Research On Dispersion Curves And Quality Factor Of Rayleigh Waves In Viscoelastic Media

Under normal circumstances,the Earth is composed of inhomogeneous viscoelastic media.Because of the dispersion loss and attenuation of seismic waves caused by the inhomogeneity and viscoelasticity of different formations,the original energy of Rayleigh waves will gradually decrease during it is propagating in the ground.With the increase of the propagation distance,the phase distortion of the Rayleigh wave will occur.And its amplitude also will gradually decrease.These changes of Rayleigh waves make it very difficult to process and analyze the seismic data.Attenuation coefficient and quality factor are the most important parameters that describe the viscoelastic properties of a medium.Therefore,to consider the influence of absorption and attenuation of the formation on Rayleigh wave dispersion curves in the propagation process of Rayleigh waves will greatly improve the resolution of the original seismic data,the imaging accuracy and the inversion reliability.The laboratory results show that the quality factors(Q_p and Q_s)of shear wave(S wave)and compression wave(P wave)are very important parameters that used to describe the attenuation absorption of Rayleigh wave.In the actual production using the surface wave method and the later data processing,the most important job is to extract the dispersion curves,and then use them to invert the S-wave and P-wave velocity of the formation.However,in this inversion process,the viscoelasticity of the medium is often not considered.This will make the calculation of the S-wave and P-wave velocity inconsistent with the actual values.The imaging result will also be biased against the actual horizons.In order to solve this problem,the quality factors(Q_p and Q_s)are added to the inversion process of the dispersion curves for the viscoelastic media.First,the formula to calculate the shear wave velocity by the phase velocity in the viscoelastic media is deduced.Then some models are established to perform the forward modeling.And the dispersion curves are extracted.The extracted dispersion curves are used in the inversions under the completely elastic condition and the viscoelastic condition with the addition of the quality factor respectively.From the comparing inversion results of these two conditions,it can be seen that the inversion result under the viscoelastic condition match with the actual layer better than that under the completely elastic condition.It means that the work of this thesis has practical meaning.In viscoelastic media,the quality factor is just as important as the formation's velocity.Another work of this thesis is to use the attenuation coefficients to invert the quality factors.Then this method is tested on the theoretical model.The calculated quality factors are matched well with the value of the given quality factors.In order to verify the feasibility of this method in actual data,an actual seismic data of Baoji is used in the actual test.The calculated Qs values are in good agreement with the Qs values that measured in the laboratory.It shows that the method used in this thesis is also feasible in the practical work.