Workshop and WG minutes - St. Petersburg, Russia
ArcGP – Arctic Gravity Project
International Association of Geodesy – International Gravity and Geoid Commission
Minutes of Workshop and WG Meeting, St. Petersburg, Russia, June 7-8, 2000
The 2nd working group meeting of the IAG-IGGC Working Group "Arctic Gravity Project" was held at Znamenka, outside St. Petersburg, Russia, on June 7-8, 2000, as part of an "Arctic Science Week". The meeting was arranged by VNIIOkeangeologia, Ministery of National Ressources, St. Petersburg. Local organizing committee was headed by A. Zayonchek of VNIIO.
The meeting was split in two parts: A workshop on the polar gravity field (June 7), and a business meeting of ArcGP (June 8). During the workshop the data status of different countries and projects in the Arctic region were highlighted, data quality discussed for various methods for gravity surveys, and a brief of some Antarctic activities was also included.
- Report on workshop day, June 7
- Minutes of the ArcGP business meeting, June 8
The meeting was opened by a welcome address by V. Kaminsky, Deputy Director of VNIIO.
The chairman of ArcGP, R. Forsberg, then opened the meeting by giving a brief overview of the background of the Arctic Gravity Project. The ArcGP was initiated based on discussions held at meetings in Celle, Germany, and Kangerlussuaq, Greenland, 1998. ArcGP has at the IUGG 1999 General Assembly in Birmingham, England, been adopted by International Association of Geodesy as an official Working Group of the International Gravity and Geoid Commission. The rationale for the ArcGP is especially the developments in global gravity field mapping, where new satellite missions such as CHAMP, GRACE and GOCE in the coming years will improve the global gravity field significantly, except for the polar regions due to orbit restrictions. But also improved insight into the tectonics, geodynamics, and geoid of the Arctic Ocean region will be an important outcome of ArcGP.
The president of the International Geoid and Gravity Commission, M. Vermeer, then gave welcoming remarks on behalf of IAG and IGGC, and outlined the basic structure of IAG and the links to ArcGP. The ArcGP is in status similar to the IGGC regional subcommissions, albeit time-limited. He stressed that interest in ArcGP is both geodetical and geophysical, and that geodetic services like International Geoid Service and Bureau Gravimetrique will benefit from this initiative. He also welcomed a similar initiative for Antarctica.
The scientific contribution included the following presentations:
EGM96 global gravity field model and ArcGP data status.
Outlined the current status of the joint NGA/NASA/Ohio State Univ. Model EGM96, a global sherical harmonic expansion of the geopotential to degree 360. Gave then an overview of the current data status for data constributed to the ArcGP, and some comparisons of different data sets.
G. Demianov: GAO98 global gravity field model and Russian geodetic activities.
A main goal is to construct a precise geoid. Current global Russian spherical harmonic model is GAO98, based a.o. on gravity data from TSNIIGAIK and the GEODAS CD-ROM global gravity data, satellite altimetry and satellite orbit models to degree and order 60. A GPS/Glonass satellite geodesy network in Russia is being established to support global activities and provide basic geodetic infrastructure for Russia. Differences between PZ-90 (Glonass) and WGS84 (GPS) coordinates were illustrated. ArcGP will improve Russian geoid and solve important geodetic problems related to geoid determination. GPS/levelling will be the means to compare geoid models.
S. Maschenkov: Regional tectonic interpretation of gravity and other geodata in the Arctic.
Showed Arctic Ocean gravity compilation made from VNIIO data, Canadian and NRL data sets, as well as similar bathymetric and magnetic compilations. All fields highlight the main features of the Arctic Ocean such as Gakkel and Lomonossov Ridges. Showed depth to Moho estimate based on 3-D gravity modelling and seismic data. Showed data on extension of continental shelf up to Lomonossov ridge and showed profile examples in Canada Basin and north of Russia. Concluded an the Arctic gravity compilation project is important for understanding geologic and tectonic structure.
V. Childers: NRL aerogravity program 1992-1999.
The US Naval Research Lab has, under the leadership of J. Brozena, carried out airborne gravity measurements over the high Arctic since 1992, and over Greenland 1991-92, using long-range P-3 aircraft, equipped with numerous GPS receivers and 2 gravimeters. Major campaigns have included the Canada and Amerasian Basins. Operations have been based out of Alaska, Greenland and – since 1998 – Svalbard, in cooperation with other US and foreign scientific groups. Over the years quality of surveys has improved, primarily due to advances in GPS technology and processing. Greenland was flown 1991-92 as a high-altitude survey at 4100 m elev. The Arctic has been flown since 1992 as low-level flights with 10 n.m. line spacing. Internal cross-over errors have decreased from 4 mGal for the oldest surveys to ca. 2 mGal since 1996. Results have shown new tectonic features, such as extinct spreading axis in Canada Basin. Comparisons to satellite altimetry were around 3 mGal r.m.s. in smooth areas, 7.5 mGal in rough areas, resolution somewhat higher for airborne gravity. Raised question of a common way to define resolution.
V. O. Leonov: Russian aerogeophysical surveys of Frans Josef Land region and Antarctica.
Outlined Russian airborne activities of "Polar Marine Geological Expeditions" in Arctic and Antarctica. Airborne mapping program is based on IL-38 aircraft system with aerogravity, aeromagnetism and radio-echo sounding channels. Navigation is by INS, radionavigation/Doppler radar and altimeter/barometer. GPS MX-4400 recently installed. Horizontal navigation accuracy better than 100 m. Gravimeter is of vibrating-string type. Standard deviation of measurements is 4-6 mGal, in future 3-3.5 mGal. Magnetic results better than 5 nT. Surveys over Frans Josef Land region in three campaigns since 1993, flying out of Murmansk. Line pattern of 5 km spacing, 20 km cross-lines were flown. Operational costs of aircraft around 2000 USD/hr. Illustrated results by anomaly plots and explained tectonic interpretation/structure. Data to be made available to ArcGP on same conditions as other Russian data.
- Antarctica: Surveys since 1989 in the Weddel Sea, Queen Maud Land, and Enderby Land. Programme suspended at the moment due to lack of funds. Line spacing 20 km, accuracy 4-7 mGal, slightly higher in the interior due to lack of satellites.
B. Coakley: SCICEX submarine geophysics and bathymetry programme.
The US Navy has made a nuclear-powered submarine available for yearly scientific cruises of the SCICEX programme since 1993. More than 100,000 km of narrow-beam bathymetry and gravity have been collected. Since 1998 wide-swath bathymetry has been collected as well. SCICEX has collected data outside 200 nm limits except north of Alaska and Svalbard. Submarine traverses at 400-750 ft depth at 15 knots. Gravity is measured with a Bell gravimeter and reduced to surface, employing a 2 min filter to the data. Vertical accelerations are corrected from depth sensor information. Some errors are expected due to errors in navigation (submersion periods up to 2 weeks give INS drift). R.m.s. cross-over errors are at 2 mGal. Base ties made in origin ports (Hawaii, England, east coast US). Examples were shown of side-scan bathymetry and gravity from recent surveys (Chukchi Borderlands, Alaska Shelf, Lomonossov and Gakkel Ridges, Yermak Plateau). SCICEX programme is now suspended, but negotiations are underway with the US Navy for more cruises.
M. Veronneau: Geoid modelling of the Arctic Region.
Explained gravity status of Canadian territory. National gravity program with spacing up to 10 km completed except for a few small voids (Foxe Basin, Great Bear Lake). GSD has contributed 105,000 surface measurements and 9,500 airborne data (1998 PMAP survey north of Ellesmere Island) for ArcGP. Showed complete free-air anomaly map for North America including Greenland as part of effort to make joint North American geoid under IGGC Subcommission on North American Geoid. ArcGP data will help this project. Showed results of geoid computations and compared to other geoid models of US and Greenland, and compared geoid accuracy to data from GPS/levelling over various areas.
R. Forsberg: Greenland aerogravity project 1998-2000.
KMS has operated an airborne gravity system in a Twin-Otter since 1998, as a spin-off from the EU project AGMASCO. System is based on custom-developed INS, laser altimetry, GPS and a Lacoste and Romberg gravimeter, owned by University of Bergen, Norway. Survey program 1998-2000 to map coastal areas around Greenland, with support from NGA. The program complements earlier NRL airborne measurements of the interior of Greenland and recent surface measurements of the ice-free parts. Cross-overs with independent ship and ice surface data indicate performance at 2 mGal r.m.s. and resolution of 6 km at airspeed 130 knots, with no bias offsets, making data very well suited for geoid computations and check of older marine data. Showed improvements of new airborne data on the geoid in region north of Greenland.
D. Solheim: Svalbard gravity project.
The Norwegian Mapping Authority has made airborne gravity measurements around Svalbard 1998-99 (SAG98 and SAG99), to complete the coverage over land and the surrounding shelf regions. Land gravity program suspended since 1986 due to fatal helicopter crash. New airborne measurements have been made with support from Norsk Hydro, Oliedirektoratet, and KMS/NGA. The same system setup and airplane were used as in the KMS Greenland airborne survey. Russian participants joined 1999 flights in ocean region between Frans Josef Land and Svalbard. Comparisons of airborne to marine gravity data are good (2.6 mGal r.m.s.), over land large discrepancies between surface and airborne data exist due to problems in upward continuation of point data in rugged topography with glaciers of unknown thickness, airborne data could be inverted to infer glacial thicknesses. Compiled data set of land and downward continued airborne data to be made available to ArcGP.
T. Boebel: AWI airborne geophysics in the Arctic and Antarctic 1997-1999.
Described AWI aerogravity and aeromag campaign in the Arctic since 1997. Surveys are based on AWI Do-228 aircraft, aerogravity system spin-off from AGMASCO project, using own S-56 LCR gravimeter. Measurements made in Fram Strait region between Greenland and Svalbard, operating from Longyearbyen and Station Nord. Primary purpose of measurements to map plate boundary features between Spitsbergen and Greenland. Showed free-air plots from 1997-98 showing Gakkel Ridge anomaly, and corresponding bathymetry and magnetics. AWI also makes shipboard gravity in region by icebreaker Polarstern.
- Antarctica: EMAGE aerogeophysical program 1996-2000..2003 to map offshore gravity and magnetics on the shelf in region around German Neymeyer station at a spacing down to 5 n.m.. Scientific objectives are to study the type of continental margin and spreading anomalies. Aerogeophysics data to be collected along with support for EPICA European drilling project in Queen Maud Land, 1999-2004. New project WEGA proposed to map larger parts of East Antarctica 2003-?. AWI activities will complement existing Russian data, AWI has these data.
M. Vermeer: Gravity surveys of Finland and the Baltic Sea.
Showed Finnish gravity coverage. Finland has very dense coverage of gravity (2-3 km spacing) over all of its national territory. A subset of these data at 5 km resolution provided for ArcGP. Data include winter ice measurements off the coasts. Joint Nordic airborne survey using KMS Twin-Otter system has completed coverage of Baltic Sea. Airborne data versus recent ship data showed 1.8 mGal r.m.s. difference. Data to be used for updated geoid model of Nordic region.
R. Forsberg: Gravity from satellite altimetry over the Arctic Ocean – status and future possibilities.
The principle of gravity recovery from satellite altimetry was outlined. Advances in retracking of satellite altimetry waveforms have made data more exact. On behalf of S. Laxon, UCL and D. McAdoo, NOAA, recent ERS ice-satellite altimetry results from the Arctic Ocean south of 81N were presented, included a detailed gravity map of the Russian shelf. Two other solutions covering the Arctic Ocean are the solution by BGI, France (G. Balmino and M. Sarrailh), based on two retracked ERS-1 repeats only, and the KMS99 field of O. Andersen, which is based on non-retracked ocean-mode ERS-1 and ERS-2 data 1993-99, giving many return in leads and over thin ice in the Arctic. Comparisons over ice-free areas show r.m.s. gravity errors of 5-8 mGal, and 5-15 mGal for comparisons to ice-breaker data off NE Greenland. KMS99 seem to be best in open ocean region, UCL/NOAA most recent field best over ice. Comparisons of this field and NRL data in the Canada basin have given fits at 3-4 mGal r.m.s. Gravity from satellite altimetry up to 88N will be possible using new satellite missions: The NASA ICESAT/GLAS laser altimetry mission, to be launched 2001, and ESA’s CRYOSAT radar mission 2003. With these data all current data voids in the Arctic will be covered.
A. Zayonchek: Integrated Interpretation of Gravity and Other Geodata in the North
Eurasian Shelf for Russian State Regional Geological Mapping Program.
Showed plots of the Bouguer gravity and magnetic anomaly for region north of Russia, to study structure of continental shelf. Complex geophysical models shown along seismic profiles. Showed estimated depth to Moho and tectonic scheme/zonation map, and showed geophysical model along Gakkel Ridge seismic line collected by BGR.
R. Macnabb: The new International Bathymetric Chart of the Arctic Ocean – building a DEM for
calculating Bouguer corrections over ocean and continent.
Outlined current status for a new grid of Arctic Bathymetry. Primary purpose to make new GEBCO map, to replace existing arctic bathymetry map GEBCO sheet 5.17 from 1979, based on very limited data. 11 organizations from 8 countries involved in geographically allocated data collections and grid preparation. Significant new data from SCICEX and older US/British submarine cruises as well as from Russian maps. New map product developed on GIS system at University of Stockholm (M. Jakobsson), based on internal 2.5 x 2.5 km grid. A prototype "beta-version" map displayed. Plans for 2000+ to incorporate additional data sets and complete project documentation prior to formal distribution of grid. Data well suited for ArcGP to convert from Free-air to Bouguer anomalies, and grid manipulation and visualisation tools already set up to handle this.
The chairman closed the session of workshop session by thanking the presenters, and gave a special thanks to the hard work of the professional simultaneous Russian/English translators.
- Status of data submitted to date
- Comparisons of different data sources
- Strategy for data merging of data sets with highly different noise characteristics
- Grid specification issues, final formats
- Strategy for obtaining missing data
- Time schedule
- Publication policy of joint papers and reports
- Web site issues and data distribution
- Auxillary grid issues: Bouguer anomalies, ice thickness DEMs
The agenda for the business meeting was:
The business meeting started with an outline of the ArcGP goal: To collect the necessary data to compile a public-domain 5’ free-air gravity grid for the area north of 64N by 2001. Original data are not to be made available, to avoid problems with national secrecy requirements. A website and ftp box have been set up at the ArcGP processing center at NGA. Parts of the contributed data also reside at KMS, notably for comparisons and data merging method development.
Status of submitted data:
National agency contributions:
Finland – OK, 2.5’ data for making 5’ mean values provided.
Sweden – OK, 5’ grid data computed at KMS from data in joint Nordic geoid data base.
Iceland – OK, original data provided to KMS.
Norway – OK, data to be provided, problems with downward continuation over Svalbard.
Greenland – August 2000 survey along coast to be included, new downward continued 5’ data grid available late 2000. Otherwise all data available.
Canada – point data provided to ArcGP web site, need to redo recent Ellesmere Island survey to update processing of orthometric heights.
Alaska – covered from NGA archives, including USGS helicopter surveys.
Russia – collected marine Arctic data 1960-1992 through aircraft-supported ice surveys. Data are high-quality point measurments using 3 gravimeters and 2-3 base stations, positioned by differential navigation, lately satellite navigation, navigation accuracy 50 m to 1 km. Colocated with depth measurements. Some limited submarine data also exist. Most surveys have been carried out by Russian Navy (HDNO/GUNIO), with additional activities by ministeries. Land data for continental Russia exist throughout, and 10’ mean data for European Russia have already been provided for ArcGP by G. Demianov.
Rear-admiral A. Makorta, HDNO stressed the monumental effort involved in the many decades of Russian gravity work across the entire Arctic Ocean basin. Admiral Makorta offered that HDNO would make available their data to compile the final ArcGP grid product, and at the same time incorporate all western available gravity data into the product. HDNO would in addition to compiling the final ArcGP grid product also be willing to print the final gravity map.
The chairman (R. Forsberg) thanked A. Makorta for the offer to take over the ArcGP processing and welcomed the initiative, which would be a major step forward in the cooperation between Russia and the international scientific community on gravity field matters. The chairman suggested that with the well-advanced progress of ArcGP, the current grid product be finished at NGA/KMS in the short term, by incorporating available 5’ data in the last data voids. This interim "beta-version" grid could then be transferred to HDNO for comparisons with the superior-quality original russian data, especially to quantify errors in satellite altimetry and airborne gravity, and to isolate possible problems in base ties with SCICEX submarine data. Updated original data (or compiled subgrids) could then be transferred to HDNO for production of the final ArcGP grid and map. The scheme would be subject to negotiations with the major data holders, notably NGA.
The status of other major data providers:
Alfred Wegener Institute, Germany – four seasons of airborne gravity available, 1997-98 already provided, 1999-2000 to be processed this fall. Polarstern data also provided.
SCICEX data – all data up to 1998 have been provided through ArcGP ftp box, 1999 data have some small residual problems. B. Coakley would reprocess the last data and forward to ArcGP soon. He would also make an effort to check base ties of earlier submitted data sets.
Naval Research Lab – all Arctic Ocean data 1992-99 provided, Greenland 1991-92 data to be downward continued and merged with surface data. With current improvements of NRL processing, the 1991-92 Greenland airborne data could be improved by reprocessing. No decision has been taken on this issue yet.
Other data sources discussed were Sweden (plans cruise to North Pole with icebreaker "Oden" 2001. Might be too late to incorporate gravimeter onboard, and get data to ArcGP), incorporation and check of historical ice-island data (e.g, T-3, Arlis and Fram).
Comparisons of different data sets
R. Forsberg presented a number of comparisons between SCICEX, NRL, Danish/Norwegian, German and airborne Russian (PMGE) gravity data in the Greenland-Svalbard-Frans Josef Land region, and around the North Pole. Surface data (high-quality marine gravimetry, Lincoln Sea ice data and LOREX ice-camp data) show good agreement to NRL and other airborne data, with no major bias differences (< 2 mGal) and r.m.s. differences mostly in range 2-4 mGal. PMGE and KMS data compare well (4 mGal r.m.s.) in overlap region west of Frans Josef Land, when transforming Russian gravity reference system to absolute/GRS80 western system using transformation parameters provided by Tsniigaik. Some problems were encountered with SCICEX data, suspected to be due to base tie errors.
Stategy for merging different data sources
The method of least squares collocation/Kringing was discussed, and generally agreed to be suitable, since it allows different standard deviations to be assigned to data. Data sets displaying bias problems would be "draped" top other data sets using related methods. It was recommended that GIS visualisation methods be utilized in the data preparation process to identify errors and problems.
Grid specification and formats
It was accepted that the decision of the Birmingham WG meeting is in force: The basic grid will be a 5’ free-air grid, displaced 2.5’ relative to the integer degrees, so that the final grid from 64N to the North Pole will consist of even number of points in both directions. An expansion of the grid will be made in the Atlantic region to include all of Greenland and Iceland.
It was discussed if a cartesian x-y grid should be adopted instead, e.g. on a polar stereographic projection. It was agreed that an internal working x-y grid would be a good idea, especially for GIS applications, but that the final grid should be given as a geographic grid, to easily merge the ArcGP grid into other regional or global grids, which are more or less invariably in geographic coordinates. A geographic grid is also natural due to the decision of a southern limit of 64N.
Strategy for obtaining missing data
Since the main data voids are covered by Russian data, negotiations should be undertaken to fill these voids. With the new HDNO offer to prepare the final ArcGP grid, it is hoped that good progress be made in talks to Russian data holders. Western data holders should eventually give permission for direct data exhange with Russia, preferably giving HDNO permission to use the original point data as part of an exchange agreement.
No other major data sources are likely existing. NGA would investigate to what degree major oil companies could make data off Alaska available.
Early 2001 would be deadline for submission of data for inclusion in the ArcGP initial "beta" grid. Flexibility would be essential, if new data surfaces, and the developments in the HDNO cooperation. Grid and initial publications to be made ready by mid-2001. Final grid and map compilation by late 2001/early 2002, likely by HDNO.
The ArcGP results should be presented at the major international meetings by the active members of the ArcGP WG, and not just the chairman or co-chairman. On the recommendation of R. Macnabb it was decided to make available a set of "standard overheads" for such presentations. Major benchmark meetings would include the IAG General Assembly in Budapest, AGU and EGS meetings, and the IAGA General Assembly in Vietnam, all 2001. A main joint paper could be submitted to Journal of Geodesy. An article in EOS would also be desirable.
Web site issues and data distribution
The publication of the final grid product would be made both on the web and on CD-ROM. A printed report and map would follow, with all members of ArcGP acknowledged.
The current ArcGP website is maintained by NGA at http://earth-info.nga.mil/GandG/wgs84/agp.
The web site will be updated with minutes and status reports as the project progresses. The web site will also be used as main distribution channel for distributions of the final grid.
Auxillary grid issues: Bouguer anomalies, ice thickness DEM
It was decided that auxillary grids of Bouguer anomalies and geoid heights should be included on the final web/CD-ROM distributed product.
To compute Bouguer anomalies the IBCAO project bathymetric grid may readily be used on the ocean. Available 5’ land DEM heights are similarly available on land, both at NGA and from international databases. However, a 5’ grid of glacier thicknesses would be required to produce reliable Bouguer anomalies over glaciers. Thickness data are available over Greenland and Iceland, but for other regions such as Svalbard and Ellesmere Island no data are available. Data exist for part of Russia (Frans Josef Land), but is not yet available. It was decided to start a subproject to compile a 5’ ice thickness grid to be used for the compilation of the Bouguer anomaly grid. 5’ grid cells on glaciers with unknown thickness will be specially flagged. KMS will take the lead on this special task.
It was similarly decided to include an Arctic geoid model based on the free-air gravity grid, using spherical FFT methods. GSD, KMS, and Tsniigaik will cooperate on this subproject when basic freeair grid is ready, including comparisons to GPS on tide gauges.
Next ArcGP meeting
It was decided to have a next ArcGP working group meeting in Ottawa, Canada, in connection with the meeting of the Canadian Geophysical Union and Subcommission of the Geoid of North America, May 14-18, 2001. The ArcGP business meeting will take place on one of the days of the week, and it will be attemted to organize a special "Arctic Geophysics" session during the CGU meeting, where many ArcGP relevant presentations will be possible. Organizer will be Marc Veronneau, GSD.
The particpants in the ArcGP express a great thanks to VNIIO and the organizing committee under the leadership of A. Zayonchek for arranging an efficient meeting, and an excellent general programme of the visit.
Copenhagen, July 5, 2000
Rene Forsberg, KMS
Participant list (addresses of ArcGP members available on ArcGP homepage).
A preliminary free-air grid plot and coverage map of different sources underlying free-air map.
Participants in ArcGP meeting, St. Petersburg, June 7-8, 2000:
W. Roest (Geological Survey of Canada, Ottawa)
R. Macnabb (Geological Survey of Canada, Halifax)
M. Veronneau (Geodetic Survey Division, Ottawa)
R. Forsberg (KMS; chairman of ArcGP)
M. Vermeer (Finnish Geodetic Institute; president of International Gravity and Geoid Commission)
T. Boebel (Alfred Wegener Institute)
J. Kaminskis (State Land Survey)
D. Solheim (Statens Kartverk)
Harald Brekke, Oddny Svendsen (Oliedirektoratet)
V. Childers (Naval Research Lab, Washington DC)
B. Coakley (Tulane Univ., Luisiana)
G. Demianov (Tsniigaik, Moscow)
K. Astafurova , A. Cheruych, V. Glebovski, V. Kaminsky, S. Paukki, S. Mashenkov, A. Zayonchek (VNIIO, St. Petersburg)
A. Makorta, A. Oparin, N. Nesterov (Head Department of Navigation and Oceanography, Russian Navy, St. Petersburg)
V.O. Leonov (Marine Geological Research Expedition, Lomonossov)
V. Kamyanskoy (Min. of National Ressources, Moscow)
V. Polikepov (All-Russian Inst. For Exploration Geophysics, Moscow).
phone (314) 676-9127, DSN 846-9127
Document last modified