ICAG2001: 3ª Asamblea Hispano-Portuguesa de geodesía y geofísica

Preliminary Results of the International Comparison of Absolute Gravimeters ICAG2001 with the FG5#211.

Enrique Rodríguez Pujol(1), María Fernández de Villalta(1), Eduardo Núñez Maderal(1)

(1) Instituto Geográfico Nacional. General Ibáñez Ibero 3, 28003 MADRID.

Summary

Since 1981 Institutions from several countries gathered to measure absolute gravity every four years in Sèvres at the Bureau International des Poids et Mesures (BIPM). Different absolute gravimeters try to obtain a reference pattern in the measurement of g. In July 2001 took place the sixth International Comparison of Absolute Gravimeters ICAG2001. In this article we show the preliminary results with the absolute gravimeter FG5#211 with which the National Geographic Institute of Spain participated for the first time.

Antecedents

Since 1960 when J. E. Faller developed his first prototipe, new generations of devices to measure the absolute value of gravity aceleration are being built. These equipment measure the gravity aceleration from pairs of time and distance of a free falling body, reaching precisions of 1 µGal (1µGal = 10-8 m/s2). After adquisition of FG5#211 absolute gravity meter device (built by the United States company Micro-g Solutions, Inc.) late year 2000, first goal planned by Instituto Geográfico Nacional was its international validation of data quality to be obtained: to attend to the Sixth International Intercomparison of Absolute Gravimeters (ICAG2001) held in Sèvres, París, July 2001. This intercomparison takes place every four years in the Bureau International des Poids et Mesures (BIPM) since 1981. Prime objective of ICAG2001 will be, as previously meetings, to find the most probable g value for a site using different gravimeters. We seek the accuracy in gravity value using the most accurate devices in gravity measure ever made. This absolute gravity equipment includes high precision time and distance measured components, which will be calibrated with higher standard patterns in BIPM.

Introduction

With FG5#211 we intend to observe a new network called "Red Gravimétrica Española de Absolutas de Orden Cero (REGA)" which will hopefully consist of at least 20 peninsular and 11 insular stations (una en cada isla principal). First observation has been made in a pie close to the one used by Joaquín Barraquer in his absolute 1883 measurement with two Repsold pendulum gravimeters. IGN ( Gravimetry Laboratory), San Pablo de los Montes, Sonseca and two stations in Centro Español de Metrología have already been measured. The principal aplications of absolute measurements of gravity can be included in Geodesy, Geophysics, Earth Geodynamics and Metrology fields. The FG5 is an absolute gravimeter based on free fall mesurement technique. A corner cube falls in a chamber where previously exists a high vacuum. An in-line Mach-Zender interferometer, different from previous designs, providing a higher measurement stability is used. FG5 wears an inertial reference system called SuperSpring that isolates measurements from floor high frequency vibrations. Use of a length standard HeNe iodine stabilized laser in frequency (absolute frequency precision of 1 part in 1010) and time, and an atomic clock (uncertainty of 5 parts in 1011), provide best results ever reached, with an individual instrumental uncertainty of a measurement of 1.1 µGal (T.M. Niebauer, et al., 1995). FG5 is an equipment built to be tranportable, with an aproximate weight of 240 kg delivered in eight boxes. Absolute gravimeter comparisons date from 1976 in Sévres, having took place five times since then. Since 1981 IAG recognize the need to celebrate periodic absolute gravimeter comparisons to detect posible sistematic errors and define the accuracy level of methodology. Final results are reduced by relative observations to point A0 at 90 cm height.

Sixth Intercomparison ICAG2001

One of the main points in order to attend to ICAG2001 with FG5#211 is to maintain the proper vacuum level inside the free fall chamber. Vacuum pressure level is maintained by an ion pump under 1 x 10^-4 Pa (Niebauer T.M. et al (1995), thus it must be connected to a power suply 24 hours a day.

Table 1 shows the working groups who atended to ICAG2001. BIPM gravimeter (FG5#108) measured 24 hours a day during the whole campaign to get a continous record in order to detect gravity field anomalies. El esquema de medidas consistirá en observar una red de 5 puntos (puntos B, B1 y B2 en Pavillon du Mail, A y A2 en el Observatorio) con los 14 gravímetros participantes.

Based on prepared agenda and in processing conditions for observations, our work consisted of measuring in three points: A y A2 in Observatory and B in Pavillon du Mail.

The main objective of ICAG2001 is to reach an experimental estimate of accuracy level of absolute gravimeters. This estimate is obtained when giving a mean solution with al measurements reduced to a point and to a particular height (basic ideas of processing are showed in Becker M. 1990).

Processing "g" software by Micro-g Solutions Inc. has been adopted. In order to observe during 24 hours (minimum time condition in every point), equipment is programmed to save 24 sets of 100 drops each one, to obtain an statistical consistent result.

Gravity aceleration value aj, observed in time Tj, is corrected to obtain gravity gj in each fall in this way (base in the data processing satandards for "International Absolute Gravity Basestation Network" IAGBN, International Gravity Comisión, Boedecker 1988 and following Sasagawa et al. notation 1995):

where two first terms d_gMT y d_gCO, are earth tide and ocean load tide corrections, corrected by using ETGTAB software (L. Timmen y H.G. Wenzel, 1994) using potencial model of Tamura 1987 and predictions of earth tide (semiempirical model) and observed parameters in Sèvres provided by International Centre for Earth Tides (ICET), shown in Table 2.

Next term d_gMP corrects pole movement using daily pole coordinates x, y published by IERS (International Earth Rotation Service). For each term dg_ATM a correction of -0.3 µGal/mbar is applied for pressure changes over a standard pressure model depending on site height. Last term, dg_DATUM is reduction to a common height datum.

In order to apply this reductions to a common datum relative gravimeters are used, which also participate in ICAG2001, providing vertical gravity gradients for every observed site. This values are applied in the absolute gravity processing (Robertson L. et al. 2001).

During la semana de trabajo en el BIPM, comparisons against BIPM patterns of FG5#211 laser and clock were carried out. FG5#211 uses He-Ne WEO Model 100 S/N 176 with l"633 nm. Compared to BIPM4 stabilized laser, it has reached system requirements (Niebauer T.M. et al. 1995) with a mean uncertainty in compared frequencies of 3 parts in 10^-11.

For the rubidium atomic clock an Standford Research System 620/1 has been used, stabilized with a BIPM cesium oscilator as source of reference over the rubidium clock. Final result is clock frequency determination f = 9.999 999 995 05 MHz with an standard deviation of 0.15 mHz.

Preliminar results

In Table 3 results obtained with FG5#211 are shown. Site B value is 980 928 012.40 µGal with an uncertainty of 1.26 µGal. This value has been obtained from 78 sets of 100 drops per set, being 1.294 m the instrumental height. Vertical gradient at this site is 2.957 µGal/cm which serves to transfer observed gravity to the common reference height of 0.9 m.

Thus there are redundant measurements in every site. In column (h / m) effective measurement height (Niebauer T.M., 1989) of FG5#211 and followed by the gravity value observed at this height and its standard deviation u(set) of the mean in that set. We include as wel the vertical gradients of gravity observed with Lacoste Romberg relative gravimeters that attend to ICAG2001. Last two columns represent final result of g reduced to DATUM 0.9 m.

Press here to display the table

• References

1. Becker M. (1990): Adjustment of microgravimetric measurements for detecting local and regional displacements. Proc. Symp. No. 103, Agosto 1989, Edimburgo (Editado por R. Rummel y R.G. Hipkin), 1990, 149-161
2. International Gravity Commission - Working Group II " World Gravity Standards " ; (Chairman Gerd Boedecker), Bull. Inf. Bur. Grav. Int., 1988, 63, 51-57.
3. Niebauer T.M. et al (1995): A new generation of absolute gravimeters. Metrologia, 1995, 32, 159-180.
4. Niebauer T.M. (1989): The effective measurement height of Free-Fall absolute gravimeters. Metrologia, 1989, 26, 115-118.
5. Robertson, L. et al (2001): Results form the Fifth International Comparison of Absolute Gravimeters, ICAG97. Metrología, 2001, 38, 71-78.
6. Sasagawa G.S. et al.: Analysis of data form the 1994 International Comparison of Absolute Gravimeters with a single computational protocol. Metrología, 1995, 32, 185-192.
7. Timmen L., H.G. Wenzel: World wide synthetic paramenters available on Internet. Bur. Grav. Int., 75, Diciembre 1994.