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AN ABSOLUTE GRAVIMETER COMPARISON IN AUSTRALIA TOWARDS ESTABLISHMENT OF A PRECISE REFERENCE FRAME FOR MEAN SEA LEVEL CHANGE MONITORING IN SOUTHWESTERN PACIFIC

M..Gladwin1, C.P. Smith1, M.Murakami2 B. Murphy3, M. Morse4

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Queensland Center for Advanced Technologies, CSIRO Australia 2643 Moggill Road, Pinjarra Hills Queensland, P. O. Box 883 Kenmore QLD 4069, Australia Geographical Survey Institute Kitasato-1, Tsukuba-shi, Ibaraki-ken, 305 Japan

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Australian Surveying and Land Information Group Scrivener Building, Dunlop Court, Fern Hill Park

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Australian Geological Survey Organization Cnr Constituteion Avenue and Anzac Parade, Canberra GPO Box 378, Canberra, ACT 2601, Australia Abstract A series of absolute gravity measurements were made at three sites in Australia as a first step of on-going bilateral collaboration between Japan and Australia for the purpose of the monitoring of mean sea level changes in the southwestern Pacific region. Three FG5 absolute gravimeters were employed for the gravity measurements. Two instruments were transported from the Geographical Survey Institute (GSI), Japan, and the third is owned by CSIRO. The formal errors of the determined gravity values are about 1 microgal. Semidiurnal and diurnal gravity changes of the amplitude of 2-6 microgals due to ocean tide loading were detected. These data indicate that the measurement accuracy of absolute gravimetry is far superior to accuracies currently attainable by GPS. Long term vertical crustal movements stable to 3 mm are achievable provided effects due to long term fluctuation of water table can be adequately documented. I. Introduction The purpose of the project is to establish and maintain a precise geodetic reference framework in the southwestern Pacific region for long-term monitoring of mean sea level changes induced by global warming. Tide gauge records, traditionally used to monitor sea level changes are usually contaminated by local vertical crustal movements. Absolute gravimetry, which has superb sensitivity in detecting the vertical crustal motion, can

provide an excellent determination of vertical crustal movements for use as a reference for tide gauges. The measurements may also be used for establishment and maintenance of a series of sites which may become part of the International Absolute Gravity Base Net (IAGBN). Good agreement among FG5 instruments has been demonstrated (Niebauer et al., 1995), and this experiment provided an opportunity for further intercomparison of independently maintained FG5 absolute gravimeters. A site at Mt. Stromlo observatory was chosen for the intercomparison, and the position of this site was measured using GPS and the measurements were supported by levelling and local meteorological data. A. Measurement Results Data were processed in accordance with the IAGBN standard procedure (Boedecker, 1991), which means that corrections are applied for 1)light travel time, 2)earth tide with an amplitude factor of 1.164 for the tides varying with time, 3)earth rotation changes evaluated by IERS observed parameters, 4)atmospheric pressure using the response coefficient of 0.30, 5)instrumental height reduction to a marker using vertical gravity gradient measured by LaCoste&Romberg gravimeters. All instruments were equipped with high speed comparitors for the measurements as recommended by Niebauer et al., 1995. Gravity values were transferred to one reference point on the floor using a gradient of 2.9558 microgal/cm measured by two LaCoste & Romberg "D" relative gravimeters. The absolute error estimate for the FG5Õs is quoted at 2 microgals (Niebauer et al. 1995). In fact, the three instruments agree to better than 1 microgal averaged over a three day period, when the height corrections of approximately 7mm (2.1 microgals) between the monuments is included. Table 1. Absolute gravity measurements by three instruments at Mt. Stromlo. gravimeter period no. of measurements mean gravity value on Mt Str PT (mgal)* std. dev. of a single drop (mgal) #104 02/08/9602/11/96 13354 979549.9230 0.015 #201 02/08/9602/14/96 22959 979549.9235 0.012 #110 02/08/9602/13/96 16236 979549.9228 0.012

* transferred to the same reference point, Mt. Stromlo PT

A. Gravity Change due to Ocean Tide Loading During the observation at the three sites, signals of gravity change due to ocean tide loading were detected. Figure 1 shows a time series of observed gravity at Mt. Stromlo. The data were corrected for solid earth tide (Cartwright and Edden 1973) to identify the ocean load corrections at the site. These residuals due to the ocean load determine the effective accuracy of short term observations of absolute gravity at a tidal reference site.

Mt Stromlo 9-13 February 1996 9.79550100

9.79549850

9.79549600

40.5

41

41.5

42

42.5 43 Julian Day

43.5

44

44.5

9.79550100

9.79549850

Mean

9.795499201 ms-2

= 12.41 µ Gal 9.79549600 40.5 41 41.5 42 42.5 43 Julian Day 43.5 44 44.5

Figure 1: Data obtained from the CSIRO FG5 both before and after removal of the solid earth tides. The variation in signal on the lower plot is the ocean load tide.

I.

Conclusions

The measurement results indicate the capability of absolute gravimeters to detect the change in gravity of 1 microgal, which corresponds to the change in height of 3 mm.

Although the instruments have been demonstrated reliable at better than 1 microgal, long term comparative measurements (e.g., annual repeated measurements), may be contaminated by environmental factors such as pressure and more particularly, ground water variations. Therefore, combination of gravimetry with other observations including water table observation and positioning by GPS or VLBI will be required to identify the cause of gravity change. GPS is another important tool for mean sea level monitoring. However, while the gravimeters can give long term repeatability to better than a few mm, the precision of vertical positioning by GPS does not reach that level. For instance, Ananga et al. (1995) studied the stability of tide gauge bench marks and the precision of height measurements by GPS in southern Australia. They showed that the precision of the height measurements were in the range of ± 4.2 cm, and probably dominated by the GPS uncertainties. This is larger than attainable with absolute gravimetry by an order of magnitude. Hence, until GPS vertical measurement precision can be significantly improved, it will be advantageous to combine these two techniques to improve the quality of sea level monitoring by calibrating GPS tide gauge data to simultaneous GPS data at nearby absolute gravity reference sites for long term stability. Acknowledgment Part of this study was supported by the Science and Technology Agency, Japan in fiscal year 1994 and 1995. References Ananga, N., R. Coleman and C. Rizos, Geodetic Monitoring of Tide Gauge Bench Marks with GPS, Jour. Geod. Soc. Japan, 1995, 41, 91-97. Boedecker, G., IAGBN: Absolute Observations Data Processing Standards, BGI-Bull. dÕInformation, 1991, 69, 25. Cartwright, D.E., and Edden, A.C., Corrected Tables of Tidal Harmonics, Geophys.J.R.Astr.Soc., 1973, 33, 253-264. Francis, O. and P. Mazzega, Global Charts of Ocean Tide Loading Effects, J. Geophys. Res., 1990, 95, 11411-11424. Ma, X. C., C. Shum, R. Eanes and B. Tapley, Determination of Ocean Tides from the First Year of TOPEX/POSEIDON Altimeter Measurements, J. Geophys. Res., 1994, 99, 24809-24820. Niebauer, T and G. Sasagawa, J. Faller, R. Hilt and F. Klopping, A New Generation of Absolute Gravimeters, Metrologia, 1995, 32, 159-180.

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