Event: EGU General Assembly 2020 (online)
Presentation by Anne-Karin Cooke, Cédric Champollion, Pierre Vermeulen, Camille Janvier, Bruno Desruelle, Nicolas Le Moigne, Sébastien Merlet
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.
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Event: EGU General Assembly, Vienna (Austria) 2019
Presentation by Anne-Karin Cooke, Cédric Champollion, Pierre Vermeulen, Nicolas Le Moigne and Sébastien Merlet
Ground-based gravity measurements can provide accurate constrains on the water storage dynamics of subsurface reservoirs. At the scale of the measurements time-lapse gravity experiments allow to monitor the water mass balance taking into account both the saturated and the unsaturated zone.
One major characteristic of the gravity measurement is the integration of all water masses across scales: gravity variations can be the effect of continentalscale soil humidity or aquifer (such as seen by GRACE measurements) and of local effects (such as the umbrellaeffect of a building or reservoir heterogeneities). The vertical gravity gradient is similar to gravity while showing a higher sensibility to local masses. The interest of the gradient for subsurface features such as caves is well known.
The objective of the presentation is the specific potential of gravity vertical gradient monitoring for water reservoir studies. The study is first based on existing measurements of gravity gradients time series (with a relative spring gravimeter): can significant hydro-logical signals be detected? Then based on numerical simulations, the potential of future ground-based for reservoir monitoring is investigated.
The capability of the gravity gradient method to detect heterogeneities (contrary to theclassical gravity) is evident: for example the classical Bouguer plate approximation often used to convert gravityinto equivalent water height (even in heterogeneous area such as karst) has no effect on the gravity gradient, hence new models need to be applied that combine gravity and gravity gradient measurements.
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Event: ENIGMA Summer School, June-July 2018
Poster by Anne-Karin Cooke, C. Champollion, C. Janvier, P. Vermeulen, N. Le Moigne, S. Merlet
More about ESR8 research project