Gravimetry
Gravimetry is sensitive to mass changes, and provides intergrative measurements of water storage changes (WSC). Within the GHYRAF (Gravimetry and Hydrology in Africa) program, we installed and get data from a relative superconducting gravimeter (SG), an FG5 ballistic absolute gravimeter and a CG5 relative microgravimeter in sudanian hard-rock basement in West Africa (AMMA-CATCH observatory, Upper-Ouémé catchment).
Hybrid gravimetry
Combining the different gravimeters leads to the concept of hybrid gravimetry (Hinderer et al., in Press, Hector et al., 2015, Hinderer et al., 2015).
While temporal changes of the gravity (hence WSC) are being monitored at base stations by an SG (drift-controlled and calibrated by absolute gravity measurements), space gravity differences between base stations and stations of a network are being monitored by repeating microgravity surveys through time. Base stations gravity changes are added up to the space difference to provide us with space-time patterns of gravity changes, as in Hector et al., 2015, for instance. In this study, we compared hybrid gravity data with WSC derived from neutron probes data (a soil moisture probe that has to be taken down a drill hole and can sample any depths, on a 0.3m radius, roughly), which showed an interesting good match.
Processing
Gravimetry is integrative by nature, and supposes the correction of global effects (tides, pole motions, atmospheric effects, hydrological loading), as well as local effects (atmospheric, vertical motions) to obtain residuals which can be attributed to local WSC. In my research, I try to improve some of these corrections, to study their impact on the estimates of WSC, and to formulate recommandation to set up new instruments for hydrology (Hector et al., 2014, Hinderer et al., 2014).
At present, I evaluate the conjunctive use of hybrid gravity data and a critical zone model to correct for hydrological effect in SG time series.
Superconducting gravimeter:
This is the most accurate sensor for time variable gravimetry. It continuously (1sc) records relative gravity changes with a precision of about 1nm/s², that is, 2.5mm of equivalent water thickness (following the Bouguer assumption). It is made of a levitating sphere within coils in supraconducting materials to limit electrical resistance, and therefore needs to be cooled down (cryogenic) using a compressor. The instrument is drifting and needs calibration using an absolute gravimeter.
Absolute gravimeter:
It provides absolute point measurements of the gravity field, by measuring the acceleration of a falling body in a vacuum chamber. Precision is up to 20nm/s² (50mm H20)
Relative spring microgravimeter:
These portable gravimeters allow to derive gravity differences from spring elongation differences between measurement stations. Because they are drifting, they should be operated in loops, by repeating measurements at some control stations to control the drift value. Precision is highly variable but can be down to 20nm/s² (50mm H20).
GRACE satellites:
By continuously measuring their small distance variations, two satellites give us access to monthly solutions of the Earth gravity field, at resolutions of a few hundreds of kilometers.
Gravimetry for geothermics
[story to come]
References:
Hinderer, J., Hector, B. A. Mémin, M. Calvo. Hybrid gravimetry as a tool to monitor surface and underground mass changes; Proceedings of IUGG Prague, Czech Republic, 22 june-2 July 2015., Series International Association of Geodesy Symposia, Vol. Inconnu, In press.
Hector, B. and Hinderer, J.: pyGrav, a Python-based program for handling and processing relative gravity data, Computers & Geosciences, doi:10.1016/j.cageo.2016.03.010, 2016.
Hector, B., Séguis, L., Hinderer, J., Cohard, J.-M., Wubda, M., Descloitres, M., Benarrosh, N. and Boy, J.-P., 2015: Water storage changes as a marker for base flow generation processes in a tropical humid basement catchment (Benin): Insights from hybrid gravimetry, Water Resour. Res., n/a–n/a, doi:10.1002/2014WR015773.
Hinderer, J., Calvo, M., Abdelfettah, Y., Hector, B., Riccardi, U., Ferhat, G., and Bernard, J.-D., 2015, Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben: Geothermal Energy, v. 3, p. 16, doi: 10.1186/s40517-015-0035-3.
Hector, B., Hinderer, J., Séguis, L., Boy, J.-P., Calvo, M., Descloitres, M., Rosat, S., Galle, S., Riccardi, U., 2014. Hydro-gravimetry in West-Africa: first results from the Djougou (Bénin) superconducting gravimeter. J. Geodyn. 2014.
Hinderer, J., Hector, B., Boy, J.-P., Riccardi, U., Rosat, S., Calvo, M., Littel, F., 2014. A search for atmospheric effects on gravity at different time and space scales. J. Geodyn. 2014.
Hinderer, J., Rosat, S., Calvo, M., Boy, J.-P., Hector, B., Riccardi, U., Séguis, L., 2014. Preliminary Results from the Superconducting Gravimeter SG-060 Installed in West Africa (Djougou, Benin), in: Rizos, C., Willis, P. (Eds.), Earth on the Edge: Science for a Sustainable Planet, International Association of Geodesy Symposia. Springer Berlin Heidelberg, pp. 413–419. 978-3-642-37222-3.
Hector, B., Seguis, L., Hinderer, J., Descloitres, M., Vouillamoz, J.-M., Wubda, M., Boy, J.-P., Luck, B., Le Moigne, N., 2013. Gravity effect of water storage changes in a weathered hard-rock aquifer in West Africa: results from joint absolute gravity, hydrological monitoring and geophysical prospection. Geophys. J. Int. 194, 737–750.