Studies of Terrestrial Gravity Using Superconducting Gravimeters

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This project consists of two separate studies using high precision gravity data collected by the network of superconducting gravimeters (Global Geodynamics Project, or GGP). The first is a comparison between the high precision monthly gravity field that is now available from the GRACE (Gravity Recovery and Climate Experiment) satellite mission and the 7 ground-based GGP stations in Europe. The intellectual merit is in using for the first time both data sets, even though they differ considerably in spatial and temporal resolution, to identify the time variation of the hydrology over central Europe. Studies done by the author and his colleagues have demonstrated that both data sets have a common signal that appears to correlate well with the predicted moisture content of the ground. This correlation will be further explored to determine where the signal comes from and to what extent the ground stations are able to validate the satellite-based models. This is a unique study because of the independence of the two observational techniques. The broader impact of the GRACE experiment is that it will provide extremely useful data for a wide variety of geophysical problems such as the accurate assessment of continental water storage for long-term planning of global water resources.The second study will use all the GGP data (20 stations), over the last 7 years, to search for several signals from the Earth's deep interior, particularly the Slichter triplet. This triplet arises from the oscillations of the Earth's inner core (IC) within the fluid outer core, when excited by an energetic source such as an earthquake; the splitting into 3 modes comes primarily from Earth rotation. The motions are extremely small - about one mm or less, but in principle detectable using the GGP gravity array. The intellectual merit of the study is to find the density jump between the inner and outer cores at the boundary (ICB). This jump is crucial for assessing the energetics to the Earth's magnetic field generation, i.e. how much energy can come from the latent heat of freezing of the IC as it grows in time. Recent estimates of the ICB density jump have varied between 0.3 and 0.8 Mg m-3, using methods such as differential scattering of seismic body waves from the ICB and from the frequencies of the normal modes of the Earth. The broader impact of this project is in graduate education, through student support for 3 years. This new study will strengthen considerably the graduate student program at St. Louis University.