Closure of microcracks in rock samples under confining pressure

Velocity and Q-value of P- and S-waves and linear strain had been measured in three orthogonal directions,under different confining pressures,for several rocks with marked foliation and strong lattice preferred orientation. The effective Poisson's ratio,crack density, crack porosity and cia value (a is the length and c is the width of the coin-shaped microcrack) of microcracks are calculated from the measured data by O' Connell's model. It is resulted that the closure of microcracks normal to the foliation plane is the fastest: but the corresponding cia value does not decrease,but increase,with the increase of confining pressure. It may be caused by the roughness and rugosity of the crack faces,which can make one crack into several smaller cracks with significantly shorter lengths but widths not markedly smaller, during the progressive closure of microcracks with increasing confining pressure. So, cia value increases with confining pressure. By measuring P- and S-wave velocity simultaneously, we can get a general idea on the shape and spatial distribution of microcracks in the sample, basing on the calculated crack sapect ratio c/a.