Using P-wave velocity logs with petrofabric effects to map natural and blast-induced fractures in hard rocks

A challenging task in environmental geophysics is to locate fractures near a leaching stope in an underground mine. Existing methods for interpreting sonic logs do not incorporate petrofabric effects. The petrofabric effects are variations of P-wave velocities caused by textural variations in the li thology. This paper describes a new concept of using the petrofabric effect s in the logs to determine anomalies of natural and blast-induced fractures in hard rocks. Full-waveform. acoustic logs were acquired near an undergro und stope at the Colorado School of Mines Experimental Mine, Idaho Springs, Colorado. Data acquisition occurred once before the stope was blasted and twice after the blast event. Laboratory studies show that the petrofabric e ffects range from 4 to 15 percent. This variation depends on rock types. To interpret location of fractures, variation envelopes of petrofabric effect s were placed in P-wave velocity logs. P-wave velocities that are lower tha n lower limits of the variation envelopes indicate natural and blast-induce d fractures. Results show that the blasting broke the entire rock mass with in 6 ft from the stope Is perimeter. The use of petrofabric effect interpre tation improves effectiveness of P-wave velocity logs in identifying fractu res.