The Forward Modeling And Character Analysis For Complex Media Based On Anisotropic Media

This paper mainly studies the snapshots simulation on anisotropic,viscoelastic,homogeneous,inhomogeneous media, the seismogram and wavefield characteristics in time-domain and frequency-domain. Because monoclinic anisotropic media with two-sets of non-orthogonal vertical cracks have wide distributions. We study the characteristics of viscoelastic anisotropy and inhomogenity by combining with viscoelasticity and inhomogenity, which have important significance to fracture reservior. This may be helpful to us to know this type reservoir well further. Besides, the discontinuity caused by cracks is another basic characteristic. The coherence algorithm based on eigenvalue structure is an effective method to analyze the media discontinuity and may have the very potentiality to reservoir detecting. In this paper, we review this algorithm and present an improved method with higher resolution.1,The homogeneous anisotropic theory is the base of this paper.By solving the Christoffel equation of plane wave in transversely isotropic media,orthotropic media,monoclinic anisotropic media, we can obtain three-types of representative phase velocity and group velocity of qP,qSV,qSH wave. Using Thomsen coefficients, we can construct the relation between the stiffness coefficient matrix,the physical parameters characteristics and the anisotropic parameters, and make analysis on anisotropic parameters by theoretical phase velocity and group velocity curve. We especially discuss (1) the variation of phase velocity and group velocity caused by Thomsen coefficient in transversely isotropic media. (2) the orthotropic media properties external the axis of symmetry plane. (3) the variation-law of Thomsen coefficients and monoclinic anisotropic properties. We verify predecessors'discussions and hold that the variations of phase-velocity and group-velocity with different Thomsen coefficients are different; the same coefficient has different effects on different-types waves; the variations of Thomsen coefficients are caused by the variations of crack parameters—crack distribution density,aspect ratio,angle, and simultaneously effect the stiffness coefficient matrix and velocity curve, and the law of this influence degree from strong to weak is crack distribution density,aspect ratio and angle.2,Forward modelling is very important for understanding wave phenomen on theoretical media. We make the model simulation on homogenous and the layered homogeneous anisotropic media. The main innovation is that we introduce the viscous factors into wave equation, and simulate the snap-shots and seismogram based on the 2D-2C transverse isotropic equation and the multi-azimuth 2D-3C on the orthogonal and monoclinic equation, with more detailed discussions on the later two. In order to understand the rule of wave propagation, we change Thomsen coefficients in VTI media regularly and summarize the characters howδ,εeffect qP,qSV and qSH wave together. On the side of wave propagation, Differences in multi-azimuth snap-shots between water-filled and dry cracked type give an comprehensive anisotropic knowledge, such as water-filled and dry cracked media mainly effect qP or qS wave separately and the rules of coupled-waves. On the side of record analysis ,we study the seismogram of above media especially for water-filled cracked media. Moreover, we compare elastic and viscoelastic case for everything mentioned above at the same time. The amplitude are in the case of obvious attenuation caused by dilatational and share anelastic quality factors. Time-frequency analysis such as STFT and instantaneous characteristics based on complex continuous wavelet transformation are also done for single trace data and full wave field seismogram data.The results show that response in time- frequency domain cased by two quality factors and observation at multi-azimuths are observable. First, frequency responses distribution are similar to the ones in time domain; Second, spectrum of single trace slope towards the negative direction of time axis and frequency components instantaneous spectrum of full wave field seismogram except first arrival are attenuated obviously.3,The discontinuity generated by crack zone is one of the basic features of the fracture reservoirs. Conventional deterministic analysis methods are not adaptable to these cases. However, random meida theory based on statistics shows its advantages. Study focused on propagation and seismic responses of nonhomogeneous monoclinic media which has natural connection with fracture reservoir are important meanings for revealing the intrinsic characteristic of fracture reservoirs.The random mechanism, based on the algorithm proposed by Thomas(2001), is discussed . This algorithm makes use of a zero-mean stable spatial random process to characterize the inhomogeneous properties within small scales, thus the spatial disturbance of elastic parameters within small scales can be described by few statistics, which are auto correlation function,correlation length,mean,standard deviation and roughness coefficient. This algorithm is randomized one or more parameters in monoclinic media, and then the random parameters model can be obtained. Choosing the monoclinic media with crack is because such media has abundance description parameters and the definite physical meaning, and the parameters include P-wave velocity, S-wave velocity, density, the distribution density of two cracks, aspect ratio, crack angle and the P-wave velocity, S-wave velocity, density of crack infillings.There are two ideas mentioned here for the aim, the first one is fixing the crack parameter, introducing the random mechanism into the background media parameters, then watching the seismic response change coursed by observed variance, scale, and random parameter. The other one is fixing the background media parameters and supposing the scale is small, then introducing the random mechanism in crack parameter—changes of crack parameters coursed by crack density, aspect ratio and angle, which can also course the seismic response. After 2D-3C simulating the snapshots and seismogram, we can summarize the following conclusions: the wave fields in random media present multi-scattering properties; the distributions of groundback media are dominant to the inhomogeneous disturbance which is weaker for the crack media case; disturbance cased by scales are detailed and local, but the ones caused by variance are diagrammatic and global, moreover the smaller the size and the larger the variance of the media, the stronger the randomicity and scattering of wave field; the differences caused by randomized crack density, aspect ratio and angle on wave field will strong.Furthermore, this article also introduces the higher order spectral analysis- Wigner Trispectrum. This method is one of the most representative method about higher order statistical time frequency analysis. It can analyze non Gaussian process and suppress gaussian noise, what's more, higher order Wigner distribution contains a lot of information, it can inhibit a part noise and enhance the centralization of time-frequency distribution, and it is very useful for the low snr record which provided in the article. If we analyze the wigner trispectrum of the single record based on elastic, viscoelastic random isotropic background and random crack parameters, we can find that the existing of viscoelastic factor makes the spectrum lean to the left of the main frequency of the wavelet, but it is opposite with elastic situation. The response of higher order spectrum which coursed by random crack parameters embody principles in time domain, but higher order spectrum's high sensitivity to the wavefield disturbance embodies the difference of the 3C record by its Distribution, it also embodies the difference of wavefield changes coursed by crack density, crack angle and crack aspect ratio.4,Theory,method and improvement of coherence technique based on eigenvalue structure. The main principle of coherence technique is calculating the coherence of every sample points in every trace of 3D data volume after migration, then forming a 3D data volume which represents coherences. That is, calculating the coherence of all data in the time window, and endowing the result to the middle point in the time window. It can help us know the geological structure in the work area effectively by 3D coherence data volume. So it can improve the execution efficience and precision, and reduce the period of exploration.Coherence technique based on eigenvalue structure, also called C3 algorithm, has three basic steps: computing covariance matrix of time slice,sorting the biggest eigenvalue and getting coherence at the center point. It emphasizes discontinuity in the media and has the best horizontal resolution, and is very adaptable identifying structures such as faults,channels even cracks. But it is noe usefull when the dip information are included. The improved method presented here computes covariance matrix line by line crossed at the center point in profiles instead of layer by layer in the fundamental algorithm. Dip information is introduced by this method and coherence is obtained finally. The advantage of improved algorithm is optimizing the biggest eigenvalue along dip direction, so it is meaningfull to improve the trueness of coherence. The experiment results show that show the method has better resolution in both horizon and dip direction. It may provide new way for developing effective methods for the cracks detection in the future.