This session is intended to gather contributions dealing with the links between heterogeneity and anisotropy in geomaterials at different scales.
For a given physical property, depending on the scale of investigation, geomaterials may appear strongly heterogeneous and difficult to describe by simple mathematical functions, or homogeneous if the scale of investigation is large enough to average all local effects. Anisotropy and heterogeneity as scale dependent characteristics are ubiquitous in rock physics problems. Depending on the focus of a study, observation scale may be chosen in order to optimize either one or the other, knowing that (1) as scale changes, several heterogeneous (resp. homogeneous) states may be reached, and that (2) to a given working scale will correspond a given measurement method. In this context, what can be called "adequate" working scale and what are the limitations of models using homogenization techniques? For example, when only few data are acquired along different directions, what may be used to decide if the observed anisotropy is representative of the medium investigated or if it results from the presence of some heterogeneity? When the scale of investigation makes the measured property sensitive to both heterogeneity and anisotropy, is it possible to extract the background anisotropy while minimizing disturbances due to heterogeneities?
Finally, we can wonder if it is always worth to worry about these characteristics. For instance, to what extent may heterogeneities control the deformation mechanism, and therefore the scale at which events shall be apprehended? In some cases, the effect of small scale heterogeneities only influences the small scale behaviour, but these effects vanish out when the system is zoomed out, and the large scale rheology is only controlled by large scale features. In other nontrivial situations, collective effects arise at large scale from the small scale heterogeneities, which are then essential to model the mechanics of the problem. Examples of such nontrivial behaviours can be found in friction or fracture problems, in strain localization in ductile or fragile systems, or in front dynamics for multiphasic or reactive flows in disordered porous media. Likewise, what is the real effect of anisotropy on the orientation of strain localization features as compared to the one of anisotropic stress state? In other words, how far do we typically stand from a conceptual isotropic homogeneous material?
As many domains are concerned by this issue, such as rock mechanics, poroelasticity, hydraulic and electric transport properties, or magnetism, we encourage contributions with well defined objects and goals. Pedagogic efforts will also be very welcome in order to maximize the impact of each presentation.

Sollicited speakers:
- Jose ANDRADE, Northwestern University, USA
- Christophe BARNES, Cergy-Pontoise University, France
- Patrick BAUD, Strasbourg University, France
- Alex HANSEN, Norwegian University of Science and Technology, Trondheim, Norway
- Jörg RENNER, Ruhr-University Bochum, Germany