International Association of Geodesy

International Association of Geodesy
Inter-Commission Project: Global Geodynamics Project (GGP).
related to Commission 3 Earth Rotation and Geodynamics
Chair: David Crossley

Directorate
D. Crossley (Chair), J. Hinderer (Secretary).

Terms of Reference

The GGP project began on 1 July 1997 and Phase 1 ended on 1 July 2003. A continuation of the project, GGP Phase 2, was approved to continue until July 1, 2007. The main purpose of GGP was, and remains, to record the Earth's gravity field with high accuracy at a number of worldwide stations using superconducting gravimeters (SGs). An important requirement is the frequent monitoring of absolute gravity at each site to co-determine secular changes. Phase 2 envisages project in which SGs are deployed in regional arrays for limited time periods.

A list of publications related to GGP and SGs is available at the GGP website, as are a number of newsletters published for the benefit of the community. The main website is http://www.eas.slu.edu/GGP/ggphome.html.

The data is being used in an extensive set of studies of the Earth, ranging from global motions of the whole Earth such as the Chandler wobble to surficial gravity effects such as atmospheric pressure and groundwater. The SG stations are run independently by national groups of scientists who send data each month to the GGP database at the International Centre for Earth Tides (ICET) in Brussels.

GGP data is recorded and processed to standards agreed between the SG groups. For some of the GGP sites, the most recent data is temporarily restricted and will become available one or two years after collection. For other GGP sites, the data is available as soon as it has been sent to ICET, without restriction. Interested scientists can contact ICET, or the GGP website, for details. Useful site links and some technical terms involved in gravimetry are also on this site.

GGP has recently endorsed a joint operation between ICET and GFZ as a means of developing the database of SG measurements. ICET provides the front-end organization to which the data is sent, and GFZ provides the technical aspects of maintaining and developing the database. GGP will thereby contribute data to ICET, for as long as ICET remains a service of the Earth Tide Commission and FAGS.

Organization

The activities of the GGP are coordinated by a directorate consisting of a Chairman (Crossley) and Secretary (Hinderer). The directorate guides the members who are responsible for all aspects of the GGP such as setting the timetable for the project, setting standards for the data acquisition systems and data exchange protocols and recommending procedures for the database operations. The membership agrees to meet at least once a year, either independently of in conjunction with an appropriate scientific meeting.

Recent Evolution

GGP (the Global Geodynamics Project) is currently an inter-union, interdisciplinary project endorsed by SEDI (Study of the Earth’s Deep Interior). We have conducted a survey of its members on possible affiliation with IAG, as discussed at the GGP Business Meeting in Sapporro, Japan, on July 6, 2003. Only 15 of approximately 80 members on the mailing list replied to our survey. We interpret this response to indicate that the majority of members do not object to the ideas proposed in our survey.

The following items constitutes GGP Proposal to Commissions 2 and 3 of the IAG:

1. Identity. GGP considers itself to be an unrestricted international scientific project that also provides a service to the community. Therefore any affiliation with IAG needs to preserve these two aspects and to allow GGP to continue its current scientific and administrative structure.

2. Affiliation. GGP would like to be affiliated with IAG as an Inter-Commission project, on the understanding that the definition of project (IAG Bylaw 1.2.3) in no way limits the time period over which GGP can operate. We have voted to seek affiliation under the general scientific directives of both Commission 2 (the Gravity Field) and Commission 3 (Earth Rotation) because the mandate of GGP encompasses the terms of reference of both Commissions.

3. Reporting. GGP prefers a mechanism whereby it reports only to one commission, in this case Commission 3 (Earth Rotation), on the assumption that there will be close communication between the commissions on matters concerning GGP.

4. IGGOS. GGP would like to establish a membership within the IGGOS framework and to participate in that organization for the purpose of the exchange of worldwide gravity data.

Scientific Objectives

GGP monitors 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 were originally planned over a time span of 6 years at a small number of permanent observatories where a superconducting gravimeter (SG) had been installed. The 6-year period was chosen as the minimum length of data required to separate annual and 14 month Chandler wobble components in the gravity record. A pilot phase of GGP commenced 2 years earlier, in July 1995, so GGP is effectively in its 9’th year of operation.

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?

Benefits

The aims of GGP are twofold:

To reassure users of SG data that extreme care has been taken in the sampling and pre-processing of the available data and that all pre-processing steps and other site-specific information such as atmospheric pressure, environmental data and a record of all site disturbances are available to users, and

To enable global signals to be extracted by various stacking procedures that would not be possible with single station recordings.

Study Groups

The GGP is open to all organizations with access to the appropriate instrumentation, i.e. a SG. Each independent organization that manages a SG will be called a SG group; there may be several SG groups in any one country. SG groups seek their own sources of financing.

The GGP Agreements encourage SG groups to (a) upgrade existing SG facilities to a common standard of data acquisition, (b) participate in continuous gravity observations by maintaining the SGs in good operating conditions at fixed locations and (c) exchange raw gravity (and other important supplemental) data through the Internet.

Global Data Acquisition and Distribution

As described later, the scientific goals of the GGP include a wide range of signals from periods of seconds to years, covering 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.

Many of the Earth parameters of critical interest in global dynamics exist in gravimetric signals at or below the ambient noise level. Examples include internal gravity waves in the fluid core and post-glacial uplift and plate motions. The SG has a frequency-domain sensitivity at the nanogal level and many periodic signals of interest are expected to be in this range. Because the time-domain variability of 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 until confirmed with similar signals from other instruments. For many purposes, these instruments must be distributed widely around the Earth because global gravimetric signals have theoretically predictable spatial and temporal global variations.

Access to worldwide gravimetric data is essential for progress in global geodynamics for several reasons. First, SG data can be used to recover long-period free oscillations of the Earth with unprecedented precision. In real time, an array of SG instruments, as considered in GGP Phase 2, would provide a means for detection of slow and silent earthquakes, co-seismic slip, and tectonic signals. 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 that 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. GGP is striving to make such data available as rapidly as possible to the scientific community, so that all the above tasks can be accomplished.

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 stability of SGs is variable, with the best instruments having instrument drift as low as about 1 microgal per year. Wherever this level of stability can be maintained by even a small number of SG stations in a regional network, particularly where confirmed with absolute gravimeters, then GGP will provide useful data for these tectonic problems.

In the past an individual with access to his/her local instrument and a computer could make major progress in the solution of both analytical and data analysis problems. However, the complexity of many 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. GGP has responded to this need by agreeing to a monthly transfer of data from all instruments to the ICET / GFZ database in Brussels. This data represents the success of the overall project.

Specific Tasks of GGP

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.

1. Earth tides and the nearly diurnal free wobble: the estimation of precise tidal parameters (e.g. gravitational delta factors) can contribute to the development of better models for correcting for ocean loading phenomena. In addition, the stacking of global delta factors provides important information on the diurnal free wobble of the Earth which is essential for theoretical work on the structure of the Earth's core.
2. Core modes: the search for internal gravity waves in the Earth's liquid core necessitates global, long-period, long-duration recordings to separate local gravity variations from a global coherent signal. If we are able to detect these waves, this will give direct information on the mechanical equilibrium of the fluid in the core, and thus information on the operation of the geodynamo.
3. Atmospheric interactions: stacking global gravity and pressure data is essential to clarify the nature of the long period phenomena in the atmosphere and for evaluating the effects of global atmospheric surface pressure and mass redistribution on the Earth's gravity field.
4. Hydrology: it has become clear during the first phase of GGP that rainfall, soil moisture, snow cover, and groundwater variations can all affect local gravity. The study of hydrology is therefore a fruitful area for GGP. With the advent of new satellite missions (CHAMP, GRACE, GOCE) it is possible to look at common signals in both ground and satellite data sets, particularly using the European stations of the GGP array.
5. Earth rotation and polar motion: the measurement of the gravity effect of polar motion (orientation of the Earth's rotation axis) requires a global coverage of stations. It should be possible to continuously monitor the location of the rotation pole on the time scale of minutes and therefore provide an independent verification of the same measurement now made with space techniques; connections with the International Earth Rotation Service (IERS) service here will be valuable.
6. Gravity changes due to tectonic motions: the monitoring of long-term changes due to tectonic motions, sea-level changes affecting the survival of coastal cities, post-glacial uplift and the deformation associated with active tectonic events.
7. Enhancing absolute gravity measurements: SGs are a valuable aid to international programs for the determination of absolute gravity values on a global scale as they provide a short-term, relative gravity reference level and they ‘fill in’ the gravity field behavior between AG measurements.
8. General research tool: GGP provides a high quality, continuous global data set will be a valuable resource for future geodetic and geophysical studies that involve the Earth's gravity.

GGP and Geodesy

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 that contain very high quality instrumentation. There are two primary areas in common between GGP and other geodetic programs:

1. Space techniques. Two space techniques that require detailed models of Earth deformation are satellite tracking and Very Long Baseline Interferometry (VLBI). At the proposed sub-centimeter level of accuracy, for projects in the 1990's such as the current Satellite Laser Ranging (SLR) and the proposed Geodynamics Laser Ranging System (GLRS) mission, precise knowledge of the Earth's dynamics, including resonances in the liquid core, are required. A global net of SGs will give the required information on dynamics of the liquid core.

2. Sea level changes. A satisfactory solution to the problem of defining the origins of sea-level changes requires input from different sources. The necessity of differentiating between the effects of height variations caused by post glacial rebound or plate tectonics and changes in sea level resulting from global warming demands the establishment of a global geodetic/geophysical observatory network, such as FLINN (Fiducial Laboratories for an International Natural Science Network), an IUGG-sponsored project initiated at the Coolfront Workshop in 1989. A central feature of such a network is the monitoring of the gravity field at a smaller group of fiducial stations equipped with SGs as well as precise positioning instrumentation (e.g. SLR, VLBI or GPS) and having accurate connections to the reference tide gauges.