Faults and matrix deformations in chalk: contribution of porosity and sonic wave velocity measurements

Measurements of sonic wave velocity and of porosity have been carried out on the Campanian so-called "craie blanche" (white chalk) of the Mons Basin. They have been made on cores collected perpendicular to normal fault planes, at different distances, between 0 (fault plane) and 30 cm. The applications of the continuity index, deduced from the sonic velocity, to these measurements allow us to estimate the amount of microcracking. A new evaluation of the used coefficients, after the physical characteristics of the intact material, has been necessary. The systematic comparison of the values of both velocity and porosity highlight the matrix transformations (decreasing of porosity by calcitic cementation) in a narrow band of about 10 cm on both sides of the fault planes. Depending on the stratigraphic level, the porosity of the intact chalk ranges between 40 and 44% and the sonic wave velocity between 2300 and 2600 m/s. Close to the fault planes, the porosity is about 30 to 35% and the sonic wave velocity about 2700 to 2900 m/s. Along the fault planes, the rock material has an anisotropic behaviour, due to both vertical tectonic microcracking and calcitic barriers resulting from cementation.