In February 1996 absolute gravity determinations were begun at Mount Stromlo by three FG5 absolute gravimeters, two belonging to the Geographical Survey Instiute of Japan, and one from the CSIRO Division of Exploration and Mining. These observations were organised and supported by AUSLIG with additional financial support from the National Tidal Facility and AGSO and form the basis of ongoing gravity studies at the site.
In January 1997 a Superconducting Gravimeter was installed in a basement lab adjacent to the absolute benchmarks by the National Astronomical Observatory of Japan and has been operating continuously since then. The instrument is operated in collaboration between the Geodynamics group in the Research School of Earth Sciences at ANU and NAO Mizusawa.
In 2008 an FG5 Absolute Gravimeter and several portable tidal gravimeters were acquired under the AuScope gravity program. The FG5 is used for maintaining a national network of absolute gravity reference stations, for calibrating and testing relative instruments including the SG, and for other geodynamics projects requiring high precision absolute gravity. The gPhone relative tidal gravimeters are being used to map the ocean load tide around Australia.
RSES operates a Superconducting Gravimeter at Mt Stromlo Observatory, continuously monitoring small changes in gravity caused by tidal, environmental and seismic effects.
The Mount Stromlo gravimeter is a GWR Instruments manufactured Compact Tidal Superconducting Gravimeter designated CT031. Operating at liquid helium temperatures, the instrument is built around a superconducting niobium sphere levitated in a magnetic field in an evacuated chamber. Gravity fluctuations are determined from changes in the feedback coil current required to precisely centre the sphere in its chamber. The SG is capable of detecting variations in the strength of Earth's surface gravity at the order of one part in 10^12 . The SG is extremely stable over long periods, and therefore ideal for normal mode, tidal and longer period observations.
SG-CT031 is part of a world-wide array making precise observations of global gravity signals. Data is shared with other observers in the Global Geodynamics Project (GGP) network through The ICET data archive facility and used to study the dynamics of the Earth's deep interior and infer details of its internal structure. Continuous observations have been conducted at the site since 1997 with occasional interruptions, the most serious of which was the 2003 fire which destroyed the building and interrupted the record for two months. Originally installed by the National Astronomical Observatory of Japan, the instrument was transferred to ANU March 2008 and became part of the AuScope gravity program.
Through the AuScope program RSES has acquired aMicro-g LaCoste FG5 absolute gravimeter. The FG5 gravimeter operates by measuring the free fall of a corner cube in a vacuum chamber using a highly stable laser interferometer and a Rubidium clock. It is capable of 2-3 microgal accuracy in 24 hour site occupations. The FG5 will be used initially for monitoring the stability of core benchmarks across the Australian continent, providing reference points for ties to sea level monitoring stations and for instrument calibration.
Tidal gravity: gPhones
A gPhone is a metal spring relative gravimeter manufactured by Micro-g LaCoste. It is derived from geophysical survey instruments but specifically optimised for measurements in the tidal frequency band by component screening, improved the temperature stability with double oven controls and better vacuum seals for improved pressure stability. In addition the gPhone is equipped with a precise GPS locked Rubidium clock for accurate timing of the phase of tidal constituents.
Three gPhones are being used to improve the accuracy of ocean tide loading models around Australia. One has just finished observing at Jabiru in the Northern Territory and two are deployed near Katherine and Tennant Creek.
Use of the AuScope funded FG5 absolute gravimeter is open to all members of the AuScope Community. Prospective users should request access by submitting the application form below.
Access to the instrument will be granted subject to the availability of trained support staff and the demands of core observing program
It is a condition of use that any publication of results produced through the use of the AuScope equipment should contain an acknowledgement of the form:
"Data have been obtained using facilities supported through the AuScope initiative. AuScope Ltd is funded under the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Commonwealth Government Programme."
To apply for access to the instruments for research projects please complete the application form below and email or fax it to:
AuScope Gravity Program
Research School of Earth Sciences
Australian National University
Canberra ACT 0200
Fax 02 6257 2737
Establishment of an Australian High Precision Absolute Gravity Network (AHPAGN)
A network of ten absolute gravity stations will be established around 100 km from the coast for measurement of reference frame height variation and to complement existing core geodetic sites. New absolute gravity sites will be chosen based on ideal geodetic site selection criteria to allow the highest accuracy measurement of the vertical component to propagate to infrastructure dependent on the new AuScope network. Absolute gravity is an independent measurement technique for monitoring the vertical datum, and can be used to confirm CGPS network vertical rates. Many existing geodetic sites are at non-ideal locations from an absolute gravity perspective and close monitoring of continental deformation into the future requires a new absolute gravity network where ideal absolute gravity can be obtained. These sites will be complemented with CGPS. The time series gravity data will provide independent confirmation of the vertical deformation rates of the continent.
Ocean load tide measurement at Australian geodetic sites
The ocean load component of the Earth tide varies from point to point depending on the regional ocean tide variation and the position of the observation relative to the coast. It is believed in some parts of Australia the ocean loading modelled values do not sufficiently represent the actual geophysical signal and the errors in the ocean loading terms are causing large residuals in processed GPS data. This project involves measurement of the ocean load component of the diurnal and semi-diurnal Earth tides at sites across Australia at a density sufficient to map its spatial variation. It is expected to require 3 - 6 month observations to obtain the high precision spectra at several tens of stations. Gravity measurements of this tidal component at locations across the continent will be compared to model predictions in order to determine the best fitting deformation model to use in correcting GPS and VLBI observations.
Operation & Calibration of Superconducting Gravimeter CT031
The Superconducting Gravimeter at Mt Stromlo provides a continuing high precision record of the time variation of gravity at the Australian site of the worldwide GGP network for studies of Earth deformation and internal structure. Data is shared with other observers in the Global Geodynamics Project (GGP) network through The ICET data archive facility. Operation of the site requires routine maintenance, annual refilling of the liquid helium dewar and refurbishment of the coldhead. Regular calibration against the FG5 absolute gravimeter is also necessary and is performed by simultaneous observation on adjacent piers through a number of tidal cycles. The records are cross correlated, making allowance for characteristics of the different instruments, to determine a reference gravity value and a scale factor for the SG. This permits collection of the most sensitive record of tidal and free oscillation gravity signals in this part of the world.
Equipment maintenance and quality control
The value of the other projects depends critically on the fact that accuracy and repeatability of the observations can be demonstrated and that the individual system parameters traced to primary standards. Repeat observations at a well calibrated and documented site and inter-comparison with visiting absolute gravimeters are used to confirm operational accuracy. In addition, testing of the effects of changes to the equipment configuration and regular monitoring of equipment parameters makes it possible to detect anomalies and systematic trends which may require correction.
Absolute gravity ties between AG benchmarks in the ACT
The objective is to update the AG ties between AG benchmarks at Mt Stromlo gravity staion, the reference site at the nearby Stromlo seismic vault and the AG mark at Tidbinbilla DSCC. The 2003 fire at Mt Stromlo, which destroyed the building housing the SG, disrupted monitoring of absolute gravity at the original benchmarks and the prospect of further changes during rebuilding led to the establishment of a reference AG site 1km from the gravity station. Several AG measurements have been taken at this site and the other nearby AG mark at Tidbinbilla DSCC since the fire. Now that reconstruction is complete we need to close the loop by completing the ties between the sites again.