Test on Elastic Anisotropic Coefficients of Gas Coal

Abstract

AbstractGas coal is abundant in fractures and characterized by anisotropy. Determining the type and magnitude of anisotropy of gas coal is of great 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 (Vp) and the horizontal and vertical components of S‐wave velocity (VSH, VSV) in three directions parallel to, perpendicular to, and an angle of 45° to the coal seam. For the transverse isotropic medium model with a vertical symmetry axis, the average values of anisotropic coefficients are |ε| ≤ 0.2, |γ| ≤ 0.2 and |γ| ≤ 0.2, showing that the gas coal is a weakly anisotropic medium. Based on that the average value of the anisotropic coefficients in the transverse anisotropic medium model with a 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 the value of the 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 a little. In this case, the direction and coefficient of elastic anisotropy of gas coal can reflect the direction and the density of cracks.