FRACTURE DETECTION IN CRYSTALLINE ROCK USING ULTRASONIC REFLECTION TECHNIQUES (CALIFORNIA)

This research was initiated to investigate using ultrasonic seismic reflection techniques to detect fracture discontinuities in a granitic rock. Initial compressional (P) and shear (SH) wave experiments were performed on a 0.9 x 0.9 x 0.3 meter granite slab in an attempt to detect seismic energy reflected from the opposite face of the slab. It was found that processing techniques such as deconvolution and array synthesis could improve the standout of the reflection event.;Subsequent laboratory experiments showed that the shear velocity of the granite increases from 10-20% when going from a dry to water-saturated condition, and that the shear attenuation constant decreases by a factor of three under these conditions. An artificial fracture was created by shimming two granite slabs to give a 1.5 millimeter air gap. P and SH reflection data recorded prior to and after filling the gap with water confirmed previous theoretical predictions concerning reflection from a fluid-filled fracture. While keeping the gap water-filled, advances in the SH reflection arrival time monitored over the following week indicated that water was absorbed a minimum of 10 centimeters into the granite slab. The most likely interpretation of the SH field data is that the later-arriving events are Love waves that result from a low velocity surface layer created by drying of the granite during the hot, dry California summer.;During the summers of 1979 and 1980 SH reflection experiments were performed at a granite quarry near Knowles, California. The purpose of this study was to use SH reflection methods to detect an in situ fracture located one to three meters behind the quarry face. These SH data were later analyzed using methods similar to those applied in the laboratory. Interpretation of the later-arriving events observed in the SH field data as reflections from a steeply-dipping fracture was inconclusive.