alternation, get the quality factor values of the low velocity layer in the surface by means of building region's empirical formula. In this method, sectional analysis and differential fit and calibrating procedure are used, and the result shows that Q can be concordant with real layers. This method can get the relational expression of the unconsolidated layer's Q and velocity just like that of the consolidated layer. The spectral ratio method is improved by means of iterative process and artificial interpose in order to abate the different response of same instrument. In practice, the result may be misleading and not reliable for the low S/N ratio. At last, main factors of accuracy affected for Q estimation are analyzed.