Microseismic monitoring of hydraulic fractures in block cave mines

Abstract

Hydraulic fracturing can be used for preconditioning in block cave mining. Microseismic monitoring is one of the techniques used to determine the orientation, extent and time evolution of the hydraulically-generated fractures. Data are recorded continuously at significantly higher frequencies than currently used in routine seismic monitoring in mining environments, and the data analysis is accordingly more complicated. Microseismic monitoring was performed as part of the hydraulic fracturing programme at Ridgeway Deeps Block Cave. The seismic data that are recorded during a typical fracture session have been described in detail. Frequency filtering and time domain methods are used to isolate the individual seismic events. A straight-ray method is used to estimate source location. A group of seismic events is then relocated with a double-difference method and the locations are used to describe the hydraulic fractures. Several hundred events from several different fracture sessions were located using this technique. For many fracture sessions, the locations are clearly planar, and estimates of strike and dip could be obtained. Fracture planes from consecutive fracture sessions were compared. The time evolution of individual fracture sessions was analysed and used to provide estimates of propagation rates. Criteria have been discussed for applying the technique to other hydraulic fracturing programmes.