气煤弹性各向异性系数实验测试

气煤裂隙丰富，具有弹性各向异性，确定气煤弹性各向异性类型与大小，对煤田各向异性研究具有重要意义. 本文在对气煤煤样孔隙率测试的基础上，测量了顺煤层面、垂直于煤层面、与煤层面成45°三个方向上纵波速度（VP）和横波速度（VSH、VSV），计算弹性常数. 对于具有垂向对称轴横向各向同性模型，各向异性系数平均值|ε|≤0.2、|δ|≤0.2和|γ|≤0.2，表明气煤是弱各向异性介质；对于横向对称轴横向各向异性模型,各向异性系数平均值ε(V)≈δ(V)，按照Thomsen观点气煤各向异性是由椭圆裂隙引起弹性各向异性.本文还讨论了各向异性系数与孔隙率的关系，表明各向异性的大小与孔隙率之间有一定的关系. 因此当煤层厚度与其顶底板岩性不变或变化很小，可以认为气煤各向异性主要由裂隙引起的，气煤各向异性大小和方向能够反映煤的裂缝密度和方位.

Test on elastic anisotropic coefficients of gas coal

Gas coal is abundant in fractures and is characterized by anisotropy. Determining the type and magnitude of anisotropy of gas coal is of important significance for studying the anisotropy of a coal field. The porosity of gas coal samples was tested. On the basis of this, we measured the P-wave velocity (V_P) and the horizontal and vertical components of S-wave velocity (V_SH,V_SV) in three directions parallel to, perpendicular to, and an angle of 45° to the coal seam，and calculated the elastic constants. For the transverse isotropic medium model with vertical symmetry axis, the average values of anisotropic coefficients are |ε|≤0.2, |δ|≤0.2 and |γ|≤0.2, showing that the gas coal is a weak anisotropic medium. Based on that the average value of the anisotropic coefficients in the transverse anisotropic medium model with horizontal symmetry axis is ε(V)≈δ(V), Thomsen thought that the anisotropy of gas coal is caused by elliptical cracks. The relationship between the elastic anisotropic coefficients and the porosity of the gas coal samples is discussed, indicating that value of anisotropic coefficient is related with porosity to a certain degree. So, the anisotropy was thought mainly to be caused by cracks when the thickness of the coal seam and the lithology of adjacent rock strata do not change or only change slightly. In this case, the direction and coefficient of elastic anisotropy of gas coal can reflect the direction and the density of cracks.