ENIGMA fellow Anne-Karin Cooke successfully defended her thesis “Characterisation of a new mobile absolute quantum gravimeter: Application in groundwater storage monitoring” on Friday 30th October 2020 virtually from Montpellier University. Congrats!
Abstract Gravimetry studies the variations of the Earth’s gravity field which can be linked to mass changes studied in various disciplines of the Earth sciences. The gravitational attraction of the Earth (referred to as g) is measured with gravimeters. Quantum gravimeters provide the possibility of continuous, high-frequency absolute gravity monitoring without instrumental drift while remaining user-friendly and transportable. We assess the performance of the absolute quantum gravimeter device AQG#B01, developed by Muquans, in view of its precision, stability, repeatability and of future terrain deployment. The measurement of g was found to not be influenced by temperature and tilt changes. Furthermore, the potential use of monitoring the vertical gravity gradient in hydrogeophysical applications was assessed experimentally and using hydrogravimetrical modelling. Time-lapse vertical gravity gradient data can deliver additional information that can be used to constrain the subsurface water distribution.
We use Approximate Bayesian Computation and the Kullback–Leibler divergence measure to quantify to what extent horizontal and vertical equivalent electrical conductivity time-series observed during tracer tests constrain the 2-D geostatistical parameters of multivariate Gaussian log-hydraulic conductivity fields. Considering a perfect and known relationship between salinity and electrical conductivity at the point scale, we find that the horizontal equivalent electrical conductivity time-series best constrain the geostatistical properties. The variance, controlling the spreading rate of the solute, is the best constrained geostatistical parameter, followed by the integral scales in the vertical direction. We find that horizontally layered models with moderate to high variance have the best resolved parameters. Since the salinity field at the averaging scale (e.g., the model resolution in tomograms) is typically non-ergodic, our results serve as a starting point for quantifying uncertainty due to small-scale heterogeneity in laboratory-experiments, tomographic results and hydrogeophysical inversions involving DC data.
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.
The freshwater‐seawater mixing zone is a critical region for chemical activity. Yet little is known about the influence of ever present spatial heterogeneity on the dynamics of mixing and calcite dissolution, which play a key role in the understanding of karst development. We analyze the impact of different heterogeneity structures and strengths on the local and global response of mixing and dissolution rates across the saltwater freshwater mixing zone. We find that the initial heterogeneity structure significantly impacts observed dissolution and mixing patterns, which sheds some new light on karst propagation in coastal aquifers.
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.