Effect of Strain Amplitude and Frequency on Compressional Amplitudes in Sandstones

A series of experiments have been carried out on sandstone samples with wide range of porosities and permeabilities in order to investigate the influence of strain amplitude and frequency on compressional and shear velocities and attenuation of sedimentary granular rocks. Low frequency velocity data was obtained from uniaxial stress cycling measurements that were conducted at a frequency of 10-2 Hz at different levels of strain amplitude from 10-5 to 10-2. Attenuation information was obtained at 10-2 Hz from the stress-strain hysteresis observed in the cycling measurements. Compressional velocity data was collected at various stresses using four different transducer/receiver sets with frequencies 50 KHz, 100KHz, 180 KHz and I MHz. The strain amplitudes for all four types of ultrasonic measurements are in the range of 10-7 - 10-6. Shear velocities were simultaneously recorded at 1 MHz. Compressional wave amplitude data were collected at a few selected stresses in order to obtain attenuation at ultrasonic frequencies. Velocities calculated from the static moduli (10-2) increase as average stress is increased. The dry velocities are very sensitive to strain amplitude. As strain amplitude increases, both compressional and shear velocities decrease. When the velocities calculated from the static moduli are plotted together with the velocities measured using the 50 KHz, 100 KHz, 180 KHz and I MHz transducers, velocity dispersion is clearly observed. However, differences between static velocities and velocities measured from 50 KHz to 1 MHz can not be explained on the basis of the frequency dependence alone. Differences between the strain amplitudes of the measurement techniques is believed to be one of the major causes of dispersion observed in the dry rocks used in this study.