Mount Stromlo Gravity Station Gravimetry at the ANU

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1 ANU COLLEGE OF SCIENCE RESEARCH SCHOOL OF EARTH SCIENCES Canberra ACT 0200 Australia Mount Stromlo Gravity Station Gravimetry at the ANU Background Terrestrial observing has been conducted at Mount Stromlo since its early days. A Photographic Zenith Tube (PZT) operated from 1961 to 1990 providing astronomical calibration of quartz clocks and later, combined with the stable atomic clocks to enable precise monitoring of the variations in Earths rotation rate. However very few gravity measurements were made in the area until 1996 when a combined observation was organised by AUSLIG with additional support from the National Tidal Facility and AGSO. On this occasion, gravity determinations were made simultaneously over five days by three FG5 absolute gravimeters, two belonging to the Geographical Survey Institute of Japan, GSI, and one from the CSIRO Division of Exploration and Mining. These measurements established the first absolute gravity benchmarks at the observatory. The Japanese instruments were also operated for several days at Tidbinbilla and at a purpose built facility at Mt Pleasant in Hobart and details of this campaign can be found on the AUSLIG CD : "Australia 1996 Absolute Gravity Campaign". The Superconducting Gravimeter In January 1997 a highly sensitive Superconducting Gravimeter (SG) was installed in a basement lab adjacent to the absolute benchmarks by the National Astronomical Observatory of Japan as part of its Ocean Hemisphere Network. It has been operating continuously since then with only minor data gaps. The SG is the most sensitive form of gravimeter, and is capable of detecting periodic variations in the strength of Earth's surface gravity at the order of

2 one part in The instrument is operated in collaboration between the Geodynamics group in the Research School of Earth Sciences at ANU and NAO Mizusawa. The Mount Stromlo SG CT031 is a Compact Tidal Superconducting Gravimeter manufactured by GWR Instruments in California. At the core of the instrument is a superconducting niobium sphere cooled to liquid helium temperatures and levitated by 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 a relative gravimeter but one that is extremely stable over long periods, and therefore ideal for normal mode, tidal and longer period observations. Calibrations In order to get the best performance it is necessary to calibrate the SG and to measure any instrumental drift periodically by making absolute gravity determinations at the site. The absolute gravity determinations made by the three FG5 instruments in 1996 provided a starting point but predate the SG installation. Over the following decade there have been several calibrations, two using the CSIRO FG5, two using the INSU (Strasbourg) FG5, and one using the Kyoto University FG5. For best results, absolute calibrations should be made every 6 months or so, but over that period we only managed one every 18 months. In addition to calibrating the SG itself, it is also necessary to separate the secular variation of gravity at the site both to determine the instrumental drift and to enable intercomparison with other sites around the world. These AG measurements therefore also provide a record of the long term stability of the site. Local ties to nearby reference marks have been made using Lacoste-Romberg gravimeters by Geoscience Australia. The Lacoste-Romberg is another relative instrument but one that is more portable and can be separately calibrated at another site. The local ties provide insurance in case there is a significant change in the mass distribution near the primary mark that cannot be otherwise quantified. Tides The changes in surface gravity caused by the changing relative positions of the Earth, Sun and Moon raise tides in the solid Earth as well as the more familiar ocean tides. In fact the ocean

3 tides also deform the solid Earth and contribute an additional ocean load component to the Earth tides at continental sites. Tides with periods from hours to years are seen in sensitive gravity records and their amplitude and phase contain information on the internal properties of the Earth. The exceptional long term stability of the SG makes it an ideal instrument for observing the longest period tides. Free Oscillations The Earth has a wide range of normal modes of free oscillation which are excited like the vibrations of a drum by sudden impulses. Large earthquakes are known to trigger these oscillations and the measurement of their frequencies in seismic records has given us a much clearer picture of the internal structure of the Earth. The Canberra SG was instrumental in demonstrating that these oscillations are also generated continuously at a low level, probably by processes operating on the surface of the Earth. These background free oscillations constitute the steady hum of the planet. mode. 1mHz is about 17 minutes After the 2004 Boxing Day earthquake in Sumatra, the SG was able to observe and record free oscillations of the Earth continuously for over three months. The quality of that record can bee seen here in comparison with the spectrum of oscillations produced by an earlier large earthquake. Each peak represents a different frequency of vibration corresponding to a particular shape of deformation. 0S0 is the spherically symmetric mode while 0S2 is the football Beyond the range of mantle oscillations so far observed are oscillations of the inner core of the Earth (Slichter modes) and possibly oscillations in the liquid outer core where the Earth s magnetic field is generated. Very large magnitude deep earthquakes have the potential to excite these oscillations to a level that the ultra sensitive SG may be able to detect and detection and characterization of these oscillations remain a long term goal of the instrument.

4 International Role SG-CT031 is now 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 a data archive facility at the International Centre for Earth Tides (ICET), and is used to study the dynamics of the Earth's deep interior and infer details of Earth s internal structure. Results from the SG, along with absolute gravity determinations at the site, also provide a more accurate reference frame for secular gravity change and vertical movement measurements in Australia and contribute to studies of the mechanisms of relative sea-level change around Australia. FG5 Absolute Gravimeter In 2009 ANU acquired an FG5 Absolute Gravimeter through the AuScope Geospatial infrastructure program. In addition to monitoring absolute gravity and crustal stability at a network of stations around Australia (the Australian High Precision Absolute Gravity Network, AHPAGN). Annual measurements are taken at Perth, Wagin, Yarragadee, Ceduna, Alice Springs, Darwin, Townsville, Tidbinbilla, Mount Stromlo, Melbourne and Hobart and additional sites are being constructed. At the Mount Stromlo gravity station, the FG5 observes on new benchmarks established in the recently refurbished Reynolds Dome and its presence now allows calibration of the Canberra SG at a more desirable frequency than previously possible. gphone Tidal Gravimeters The Mount Stromlo gravity station is also the base for an Australia wide observing program attempting to measure the variation of long term ocean tide deformations affecting continental sites. The magnitude of these deformations is such that that they now affect the precise GPS height measurements used in environmental and long term sea level monitoring so their evaluation is an important step in improving the accuracy of these measurements. The three gphones are deployed at field sites for 6-9months to collect an accurate record of the ocean load tide. They have so far been run at Yarragadee, WA, and Jabiru, Katherine and Tennant Creek in the NT and are now running at Broome and Karratha in WA.

5 Related Material ANU/AuScope Gravity program: GGP Web Page: Data Centre: ggp.gfz-potsdam.de Network of Superconducting Gravimeters benefits a number of disciplines, D. Crossley et al., EOS, Trans. AGU 80, , On the observations of gravity tides and Earth's free oscillations with a superconducting gravimeter CT#031 at Mt. Stromlo, Australia. T. Sato et al., Proceedings of the 13th International Symposium on Earth Tides, Brussels Incessant excitation of the Earth s free oscillations: global comparison of superconducting gravimeter records. K. Nawa et al., Phys. Earth Planet. Interiors, 120, , The planet that hums. R Coontz, New Scientist, 11 September 1999 High-resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data, S. Rosat et al, Geophys. Res. Lett., 32, 3, L13304, 2005 doi: /2005gl Absolute gravity measurements and calibration of SG-CT031 at Canberra, M. Amalvict et al. J. Geodetic Soc. Japan, 47, , Phoenix rises from the ashes, ANU Reporter 34, 1, 5, 2003