Anisotropic AE Attenuation in Mapping of Composite Specimen Progressive Failure under Unconfined Loading

The mapping of failure that uses acoustic emission (AE) is an advanced approach when tracking the location of cracks in rock materials under progressive stress. However, attenuation of the acoustic wave significantly affects the identification of the acoustic source. The isotropic attenuation model suggests a constant attenuation in all rock material directions, which provides unrealistic failure mapping for anisotropic rocks. This study investigated the impact of high attenuation on the mapping failure of a composite (sandstone–shale–sandstone), intact sandstone, and shale specimens that used AE under axile stress. The results proved that wave attenuation is the primary obstacle when obtaining a compatible failure map to the real specimen failure. Multiple setups of AE sensors were implemented to quantify the attenuation values for a wave that propagated across composite joints and shale bedding planes. The amplitude and energy attenuation (EA) values increased by 85% and 47%, respectively, when the bedding plane of the shale was from 0° to 90°, which reflected anisotropic behavior. The energy and amplitude reduction reached 99% and 39%, respectively, by propagating across a single joint. In addition, behind a double joint, no signal was received. However, the application of a load perpendicular to the joint interface improved the acoustical–wave characteristics. Therefore, when the rock material anisotropy was increased, there was lower accuracy when mapping specimen failure, because the attenuation varied at each angle between the source and sensor. In addition, the EA method suggested in this study presented hits location more precisely compared with the P-velocity (Vp) method. These findings could contribute to the development of an anisotropic attenuation model for field and laboratory application of AE.