Variants of the strain-amplitude dependence of elastic wave velocities in rocks under pressure

The dependence of the compressional and shear wave velocities on strain amplitude in the Madra dolomites is studied. The experiments were performed using an ultrasonic pulse transmission technique (f = 1 MHz) with a uniaxial pressure of P = 1-60 MPa. An amplitude dependence of wave velocity in the range epsilon(d) similar to (1-3) x 10(-6) under a pressure of 5-20 MPa is found. The compressional velocity depends on strain amplitude but the shear velocity does not depend on amplitude change. Determined by the first arrival time, the compressional velocity increases with amplitude. However, if the compressional velocity is determined by the measurement of peak time, the velocity decreases with amplitude. For the upward and downward pressure, the curve V-p(P-ax) exhibits open hysteresis, yielding a residual component of velocity. An intersection of the branches of the hysteretic loop is observed in more porous dolomites in comparison with less porous dolomite. This intersection is manifested most in the residual components of the dynamic bulk modulus and Poisson's ratio. The dynamic parameters can be used as is supposed as additional criteria in a geological interpretation.