CONSEIL DE L'EUROPE

Cahiers du Centre Européen de Géodynamique etde Séismologie

VOLUME 3

Proceedings of the Workshop:Non Tidal Gravity Changes

Intercomparison between absolute andsuperconducting gravimeters

September 5th to 7th 1990Walferdange

Grand-Duchy of Luxembourg

Edited by:C. Poitevin

Luxembourg, 1991

TABLE OF CONTENTS

List of Participants

Flick J. : Presidential Address (ECGS)

Poitevin C.IGC Working Group V :

Monitoring of Non Tidal Gravity Variations. 3

SESSION 1 : Absolute GravimetersChairman W. TORGE

Torge W. :The present state of absolute gravimetry. (Invited paper) 9

Elstner C. :On the results of absolute gravity measurements at Potsdamin the period 1976-1990 23

Niebauer T. M., Faller J.E. :Absolute gravimetry: Environmental noise limits. 39

Tsubokawa T. :

Absolute and superconducting gravimetry in Japan. 47

Ducarme B., Maëkinen J., Röder R., Poitevin C. :Intercomparison of the absolute gravimeters JILAG-3 andJILAG-5 at Brussels with reference to the superconductinggravimeter TT3O. 73

SESSION 2 :Superconducting gravimeters Chairman J.M. GOODKIND

Goodkind J.M.:The superconducting gravimeters : principle of operation,current performance and future prospects. (Invited paper) 81

Goodkind J.M., Young C., Richter B., Peter G., Klopping F.:Comparison of two superconducting gravimeters and anabsolute meter at Richmond Florida. 91

Richter B.D.:Calibration of superconducting gravimeters. 99

Sato T., Tamura Y. :The circumstances of the observations with the SCG ofNAO. (Report). 109

Hinderer J., Florsch N., Maëkinen J., Legros H.:Intercomparison between an absolute and a superconductinggravimeter in Strasbourg : Calibration capability. 121

Bower D.R.,Liard J.,Crossley D.J., Bastien R. :Preliminary calibration and drift assessment of thesuperconducting gravimeter GWR12 through comparisonwith the absolute gravimeter JILA 2. 129

Hsu H.T. :Detection of non-tidal gravity variation in China. 143

De Meyer F., Ducarme B. :Modelisation of the non tidal gravity variations at Brussels.145

Goodkind J.M., Young C.:Gravity and hydrology at Kilauea volcano, the geysers andMiami. 163

Aldridge K., Crossley D.J., Mansinha L., Smylie D.E. :GGP The Global Geodynamics Project (Working Version,April15, 1991). 169

SESSION 3:Non tidal gravity changes : theoretical aspects

Chairman J. HINDERER

Hinderer J., Legros H. :Global Earth dynamics and non-tidal gravity changes.(Invited paper) 197

Delcourt-Honorez M. :Total effect of groundwater and internal fluid pressurevariations on gravity. 207

Maëkinen J., Tattari S.:Subsurface water and gravity. 235

Achilli V., Baldi P. Focardi S., Gasperini P., Palmonari F.,Sabadini R .:The Brasimone experiment : A measurement of thegravitational constant G in the 10-100 m range of distance.241

Conclusion 247

The ICC-Working Group V :Monitoring of Non-tidal Gravity Variations.

C. POITEVINChairman IGC-WG5Centre de Géophysique InterneObservatoire Royal de BelgiqueAvenue Circulaire, 3B-1180 BruxellesBelgium

This presentation is an updated version of the reportdistributed at the ICC Special Meeting of August 5th, 1989during the General Meeting of the JAG in Edinburgh, U.K.A complete quadrennial report will be prepared for the XXthIUGG General Assembly in Vienna in 1991.

On August 20th, 1987 during the XIXth IUGG GeneralAssembly in Vancouver, the International GravityCommission approved the creation of the ICC-WorkingGroup V: "Monitoring of Non-tidal Gravity Variations

A resolution, first discussed by the present WG-membersduring a preliminary meeting in Vancouver, has beenadopted by the JAG as Resolution n 4 (Bul. Geod. Vol. 62,n3, p. 278). It supports the work of IGC-WG5. According tothe JAG rules, the resolution has been sent officially at that

time to all the concerned institutions.

The Terms of Reference of IGG-WG5 are "to link togetherthe existing and future superconducting gravimeters in anetwork monitored by absolute gravimeters in order to studyresiduals, after removal of the tides, for geophysicalinterpretation, leading to the monitoring of non-tidal gravityvariations at a global scale".

THE PRESENT STATE OF ABSOLUTEGRAVIMETRY

Wolfgang TorgeInstitut für ErdmessungNienburger Str. 6D - 3000 Hannover 1Fed. Rep. of Germany

ABSTRACT. After about 40 years of development, absolutegravimetry based on the free-fall principle has now reachedan operational state. More than ten transportable instrumentsof different design are available worldwide now, employinglaser Interferometry, short time measurement devices,vacuum and vibration absorbing techniques, and on linecomputer control. An accuracy of +0.1 µms-2 or better canbe expected in field work, for the mean value of some 100 tofew 1000 individual experiments. Thus, absolute gravimetrycan be used to establish large-scale gravity control, and tosupport relative gravimetry with respect to absolute datum,calibration, and drift control. For a more efficientemployment of absolute techniques, repeatability andaccuracy should be improved, to the order of a few 0.001 and0.01 µms-2, respectively.

As an example for the present state-of-the-art of free-fallgravimeters, the main characteristics and error sources of theJILAG-3 gravimeter system are presented. This advancedFaller-type instrument is operated by Institut für Erdmessung(IFE), Universität Hannover, since 1986. After laboratoryand field tests and hard- and software improvements, theinstrument has been used for about 110 absolute gravitydeterminations, on about 70 different sites in central andnorthern Europe, Iceland, Greenland, South America, andChina. While some of these stations contribute to the globalabsolute gravity net, most of them serve for regional andlocal gravimetric control, especially for geodynamicinvestigations at tectonic plate boundaries, and in intraplateseismic or uplift/subsidence areas.

Based on the analysis of the station specific drop-to dropscatter, on long-term repetition measurements in Hannoverand Clausthal (stable bedrock site), and about 40comparisons with the results of other absolute or relativegravimeters, it has been tried to derive realistic estimates forthe repeatability and accuracy of the JILAG-3 system.Present day repeatability may be characterized by standarddeviations of +0.01 to 0.08 µms-2, for time intervals of a fewdays to a few years. The corresponding figures for theaccuracy of an absolute gravity determination (mean value)are +0.05 to 0.08 µms-2. Among the main error sources arelaser frequency changes, floor recoil effects, model errors ofair pressure and earth tide reductions, and unmodeled soilmoisture and ground water variations; strategies for attackingthese problems are outlined.

On the results of absolute gravity measurements atPotsdam in the period 1976 - 1990

Cl. ElstnerCentral Institute for Physics of the Earth Potsdam

Abstract

In the Central institute for Physics of the Earth at Potsdamseveral determinations of the absolute value of gravity wereperformed by the aid of the soviet absolute gravimeterGABL (Gravimeter absolute Ballistical Laser (equipped))and the JILAG3;. instrument of the University of Hannover.

The results of seven measurements are presented. Thecomparison and discussion of these data yields to thesuggestion on temporal gravity variations.

Absolute gravimetry: Environmental noise limits

T. M. Niebauer* and J. E. FallerJoint Institute for Laboratory Astrophysics,Boulder Colorado, USA

A continuous one month absolute gravity record wasobtained in 1986 using one of the newly developed JILAGabsolute gravimeters. The data were corrected for the effectsof earth tides, local and global air pressure variations, andocean loading. The disagreement between the raw gravitydata and calculated environmental effects was about 2.0 µGalrms. The rms amplitude of every Fourier transform peak wasless than 0.5 µGal and the background noise amplitude was

about 0.05 µGal above about 3 cycles/day in each bin of 1cycle/month. This corresponds to a linear noise spectraldensity of about 80 µGal/VHz. The gravimetric factor, foundby fitting the gravity data to the tidal potential, agrees withthe theoretical value to 0.3% where the predominantuncertainty was given by possible errors in the ocean loadmodel. The observed precision of the JILAG type absolutegravimeters is consistent with environmental gravity noiselimitations over a one month continuous observation period.

Absolute and Superconducting Gravimetry in Japan

T.TsubokawaNational Astronomical Observatory, Mizusawa, 023 Japan

Abstract: Two institutes in Japan, National AstronomicalObservatory Mizusawa (NAOM) and Geographical SurveyInstitute (051) have independently developed their absolutegravimeters.

NAOM has 4 sets of absolute gravimeters of which one is ofSakuma 5 original stationary type. The other 3 sets are oftransportable type employing a simple free-fall method. Theset of the first generation has been working in the laboratorysince 1978 and at 9 stations mainly in Tohoku District,north-eastern Japan, since 1984. The stability of single dropmeasurement as good as 10 µGals was attained with this set.The set of the second generation was accomplished inSeptember 1989 and took part in the Third InternationalComparison of Absolute Gravimeters held at the BIPM inNovember 1989. The result of the NAOM gravimeter wasvery close to the mean value from the other 9 participants.The systematic difference of 20 µ4Gals was found betweenthe gravimeters of the 1st and the 2nd generations. One moreset is of a rotating vacuum-pipe type, which has nocomplicated mechanism inside the vacuum-pipe so that alarge number of measurements can be made.

On the other hand, GSI has conducted absolutemeasurements of gravity with its own gravimeter at 11fundamental stations in Japan since 1982, for the purpose ofrevising the Japan Gravity Standardization Net 1975(JGSN75). The GSI apparatus is that revised from thecommercial version of Sakuma's transportable type,especially in the data acquisition unit. Direct comparisons ofthe 051 with the NAOM absolute gravimeters have beencarried out at 4 stations since 1987. Systematic discrepancyas much as 6O µGals was found between the 051 and theNAOM 1st generation gravimeters, The 051 is planning toperform absolute gravity measurements with the 081

apparatus at Syowa Station (Antarctica) , one of the IAGBNcategory A stations, in 1991, whereas the NAOM has thesimilar plan with a NAOM gravimeter at the same station in1992.

As for the superconducting gravimetry, 4 sets ofsuperconducting gravimeters of GWR 11-70 are working atEsashi (NAOM), Kyoto (Faculty of Science, KyotoUniversity, 2 sets) and Kakioka (Ocean Research Institute,University of Tokyo) for the purpose of both clarifying thefine structure of gravity earth tides and monitoring non-tidalsecular gravity changes. We have a plan to install one moresuperconducting gravimeter also at Syowa Station in 1991.

On the other hand, GSI has conducted absolutemeasurements of gravity with its own gravimeter at 11fundamental stations in Japan since 1982, for the purpose ofrevising the Japan Gravity Standardization Net 1975(JGSN75). The GSI apparatus is that revised from thecommercial version of Sakuma's transportable type,especially in the data acquisition unit. Direct comparisons ofthe 051 with the NAOM absolute gravimeters have beencarried out at 4 stations since 1987. Systematic discrepancyas much as 6O µGals was found between the 051 and theNAOM 1st generation gravimeters, The 051 is planning toperform absolute gravity measurements with the 081apparatus at Syowa Station (Antarctica) , one of the IAGBNcategory A stations, in 1991, whereas the NAOM has thesimilar plan with a NAOM gravimeter at the same station in1992.

As for the superconducting gravimetry, 4 sets ofsuperconducting gravimeters of GWR 11-70 are working atEsashi (NAOM), Kyoto (Faculty of Science, KyotoUniversity, 2 sets) and Kakioka (Ocean Research Institute,University of Tokyo) for the purpose of both clarifying thefine structure of gravity earth tides and monitoring non-tidalsecular gravity changes. We have a plan to install one moresuperconducting gravimeter also at Syowa Station in 1991.

INTERCOMPARISON OF THE ABSOLUTEGRAVIMETERS JILAG-3 AND JILAG-5 AT

BRUSSELS WITH REFERENCE TO THESUPERCONDUCTING GRAVIMETER TT3O

Ducarme B., Makinen J., Röder R., Poitevin C.

Abstract In 1989-1990 repeated absolute gravitymeasurements have been performed at Brussels withJILAC-3 and JILAC-5 instruments.

During the same period JILAC-3 and JILAC-5 have beenalso intercompared at Clausthal and Paris. The results showclearly an offset of the order 10 µgal (table 1).

In Brussels these values should be compared to an earliermeasurement of JILAC-3 in 1987. The discrepancy is largerthan 10 µgal although the g value in Brussels should bestable as no important water-level fluctuations did occur inthe meanwhile.

The data of the superconducting gravimeter corrected for thepolar motion exhibit an annual wave with an amplitude ofthe order of 5 µgal (extrema in March and September).Unhappily the epochs of the 1989 measurements do notcorrespond to these extrema.

However we tentatively tried to apply an annual term on theabsolute observations at Brussels and Clausthal (table 2). ForJIIAC-5 at least this correction decreases the standarddeviation in both stations.

KEYWORDS: Absolute gravimeters, Superconductinggravimeters, non tidal gravity variations.

THE SUPERCONDUCTING GRAVIMETERSPRINCIPLES OF OPERATION, CURRENT

PERFORMANCE and FUTURE PROSPECTS

John M. GoodkindUniversity of Ca1ifornia, San Diego

TYPE I SUPERCONDUCTIVITYTemperature dependent penetration of a magnetic field

Superconductivity was discovered when it was observed thatthe electrical resistivity of certain metals becameunmeasurably small below a critical temperature, Tc. Thestudy of these materials later revealed that a simplyconnected sample, placed in a magnetic field, would expelthat field from its interior when its temperature wasdecreased below Tc. This second property ofsuperconductors is called the Meisner effect after itsdiscoverer. (A useful general reference on superconductivityis:

Tinkham, 1975.

COMPARISON OF TWO SUPERCONDUCTING

GRAVIMETERS AND AN ABSOLUTE METER ATRICHMOND FLORIDA

John M. Goodkind and Conrad YoungUniversity of California, San Diego

Bernd RichterInstitute for Applied Geophysics, Frankfurt Germany

George Peter and Fred KloppingNOAA-NGS Rockville, Maryland

INTRODUTION: In a joint effort by NOAA-NGS, UCSD,and 'FAG two superconducting gravimeters have beenoperating at the Naval Observatory in Richmond, Floridasince December 1989. In March 1990, the NOAA group ranone of their absolute meters at this site for 6 days so that thesignals from all three instruments could be compared. Apreliminary analysis by one of us (JMG) of a seven monthrecord of the two superconducting gravimeters and the sixday record from all three is reported here. A completediscussion of these results including a more completeanalysis will be provided in future publications.

Calibration of Superconducting Gravimeters

Bernd D. RichterInstitut fuer Angewandte GeodaesieRichard Strauss Allee 11D-6000 FRANKFURT I Main

Abstract: A calibration system for superconductinggravimeters has been developed on the base of additionalartificial accelerations. The first experiments demonstratethat the derived calibration factor agrees with the results ofother methods within the error bounds. Investigations of thefrequency transfer function allow the extrapolation of theresults to lower frequency domains. The first experimentshows disadvantages in the present setup and leads toimprovements for a final design.

The circumstances of the observations with the SCG ofNAO

Tadahiro Sato and Yoshiaki TamuraNational Astronomical Observatory of Japan

This is a status report of the observations with the superconducting gravity meter (SCO) at the Esashi Site of NAC.

The observation started in February, 1988 and continueduntil Oct. 1989. Now the instrument is under repairs toimprove the assembly of the sphere and the coils. Therefore,this report is based on the 17 months data obtained from thebeginning of February, 1988.

We describe:(1) installation, (2) environment of theobservation, (3) data acquisition system, (4) drift, (5) noiselevel and (6) the scientific programs of NAO concerned withthe SCG.

INTERCOMPARISON BETWEEN AN ABSOLUTEAND A SUPERCONDUCTING GRAVIMETER IN

STRASBOURG: CALIBRATION CAPABILITY

J.Hinderer1, N. Florsch2, J. Ma~kinen3 and H. Legros1

(1) Laboratoire de Géodynamique, Institut de Physique duGlobe, 5 rue René Descartes,67084 Strasbourg Cedex, France(2) Laboratoire de Géophysique Appliquée, Université P. etM. Curie, 4 place Jussieu, 75252Paris Cedex 05, France(3)Finnish Geodetic Institute, Ilmalankatu lA, SF-00240,Helsinki, Finland

Introduction: The problem of accurately calibrating asuperconducting gravimeter is of fundamental importance forany geophysical interpretation of the high quality dataprovided by this instrument. There are several well-knownmethods based on mass attraction or inertial acceleration thatcan be used to estimate the conversion factor (calibration)which transforms the 'gravity' output voltage (in Volt) fromthe feedback system of the relative meter in true gravityvariations (in µgal). Usually, most of the relative meters(including the superconducting ones) are calibrated from thecomparison with a parallel registration of another (or severalothers) relative gravimeters which are themselves preciselycalibrated on a calibration line (e.g. Wenzel et al. 1990). Wereport here on the possibility of calibration of asuperconducting gravimeter by using a parallel registration ofa continuous set of 24 hours of absolute gravity observationsmade with a free-fall gravimeter.

PRELIMINARY CALIBRATION AND DRIFTASSESSMENT OF THE SUPERCONDUCTING

GRAVIMETER GWR12 THROUGH COMPARISONWITH THE ABSOLUTE GRAVIMETER JILA2

Don R. Bower, J. Liard1, D. J. Crossley' and R. Bastien11Geophysics Div., Geological Survey of Canada, Ottawa,Ont.,K1A 0Y32Dept. of Geological Sciences, McGill University, Montreal,Que., H3A 2A 7

Abstract: GWR12 began operation November 7, 1989 at theCanadian Superconducting Gravimeter Installation nearOttawa (Cantley, Que.) and has operated continuously sincethen with a nearly linear drift of approximately -0.5 µGal/day. A calibration factor was determined by comparisonin the tidal bands of the GWR12 data with an extensiveseries of hourly JILA2 measurements. After tides have beenremoved comparison of GWR12 and JILA2 data at irregularintervals over a 4-month period shows good correlation whena linear trend of -0.49 µGal/day is removed from the GWR12data. These data appear also to be related to changes in thewater level in a nearby deep well. In the tidal bands an rmserror of 0.4% was achieved in determining the gravimetricfactors for M2, Ol and K1 from the absolute measurements.Comparison of the GWR12 and JILA2 data, corrected forocean tide and local atmospheric pressure effects, with thetheoretical tidal gravity reveals the following: 1. BothGWR12 and JILA2 show the same anomalously large (byabout 2%) ~2 tidal gravity. 2. Tidal gravity measured byJILA2 at other frequencies is 0.6% larger than the theoreticalfor this site. 3. The sensitivity of the GWR12 gravimeter atthe tidal frequencies M2 and Ol has remained constant withinthe experimental error (0.1%) throughout the eight months ofoperation.

DETECTION OF NON-TIDAL GRAVITY VARIATIONIN CHINA

Hsu HoutseInstitute of Geodesy and GeophysicsChinese Academy of Sciences54 Xu Dong Road, Wuchang, Hubei (430077)People's Republic of China

Abstract: In recent years, the Chinese Academy ofMetrology has successfully developed the second generationabsolute gravimeter (NIM-Il), and measured the absolutegravity values in Beijing, Haerbin and Kunming. At the sametime, in 1990 China cooperated with Hanover Univ. WestGermany and Finland Geodetic Institute to do repeatedmeasurements in the above mentioned places using U.S.meter (JILAG).

Modelisation of the non-tidal gravity variations atBrussels

F. De Meyer, B. Ducarme

Abstract: A superconducting gravimeter GWR is recordingat the Royal Observatory of Belgium (Brussels) since April1982. The data are split in two sections. In the first one (fourand an half years), the long-term trend is difficult to modeldue to instrumental perturbations. When a reliable driftmodel is built, periodical gravity changes show up at theannual and Chandlerian (430 days) periods. The second part(three years) exhibits a much better signal to noise ratio.Unfortunately the length of the observations does not yetallow a complete separation of the two contributions and acorrect evaluation of the amplitude factor 6 associated withthe Chandler term. The amplitude of the anomalous annualterm reaches 5 microgal (50 n m s-2),

Keywords: Superconducting gravimeter, polar motion,instrumental drift, modelisation.

GRAVITY AND HYDROLOGY AT KILAUEAVOLCANO, THE GEYSERS AND MIAMI

John M. Goodkind and Conrad YoungUniversity of California, San Diego

Introduction: In order to use gravity to measure geophysicalphenomena such as vertical crustal motion, otherenvironmental influences on gravity must be identified andmeasured. Of these, the effect of ground water has beenstudied least. I discuss here recent data from The HawaiianVolcano Observatory (HVO) on Kilauea Volcano and fromMiami, Florida. The hydrology of these two locations differsubstantially. However, the observed response to rainfall inHawaii is similar to that observed in earlier work at TheGeysers geothermal field in California. The major features ofthe response at Hawaii and The Geysers are explained interms of models that consist of a small number of discreteaquifers at different elevations which drain from higher tolower. The terrain at Miami is exceptionally flat andhomogeneous and the hydrology is influenced by nearbycanals rather than flow between aquifers. The analysispresented here is preliminary and more detailed work will bepublished in the future with coworkers in Hawaii and Miami.

Global Earth Dynamics and Non-Tidal Gravity Changes

J. Hinderer and H. LegrosLaboratoire de GéodynamiqueInstitut de Physique du Globe5 rue René Descartes, 67084 Strasbourg CedexFrance

Introduction: The spectrum of vertical gravity changes atthe Earth's surface clearly shows effects which vary fromlocal to planetary scales as well as from short periods (lessthan 1 hour) to secular (several decades) variations. We showhere how gravity disturbances that can be observed at theEarth's surface with high-precision gravimeters can berelated to planetary-scale dynamics of the Earth. For this weconsider the elasto~gravitational deformation of an Earthmodel with fluid parts (outer core, superficial layer) with thehelp of a Love number formalism which has been developedelsewhere (Legros 1987; Hinderer and Legros 1989). Therelationships between gravity changes and importantgeodynamic processes of global extent relative to the wholeEarth or occurring only in the fluid parts (outer core,atmosphere, ocean) are discussed and some numericalapplications are given.

TOTAL EFFECT OF GROUNDWATER ANDINTERNAL FLUID PRESSURE VARIATIONS ON

GRAVITY

Dr. Micheline DELCOURT-HONOREZRoyal Observatory of BelgiumCentre de Géophysique InterneAvenue Circulaire, 3B-1180 BrusselsBelgium

Abstract: We present a theoretical study of the total effect ofgroundwater and internal fluid pressure variations on gravityi.e. the effect of the land surface displacement and theattraction variation effect. We comment the numerical valuesof the effect we obtain on the superconducting gravimeterrecords at the Royal Observatory of Belgium.

SUBSURFACE WATER AND GRAVITY

J. MäkinenFinnish Geodetic Institute, Helsinki, FinlandS. TattariWater Research Institute, Helsinki, Finland

Abstract. We present results from parallel measurements ofgroundwater level, soil moisture content, precipitation, andgravity. Soil moisture turns out to be as important a source ofgravity variation as groundwater. Precipitation alone explainspoorly subsurface water storage variations, other variableslike evapotranspiration must be accounted for. The attractionof snow and gradient effects are discussed.

The Brasimone experiment: a measurement of thegravitational constant G in the 10-100 m range of

distance.

V. Achilli1, P. Baldi2, S. Focardi3,P. Gaspevini4, F. Palmonavi3, R. Sabadini3

University of Roma "Tor Vergata"1, Udine2, Bologna3

I.N.G., Roma4

Summary: An experiment based on the measurement of thevariation of local gravity induced by periodic filling of apower storage plant basin, is described. A superconductinggravimeter will be installed in a tunnel under a lake, wherewater masses of 106 tons are normally displaced every day,in order to store energy in hours of low demand , by pumpingwater from a low-level basin located at a distance of 5 Km;the measurable effect, due to a maximum variation of 7 m inthe water level, is 300 10-6 gal. The purpose of theexperiment is to provide evidence for deviations fromNewton's gravitational law.

List of Participants

Achilli ViadimiroDip.Ing.C.-Univ.Roma"Tor Vergata"Via E.Carnevale1-00173 Rome ItalyTEL 39 6 24990575 FAX 39 6 24990586TELEX 611462 univrm

Alms RainerGeol . Inst.Univ.BonnNussalle 8D-W/5300 Bonn 1 F.R.G.TEL 49 22873258

Baker TrevorProudman Oceanographic Lab.Bidston Observatory

GB-L43 7RA BirkenheadGreat BritainTEL 44 51 653 8633FAX 44 51 653 6269TELEX 628591 ocean b

Baldi PaoloIst.sc.terra-Univ.UdineV.le Ungheria 431-Udine ItalyTEL 39 51 243586 FAX 39 51 250106

Crossley OavidMc Gill Univ.-Opt.Geol.Sc.University street 3450CNO-H3A 2A7 Montreal CANADATEL 1 514 398 4886FAX 1 514 398 4680EMAILdavid-C@geosci . lan.mcgi 11 .ca

d'Oreye NicolasObservatoire Royal de BelgiqueAv.Circulaire 3B-1180 BruxellesTEL 32 2 3730 211FAX 32 2 3749 822TELEX 21565 obsbel b

de Freitas SilvioUniv.Federale do ParanaCentro Politecoico Oept.Geosc.81504 Curitiba BrazilTEL 55 41 2662122FAX 55 41 2245170TELEX 415100 ufrr br

Degryse KarinKoninklijke Sterrenwacht Belgie.Ringlaan 3B-1180 BrusselTEL 32 2 2720 266FAX 32 2 3749 822TELEX 21565 obsbel b

Dehant VeroniqueObservatoire Royal de BelgiqueAvenue Circulaire 3B-1180 Bruxelles BelgiumTEL 32 2 3730 246

FAX 32 2 3749 822TELEX 21565 obsbel bEmail: veroniq@titan.oma.be

Delcourt MichelineObservatoire Royal de BelgiqueAvenue Circulaire 3B-1180 BruxellesTEL 32 2 3730 211FAX 32 2 3749 822TELEX 21565 obsbel b

Ducarme BernardObservatoire Royal de BelgiqueAvenue Circulaire 3B-1180 BelgiqueTEL 32 2 3730 248TELEX 21565FAX 32 2 3749 822EMAIL bernard@astro.oma .be

Elstner C.Ak.Wiss.OOR-Zentralinst.Pysik ErdeTelegrafenweg AllD-0/1561 Potsdam

Fajklewicz Zb.Acad.Mining-Metalurgy/Inst.Geoph.Al.Mickiewicza 30 Pavilon A-0P-30059 KrakowTEL 48 338100TELEX 322203 agh p1

Faller JamesUniv.of ColoradoJila Campus Box 440USA-80309 Boulder,COTEL 1 303 492 8509FAX 1 303 492 5235TELEX 755842

Flick JeanMin Aff.CulturellesRue Louis XIV 25L-1948 LuxembourgG.D.LuxembourgTEL 352 44 16 52FAX 352 45 89 40TELEX 47096

Francis OlivierObservatoire Royal de BelgiqueAv.Ciculaire 3B-1180 BruxellesTEL 32 2 3730 251FAX 32 2 3749 822TELEX 21565 obsbel b

Goodkind JohnUniv.of California,San OiegoOept of Physics, B-019USA-92093 La Jolla,Calif. USATEL 1 619 534 2716FAX 1 619 534 0173TELEX 9103371271

Hinderer JacquesInst.Phys.du GlobeRue Descartes 5F-67034 StrasbourgFranceTEL 33 88 60 50 63FAX 33 88 61 67 47

Hsu H.T.Inst.Geod/Geoph. -Chinese Ac.ScXu Dong Road 54RCP-Wuhang HubeiChinaFAX 86 18011095

Marson IginioIst.Miniere e Geofisica Appi.Via Valerio 10I-34127 TriesteItalyTEL 39 40 568 201TELEX 460014 fisicaFAX 39 40 560 3176

Mller MatthiasInst. Physical GeodesyPetersenstr, 13D-W/6100 DarmstadtF.R.G.TEL 49 6151 163900FAX 49 6151165489

Mkinen JaakkoGeodeettinen Laitos

Ilmalankatu 1aSF-00240 HelsinkiFinlandTEL 358 0 410433FAX 358 0 414946EMAIL: geodeet@finpun.bitnet

Niebauer Timothy M.AXIS Inst.Shop FacilityLee Hill Orive 135USA-Boulder CO 80302 U.S.A.TEL 1 303 442 4753FAX 1 303 442 6809

Paquet PaulObservatoire Royal de BelgiqueAv.Circulaire 3B-1180 Bruxelles BelgiumTEL 3223730 266FAX 322 3749 822TELEX 21565 obsbel bEMAIL paulpaq@astro.oma.be

Poitevin ChristianObservatoire Royal de BelgiqueAvenue Circulaire 3B-1180 BruxellesBelgiumTEL 32 2 3730 294FAX 32 2 3749 822Telex 21565 obsbel bEmail: chrisp@astro.oma.be

Remmer OleKort-og MatrikelstyrelsenGamlehave Alle22DK 2920- CharlottenlundDenmarkTEL 45 31 631833FAX 45 31 631824Telex 15184 Seismod

Richter BerndInst.Fr Angewandte GeodasieRichard-Strauss Allee 11D-W/6000 Frankfurt/Main F.R.G.TEL 49 6963 33225FAX 49 6963 33425TELEX 0413592 ifag d

Sarrailh MichelBureau Gravim. Intern./CNESAvenue Ednuard Belin 18F-31055 Toulouse CedexFranceTEL 33 61 33 28 93FAX 33 6125 30 98TELEX 530776f

Smeets IngeborgAstronomisch Inst.K.U.LeuvenCelestijnenlaan 200 BB-3030 HeverleeBelgiumTEL 32 16 200 656TELEX 23674

Smets EdouardDir.Bruggen & WegenLange Lozanastraat 134B-2018 Antwerpen Belgium

Torge WolfgangInst. Erdmessung-Univ. HannoverNienburgerstr. 6D-W/3000 Hannover 1 F.R.G.TEL 49 5117 622795FAX 49 5117 624006TELEX 923868 unihn d

Tsubokawa TsuneyaNat.Astron.Obs. -MizusawaHoshiga oka, 2-12J-023 Mizusawa, Iwate JapanTEL 81 19724 7111FAX 81 19723 5156TELEX 837628

Van Ruymbeke MichelObservatoire Royal de BelgiqueAv.Circulaire, 38-1180 Bruxelles BelgiumTEL 3223730 211FAX 3223749 822TELEX 21565 obsbel b

Wenzel Hans-GeorgGeod. Inst.Univ.KarlsruheEngelenstr 7DO-W/7500 Karsruhe 1

F.R.G.TEL 49 7216 082317FAX 49 7216 94552

CONCLUSIONS OF THE WORKSHOPECGS - IGC WG V

NON TIDAL GRAVITY CHANGESWALFERDANGE (G.-D. LUXEMBURG) 5-7 Sept.1990

These conclusions have been submitted to all participants tothe workshop for correction, improvement and approvalbefore publication in the proceedings: Cahiers du CentreEuropéen de Géodynamique et de Séismologie Vol. n0 3.

The repeatability of absolute gravity measurements in a samesite seems to be around +/-5µ gal.

That is not enough to put real constraints on the modelizationof the long term (secular?) drift of superconductinggravimeters in a stable site.

The calibration of superconducting gravimeters by means ofabsolute instruments reaches now 0.5%.

Superconducting sites are very interesting for absolutemeasurements as all geophysical parameters of concern arecontinuously monitored and short term (up to 1 year?)gravity variations clearly detected by the superconductinggravimeters.

The effect of large water table atmospheric pressure localgeophysical parameters variations suspected in the residuecurve of superconducting gravimeters can be elucitated ifabsolute gravity measurements are available before and afterthe events (!!!) and vice-versa

What is to be done?

to encourage development of:

1. Absolute gravimeters with +/- 1µgal precision in order to:constraint the drift of superconducting gravimeters;

· obtain a calibration of superconducting gravimeters at thelevel of 0.Ox %

2. A network of superconducting gravimeters:

an annual wave (~ 8µ gal p.p.) has been detected by most ofthe superconducting gravimeters.

Existence of this wave could also be confirmed by absolutemeasurements at the extrema of this wave.

. induced effect of polar motion: To determine the 6 factor atthis frequency.

From the computational point of view, the calculation of theatmospheric pressure correction should be investigated inmore details. For long period phenomena it should benecessary to compute mean dayly effects based on a realtime global atmospheric model.

Two projects have been introduced during the meeting andhave to be considered carefully:

1. A proposal to submit to the EEC to obtain funds to supportthe purchase of a new Faller's type absolute gravimeterswhich will be at the disposal of the european scientificcommunity for geodynamical purposes in the frame ofactivities of the ECGS;

2. G G P

Global Geodynamics Project Exchange of rawsuperconducting data through an international centre.

BACK TO PUBLICATIONS