The International Gravity Commission (IGC) has fulfilled the duties through the actions of the members in the framework of the overall Commission activities and through the action of four Working Groups: WG-2 World Gravity Standards, WG-6 Intercomparison of Absolute Gravimeters, WG-7 Monitoring of Non Tidal Gravity Variations, WG-8 Relative Gravity network for 1997 Absolute Gravimeter Intercomparison.
According to the bylaws, the IGC has organised the General Assembly as a joint meeting with the International Geoid Commission. 113 participants have attended the meeting, which was held in Trieste from September 5 to September 12, 1998 and has strengthened the relationships between the two Commissions of Section III. To enlarge the attendance to the meetings of the two commissions, a midterm meeting has been co-sponsored as well. The meeting (Gravity, Geoid and Marine Geodesy) was held in Tokyo from September 30 to October 5 1996 and has been attended by about 200 participants from 27 countries. The idea to organise midterms meeting in different continents has proved to be positive since it allows a wider participation of scientists and will be continued also for the future. A second midterm meeting of the two commissions has been in fact already announced for the year 2000 in Canada.
Relevant progress has been made also in two regional gravity initiatives. The IGC has given the scientific support to the realisation of an international project aimed at the establishment of absolute gravity sites in many countries of Central Europe (Czech republic, Croatia, Estonia, Hungary, Latvia, Lettonia, Poland and Slovenia). Secondly, continuing the activities in training young scientists from Africa initiated several years ago, the IGC has supported a training course in gravimetry, held in Cairo (Egypt), which was well attended by scientists from Arab and African countries. Previous courses have been held in the Ivory Coast and Nigeria.
National representatives of member countries have reported ongoing activities. To summarise, the focus of the researches is on absolute gravimetry (establishment of reference sites) on absolute and superconducting gravimetry (to study time-dependent gravity variations induced by sea level variations, seismic and volcanic activities, ground level variations, post glacial rebound) and on national networks. A vast variety of regional gravity surveys has been reported as well.
The activities of WG-2 World Gravity Standards (Chairman G. Boedecker) have been particularly devoted to the establishment of an International Absolute Gravity Basestation network (IAGBN). The Network includes two subsets: IAGBN-A was intended as a closed set of stations for which 36 sites had been selected to support both the needs of reference sites and studies on global changes. For these reasons many of them were proposed for collocation at space geodetic sites. In the IAGBN catalogue 20 IAGBN-A stations have been already included. The remaining sites are situated in Siberia, Africa and on remote islands. Activities to promote the establishment of these sites are in progress. IAGBN-B has been created as an open collection of stations where absolute gravity has been observed that are not merely experimental observations but warrant a good standard. WG-2 has been also active in the field of national gravity base networks with assistance in the establishment of the national nets of Iran, Ghana and Egypt. WG-2 has also launched a "call for interest" for the adjustment of a Unified European Gravity Network, which is aimed at the unification of the national networks of European countries.
WG-6 Intercomparison of Absolute Gravimeters (L. Robertsson) has held the fifth international intercomparison in Sevres in autumn 1997. Fifteen absolute gravimeters participated. As the accuracy of absolute gravimeters is now starting to exceed the precision of relative gravimeters, the comparison was made by both a relative network and by having each absolute meter measure at multiple locations. A relevant contribution to the success of the intercomparison campaign has been given by the activity of WG-8 Relative Gravity network for 1997 Absolute Gravimeter Intercomparison (Chairman M. Becker). Twenty-five participants using twelve relative gravimeters made transfer and gradient measurements. A calibration line and the BKG moving platform were also used. The final results of both absolute and relative intercomparison have shown that absolute gravimetry is able to determine the gravity acceleration with an accuracy of 3.3 mgal.
WG-7 Monitoring of Non-Tidal Gravity Variations (Chairman
B. Richter) has presented a project entitled "a gravity network for ground
truth for satellite missions". The project targets Europe with an action
plan for the next two years to a) improve the network of superconducting
gravimeters in Europe; b) recompute the existing data sets in a uniform
manner; c) collect and analyse data sets from European Stations.