Study On Traveltime Approximation And Inversion Method In Arbitrary Anisotropic Media

Anisotropic media is widely existed in the earth's crust,which is mainly manifested in the vary of seismic wave propagation's velocity and attenuation with various direction.In seismic data,the shear wave splitting which is usually observed is caused by the anisotropy of the underground medium.Seismic anisotropy is of great significance to understanding the internal structure of the earth,exploring complex oil and gas reservoirs,and predicting geological disasters.In the exploration of oil and gas reservoirs,seismologists often need to obtain information about the underground medium based on the observed seismic wave records.One of the main tasks is estimating velocity-anisotropy models adequate for high-accuracy,high-resolution seismic imaging,interpretation,and characterization of hydrocarbon reservoirs.Transverse isotropy models,which is the earliest and most widely studied anisotropy model,have been successfully used in the industry for more than a decade,setting the industry standard.In recent years,high quality,high density,multi-azimuth seismic data acquired today often clearly reveal,lower anisotropy symmetry in the subsurface,as a consequence,we often have to reach beyond conventional approaches while striving for a step change in our seismic imaging and characterization capabilities.Recently,anisotropic with lower symmetry like orthorhombic anisotropic media and triclinic anisotropic media is more and more concerned.Moreover,in rock experiments,seismic data could be used to estimate relatively complicated anisotropy of rock samples.Great advantage of these experiments is that laboratory measurements can provide a sounding of a sample from all directions,and thus give a chance to recover the complete stiffness tensor.This,however,requires to record not only P wave,but also both S waves.The use of S waves brings an additional and independent information useful in the inversion,but it also introduces problems.S waves usually arrive in the P-wave coda,and thus their detection is less accurate.There are generally two S waves propagating in anisotropic media.In some directions,they arrive separately,in other directions they arrive coupled,indistinguishable.Moreover,in different directions,they may arrive in a different order or multiply.All the above-mentioned problems must be considered when dealing with S waves.Our intention here is to offer a novel practical approach to arbitrary anisotropy with lower anisotropic.In the subsurface or rock samples whose anisotropy symmetry is low,we give cost-effective data processing for the relative seismic data.Besides introduce the basic theory of anisotropy,this article focuses on the traveltime approximation and inversion in arbitrary anisotropic media.For different problems in seismic exploration and rock experiments,we study related traveltime approximation based on Weak Anisotropic(WA)parameters.Vertical Seismic Profile(VSP)data and reflected wave travel time data are the most used seismic wave information in seismic exploration,we give relative inversion process according to the characteristics of different seismic data.This article first reviews the propagation theory of elastic waves in anisotropy media and then introduces its rock classification,and then,introduces the wave equation of elastic wave in anisotropic medium and the formula of phase velocity and group velocity of elastic wave.Finally,we introduce WA parameters and study their physical insight,we compare the WA parameters with Thomsen's parameters in VTI media.We derive the travel time formula of reflected waves in arbitrary 3D anisotropic media.We use the weak anisotropy approximation and WA parameters to derive the reflected P-wave traveltime formula in anisotropy.We use phase slowness approximation and phase velocity squared approximation to derive the travel time formula,and used anisotropic media models with different strengths and symmetry to test the accuracy of the deduced approximation.Finally,we deduced the conversion formula of WA parameters between different coordinate systems,and based on this conversion formula,deduced the reflected P-wave travel time formula in threedimensional anisotropy.We study the inversion methods for VSP data and reflection seismic data.We performed a first-order linear change of the WA parameters,and obtained anisotropy(A)parameters which are more suitable for inversion.We designed an inversion process for VSP data to invert 15 A parameters related to P-wave propagation.We use artificially synthesized traveltime data and add actual noise to simulate actual data for testing,which confirmed the inversion method's effectiveness and study the influencing factors.Through the A-parameter results calculated by the inversion and the travel time approximation formula used,we can reconstruct the phase velocity surface,so as to further analyze the type and direction of the anisotropic symmetry of the medium.Regarding the reflection P-wave traveltime inversion,we designed the inversion process for the two cases where the layer thickness is known and unknown.We use artificially synthesized travel time data to add Gaussian random noise for testing.We study the inversion methods for rock sample seismic data.We introduce the concept of common S waves,inverted all 21 A parameters,and reconstructed the phase velocity surfaces of P waves and common S waves based on the inversion results.We test the rock sample data collected in the laboratory,and compared it with the results of other studies,proved the rationality of the results,and analyzed its influencing factors.