Towards physical modeling of local-scale hydrological contribution of soils for
precise gravimetric corrections in Strasbourg
1
2
1
Laurent Longuevergne , Gilbert Fehrat , Patrice Ulrich2, Nicolas Florsch , Jacques
Hinderer2
1 UMR Sisyphe, 4 Place Jussieu, 75252 Paris, France
2 EOST, 5 rue René Descartes, 67084 Strasbourg, France
Two questions must be answered in order to physically model the impact or water
mass redistribution on gravimetric data. The first question is: what is the amount of
water? This question implies the definition of a hydrological system of study to focus
on.
The mass balance equation within this system is then calculated with
measurements and/or models. The second question is: how is distributed this amount
of water within the hydrological system?
These questions imply a methodology that is applied to the gravimetric observatory in
Strasbourg. In this work, we focus on the soil layer that is above the gravimeter.
Indeed, previous studies (Llubes et al. 2004) showed the capacity of these loess soils
to store a 200-mm full layer of water per meter of soil (equivalent to a 80 nm.s-2
variation).
Two multi-depth frequency-domain reflectometer (FDR) probes have been installed.
They are monitoring the variation of the water content of the entire soil thickness
with a 5-minute sampling since August 2005. The geometry of the system has also
been detailed: a local digital elevation model (DEM) has been established thanks to
differential GPS. The geometry and heterogeneity of the soil layer have been
evaluated thanks to in-situ geophysical and geomecanical prospections.
These first 5 months show that the local soil contributes actually as much as 80 nm.s-2
to the gravity variations on a wide frequency band.