摘要

摘要
本论文通过煤弹性各向异性系数测试、各向异性方位理论分析与正演模型，形成了系统的P波方位各向异性煤裂缝评价技术。

煤弹性各向异性参数和各向异性系数测试，证明了煤具有弱各向异性，煤为正交各向异性介质；煤样多方位速度大小，表明当纵波传播方向由平行于各向异性方向变化到垂直于各向异性方向时，P波速度由大逐渐变小，速度在平面上呈现扁椭圆状。

在各向同性介质中，煤、岩样速度测试与正演模拟表明，二维AVO分析技术在煤田地震勘探仍然适用，同样可以评价煤层裂缝密度相对变化；在对称轴水平的横向各向同性介质中，正演模拟煤层P波方位各向异性变化，研究其方位各向异性特征，并讨论了垂直裂缝模型在固定偏移距上，P波反射振幅响应A与炮检方向和裂缝走向之间夹角 余弦呈近似线性关系，线性关系的截距M是与炮检距有关，斜率N与炮检距、裂缝特征有关。

在以振幅变量N进行裂缝密度分析时，要结合平均振幅M分析，取M
N/作为裂缝密度直接指示。

采用直接解方程组与最小二乘法反演煤层裂缝密度与方向的数值计算方法；讨论了薄层调谐厚度、观测系统、向斜对方位各向异性分析可能存在陷井，以及对煤层裂缝预测的影响。

将P波方位各向异性分析方法应用到煤田三维地震勘探中，实际资料试算表明，煤层裂缝密度、方位与勘探区主要构造特征相吻合，证明P波方位各向异性方法预测煤层裂缝方法可行。

关键词: 各向异性系数；测试；煤层裂缝；正演模拟；P波
Abstract
By the test of the elastic anisotropic coefficients, analysis of the azimuthal anisotropic theory and forward modelling, a systematical evaluation technique for the azimuth anisotropic coal cracks of P wave has been formed. And the test results of the anisotropic coefficients and the anisotropic constants of the coal elasticity proved that coal possesses a weak anisotropy, and coal is the orthogonal anisotropic medium. The size of the multi-azimuth P wave velocity indicated that the velocity is changed from big to small when the direction of the wave propagation is changed from parallel to perpendicular with the anisotropic direction in the coal samples, and they show the flat elliptic shape in the plane. In the isotropic medium, by test the velocity to the samples of the rock and the coal, and modelling, the two-dimensional amplitude versus offset analysis technique is still suitable to the coal seismic exploration, and the relative crack density can be evaluated. In the transversely isotropic medium, the forward modelling of the reflection coefficients of P wave were done, the azimuthal anisotropic characteristics of the coal with the vertical crack was discussed in fixed offset. The reflection amplitude response A of P wave included angle cosine between the crack strike and the axis azimuth shows an linear relation approximately, and the intercept of the linear relation M is relevant to the offset, the slope N is relevant to the offset and the crack features. When analyzing the crack density use the amplitude variety N , combined with the average amplitude M analysis , N/M is taken as a direct instruction of the crack density. And the numerical calculating methods such as the least-squares method and the equation groups of direct solution were used to invert the direction and the density of the crack in coal seam; the influences of the thin-layer tuning thickness, the layout and the syncline on the possible existing trap in the azimuthal anisotropic method and the results on prediction cracks were analyzed.
The azimuthal anisotrpic analysis method of P wave has been used in the coal three-dimensional seismic exploration. The actual calculated data indicated that the azimuth and the density of cracks are corresponding to the main structure characteristics of the exploration area, and it is proved that the azimuth anisotropy analysis of P wav to determination the cracks is feasible.
Key words: anisotropy coefficients; test; coal cracks; forward modeling; P wave
Detailed abstract
By means of the forward modelling, theoretical analysis of the azimuthal anisotropy and the physical test for anisotropic coefficientss of coal, a systematical evaluation technique for the azimuth anisotropic coal crack of P wave has been formed. The main achievements are as follows:
1. Expressing the physical meaning of crack and proposing the evaluation methods for the geologic bodies with different dimensions in the coal seismic exploration
The various scale and the different dimensions of the cracks show the different forms, and there are the different reflecting seismic characteristics of the spatial. On the basis of the isotropic elastic medium, the analysis technique for the seismic multi-attributes, the coherence cube, the dip and the azimuth expressed the characteristics of the kasrt cave and the fault (fault throw ≥3m ); the seismic response on the basis of the anisotropic medium reflects the wave length or the average effects of the fissure and the cracks in CMP, that is, according to the comprehensive effects of both fissure and crack in a CMP, there exists the statistic property of the geological body.
2. Still suitability of two-dimensional amplitude versus offset (A VO) analysis technique , with forward modelling and testing velocities of both the rock and the coal under the condition of isotropic medium
The velocities of perpendicular (Z) and parallel (X ,Y ) to the coal seam of sand-mud stone, mud stone of the roof and floor as well as the coal samples indicated that the supersonic velocities of both P and shear wave in identical directions had a better coherency, but the coherencies for the velocities of the different kinds of wave and the different directions of the coal samples were more poor than that of the sand-mud stone and the mud stone, i.e., the anisotropy of the coal is large, and the anisotropy of the sand-mud stone and the mud stone are weak , the coherency of the densities of both rock samples and coal samples with the velocities of P and shear waves is rather poor.
In the isotropic medium, according to Zoeppritz equation, the reflection coefficients can be accurately calculated. The forward modelling was carried out by the equation, in the same time, the using conditions of the conventional A VO formula by Aki, Richards and Shuey (1985) were also analysed in the coal seismic exploration. It could be observed that the incidence angle of the conventional A VO of the coal seam roof reflection should be 10-<C θθ°( C θ is critical angle) , and the incidence angle of the A VO analysis to the floor reflection should be 30<θ°, however, they are not absolute quantity of the reflectivity and reflect only the relatively changing quantity.
The breakage degree of coal is changed from high to low and Poisson ’s ratio is increased. In the roof of the coal seam the positive Poisson ’s ratio difference is formed, on the contrary, in the floor the negative Poisson ’s ratio difference is formed. If Poisson ’s ratio of both floor and roof of the coal seam is invariable, the crack density determines Poisson ’s ratio difference in both floor and roof, i.e., under the condition of the isotropic medium, A VO can evaluate the relative variety of the crack density in the coal seam. And at a fixed azimuth angle two-dimensional A VO analysis technique is still suitable for the weak anisotropic
medium.
The forward modeling reveal that P wave reflectivity of the coal seam base on the reflectivity in its top mainly, so A VO analysis should be at the angle of incidence 60～600.
3. Finishing the test of the anisotropic coefficients and the anisotropic constants of the coal elasticity and verifying the weak anisotropy of coal as well as proposing coal as the orthogonal anisotropic medium
The test of the elasic anisotropic coefficients of the coal is finished first of all, and a set of the testing methods for elastic anisotropic coefficients of the coal was established. The measurement of the P and shear wave velocities ( P V , SH V ,SV V ) according to directions of the perpendicular, parallel and 45°to the coal seam was carried out. The velocity parallel to the seam plane is bigger than that of the same wave in the direction of perpendicular, parallel and 450 to the seam plane. The testing results were shown that the anisotropy of the vertical axis of symmetry (VTI ) was bigger than that of the horizontal axis of symmetry (HTI ), as the coal anisotropic coefficients are ε≤0.2 , δ≤0.2 and γ≤0.2, the coal is the weak anisotropic medium. The mean value of the anisotropic coefficients of the coal samples in HTI model was )()(V V δε≈, Thomsen thought that the anisotropy of coal is caused by the elliptially cracks, the coal has both the horizontal bedding anisotropy and the anisotropy of the cracks, that is, the coal is the orthogonal anisotropic medium. If the thickness of the coal seam ( rock property ) is no or smaller change, the direction and the size of the coal elastic anisotropy indicate the direction and the density of the coal cracks with the horizontal axis of symmetry.
The size of the multi-azimuth P wave velocity shows that if the propagation direction of the P wave is changed from parallel to perpendicular with the anisotropy direction, is changed from big to small progressively and the velocities in the plane show the flat elliptic shape in the coal samples. And for the shear waves SV and SH , their propagation direction is perpendicular to the vibrating direction, they are splitting.
4. Forward modelling of P wave azimuthal anisotropy variety with the horizontal axis of symmetry and analysis the azimuth anisotropy features of the coal seam
In the transversly isotropic medium with the horizontal axis of symmetry , owing to the cracks in coal, the reflectivity changes with a change of the incidence angle, and with a change of the azimuth as well. The forward modelling results of the coal azimuth anisotropy show: (1) the coal seam is the strong reflecting layer, but the changes of the reflectivity is smaller, it is mainly caused by the azimuth anisotropy in the top of the crack medium; (2) if the azimuth angle is parallel to the crack strike, the reflectivity reaches up to the maximum absolute value. however, if the azimuth angle is perpendicular to the crack strike, the absolute value of the reflection coefficient is minimum, and the reflection coefficient is the cosine function of the azimuth angle and the period is π; (3) if the incidence angle is 0°~6°, the comparison of the reflection coefficients the anisotropy causing with the reflection coefficients, the part of anisotropy reflection coefficients is neglected as a fixed value in each azimuth; and (4) if the incidence angle is in the range of 15°~ 25°, the reflectiion coefficients in different azimuths are more obvious.
5. Analysis of the azimuthal anisotropic characteristics of P wave with the vertical crack in fixed offset and application of the least-squares method and the equation groups of the direct solution to inversion of the direction and the density of coal seam cracks
The reflection amplitude response A of P wave included angle cosine between the crack strike and the axis azimuth shows an approximately linear relation, and the intercept of the linear relation M is relevant to the offset. The slope N is relevant to the offset and the crack features. When analyzing the crack density using the amplitude variety N, combined with the average amplitude M analysis, N/M is taken as a direct instruction of the crack density. Compared with the amplitude of envelope to the dynamic feature of the seismic recording is more accurately, and is more suitable to the seismic lateral prediction. The numerical calculating methods such as the least-squares method and the equation groups of direct solution were used to invert the direction and the density of the crack in coal seam.
6. Use the azimuth anisotropic analysis method of P wave in practice of the three-dimensional coal seismic exploration and initially formed the systematical evaluation technique for the azimuth anisotropy cracks
The amplitude versus azimuth of P wave analysis could evaluate the density and the direction allgned vertical cracks of the coal seam, and this analysis could be carried out in the azimuthal gathers of before stack and stack in part , or in a certain range of the incidence angle and in a certain range of the offset as well. The data of the wide azimuth and the big offset were very sensitive to the anisotropy of the cracks.
In the azimuth anisotropic analysis of P wave, the focuses of data acquisition, processing are the conformance corrections of shot points and geophone stations and to take the measures of protection the amplitude. And aiming at the structure of the actual data and the setup, the possible existing traps of the thin-layer tuning thickness, the layou and the syncline were discussed.
The test calculation of the actual data indicated that the azimuthal direction and the density of the cracks is corresponding to the main structure characteristics in exploration area, and it is proved that the azimuth anisotropy analysis of P wave to determination the cracks is feasible in coal seam.。