Corinna Kroner

Comparison of different barometric pressure reductions for gravity data and resulting consequences

Abstract Different methods of barometric pressure reductions are applied on data recorded with the superconducting gravimeter at Potsdam, Germany. The results are compared regarding their success in removing barometric pressure effects in different frequency ranges. Among the corrections, pure regression methods are used as well as a combination of a physical correction using atmospheric Greens functions [Merriam, J.B., 1992. Atmospheric pressure and gravity. Geophys. J. Int. 109, 488–500; Merriam, J.B., 1995. The atmospheric pressure correction in gravity at Cantley Quebec. In: Hsu, H.T. (Ed.), Proceedings of the 12th International Symposium on Earth Tides. Science Press, Beijing, pp. 161–168; Sun, H.-P., 1995. Static deformation and gravity changes at the earths surface due to the atmospheric pressure. PhD Thesis, Cath. Univ. Louvain, Belgium] and a local reduction coefficient. This combination is required due to the lack of a sufficient number of local barometric pressure data sets. After applying the physical correction on the gravity data, the local coefficient is determined in two ways: as a fit between station pressure and the partly pressure corrected gravity and as a theoretical value calculated with atmospheric Greens functions and station pressure. The efficiency of the corrections is compared by the results of tidal and spectral analyses as well as coherences between corrected, detided gravity and barometric pressure. In comparison to the standard correction with one regression coefficient, we get similar or slightly improved corrections (depending on the frequency) when applying a frequency-dependent admittance or a correction including atmospheric Greens functions. In general, we find no or only small differences in the corrections that emerge for short periods (periods≤2 days) and bigger changes in the long-periodic range (periods>2 days). We attribute the lacking improvement in the short periods to the partly too low sampling rate of the pressure data and the necessity to have more pressure data sets from the local area around the gravimeter available. Our studies of lateral pressure variations occurring in the local zone indicate a significant lateral pressure variability. A model calculation regarding the influence of these deviations on the pressure reduction yields the necessity of a better consideration of them in the correction. We conclude that one step towards an improved reduction of barometric pressure effects is to redo corrections that include atmospheric Greens functions in combination with regional hourly pressure data as well as data from a local pressure network surrounding the gravimeter location.

Modeling the hydrological effect on local gravity at Moxa, Germany

Abstract A superconducting gravimeter has observed with high accuracy (to within a few nm s−2) and high frequency (1 Hz) the temporal variations in the earth’s gravity field near Moxa, Germany, since 1999. Hourly gravity residuals are obtained by time averaging and correcting for earth tides, polar motion, barometric pressure variations, and instrumental drift. These gravity residuals are significantly affected by hydrological processes (interception, infiltration, surface runoff, and subsurface redistribution) in the vicinity of the observatory. In this study time series analysis and distributed hydrological modeling techniques are applied to understand the effect of these hydrological processes on observed gravity residuals. It is shown that the short-term response of gravity residuals to medium- to high-rainfall events can be efficiently modeled by means of a linear transfer function. This transfer function exhibits an oscillatory behavior that indicates fast redistribution of stored water in the upper...

Evaluating catchment-scale hydrological modeling by means of terrestrial gravity observations

residuals are derived by filtering and reducing for Earth tides, polar motion, barometric pressure variations, and instrumental drift. These gravity residuals show significant response to hydrological processes (precipitation, evaporation, surface and subsurface flow) in the catchment surrounding the observatory. We can thus consider the observed gravity change as an integrator of catchment-scale hydrological response (similar in nature as discharge measurements), and therefore use it to constrain catchment-scale hydrologic models. We test a set of simple water balance models against measured discharge, and employ observed gravity residuals to evaluate model parameters. Results indicate that a lumped water balance model for unsaturated storage and fluxes, coupled with a semidistributed hydraulic groundwater model for saturated storage and fluxes, successfully reproduces both gravity and discharge dynamics.

Hydrological signals in gravity — foe or friend?

Although hydrological effects on gravity are known nearly as long as the influence of barometric pressure, they are not as well understood as the latter. The improvement of gravity data quality during the last years adds weight to the importance of understanding the hydrological influence. Moxa observatory is one station at which studies regarding hydrological effects are carried out. From soil moisture, water level and meteorological observations the effects of different hydrological contributors including snow can be modelled and compared to the gravity residuals of the superconducting gravimeter (SG). The total peak-to-peak amplitude amounts to 35 nm/s2. Contributions from the various areas around the observatory partly compensate due to the hilly morphology. The comparison between residuals and computed total hydrological effect yields a good agreement, but also shows that not all hydrological influences have been taken into account. A significant additional hydrological influence is due to the hill flank near the SG.

Comparison of Superconducting Gravimeter and CHAMP Satellite Derived Temporal Gravity Variations

The operational Superconducting Gravimeter (SG) network can play an important role for validation of satellite-derived temporal gravity field variations. A comparison shows a quite good agreement between SG and CHAMP results within their estimated error bars. It could be proved that the SG-derived temporal gravity variations are representative for a large area within the µgal accuracy, if the local gravity effects are removed.. The long-periodic tidal waves are well determined by ground measurements, therefore they can be applied as a reference for validation. For further validation, field SG measurements should be carried out in representative areas with large gravity variations (e.g. Amazon area).