Preface to the Global Geodynamics Project

I review some features of the Global Geodynamics Project during its initial phase of operation 1997–2003. These fall into three categories: (1) instrumental—the development of the superconducting gravimeters during this time; (2) organizational—how the various SG groups have come together to improve their installations and provide their data; and (3) scientific—what have been some of the notable observational achievements of the project. This overview is then followed by a brief description of some of the new tasks for GGP Phase 2 (2003–2007). © 2004 Elsevier Ltd. All rights reserved. 1. Historical introduction The Global Geodynamics Project (GGP) was conceived during the same time that Study of the Earth’s Deep Interior (SEDI) was being planned at the ISECALM (initial acronym for SEDI) Workshop in Trieste, 1986. At that time a number of different areas of geophysics came together to initiate a cooperative and concerted effort to make progress on problems of the deep interior. Melchior11 spoke of the recent improvement between spring gravimeters and superconducting gravimeters (SGs) and Smylie discussed the possibility of detecting core dynamics using the SGs. Smylie and Crossley carried the enthusiasm of the workshop back to Canada where eventually the theoretical geodynamics group (Aldridge, Rochester, Mansinha, Merriam and others) decided to acquire one of the ‘new’ SGs. The instrument was funded in 1987 and installed in Cantley, Quebec, in 1989. Subsequently the Canadians put forth a proposal for the ∗ Tel.: +1 314 977 3131. E-mail address: [email protected]. 1 Sadly, Paul Melchior passed away recently. 0264-3707/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2004.07.002 226 D. Crossley / Journal of Geodynamics 38 (2004) 225–236 existing worldwide SG stations to form an international project (GGP) to cooperate on data collection and exchange (Aldridge et al., 1991). At that time there were 11 instruments operating, all of them the original full-size TT models: one each in Strasbourg, Wettzell, Brussels, and Wuhan, four in Japan, two in Richmond, Florida (including one of John Goodkind’s UCSD instruments) and the Cantley instrument. In 1990, at the Walferdange Workshop on non-tidal gravity changes—intercomparison between absolute and superconducting gravimeters, we noted “. . . despite all good intentions, data sets of substantial length and quality have rarely been obtained in the 9 years of recording” (Aldridge et al., 1991). The reasons for this were varied, but up to then gravimeter data had been collected almost entirely for local experiments and projects and record continuity was of secondary importance. In our first GGP proposal, we used the term global geodynamics to signify that the signals we (the Canadians) were most interested in were global in nature, i.e. oscillations of the inner core and within the liquid core, polar motion and wobbles. The scope of the final proposal included many more projects than the deep Earth, spanning a whole range of deformations from seconds to decades. It was of course recognized that the measurement of gravity at a single location is dominated by the Earth tides and includes local (and regional) environmental effects such as the atmosphere and hydrology. Between 1991 and 1997 there were extensive discussions within the Earth tides community on how to fulfill the aims of the original proposal, and some of the original ideas (such as a network of SGs within Canada and a data center in Canada) did not survive. Nevertheless the international community was able to agree on the main points of the project and engage in a pilot project to test the data exchange formats. In 1987 GGP was endorsed by SEDI as one of its ‘projects’, and as such GGP came under the umbrella of the IUGG in a loose way. The official start of GGP was not to be until 1 July 1997, with about 17 instruments participating, and the subsequent history of the project is well documented in the series of 13 GGP Newsletters (http://www.eas.slu.edu/GGP/ggpnews.html). Since 1997 the progress of GGP has been steady and productive, and in mid-2003 GGP began a second phase of continuing observations that will last up to the next IUGG in 2007. Notable among the recent developments was the incorporation of GGP into the structure of the International Association of Geodesy (IAG); GGP is officially as an inter-Commission project, responding to Commission 3 (Earth Rotation and Geodynamics) and Commission 2 (The Gravity Field). In the future GGP will become part of integrated global geodetic observing system (IGGOS). Another point of interest is that the Japanese component of GGP has developed into a strong sub-organization with a GGP Japan Center at the National Astronomical Observatory in Mizusawa, administered in cooperation with the Ocean Research Institute, Tokyo.