f In-Situ velocity and attenuation measurements for fracture studies in granites

The exploration of ornamental rocks is today a major industry. In some areas, outcrops of granite massifs have become explored nearly to exhaustion. This opens the need to employ geophysical methods to determine the quality of a particular type of granite beneath the weathered zone. Evaluation of in situ rock mass characterization as been attempted for several decades using seismic methods. It is also recognized that seismic velocity analysis alone does not produce unic and reliable results and that attenuation measurements should be made for a better characterization. IGM as conducted a series of tests in 3 granite quarries where geological control was possible. Using refraction data obtained from a radial set of seismic lines in each place, P wave velocities and attenuation measurements using spectral ratios were used for in situ rock mass characterization. A 24 channel EG&G ES-2401 Geometrics seismograph was utilized, with a 2.5 meter geophone spacing. The source employed was the hammer and steel plate and each shot consisted of ten stacked blows. A digital sampling interval of 0.1 millisecond was needed for a detailed analysis of the refraction data with the generalized reciprocal method. Four end shots and a split-spread shot were used per line, and three radial lines at two of the quarries were realized. In both these quarries, the degree of fracturing varied. At a third quarry, where fracturing was less intense, a single profile was carried out paralell to the main fracturing system, in order to get values of seismic velocities and attenuation of almost unfractured rock. Velocities for each line were obtained with the general reciprocal method and delay-times method optimized with ray-tracing. Attenuation results were obtained from spectral ratios between an input signal (nearest geophone to the source) and each line of geophones. Preliminary results show that this technique can be used with success to evaluate the degree of weathering and fracturing of granites.