Developing hydrogeophysics for critical zone studies, importance of heterogeneities and processes at the mesoscopic scale

The critical zone is the thin outer layer supporting life on earth. It is a compartment in which many complex and coupled bio-chemico physical processes take place. Characterizing and monitoring these processes is of utmost importance to understand and protect the critical zone. Geophysical methods are very appropriate tools to study them in situ and non-intrusively thanks to the sensitivity of measurable physical parameters to properties of interest. However, the critical zone is also a very heterogeneous medium at different scales and the quantitative use of geophysics rely on our capability to take them into account effectively. In this manuscript I present the various works that I conducted during the last ten years to better use hydrogeophysical methods by taking heterogeneities into account: from theoretical petrophysical model developments to laboratory experiments and in situ monitoring in critical zone observatories. After presenting the geophysical methods I have been using, I define the concept of mesoscopic scale heterogeneities, that is: larger than the pore scale but smaller than the resolution of the considered geophysical method. The first heterogeneities that I present are related to the pore size distribution and its impact on the water saturation, flow and solute transport properties. The second heterogeneities that I study are the presence of fractures and fracture networks in porous media. The final kind of heterogeneities that I am considering are induced by local biogeochemical reactions and their impact on geophysical signals. These new advances and on-going research projects pave the way to a more quantitative and more integrated use of geophysics to study the critical zone in its true complexity.