The vadose zone is the main host of surface and subsurface water exchange and has important implications for ecosystems functioning, climate sciences, geotechnical engineering, and water availability issues. Geophysics provides a means for investigating the subsurface in a non-invasive way and at larger spatial scales than conventional hydrological sensors. Time-lapse hydrogeophysical applications are especially useful for monitoring flow and water content dynamics. Largely dominated by electrical and electromagnetic methods, such applications increasingly rely on seismic methods as a complementary approach to describe the structure and behavior of the vadose zone. To further explore the applicability of active seismics to retrieve quantitative information about dynamic processes in near-surface time-lapse settings, we designed a controlled water infiltration experiment at the Ploemeur Hydrological Observatory (France) during which successive periods of infiltration were followed by surface-based seismic and electrical resistivity acquisitions. Water content was monitored throughout the experiment by means of sensors at different depths to relate the derived seismic and electrical properties to water saturation changes. We observe comparable trends in the electrical and seismic responses during the experiment, highlighting the utility of the seismic method to monitor hydrological processes and unsaturated flow. Moreover, petrophysical relationships seem promising in providing quantitative results.
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.
Event: AGU Fall Meeting 2018, Washington DC (USA) poster by Lara Blazevic, Ludovic Bodet, Damien Jougnot, Laurent Longuvergne
Seismic methods have been recently applied to the monitoring of spatial and temporal variations of near surface characteristics for hydrogeological purposes. The seismic signal is certainly related to mechanical properties that partly depend on porosity and saturation. The behavior of pressure (P) and shear (S) waves in the presence of water is partially decoupled, and the ratio of their propagation velocities VP/VS has been used to study water saturation changes.
However, the interpretation of the mechanical properties remains complex in unconsolidated near surface materials, limiting the quantitative description of linked hydrodynamic properties. In this study, we investigate the theories behind wave propagation velocities in poorly consolidated media and how they are affected by water content, focusing our discussion on the partially saturated response.
We present a field case where we used a Hertz-Mindlin based rock physics model to estimate water saturation from VP and VS from seismic data. The model is able to distinguish between dry and fully saturated areas at two distinct hydrological periods, but fails in identifying partially saturated areas in both cases. This work underlines the need for more elaborated models to infer hydrodynamic properties from seismic data.