ENIGMA fellow Lara Blazevic successfully defended her thesis “Monitoring spatio-temporal wate redistribution in the subsurface with seismic methods” on Friday 18th September 2020 virtually from Sorbonne University. Congrats!
Abstract The characterization and monitoring of subsurface water systems are fundamental to groundwater resources conservation and management. To this end, hydrogeophysics provides a suite of non-invasive methods to study the shallow subsurface environment and the processes occurring therein over multiple scales. Time-lapse hydrogeophysical applications are notably useful to monitor water dynamics and follow temporal variations in water content. Largely dominated by electrical and electromagnetic methods, these applications are being increasingly explored with seismic methods. The seismic signal is dependent on the mechanical properties of the medium which are in turn affected by changes in water content. Consequently, seismic responses are also influenced by hydrological properties and state variables. Nonetheless, complexities in describing the mechanical behavior of partially saturated shallow materials have limited the quantitative characterization of the subsurface and associated water dynamics by means of seismic methods. Here we investigate the evolution of seismic responses with varying water content in time-lapse field contexts, analyzing both data and inverted parameters, and compare the resulting trends with established petrophysical relationships. We show that seismic time-lapse inversions of P-wave refraction data and corresponding changes in wave propagation velocity enable the recognition of preferential water flow paths in the subsurface, highlighting the potential of seismic methods to monitor hydrological processes and unsaturated flow. Overall, qualitative agreements between seismic velocity trends and theoretical petrophysical relationships still eclipse accurate quantitative estimations of water content from inverted seismic parameters. We anticipate further time-lapse seismic field studies to help bridge the gap between qualitative and quantitative observations. In the wake of the recent advancements in seismic equipment and techniques, appropriate field-scale petrophysical relationships will play an important role in the development of seismic methods for hydrological applications.
Event: EGU General Assembly 2020 (online) Presentation by Anne-Karin Cooke, Cédric Champollion, Pierre Vermeulen, Camille Janvier, Bruno Desruelle, Nicolas Le Moigne, Sébastien Merlet https://doi.org/10.5194/egusphere-egu2020-9020
Time-lapse ground-based gravimetry is increasingly applied in subsurface hydrology, providing mass balance constraints on water storage dynamics. For a given water content change as e.g. after a precipitation event, the simplest assumption is that of a homogeneous, infinite slab (Bouguer plate) of water column causing the measurable increase in gravitational attraction. For heterogeneous subsurface environments such as karst aquifers at field scale this assumption may not always hold. The gravity signal is depth-integrated and non-unique, hence indistinguishable from a heterogeneous distribution without further information.
Exploiting the different spatial sensitivities of gravity and vertical gravity gradient (VGG) data can shed light on the following questions:
Is the subsurface water content within the gravimeter’s footprint likely to be homogeneous or showing small-scale heterogeneity?
If not, at which distance are these mass heterogeneities and how large are they?
Which monitoring set-ups (tripod heights, number of and distance between VGG measurement locations) are likely to detect mass heterogeneity of which spatial characteristics?
One year of monthly vertical gravity gradient surveys has been completed in the geodetic observatory in karstic environment on the Larzac plateau in southern France. We interpret the VGG observations obtained in this field study in the context of further available hydraulic and geophysical data and hydro-gravimetrical simulation. Finally, practical applications in view of detecting near-surface voids and reservoirs of different porosities as well as their storage capacity and seasonal dynamics are evaluated.
Event: AGU Fall Meeting 2019, San Francisco (USA) Poster by Justine Molron, Niklas Linde, Ludovic Baron, Jan-Olof Selroos, Caroline Darcel, Philippe Davy
The identification of (open) fractures in the subsurface is critical for evaluating potential routes for contaminant transport from deep disposal sites. Ground Penetrating Radar (GPR) is suitable for this task and its detection capacity depends on fracture characteristics (orientation, aperture and size) and on the dielectrical and electrical contrast between the fluid or material filling the fractures and the surrounding bedrock.
A GPR experiment was performed in the Äspö Hard Rock Laboratory (Sweden) in a tunnel located 410 m below the sea level with a length of 20 m long, a width of 4 m and a height of 4.5 m. The geological formations are fractured granite, diorite and granodiorite with negligible matrix permeability and very low transmissive fractures (10E-9 to 10E-10 m2/s for most permeable zones).
We used 160 MHz, 450 MHz and 750 MHz antennas, pulled along the clean and flat tunnel floor along parallel lines separated by 0.10 m for 160 MHz and by 0.05 m for 450 and 750 MHz antennas. This measurement set-up and antenna choices allow for a 3D identification of fractures from GPR diffractions and reflections, with different image resolutions and investigation depths reaching 10m, 8m and 5m for 160, 450 and 750 MHz, respectively.
Based on the data, we identify 15 reflections that could correspond to larger fractures with dimensions of 2 to 5 m. We compare the GPR-inferred fractures with the corelogging of three 9.5 m deep boreholes that were drilled after the GPR campaign. The strong GPR reflections in the borehole area largely correspond to the depth and orientation to the fractures identified by the Optical Televiewer (OPTV) data.
Additionally, pumping and injection tests in each borehole showed that the GPR-inferred fractures are situated in the most permeable regions. The occurrence of GPR fractures was then compared with a statistical description of the fracture network built from the intersection of boreholes and 2D trace maps from tunnel walls. Given the average size of the GPR-inferred fractures, we demonstrate that they are overall consistent with the expected fracture density below the tunnel.
Event: AGU Fall Meeting 2019, San Francisco (USA) Poster by Behzad Pouladi, Niklas Linde, Olivier Bour, Laurent Longuevergne
Subsurface characterization often relies on inversion of either pressure or tracer data. Unless data from many pumping and observation wells are available, the inversion process only resolves smooth low-resolution images of subsurface properties, which leads to less accurate subsurface ﬂow and reactive transport predictions. Furthermore, tracer tomography can be very challenging and convergence to a global minimum is difficult. Active-distributed temperature sensing technology opens up the prospect of replacing tracer test data with estimates of subsurface groundwater flux .
Here, the value of using estimated subsurface groundwater fluxes as a data source to reconstruct subsurface hydraulic properties is explored using a sequence of synthetic multivariate Gaussian aquifers with different measurement configurations. These results are compared to inversion of pressure data and joint inversion of the two data types with the inversions being based on the Principal Component Geostatistical Approach . Inversion of pressure data resulted in a smoothed reconstruction of aquifer heterogeneity capturing approximately high and low conductivity regions while ground water flux data inversion leads to higher-resolution estimates. This is reflected, for one of the considered examples, by a correlation coefficient that increases from 0.57 for the pressure data to 0.65 for the ground water flux data. The complimentary nature of the data sets is represented by a correlation coefficient that increases to 0.74 for the joint inversion of the two data types.To conclude, inversion of ground water flux whether individually or jointly with pressure data, can provide enhanced information about the heterogeneity of subsurface media compared with using pressure data alone.
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.