Event: AGU Fall Meeting 2019, San Francisco (USA)
Presentation by Lara Blazevic, Ludovic Bodet, Niklas Linde, Laurent Longuevergne, Sylvain Pasquet, Thomas Hermans, Damien Jougnot
Geophysical methods provide non-intrusive means to obtain subsurface information of relevance for agriculture, pollutant transport and critical zone processes. Electrical resistivity tomography (ERT) is routinely employed to derive water content and associated fluxes while seismic methods in hydrogeophysics have recently developed with the estimation of Poisson’s ratio from the combined use of P-wave traveltime tomography and surface-wave dispersion inversion. Here, we investigate the complementarity of such time-lapse approaches in a well-known and controlled context.
The Ploemeur Hydrological Observatory, located in Brittany (France), lies on a contact zone between granite and micaschists. The crystalline bedrock aquifer is an important source of drinking water for the nearby population and is monitored with numerous boreholes and experimental campaigns on site.
In September 2018, we carried out a two-day controlled and gradual infiltration experiment in soil overlaying the micaschists and performed eleven repeated electrical resistivity and active seismic acquisitions on two orthogonal lines crossing the 2.2×2.4 m2 infiltration area. In total, 3.3 m3 of water were injected. Adjacent to the infiltration area, time-domain reflectometry (TDR) sensors installed at different depths provided real time water content estimates during the experiment. They reveal that in the upper 0.25 m, the increases in water content may exceed 125%, and may increase by 25-50% even at 2 m depth. Our 2D and 3D time-lapse ERT inversions agree with these findings, in that we observe a decrease of up to 90% in electrical resistivity in the upper 1 m.
For the seismic data, we computed the differences in first arrival times with respect to the first reference acquisition by cross-correlating the traces and observed positive relative changes in traveltimes in the infiltration area going from 30-90%. The 2D time-lapse traveltime inversion shows a similar behavior as the ERT with P-wave velocities decreasing between 50-90% in the upper 1 m.
Our ultimate aim is to combine these results with S-wave velocities from surface-wave analyses and perform joint 3D time-lapse inversion of the dataset to better constrain water content and rock physics models in the vadose zone.