Effect of pore and confining pressures on VP in thermally pre-cracked granites

The acoustic P-waves velocity V-P is measured in a water saturated fine-grained granite as a function of pore pressure for a series of confining pressures ranging from 10 to 75 MPa. Prior to V-P measurements, different crack contents were generated in the granite specimens by means of controlled thermal pre-treatments at 450, 510, and 555 degrees C. V-P variations are interpreted as resulting from crack behaviour evolution in response to imposed pressures. An effective pressure law with an effective pressure coefficient close to 0.9 is derived for V-P from a delimited pressure domain. This behaviour is interpreted in terms of crack deformation under pressure.