STRESS WAVE ATTENUATION IN SHOCK-DAMAGED ROCK

The velocity and attenuation of ultrasonic stress waves in gabbroic ro ck samples (San Marcos, California) subjected to shock loading in the 2 GPa range were studied. From P wave velocity measurements we determi ned the damage parameter D-p and crack density epsilon of the samples and related these to the attenuation coefficient (quality factor) unde r dynamic strains of 2 x 10(-7) and at a frequency of 2 MHz using the ultrasonic pulse-echo method. A fit to the data yields the P wave spat ial attenuation coefficient at a frequency of 2 MHz, alpha(p)(D-p) = 1 .1 + 28.2D(p) (decibels per centimeter). From the relation between the attenuation coefficient and quality factor, the quality factor Q is g iven by Q(-1) = 0.011(1 + 25.6D(p))(1 - D-p)(1/2). Using O'Connell-Bud iansky theory relating crack density to velocity, the parameter in Wal sh's theory was determined based on experimental data. An approximate method is also proposed to estimate the average half-length of cracks based on the attenuation measurements.