A major challenge in water resources research is to document the spatial and temporal variability... more A major challenge in water resources research is to document the spatial and temporal variability of the hydrological processes over short time and space scales. Indeed, the quality of model predictions for resource assessment is dependent on reliable datasets, representative of the hydrological regime and its variations. This issue is particularly sensitive in Sahelian Africa, where available hydrological datasets are scarce. In this study, the variability of the water storage was investigated at the subcatchment scale (< 1km) by an intensive microgravity field campaign. Weekly microgravity surveys were carried out in a small endoreic catchment in Southwest Niger, during three months of the rainy season in 2009. Gravity measurements were performed at 16 stations located near a temporary pond, where rapid infiltration towards the aquifer occurs. The highest (63 μGal) gravity signal was measured on a station located above the pond and is well explained by the direct effect of the ...
We have used tidal gravity measurements from six stations in central Europe to investigate the re... more We have used tidal gravity measurements from six stations in central Europe to investigate the resonance in the diurnal tidal band, caused by inertial coupling between the mantle and outer core of the Earth. By the use of stacking it was possible to determine the eigenfrequency and quality factor of this eigenmode, commonly called the ‘nearly diurnal free-wobble'. We assessed the effect of systematic errors from the ocean correction to the tidal measurements employing a Monte-Carlo method. The observed eigenfrequency is 1 + 1/(434 ± 7) cycles per sidereal day, and is significantly higher than predicted by theories. The observed quality factor is (2.8 ± 0.5) × 103.
Physics of The Earth and Planetary Interiors, 1987
Motions at the top of the core are known to be responsible for the secular variation of the earth... more Motions at the top of the core are known to be responsible for the secular variation of the earth's magnetic field. If this flow is supposed geostrophic, the associated pressure field can have an appropriate geometry to exert a pressure torque upon the elliptical core-mantle boundary and, also, to alter the elastic products of inertia in such a way as to excite the earth's and core's wobbles. The authors consider some schematic excitation functions and the resulting amplitudes of the earth's and core's rotational motions. The proposed mechanism is shown to be efficient for exciting the long-period Markowitz wobble of the rotation axis and also the Chandler wobble if the variations in the pressure field have the right time scales, as indeed suggested by the available secular variation data.
Ground-based gravity observations have the potential to add useful information to the interpretat... more Ground-based gravity observations have the potential to add useful information to the interpretation of data from the new satellite gravity missions (CHAMP, GRACE, GOCE). We examine 4.5 yr of data from eight superconducting gravimeters (SGs) associated with the Global Geodynamics Project (GGP), from 1997 to 2001, and simulate the time variations that might be seen by a satellite. Signals that are removed from the gravity data before spatial averaging are the solid Earth and ocean tides, a global atmospheric loading using a vertical perfect gas law for the atmosphere, International Earth Rotation Service (IERS) polar motion and instrument drift. The 1-d gravity residuals form the basis of an interpolated minimum curvature grid that we spatially average and analyse using both surface polynomials and empirical orthogonal functions (EOFs). A clear annual component is present that, if truly regional, should be easily detectable by a satellite such as GRACE. The signal is consistent with expected continental water storage, which provides some interest for the future comparison of ground and satellite data.
This study is devoted to a preliminary comparison of the gravity records provided by two geograph... more This study is devoted to a preliminary comparison of the gravity records provided by two geographically well-separated superconducting gravimeters over a common two-year observing period. The data sets are processed for the first time following exactly the same numerical steps. After a brief description of the observed gravity and pressure signals, we perform a classical least-squares fit to tides, taking into account a single coefficient for the effect of the local barometric pressure fluctuations and a second-order polynomial approximation for the instrumental drift. The results show a totally different drift behaviour between the two instruments. The time fluctuation of the resulting residual gravity signals is shown, as well as the corresponding power spectral density. In particular, we provide detailed statistics on the spectral content of the residual gravity signals. In a subsequent step, we first high-pass filter the observed gravity and pressure signals and then apply an automatic method to detect and remove obvious spikes. A new least-squares fit then provides new residual gravity signals, the power spectral density of which is reduced in all frequency bands, especially in the subtidal band. We finally provide cross-spectral estimates of these two residual gravity sets indicating a further reduction in the power spectral density level.
Eos, Transactions American Geophysical Union, 1999
A global network of superconducting gravimeters (SGs) is compiling significant data for a range o... more A global network of superconducting gravimeters (SGs) is compiling significant data for a range of important studies spanning a number of disciplines concerned with the Earth's gravity, tides, environment, and geodetics. Among phenomena being looked at are seismic normal modes, the Slichter triplet, tidal gravity, ocean tidal loading, core nutations, and core modes. Hydrologists and volcanologists also may benefit from SG data.The network was set up by the Global Geodynamics Project (GGP),an international program of observations of temporal variations in the Earth's gravity field. Observations began in 1997 and will continue until 2003. Eighteen SGs currently are in operation in the network (see Figures 1 and 2).
The elastogravitational deformation of an earth model with a liquid outer core and a surficial fl... more The elastogravitational deformation of an earth model with a liquid outer core and a surficial fluid layer modeling the oceans or the atmosphere is considered. The surface gravity changes caused by planetary-scale dynamical phenomena can be expressed in terms of generalized gravimetric factors. Each of these factors is related to a specific boundary condition (in normal or transverse traction, in potential and derivative) which has to be considered at the different interfaces (inner core-outer core, outer core-mantle, surface) according to the problem. This formalism is applied to express in a self-consistent and uniform manner the gravity disturbances due to various sources: tides, earth rotation, loading process, inner and outer core dynamics, with special attention paid to possible resonance effects.
A major challenge in water resources research is to document the spatial and temporal variability... more A major challenge in water resources research is to document the spatial and temporal variability of the hydrological processes over short time and space scales. Indeed, the quality of model predictions for resource assessment is dependent on reliable datasets, representative of the hydrological regime and its variations. This issue is particularly sensitive in Sahelian Africa, where available hydrological datasets are scarce. In this study, the variability of the water storage was investigated at the subcatchment scale (< 1km) by an intensive microgravity field campaign. Weekly microgravity surveys were carried out in a small endoreic catchment in Southwest Niger, during three months of the rainy season in 2009. Gravity measurements were performed at 16 stations located near a temporary pond, where rapid infiltration towards the aquifer occurs. The highest (63 μGal) gravity signal was measured on a station located above the pond and is well explained by the direct effect of the ...
We have used tidal gravity measurements from six stations in central Europe to investigate the re... more We have used tidal gravity measurements from six stations in central Europe to investigate the resonance in the diurnal tidal band, caused by inertial coupling between the mantle and outer core of the Earth. By the use of stacking it was possible to determine the eigenfrequency and quality factor of this eigenmode, commonly called the ‘nearly diurnal free-wobble'. We assessed the effect of systematic errors from the ocean correction to the tidal measurements employing a Monte-Carlo method. The observed eigenfrequency is 1 + 1/(434 ± 7) cycles per sidereal day, and is significantly higher than predicted by theories. The observed quality factor is (2.8 ± 0.5) × 103.
Physics of The Earth and Planetary Interiors, 1987
Motions at the top of the core are known to be responsible for the secular variation of the earth... more Motions at the top of the core are known to be responsible for the secular variation of the earth's magnetic field. If this flow is supposed geostrophic, the associated pressure field can have an appropriate geometry to exert a pressure torque upon the elliptical core-mantle boundary and, also, to alter the elastic products of inertia in such a way as to excite the earth's and core's wobbles. The authors consider some schematic excitation functions and the resulting amplitudes of the earth's and core's rotational motions. The proposed mechanism is shown to be efficient for exciting the long-period Markowitz wobble of the rotation axis and also the Chandler wobble if the variations in the pressure field have the right time scales, as indeed suggested by the available secular variation data.
Ground-based gravity observations have the potential to add useful information to the interpretat... more Ground-based gravity observations have the potential to add useful information to the interpretation of data from the new satellite gravity missions (CHAMP, GRACE, GOCE). We examine 4.5 yr of data from eight superconducting gravimeters (SGs) associated with the Global Geodynamics Project (GGP), from 1997 to 2001, and simulate the time variations that might be seen by a satellite. Signals that are removed from the gravity data before spatial averaging are the solid Earth and ocean tides, a global atmospheric loading using a vertical perfect gas law for the atmosphere, International Earth Rotation Service (IERS) polar motion and instrument drift. The 1-d gravity residuals form the basis of an interpolated minimum curvature grid that we spatially average and analyse using both surface polynomials and empirical orthogonal functions (EOFs). A clear annual component is present that, if truly regional, should be easily detectable by a satellite such as GRACE. The signal is consistent with expected continental water storage, which provides some interest for the future comparison of ground and satellite data.
This study is devoted to a preliminary comparison of the gravity records provided by two geograph... more This study is devoted to a preliminary comparison of the gravity records provided by two geographically well-separated superconducting gravimeters over a common two-year observing period. The data sets are processed for the first time following exactly the same numerical steps. After a brief description of the observed gravity and pressure signals, we perform a classical least-squares fit to tides, taking into account a single coefficient for the effect of the local barometric pressure fluctuations and a second-order polynomial approximation for the instrumental drift. The results show a totally different drift behaviour between the two instruments. The time fluctuation of the resulting residual gravity signals is shown, as well as the corresponding power spectral density. In particular, we provide detailed statistics on the spectral content of the residual gravity signals. In a subsequent step, we first high-pass filter the observed gravity and pressure signals and then apply an automatic method to detect and remove obvious spikes. A new least-squares fit then provides new residual gravity signals, the power spectral density of which is reduced in all frequency bands, especially in the subtidal band. We finally provide cross-spectral estimates of these two residual gravity sets indicating a further reduction in the power spectral density level.
Eos, Transactions American Geophysical Union, 1999
A global network of superconducting gravimeters (SGs) is compiling significant data for a range o... more A global network of superconducting gravimeters (SGs) is compiling significant data for a range of important studies spanning a number of disciplines concerned with the Earth's gravity, tides, environment, and geodetics. Among phenomena being looked at are seismic normal modes, the Slichter triplet, tidal gravity, ocean tidal loading, core nutations, and core modes. Hydrologists and volcanologists also may benefit from SG data.The network was set up by the Global Geodynamics Project (GGP),an international program of observations of temporal variations in the Earth's gravity field. Observations began in 1997 and will continue until 2003. Eighteen SGs currently are in operation in the network (see Figures 1 and 2).
The elastogravitational deformation of an earth model with a liquid outer core and a surficial fl... more The elastogravitational deformation of an earth model with a liquid outer core and a surficial fluid layer modeling the oceans or the atmosphere is considered. The surface gravity changes caused by planetary-scale dynamical phenomena can be expressed in terms of generalized gravimetric factors. Each of these factors is related to a specific boundary condition (in normal or transverse traction, in potential and derivative) which has to be considered at the different interfaces (inner core-outer core, outer core-mantle, surface) according to the problem. This formalism is applied to express in a self-consistent and uniform manner the gravity disturbances due to various sources: tides, earth rotation, loading process, inner and outer core dynamics, with special attention paid to possible resonance effects.
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