The new IMGC-02 transportable absolute gravimeter ...

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ANNALS OF GEOPHYSICS, VOL. 51, N. 1, February 2008

Key words gravity – absolute gravimeter – meas-urements – geophysics – volcanology

1. Introduction

The best accuracy achievable in measuringthe acceleration due to gravity g is estimated tobe few microgals (1 µGal=1×10–8 m⋅s−2) andconcerns modern absolute ballistic gravimeters.The local g value is extracted from the trajecto-

ry of a free falling test body by fitting a suitablemotion model to the time-space coordinates.Such accuracy is reached by using optical inter-ferometric methods to track the test mass flight.The wavelength of an iodine-stabilized laserbeam (helium-neon) used to illuminate the in-terferometer is the standard of length while thefrequency of an atomic resonator (rubidium)used to synchronize the acquisition system, isthe standard of time.

Since 1968, starting with the technical assis-tance of the Bureau International des Poids etMesures – BIPM, the construction and mainte-nance of transportable absolute gravimeters hasbeen continued at the Istituto di Metrologia «G.Colonnetti» (now Istituto Nazionale di RicercaMetrologica – INRIM) (Cerutti, 1967, 1971;Cerutti et al., 1974). Figure 1 shows the first ver-

The new IMGC-02 transportable absolutegravimeter: measurement apparatus and

applications in geophysics and volcanology

Giancarlo D’Agostino (1), Sergio Desogus (1), Alessandro Germak (1), Claudio Origlia (1), Danilo Quagliotti (1),Giovanna Berrino (2), Gennaro Corrado (3), Vincenzo d’Errico (2) and Giuseppe Ricciardi (2)

(1) Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy(2) Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy

(3) Dipartimento di Scienze della Terra, Università degli Studi di Napoli «Federico II», Napoli, Italy

AbstractThe research carried out at the Istituto Nazionale di Ricerca Metrologica (formerly Istituto di Metrologia «G.Colonnetti») aiming to develop a transportable ballistic absolute gravimeter ended with a new version of the in-strument, called the IMGC-02. It uses laser interferometry to measure the symmetrical free rising and fallingmotion of a test mass in the gravity field. Providing the same accuracy achieved with previous versions, the in-strumental improvements mainly concern size, weight, data processing algorithms and operational simplicity.An uncertainty of 9 µGal (1 µGal=1×10–8 m·s−2) can be achieved within a single observation session, lastingabout 12 h, while the time series of several observation sessions show a reproducibility of 4 µGal. At this lev-el, gravity measurements provide useful information in Geophysics and Volcanology. A wide set of dynamicphenomena, i.e. seismicity and volcanic activity, can produce temporal gravity changes, often quite small, withan amplitude ranging from a few to hundreds of microgals. Therefore the IMGC absolute gravimeter has beenemployed since 1986 in surveying the Italian active volcanoes. A brief history of the gravimeter and the descrip-tion of the new apparatus, together with the main results of ongoing applications in Geophysics and Volcanolo-gy are presented.

Mailing address: Dr. Giancarlo D’Agostino, IstitutoNazionale di Ricerca Metrologica (INRIM), Strada delleCacce 91, 10135 Torino, Italy; e-mail: [email protected]

Vol51,1,2008_DelNegro 16-02-2009 21:27 Pagina 39

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G. D’Agostino, S. Desogus, A. Germak, C. Origlia, D. Quagliotti, G. Berrino, G. Corrado, V. d’Errico and G. Ricciardi

sion of the apparatus while the picture in fig. 2shows the present one, called IMGC-02, whichbenefits from nearly forty years of research ac-tivity in this field (D’Agostino, 2005; Germaket al., 2006). Although the mechanical action ofprojecting vertically upwards is far more diffi-cult than dropping an object, all prototypesadopted the symmetrical method, where boththe rising and falling trajectories are used to fitthe motion model. In the case of transportableapparatus this is particularly advantageous be-cause the technical solutions used to cancel outthe effect of the residual air resistance on thefalling body are not so demanding and the vac-uum system of the apparatus can be simplified.Furthermore this method is the most promising

due to its inherent high precision and its relativefreedom from systematic errors such as timingerrors. Nevertheless, there are in practice veryfew working instruments adopting the symmet-ric method. The majority of other gravimetersadopt the simple free falling method, where on-ly the falling trajectory is used (Niebauer et al.,1995). This is a matter of concern for the metro-logical community because the experimentalway to highlight any inherent systematic erroris to compare instruments adopting differentmeasuring methods and techniques. As an ex-ample, only two rise and fall systems participat-ed in the last International Comparison of Ab-solute Gravimeters – ICAG, organized byBIPM and held in 2005: the IMGC-02 and theTBG absolute gravimeter, Ukraine.

Several new features characterize the presentinstrument, which is smaller and lighter than theprevious versions. Among them there is the au-

Fig. 1. First version of the IMGC absolute gravimeter.

Fig. 2. Picture of the new absolute gravimeter(IMGC-02).


41

The new IMGC-02 transportable absolute gravimeter: measurement apparatus and applications in geophysics and volcanology

tomatic running, which allows a higher measure-ment rate and shorter time to measure the gravi-ty value. The IMGC-02, like every prototype de-veloped before, participates regularly in theICAGs, assuring the long-term reliability of therecorded gravity data. Confidential results of thelast ICAG-05, reported in fig. 3, confirm that theIMGC-02 has reached a point in its developmentwhere the measurement results agree within theexpanded uncertainty, estimated to be about 9µGal. Such an uncertainty level is achieved withone observation session that lasts about 12 h,even if the apparatus showed a measurement re-producibility of about 4 µGal at the INRIM grav-ity laboratory. At this level, the information ex-tracted from gravity measurements can be usefulin many applications of Geophysics and Vol-canology. In fact, geodynamics, seismicity, andvolcanic activity can produce temporal variationin the gravity field, often quite small, within theamplitude range from a few to hundreds of mi-crogals. For this reason, since 1986, the IMGC

absolute gravimeter has been employed in sur-veying the Italian active volcanoes in co-opera-tion with the Osservatorio Vesuviano (presentlyIstituto Nazionale di Geofisica e Vulcanologia -INGV) and the University «Federico II», both inNapoli (Italy) (Berrino et al., 1988; 1999; Berri-no, 1995). In particular, the IMGC-02 has al-ready been employed to collect absolute gravitydata in three new absolute stations placed by theINGV in the Colli Albani volcanic area (Berrinoet al., 2007).

New features and technical solutions of theIMGC-02 are described later on together withan overview of the ongoing applications inGeophysics and Volcanology.

2. Description of the apparatus

The schematic drawing in fig. 4 describesthe present instrument, 1 m high, 0.5 m wideand 0.6 m long, respectively, which consists of

Fig. 3. Results of the last International Comparison of Absolute Gravimeters (ICAG05). Errors bars representthe measurement expanded uncertainties.


42


five easily separable parts: i) a catapult in a vac-uum cylinder, ii) a Michelson interferometer ina case connected to an anti-vibrating system,iii) a laser body, iv) a photodetector and v) asupporting frame. The whole apparatus, 180kg weight including the rack housing the elec-tronic systems, a vacuum pump and mountingtools, can be transported in a small van and re-assembled by an operator within one hour. Thecomplete automation of the instrument allows ameasurement rate of about 120 launches everyhour. Thus one absolute station, consisting of arecord of about 1500 gravity data, can be creat-ed in one night. It is recommended, before theoptical alignment, to let the mechanical partsreach thermal stability and start the measure-ment after a warm up of the electronic systems.The IMGC-02 is designed to run at room tem-perature with a stability of about ±2°C.

The test mass, 40 g in weight and 6.5 cmhigh, consists of a solid cube corner reflector

rigidly fixed on an aluminium support. To avoidthe systematic error introduced in case of rota-tion of the free falling object, the center of grav-ity of the test mass is adjusted to coincide with-in 0.1 mm with the optical center of the cubecorner (Germak et al., 2002).

A purpose-built catapult throws the reflectorupwards about 20 cm in vacuum while givingit a spurious rotation and a horizontal velocity.Although intensity and direction are both ran-dom, it is estimated an average rotation of25×10−3 rad⋅s−1 and an average velocity of0.25×10−3 m⋅s−1. These motions are the majorsource of error for the IMGC-02. They add tothe measurement result two non-gravitationalcomponents: an apparent centrifugal accelera-tion and the Coriolis acceleration. The contri-bution of these effects to the combined meas-urement uncertainty is about 4 µGal.

The reference corner reflector is fixed to theinertial mass of a long-period (≅20 s) seis-mometer. This technical solution is employedto attenuate the perturbation of the gravitymeasurement due to the ground motion and themechanical shocks caused by the launching ofthe test mass. By means of this device the effectis estimated to be reduced by more than 10times. As an example, at the INRIM gravitylaboratory, the experimental standard deviationof the recorded data decreases from hundreds toa few tens of microgals (Germak et al., 2006).In normal working conditions this correspondsto an experimental standard deviation of themean value of about 1 µGal for one observationsession, which contributes to the combinedmeasurement uncertainty.

The measurement uncertainty is a combina-tion of effects due to influence factors charac-teristic of the instrument, reported in table I, to-gether with the effects characteristic of the ob-servation site, reported in table II. The overallexpanded uncertainty is about 9 µGal. It isevaluated as the combined standard uncertaintytimes the coverage factor at the 95% confidencelevel.

The present prototype adopts a unique data-processing technique to time the interferencesignal, called the local-fit method (D’Agostinoet al., 2005b) where a digital oscilloscope, syn-chronized to a reference rubidium oscillator, is

Fig. 4. Schematic drawing of the IMGC-02 ab-solute gravimeter.


43


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45


used to sample and buffer the output voltage ofthe photo detector during the test body trajecto-ry. The time-space coordinates are extractedfrom the acquired signal by fitting a suitableequation model to the interference fringes. Thismethod offers new powerful tools for post-pro-cessing the stored data because the full acquisi-tion of the interference signal opens a directway to assess the quality of the trajectory. Itshows less sensitivity to large fringe variationand makes the IMGC-02 smaller, lighter andcheaper than before since previous expensiveand bulky electronic components are avoided.Experimental comparisons show that the local-fit method is at least as accurate as traditionalmethods. From a theoretical point of view, itsapplication removes, as an additional advan-tage, the frequency-dependent time delays intiming the interference signal, even if, as al-ready mentioned, the relevant effect is can-celled out in a rise and fall system like theIMGC-02.

A new software code was developed andtested to manage the instrument during the au-tomatic working. The code runs in a personalcomputer directly plugged into the rack housingall the electronic devices. The software package

was developed with Labview platform and con-sists of two different programs: one for control-ling the instrument and storing the experimen-tal data during the measurement session (Grav-isoft M), the other for processing and filteringthe recorded data during the post-processingsession (Gravisoft PP).

The user interface panel of the Gravisoft Mis showed in fig. 5. Graphic screens supply theoperator with immediate information on theresidual noise of the least-squares fit, gravitytime series, histograms and statistical data. Af-ter the measurement session, the Gravisoft PPanalyses and displays all stored trajectories.Data are filtered according to rejecting criteriarelevant to the measurement conditions. Themost critical factor is the visibility variation ofthe interference fringes, which indicates a hori-zontal motion of the test mass. Outliers arefound by applying the Chauvenet criterion tothe parameters estimated with the least-squaresregression. Moreover there is the possibility toelaborate the data by applying geophysical cor-rections relevant to the tide and ocean loading,the polar motion and the local barometric pres-sure.

The present instrument is the first fully au-

Fig. 5. User interface panel of the Gravisoft M.


46


tomated prototype developed by INRIM. Its op-tical alignment is easier than before and the ap-paratus can work automatically without the op-erator. This makes the IMGC-02 a modern ab-solute gravimeter suitable for future applica-tions in Geophysics and Volcanology requiringthe above-declared level of uncertainty.

3. Applications in geophysics and volcanology: comparison with relativemeasurements on some Italian active volcanoes

Many gravimetric surveys have been carriedout since 1981 on the Neapolitan (Vesuvio,Campi Flegrei and Ischia) and on some Sicilian(Vulcano and the whole Aeolian Islands andPantelleria) volcanoes to investigate their dy-namics (Berrino and Corrado, 1991; 2007). Theresults show that combining absolute and rela-tive measurements is definitely the most com-plete and reliable way to define and distinguishbetween long-term and short-term variations(Berrino et al., 1999; Berrino, 2000). Some-times both repeating absolute measurementsand establishing a greater number of absolutestations in the dynamic area are necessary. Infact, absolute measurements define the refer-ence station (or «base station») and, if they arerepeated, detect its variation or confirm its sta-bility (within 10 µGal) on long time. Being ab-solute measurements directly linked to stan-dards of time and length, they are fairly inde-pendent from instrumental reference. As a con-sequence they allow performing measurementschedules lasting long time and to calibrate andcompare relative gravimeters.

Since 1986, absolute gravity measurementshave been carried out to survey the dynamics ofItalian active volcanoes (Berrino et al., 1988,1999, 2006; Berrino, 1995, 2000). The observa-tion points have been chosen in correspondenceof reference stations and on selected ones of therelative gravity networks on volcanoes.

The following absolute stations have beensettled (see fig. 6 for their location): in 1986,Napoli (reference station of the gravity net-works of Neapolitan volcanoes) and Vesuvio; in1990, Milazzo (reference station of the gravity

networks of Vulcano and Aeolian Islands) andVulcano; in 1993, the island of Pantelleria; in1995, Stromboli; in 1998, Campi Flegrei andIschia; and recently, in 2005, Sant’Angelo Ro-mano and Palestrina (both reference stations ofthe gravity networks of the Colli Albani area)and Castel Gandolfo (D’Agostino et al., 2005;Berrino et al., 2007).

Regarding the island of Pantelleria, the dis-tance from Sicily did not allow the link of thelocal relative gravity network to an external ref-erence station, therefore two absolute stationswere established, one of which was selected asa local reference, according to the dynamicssingled out since 1980 (Berrino, 1997; Behnckeet al., 2006).

Moreover, in the framework of a EuropeanProject, in 1991 two stations were establishedon Mt. Etna and Centuripe (reference stationfor this area) (Berrino et al., 1999). The refer-ence stations of Napoli, Milazzo and Centuripe,established in 1986, 1990 and 1991 respective-ly, are also ties of the «Italian Gravity Networkof Zero Order» (Berrino et al., 1995). Finally,

Fig. 6. Location of absolute stations in SouthernItaly. Squares represent the reference stations for therelative gravity network, triangles represent the ab-solute stations located on the dynamic areas. Circlesare the stations constituting the fundamental ties ofthe «Calibration line» for relative gravimeters and astation (Cosenza) settled in a seismic area.


47


in the framework of a project for the establish-ment of new absolute stations to survey seismicareas, one station in Cosenza and three stationsin Troia, Foggia and Mattinata were set up, thelatter constituting the fundamental ties of a cal-ibration line of relative gravimeters (Berrino etal., 1988; Berrino, 1995). The vertical gradientof gravity was measured in every station totransfer the absolute g value to the measure-ment height of a relative spring gravimeter. Fur-thermore, each absolute station is supplied withone or more «satellite» external stations onwhich the absolute value is transferred bymeans of several relative gravity links.

It has to be noted that absolute stations onactive volcanoes are well located in the moni-toring networks taking into account logistic re-quirements. They are chosen to be in correspon-dence with selected relative stations accordingto the dynamics of the surveying area.

Most of the stations have been surveyedmore than once. The periodical surveyingchecks long-term variations of the selected sta-tions, as well as periodic calibration and com-parisons for relative gravimeters. In order tomake comparable the absolute and the relativemeasurements, gravity readings are also cor-rected for polar motion effect, in addition to theusual reduction such as earth tide, atmosphericload and drift in the case of relative measure-ments. Moreover, the absolute measurementsare also referred to the same height then the rel-ative one taking into account the value of themeasured vertical gravity gradient.

The absolute gravity measurement at theVesuvio station was repeated four times, in par-ticular in 1994, 1996, 1998 and 2003. Always in2003, absolute measurements at the stations ofPozzuoli and Ischia were repeated. During the2003 the base station of Napoli was moved to anew point of the same building, because theroom containing the old point underwent somestructural modifications that could have changedthe gravity value. In fact a decrease of 58 µGalwas recorded at the old point whereas the linksamong the two absolute reference stations andthe external satellite station confirmed that thesatellite point was stable. For this reason the de-crease was believed only due to its structuralmodifications. This was confirmed by the com-

parison between the gravity changes from ab-solute measurements in all and that from relativeones obtained taking into account as referencestation the external satellite relative station.

Also, the absolute gravity measurementswere repeated in 1995 at Vulcano (Berrino etal., 1999) and from 1991 to 1994 twice at Etna(1992 and 1994) and once at Centuripe (1994)(Berrino, 1994; Berrino et al., 1999).

There is in general a good agreement be-tween the gravity changes revealed by relativemeasurements and the absolute gravity datacollected in repeated observations at the sameobservation site. No significant gravity changeswere measured at Vulcano, Pozzuoli and Ischia,and also on Mt Etna in spite of the large erup-tion which occurred from 1991 to 1993. As anexample the record of absolute and gravity datacollected at Vulcano is reported in fig. 7.

On the contrary, a considerable variationwas observed at the absolute station of Vesuviofrom 1986 to 1994 (fig. 8): the gravity value de-creased by about 60 µGal. This huge variationis in agreement with the gravity changes meas-ured with relative gravimeters. Moreover, thelong-term stability of the reference relativegravity station (satellite station) of Napoli wasalready confirmed taking into account data re-ferred to it. The gravity decrease also occurredwhen also: i) the seismic activity increased(1989-1991); ii) the continuous gravity meas-urements showed a significant change in thetidal parameteres from 1991 to 1994; iii) therelative gravity measurements spanning thewhole network indicated a progressive enlarge-ment in two distinct areas respectively charac-terized by a decrease (in the north-western sec-tor of the network) and an increase (in thesouth-eastern part of the network and at thebase of the cone). The absence of ground defor-mation implied a mass redistribution withoutsignificant volume changes, probably fluid mi-gration at the depth of the seismic foci, i.e. at afew kilometers (Berrino et al., 1993).

4. Conclusions

The latest development of the transportableapparatus for absolute gravity measurements,


48


called IMGC-02, fully designed at the laborato-ries of INRIM, was here discussed togetherwith achievements coming from absolute grav-ity data collected in active volcanic areas ofSouthern Italy. Like its previous versions theIMGC-02 particularly proves to be a useful toolfor applications in Geophysics and Volcanolo-gy. The instrument has reached a quite goodlevel of user friendliness in acquiring absolutegravity data, at the microgal level of accuracy,for the surveying of dynamical processes. Thiswas experimentally demonstrated by the repeti-tion of absolute gravity measurements in some

selected gravity stations on volcanoes, specifi-cally at Vesuvio and the isle of Vulcano. The re-cent measurement results, obtained at the ab-solute stations in the Colli Albani area, aretherefore added to the data set collected in thepast.

Acknowledgements

The Authors are very grateful to the refereesfor their suggestions and comments which im-proved the manuscript.

Fig. 7. Record of repeated relative (circle) and absolute (square) gravity measurements at Vulcano.

Fig. 8. Record of repeated relative (circle) and absolute (square) gravity measurements at Vesuvio.


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REFERENCES

BEHNCKE, B., G. BERRINO, G. CORRADO and R. VELARDITA

(2006): Ground deformation and gravity changes onthe Island of Pantelleria in the geodynamic frameworkof the Sicily Channel, J. Volcanol. Geotherm. Res.,150, 146-162.

BERRINO, G. (1994): Vesuvius Observatory contribution:Absolute gravity observations and gradiometry atCenturipe and Serra la Nave ( Mt. Etna) stations, inFinal Report of Science Plan ‘Volcanic Deformationand Tidal Gravity Effect at Mt. Etna’, EEC SCI-ENCE Project No. ERB40002PL900491-(90400491), 28-59.

BERRINO, G.(1995): Absolute gravimetry and gradiometryon active volcanoes of Southern Italy, Boll. Geofis.Teor. Appl., XXXVII (146), 131-144.

BERRINO, G. (1997): Gravity changes and present-day dy-namics of the Island of Pantelleria (Sicily Channel -Italy), J. Volcanol. Geotherm. Res., 78, 289-296.

BERRINO, G. (2000): Combined gravimetry in the observa-tion of volcanic processes in Southern Italy, J. Geo-dyn., 30, 371-388.

BERRINO, G. and G. CORRADO (1991): Gravity changes andvolcanic dynamics, Cahier du Centre Europeen de Ge-odynamique et de Seismologie, 4, 305-323.

BERRINO, G. and G. CORRADO (2006): 25 years of high pre-cise gravity measurements on Italian active volcanoes,in Convegno Nazionale MGMEESV (Metodi Gravime-trico, Magnetico, Elettrico ed Elettromagnetico in Sis-mologia e Vulcanologia) INGV-Sezione di Catania, 27-29 settembre 2006.

BERRINO, G. and G. CORRADO (2007): 1981-2007 gravitymonitoring of Italian volcanoes. International Sympo-sium on Terrestrial Gravimetry: Static and MobileMeasurements (TG-SMM2007), 20-23 August 2007,Saint Petersburg (Russia)

BERRINO, G., L. CANNIZZO, G. CERUTTI and G. CORRADO

(1988): Nuove misure assolute di gravità nell’Italiameridionale, in Atti del 7° Convegno Nazionale Grup-po Nazionale di Geofisica della Terra Solida, CNR,949-958.

BERRINO, G., U. COPPA, G. DE NATALE and F. PINGUE

(1993): Recent geophysical investigation at Somma-Vesuvius volcanic complex, J. Volcanol. Geotherm.Res., 58, 239-262.

BERRINO, G., I. MARSON, L. ORLANDO, L. BALESTRI, R.BALIA, L. BONCI, E. BOZZO, M.T. CARROZZO, G.CERUTTI, C. CESI, M. CIMINALE, M. CRESPI, P. DE

MARIA, F. FERRI, M. LODDO, D. LUZIO, E. PINNA and A.Rossi (1995): Rete gravimetrica Italiana di ordine zero.Stato di avanzamento, in Atti del 14° Convegno

Nazionale Gruppo Nazionale di Geofisica della TerraSolida, 453-460.

BERRINO, G., G. CERUTTI, G. CORRADO, P. DE MARIA and U.RICCARDI (1999): Gravity studies on active italian vol-canoes: a comparison between absolute and relativegravimetry, Boll. Geofis. Teor. Appl., 40 (3-4), 497-510.

BERRINO, G., F. RIGUZZI, V. D’ERRICO, G. RICCIARDI, B.TORO, M. DI FILIPPO, M. DI NEZZA, A. GERMAK, G.D’AGOSTINO and C. ORIGLIA (2007): Dynamics andStructure of the Colli Albani Volcanic District fromgravity measurements, Ann. Geophysics (submitted).

CERUTTI, G. (1967): Determinazione assoluta dell’accele-razione di gravità, Relazione Interna R34 (Istituto Di-namometrico Italiano, CNR).

CERUTTI, G. (1971): Misura assoluta dell’accelerazione digravità, Relazione Interna R59 (Istituto di Metrologia«G. Colonnetti», CNR).

CERUTTI, G., L. CANNIZZO, A. SAKUMA and J. HOSTACE

(1974): A transportable apparatus for absolute gravitymeasurement, VDI-Berichte, 212, 49-51.

D’AGOSTINO, G. (2005): Development and MetrologicalCharacterisation of a New Transportable AbsoluteGravimeter, Ph.D. Thesis (Politecnico di Torino).

D’AGOSTINO, G., S. DESOGUS, A. GERMAK and C. ORIGLIA

(2005a): Measurements of the acceleration due to grav-ity in Santangelo Romano, Palestrina and Castel Gan-dolfo, IMGC Technical Report 140.

D’AGOSTINO, G., A. GERMAK, S. DESOGUS, C. ORIGLIA andG. BARBATO (2005b): A Method to Estimate the Time-Position Coordinates of a Free-Falling Test-Mass inAbsolute Gravimetry, Metrologia, 42 (4), 222-228.

GERMAK, A., A. CAPELLI, G. D’AGOSTINO, S. DESOGUS, C.ORIGLIA AND D. QUAGLIOTTI (2006a): A transportableabsolute gravity meter adopting the symmetric rise andfalling method, in Proceedings of the XVIII IMEKOWorld Congress, Metrology for a Sustainable Develop-ment, September 17-22 2006, Rio de Janeiro, Brazil(on CD-ROM).

GERMAK, A., S. DESOGUS, G. D’AGOSTINO, C. ORIGLIA, D.QUAGLIOTTI, G. CELLI and O. FRANCIS (2006b): Mea-surements of the acceleration due to gravity at the grav-ity laboratory of the National Institute of MetrologicalResearch, Turin, Italy, INRIM Technical Report 33.

GERMAK, A., S. DESOGUS and C. ORIGLIA (2002): Interfer-ometer for the IMGC rise-and-fall absolute gravimeter,Metrologia, 39, 471-475.

ISO-GUM (1992): Guide to the Expression of Uncertaintyin Measurement, BIPM, IEC, ISCC, ISO, IUPAC, IU-PAP, OIML.

NIEBAUER, T., G. S. SASAGAWA, J. E. FALLER, R. HILT and F.KLOPPING (1995): A new generation of absolutegravimeters, Metrologia, 32, 159-180.


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