The effects of loading frequency, strain amplitude, and saturation on elastic moduli and attenuation have been measured in samples of the Topopah Spring Member welded tuff. Four different laboratory techniques have been used to determine Young's modulus and extensional wave attenuation at frequencies ranging from 10 -2 to 106 Hz. The results are compared with data acquired for Sierra White granite under the same conditions. The modulus and attenuation in room dry samples remain relatively constant over frequency. Frequency dependent attenuation and modulus dispersion are observed in the saturated samples and are attributed to fluid flow and sample size. The properties of tuff were independent of strain amplitude in room dry and saturated conditions.
The applicability of laboratory data for engineering purposes is a prime concern for design and licensing of the proposed nuclear waste repository at Yucca Mountain. Significant differences in elastic moduli and wave attenuation have been observed for a given rock, as measured with different laboratory and field techniques. These differences can be attributed to the time and strain amplitude dependencies in the anelastic response of rock to various loading conditions. Anelastic mechanisms in rock produce a phase lag of strain behind stress and affect the measured moduli and attenuation. The modulus, M, is represented by a complex quantity (Toksoz and Johnson, 1981),
[Equation available in full paper] (1)
M R and M I are the real and imaginary parts of the modulus, respectively. The phase lag, q?, is a direct measure of the intrinsic loss or attenuation, Q4 (Toksoz and Johnson, 1981),
[Equation available in full paper] (2).
Several investigators have reported frequency, strain amplitude, and saturation dependences on M and Q-1 in crustal rocks (Gordon and Davis, 1968; Tittmann et al., 1977; Toksoz et al., 1979; Winkler et al., 1979; Spencer, 1981; Dunn, 1986; Paffenholz and Burkhardt, 1989). Different laboratory techniques were used to measure these dependences. Spencer (1981) used cyclic loading technique to examine the effects of frequency from 1 to 10^3 Hz and found that moduli and Q^-1 changed dramatically with frequency in samplesaturated with different fluids. Winkler et al. (1979) utilized this same method at frequencies less than 1 Hz to examine the effects of strain amplitude. He observed that toodull decreased and Q^-1 increased with increasing strain amplitude in rock. Tittmann et al. (1977) used the resonant bar technique to examine these effects in vacuum dry and saturated samples at frequencies ranging from 1 to 200 kHz. He reported large variations in modulus and Q^-1 as function of saturation. Toksoz et al. (1979), examined the effects of fluid saturation and confining pressure on Q^-1 at ultrasonic
frequencies.
In this study, Young's modulus and QE^-1 are determined for the Topopah Spring Member welded tuff with several laboratory techniques over a large frequency band, 10^-2 to 106 Hz. The effect of strain amplitude on these properties also is examined.
