The effect of heating on the microstructural evolution of La Peyratte granite deduced from acoustic velocity measurements

The acoustic P-waves velocity VP was 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. Different crack contents were generated in the granite specimens by means of controlled thermal pre-treatment at 450, 510, 555 and 600 °C. VP variations are interpreted as resulting from crack behaviour evolution in response to imposed pressures. Data analysis is proposed by using an effective medium approach. The elastic behaviour of the cracked rock is controlled by the crack density parameter, which varies with applied pressures due to crack closure. Two crack families corresponding to intragranular cracks and cracked grain boundaries and a decreasing exponential distribution of crack aspect ratios are introduced to take into account the heterogeneity of the rock samples and allows us to calculate the acoustic velocity VP for various effective pressures. Comparison between theoretical and experimental values shows that the model leads to consistent velocity variations. Moreover, when applied to the four pre-treatment temperatures, the model gives informations on the effect of heating on the microstructural parameters of the samples, in particular the behaviour of both crack families when the quartz α–β transition is passed.