Abstract

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 lithology. This paper describes a new concept of using the petrofabric effects in the logs to determine anomalies of natural and blast-induced fractures in hard rocks. Full-waveform acoustic logs were acquired near an underground 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 effects range from 4 to 15 percent. This variation depends on rock types. To interpret location of fractures, variation envelopes of petrofabric effects were placed in P-wave velocity logs. P-wave velocities that are lower than lower limits of the variation envelopes indicate natural and blast-induced fractures. Results show that the blasting broke the entire rock mass within 6 ft from the stope's perimeter. The use of petrofabric effect interpretation improves effectiveness of P-wave velocity logs in identifying fractures.

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