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Brief Summary of the GGP
Brief Summary of the GGP
The Global Geodynamics Project (GGP) is a proposal to monitor changes in the Earth's gravity field at periods of seconds and longer. The GGP is named to indicate the application of gravity data to the solution of a number of geodynamic problems; additionally GGP may become a source for absolute gravimeter data as well as other geodynamic data.
The measurements will be taken over a time span of 6 years at a small number of permanent observatories where there is a superconducting gravimeter (SG) currently installed. The 6-year period has been chosen as the minimum length of data required to separate annual and 14 month Chandler wobble components in the gravity record. The Observation Period will commence July 1995.
The SG has been, for the past two decades, the most sensitive, stable instrument for the measurement of the vertical component of the Earth's gravity field. Each of the currently operating SGs is the focus of a national effort to provide a continuous gravity record for geodetic and geophysical research. The GGP is an opportunity for the various SG groups to participate in a global campaign to monitor the gravity field and to exchange the raw data.
Precise global measurements of the Earth's gravity field are essential to answer a number of important questions in geophysics, which we outline in more detail in the next section (a) do internal gravity waves (inertial waves if the fluid is neutrally stratified) exist in the Earth's liquid core and are their gravitational effects at the Earth's surface detectable ? (b) what is the gravity effect of the global atmospheric loading and mass re-distribution on the solid Earth ? (c) through global tidal analysis, can we refine estimates of the nearly diurnal free wobble of the Earth and models of oceanic loading on the solid Earth ? (d) what changes in gravity are associated with slow and silent earthquakes, tectonic motions, sea-level changes and post-glacial rebound ? (e) can we monitor the location of the rotation pole of the Earth on a time scale of minutes ? (f) can SG recordings of the earth's normal modes enhance the global long period seismic and spring gravimeter networks ?
The benefits of GGP are twofold :
The GGP is open to all organisations with access to the appropriate instrumentation, i.e. a SG. Each independent organisation that manages a SG will be called a SG group; there may be several SG groups in any one country.
The GGP Agreements encourage SG groups to (a) upgrade existing SG facilities to a common standard of data acquisition, (b) participate in the 6-year observation period by maintaining the SGs in good operating conditions at fixed locations and (c) exchange raw gravity (and other important supplemental) data through high speed computer links.
Funding for GGP activities is the financial responsibility of the supporting government organisations for each of the national SG groups.
It is not the intention of the GGP to restrict in any way the provision of SG data between individual SG groups and potential users, either individual or collective. Each SG group is free to provide its data as it wishes in response to normal requests. However GGP Data as a whole is available on a restricted basis to protect the investment in time and effort that SG groups have made to acquire the data and pre-process it to GGP standards.
The GGP will include an International Data Centre from which all the SG data can be accessed according to the agreements reached between participating SG Groups. The Japanese SG Groups will establish a Japan GGP Sub-Centre to exchange data with the International Data Centre. SG data will be restricted to GGP participants for the first year following collection; thereafter the data will be placed in the public domain for general access.
Let the GGP participants be in no doubt that when GGP begins, some significant changes will occur in the pace with which SG data is processed. Each group will have a one year grace period in which to process its data and make it available to other GGP participants. Then there will be a further year for the various groups to collaborate in publishing the results of the data exchange. After two years the GGP data will be available to the scientific community in general. We are convinced this agenda will stimulate a more active dissemination of this important data and that the various SG groups can rise to the challenge of preparing the data.
The activities of the GGP are coordinated by a Steering Committee with a Chairman (Crossley) and Secretary (Hinderer). The Steering Committee is responsible for all aspects of the GGP such as creation of sub-committees, setting the timetable for the project, setting standards for the data acquisition systems and data exchange protocols and coordinating activities of the Data Centre. The Steering Committee agrees to meet at least once a year, in person, preferably in conjunction with an appropriate scientific meeting.
The GGP has been endorsed by the IUGG Inter-Union Project SEDI (Study of the Earth's Deep Interior) and has been presented to the IAG. We will also present the proposal to IASPEI.
The scientific goals of the GGP include seismic, geodetic and geophysical components that range in frequency content from seismic normal modes, tides, core modes and wobble modes of the Earth to other long period variations in Earth's gravity field such as tectonic deformation (Fig. 1).
Many of the Earth parameters of critical interest in global dynamics are recoverable from gravimetric signals at or below the ambient noise level. This includes internal gravity waves in the fluid core, post-glacial uplift and plate motions. The SG has a stated sensitivity at the nanogal level and most signals of interest are expected to be in this range. Since the ambient gravity noise is usually two to three orders of magnitude greater than this, global signals identified on the record of an individual instrument at the nanogal level cannot be considered reliable unless and until confirmed with similar signals from other instruments. Furthermore, these instruments must be distributed widely around the Earth because global gravimetric signals have theoretically predictable spatial and temporal global variations.
Rapid access to worldwide gravimetric data is essential for progress in global geodynamics for several reasons. First, high resolution gravimetric data makes it possible to measure free oscillations of the Earth with unprecedented precision. In real-time, an array of SG instruments would provide a means for global detection of `silent earthquakes' (Beroza & Jordan, 1990). Second, sub-milliarcsecond orientation can be obtained through measurement by an SG network for space-based measurements such as Satellite Laser Ranging and the US-proposed GLRS project to position points on the Earth's surface to the sub-centimeter level through the use of Earth-based retro-reflectors and satellite-based lasers. Third, Earth models which incorporate core resonances require access to gravimetric data at the nanogal level to successfully account for motion in the deep interior in all of the orientation calculations. The proposed network of instruments and communication makes access to gravimetric data sufficiently rapid that all the above tasks can be accomplished.
Lastly, a number of tectonics-related problems require global gravity field data for their resolution. In particular the problems of long-term secular changes in elevation, caused not only by post-glacial rebound and sea level changes but also by active plate-tectonic related deformation, need long-term gravity variations at continental scales. The long period behavior of SGs is currently very variable, with the best instruments virtually drift-free at the level of about 1 microgal per year. If this level of stability can be maintained by even a small number of SG stations in the global network, particularly where confirmed with Absolute Gravimeters, then GGP will provide useful data for these tectonic problems. At shorter periods, the signatures of earthquake pre-cursor activity, the search for slow earthquakes and the precise monitoring of the normal modes following large earthquakes are all tasks for which the SG is particularly suited.
In the past an individual with access to a computing service could make major progress in the solution of both analytical and data analysis problems. However, the complexity of presently extant problems in global geodynamics is such that measurements on a global scale are needed to make even minimum progress. Concerted effort by cooperating scientists is needed to make any significant advance. Without uniform high precision global data it will be impossible to move toward the solution of the problems of the Earth's deep interior. High quality continuous vertical gravity values need to have global coverage to answer a number of important geophysical questions. Due to the extremely weak gravity signals (some at the nanogal level) associated with many of these phenomena, it is essential that very high sensitivity ground based observations using SGs be undertaken.
The SG is capable of recording temporal gravity variations from seconds to years and thus the GGP has application to large number of scientific tasks. As indicated above, at long periods (months - years), we highly recommend the use of a SG supplemented by an Absolute Gravimeter to fully characterize secular trends in gravity.
There are important connections between the above goals and other scientific programs of national concern. In particular, the geodetic community clearly recognizes the importance of simultaneous geodetic (positional) information and gravity changes at fiducial stations which contain very high quality instrumentation. There are two primary connections :
Additionally, GGP will be used for valuable local studies: