Encouraging the use of seismic methods for the hydrogeophysical characterization of the critical zone

The characterization, study and monitoring of hydrosystems mainly rely on piezometric and log data, e.g. on local information. Fortunately, hydrogeophysics provide appropriate tools to interpolate boreholes information and to image heterogeneities in the critical zone. When electrical and electromagnetic methods predominate in such context, we recently suggested the use of classical seismic methods not only to provide a characterization of the subsurface geometry, but also to estimate the mechanical properties of the critical zone influenced by its water content. We tested, on two critical zone observatories with distinct hydrogeological characteristics, the simultaneous estimation of pressure (P-) and shear (S-) wave seismic velocities (VP and VS, respectively) from P-wave travel-time tomography and surface-wave dispersion inversion respectively. On both sites, e.g. a fractured environment with strong discontinuities and a continuous multi-layered hydrosystem, we were able to image spatial and/or temporal variations of VP/VS ratio, whose evolution was strongly associated to the water content observed locally.