Acoustic Emission Analysis of Initiation and Propagation of Faults in Brittle Rock and Compaction Bands in Porous Rock

Abstract

Formation of shear fractures in granite and basalt samples, as well as initiation and propagation of compaction bands in Bleurswiller and Bentheim sandstones at different loading conditions were investigated by means of advanced Acoustic Emission (AE) analysis. For brittle fracture we have found that regardless of the boundary conditions and loading rates, process of brittle failure of rocks can be separated into two main stages: (1) accumulation of uncorrelated tensile cracks and (2) appearance of shear cracks interconnecting a network of previously formed tensile cracks. At the fracture initiation, the majority of all AEs could be identified as tensile type events and fractal dimension analysis confirms appearance of non-correlated and randomly distributed tensile cracks. During the second stage, fracture propagation involves significant crack-induced dilatancy, the percentage of tensile events decreased significantly and shear-type AE sources dominated during the later fracturing. In the case of porous rock, nucleation of compaction bands could be clearly demonstrated by the appearance of AE clusters inside the samples. Structural analysis of deformed specimens showed excellent agreement between the locations of AE clusters and the positions of compaction bands. AE analysis confirms progressive coalescence and growth of compaction bands perpendicular to the loading direction. First motion polarity analysis of AEs shows dominantly pore collapse type events. Therefore, detailed analysis of AE allows us recognition of the current stage of brittle rock fracture, and transition between these stages can be reliably identified by monitoring of AE and velocity characteristics. In the case of porous rock initiation and propagation of compaction bands, which significantly reduces permeability, these could also be reliably identified and monitored by advanced AE analysis.