ARCTIC GRAVITY PROJECT
Association of Geodesy International Gravity and Geoid Commission
Minutes of Workshop and WG Meeting, St.
Petersburg, Russia, June 7-8, 2000
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.
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
meeting was opened by a welcome address by V.
Kaminsky, Deputy Director of VNIIO.
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.
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.
scientific contribution included the following presentations
(presenting author shown) summary made primarily from notes taken
by S. Kenyon:
Kenyon: EGM96 global gravity
field model and ArcGP data status.
the current status of the joint NIMA/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
global gravity field model and Russian geodetic activities.
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.
Maschenkov: Regional tectonic interpretation of gravity and other
geodata in the Arctic.
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.
Childers: NRL aerogravity program 1992-1999.
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.
O. Leonov: Russian aerogeophysical surveys of Frans Josef Land region
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
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.
Coakley: SCICEX submarine geophysics and bathymetry programme.
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.
Veronneau: Geoid modelling of the Arctic Region.
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
Forsberg: Greenland aerogravity project 1998-2000.
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
NIMA. 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.
Solheim: Svalbard gravity
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/NIMA. 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.
Boebel: AWI airborne geophysics
in the Arctic and Antarctic 1997-1999.
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.
Vermeer: Gravity surveys of
Finland and the Baltic Sea.
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.
Forsberg: Gravity from satellite
altimetry over the Arctic Ocean status and future possibilities.
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 ESAs CRYOSAT
radar mission 2003. With these data all current data voids in the
Arctic will be covered.
Zayonchek: Integrated Interpretation of Gravity and Other Geodata in
Shelf for Russian State Regional Geological Mapping Program.
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.
Macnabb: The new International
Bathymetric Chart of the Arctic Ocean building a DEM for
Bouguer corrections over ocean and continent.
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.
chairman closed the session of workshop session by thanking the
presenters, and gave a special thanks to the hard work of the
professional simultanous Russian/English translators.
of the ArcGP business meeting, June 8
agenda for the business meeting was:
- Status of data submitted to date
- Comparisons of different data sources
- Strategy for data merging of data sets with highly different noise
- 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
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 NIMA. Parts of the contributed data also reside at KMS, notably for
comparisons and data merging method development.
of submitted data:
OK, 2.5 data for making 5 mean values provided.
OK, 5 grid data computed at KMS from data in joint Nordic geoid
OK, original data provided to KMS.
OK, data to be provided, problems with downward continuation over
August 2000 survey along coast to be included, new downward
continued 5 data grid available late 2000. Otherwise all data
point data provided to ArcGP web site, need to redo recent
Ellesmere Island survey to update processing of orthometric heights.
covered from NIMA archives, including USGS helicopter surveys.
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.
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.
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 NIMA/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 NIMA.
status of other major data providers:
Wegener Institute, Germany four seasons of airborne gravity
available, 1997-98 already provided, 1999-2000 to be processed this
fall. Polarstern data also provided.
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.
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.
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).
of different data sets
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.
for merging different data sources
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.
specification and formats
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.
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.
for obtaining missing data
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
other major data sources are likely existing. S. Kenyon would
investigate to what degree major oil companies could make data off
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.
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
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.
current ArcGP website is maintained by NIMA at http://www.nima.mil/GandG/agp.
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
was decided that auxillary grids of Bouguer anomalies and geoid
heights should be included on the final web/CD-ROM distributed
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 NIMA 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.
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
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
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
July 5, 2000
list (addresses of ArcGP members available on ArcGP homepage).
preliminary free-air grid plot and coverage map of different sources
underlying free-air map.
in ArcGP meeting, St. Petersburg, June 7-8, 2000:
Roest (Geological Survey of Canada, Ottawa)
Macnabb (Geological Survey of Canada, Halifax)
Veronneau (Geodetic Survey Division, Ottawa)
Forsberg (KMS; chairman of ArcGP)
Vermeer (Finnish Geodetic Institute; president of International
Gravity and Geoid Commission)
Boebel (Alfred Wegener Institute)
Kaminskis (State Land Survey)
Solheim (Statens Kartverk)
Harald Brekke, Oddny Svendsen (Oliedirektoratet)
Childers (Naval Research Lab, Washington DC)
Coakley (Tulane Univ., Luisiana)
Kenyon (NIMA, St. Louis; co-chairman of ArcGP)
Demianov (Tsniigaik, Moscow)
Astafurova , A. Cheruych, V. Glebovski, V. Kaminsky, S. Paukki, S.
Mashenkov, A. Zayonchek (VNIIO, St. Petersburg)
Makorta, A. Oparin, N. Nesterov (Head Department of Navigation and
Oceanography, Russian Navy, St. Petersburg)
Leonov (Marine Geological Research Expedition, Lomonossov)
Kamyanskoy (Min. of National Ressources, Moscow)
Polikepov (All-Russian Inst. For Exploration Geophysics, Moscow).