Experimental study on ultrasonic velocity and anisotropy of tectonically deformed coal

Tectonically deformed coal (TDC) is defined as coal formed by various tectonic stresses. As one of the key concerns in the formation and recovery of coal bed methane (CBM) from the tectonic deformation coal seams, the texture and fractures of TDC present various features in different deformation environments and as a result the physical properties change. Therefore, the identification of TDC is significant and can be obtained from the physical properties. In this study, fourteen representative TDC and two undeformed coal samples from the Suzhou mining area in the east of north China plate were collected to investigate the geophysical recognition parameters of TDC by elastic properties, such as the velocities (P and S-waves), elastic parameters (Vp/Vs and Poisson's ratio) and velocity anisotropies. The results show that the elastic properties of undeformed coal and TDC samples from different deformation environments exhibit significant difference. Normally, the velocities and elastic parameters of undeformed coal have the highest values, while the velocity anisotropy has the lowest value. In brittle deformation environment, the velocity and elastic parameters decrease while the velocity anisotropy ascends with increased deformation extent. For the TDC samples from shear deformation environment, the velocity anisotropies present the significant high values among the selected TDC samples. In plastic deformation environment, the elastic parameters gradually decrease with the deformation extent ascending, while the velocity anisotropies are relatively high. Among the elastic properties, the Vp/Vs, Poisson's ratio and velocity anisotropy are sensitive to deformation type and extent of coal samples. Furthermore, the development characteristics of coal structural fractures have a significant impact on the elastic properties of measured coal samples from different deformation environments. In summary, the results may provide reference parameters for future studies in CBM rock physics from the tectonic deformation coal seams.