Anisotropy of elastic, magnetic and microstructural properties of the Callovo-Oxfordian argillite

Abstract

We investigated the anisotropic behaviour of the Callovo-Oxfordian argillite in laboratory experiments, and analysed the rock microstructure using different imaging techniques. The starting material was a core retrieved at 460 m depth in well EST204 at the site of the French Underground Research Laboratory in the Meuse/Haute-Marne region, from which 17 small subcores were prepared in three orthogonal directions. The anisotropy of magnetic susceptibility (AMS) was studied using a KLY3S kappabridge. The technique allows one to determine the susceptibility tensor which can be represented by an ellipsoid with principal axes K max, K int and K min in decreasing order. We found that the AMS ellipsoid is consistent with an oblate fabric. We have also investigated the elastic anisotropy through the measurement of P-wave velocity in different directions, following the procedure described in our previous works [Louis, L., David, C., Robion, P., 2003. Comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions. Tectonophysics 370, 193–212; Louis, L., Robion, P., David, C., 2004. A single method for the inversion of anisotropic data sets with application to structural studies. Journal of Structural Geology 26, 2065–2072]. The anisotropy of P-wave velocity is significantly larger than the anisotropy of magnetic susceptibility. Interestingly, we found an excellent correlation between AMS and the anisotropy of P-wave velocity, with a minimum velocity V min and susceptibility K min fairly well constrained in the direction perpendicular to bedding. Both from the elastic and magnetic viewpoint, the argillite behave as a transverse isotropic medium, which is typical of a sedimentary fabric with no clear sign of tectonic fabric. Our data set on P- and S-wave velocity measurements allowed for the full determination of the transverse isotropy stiffness tensor as well as the Thomsen’s parameters within the framework of weak anisotropy. We also present some results of a study of the rock microstructure and its structural anisotropy using stereological techniques and X-ray microtomography. The microtomography technique revealed the existence of tubular structures made of dense material (pyrite), the origin of which is not clearly defined but has probably to do with bioturbation during the formation of the sediment.