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9 Seismic Velocity Determination Laboratory measurements of rock velocities at ultrasonic fre- quencies and high pressures are commonplace. Bonner and Schock (1981) provide a recent update and summarize dimension require- ments for reliable velocity determinations. For determination of compressional velocity, the length of a cylindrical specimen must be less than 6 times the diameter, or interactions with the sides of the core sample will cause the simple plane wave to become dispersive. Because the distance traveled by the wave should be at least 50 times the mean grain diameter to obtain a velocity representative of the whole rock, velocity determinations should be made on large samples (10 cm long; Table 2) of rocks with a grain size greater than 2 mm (many granites, for example). The need for core samples becomes all the more obvious when a representative average compressional velocity for a rock unit is desired. The proposed 10 km deep drill hole that will inter- sect observed reflectors and known structures offers an excellent opportunity for calibration of seismic reflection and refraction ex- periments in the southern Appalachians through determination of average compressional velocities of tectonostratigraphic units on the basis of laboratory measurements on cores. Removal of sam- ples from confining pressure and differential stress conditions at depth may induce changes, such as opening of microcracks and 36
57 loss of pore fluids, that result in decreases in seismic velocities. Laboratory measurements can be made under appropriate con- fining pressures, and core samples can be handled in such a way as to prevent dehydration. The effect of elevated temperature on velocity may be significant, but it is generally less than effects of microcracks. Because of the low geothermal gradient in this region, measurements at high temperatures will not be necessary. Anisotropy of the metamorphic rocks of the Appalachian oro- gen will affect seismic velocity determinations. In order to measure velocities in directions other than the vertical, oriented sidewall samples may be needed. These measurements could be made on the same samples used for microstructural and other studies. Con- ventional core samples may be adequate for relatively fine-grained rocks if orientation can be established from planar structures and dip logging records.
