Quantifying the physical and mechanical heterogeneity of porous volcanic rocks from the Chaine des Puys (Massif Central, France)

Volcanic structures are inhomogeneous on a number of scales. Quantifying whether such inhomogeneities result in a physical and mechanical anisotropy is important to improve large-scale volcano and volcano-geothermal modelling. It is, however, currently challenging to account for such anisotropy because there are very few experimental studies that quantify the physical and mechanical property anisotropy expected for volcanic rocks. Here, we measure the porosity, permeability, P-wave velocity, thermal properties, uniaxial compressive strength (UCS), and static Young's modulus of samples from five trachyandesite lavas from the Cha & icirc;ne des Puys (Massif Central, France) that have been cored in three arbitrary orthogonal directions. We show, using X-ray computed tomography, that the pores in the most anisotropic lava are oblate spheroids, with a major axis 2-5 times larger than their minor axis, that have a shape preferred orientation. This lava is an order of magnitude less permeable, has a P-wave velocity similar to 25% lower, and has a UCS and Young's modulus similar to 50% lower when measured perpendicular to the major pore axis. The observed physical and mechanical anisotropy is therefore the result of a pore shape preferred orientation. We conclude that, alongside large-scale sources of anisotropy such as stratified lavas, modellers should also consider incorporating a physical and mechanical anisotropy in the volcanic rocks forming a particular layer in the development of models that seek to incorporate greater complexity. Incorporating additional complexities in modelling, where and when possible, will provide the most accurate predictions and therefore the most reliable information for a given volcano or geothermal reservoir.