Two Decades of High Precision Gravimetry, GGP, and Prospects for the Future

1. Two Decades of High Precision Gravimetry, GGP, and Prospects for the Future David Crossley Earth and Atmospheric Sciences, Saint Louis University, St. Louis, USA Jacques Hinderer IPG/EOST Strasbourg, France

2. Thank you … • Cheinway Hwang, • CW Lee, • Ricky Kao, and other workshop organizers, • and financial sponsors for the opportunity to participate in the Workshop and visit Hsinchu

3. Outline SG beginnings 1967-80 Early installations 1981-89 GGP activities 1990-96 GGP accomplishments 1997-2007 covered in Workshop: • seal level, • general geophysics and geodynamics • earthquakes • ocean tides • data fusion • calibration • hydrology • AGs • GRACE • geoid height Prospects for the future

4. SG beginnings 1967-80 40 yr Prothero, W. A., 1967. A cryogenic gravimeter, Ph. D. thesis, Univ. of Calif. at San Diego, La Jolla. Prothero, W. A., and Goodkind, J. M., 1968. A superconducting gravimeter, Rev. Sci. Instrum., 39, 1257-1262. Prothero, W. A., and Goodkind, J. M., 1972. Earth tide measurements with the superconducting gravimeter, J. Geophys. Res., 77, 926-932

5. SG beginnings … Prothero and Goodkind (1972) tide removal (no pressure correction)

6. Prothero and Goodkind: first SG analysis the tides!

7. the second topic normal modes analysis! Kamchatka 7.1

8. SG beginnings - GWR Warburton, R. J., Beaumont, C., and Goodkind, J. M., 1975. The effect of ocean tide loading on tides of the solid earth observed with the superconducting gravimeter, Geophys. J. R. astr. Soc.,43, 707-720. Warburton, R. J., and Goodkind, J. M., 1977. The influence of barometric-pressure variations on gravity, Geophys. J. R. astr. Soc.,48, 281-292. Warburton, R. J., and Goodkind, J. M., 1978. Detailed gravity-tide spectrum between one and four cycles per day, Geophys. J. R. astr. Soc., 52, 117-136. 30 yr (these 3 papers should be on the reading list of all SG researchers)

9. The Gamblers The Pioneers formed GWR in 1979 as a commercial venture purchased and installed instruments John Goodkind - the inventor Richard Warburton – the innovator Richard Reineman – the backroom wizard Paul Melchior (1981) Bernd Richter (1981,85) Hou-Tse Hsu (1986) Jacques Hinderer (1987)

10. Shanghai Observatory 1981 King of Belgium Melchior

11. Early SG Installations 1981-89 • Richter – first SG installed at Bad Homburg (near Frankfurt), former wine cellar of castle

12. Models TT40 (1981) and TT60 (1985) and the first parallel recording over a period of 10 months, showing agreement to a few 0.1 uGal

13. … and inside

14. … compare to modern version 2007 MunGyung, Hsinchu …

15. a notable publication of this era by B. Richter Start of 20 yr retrospective

16. Variation of local pressure admittance amplitude and phase variations with frequency amplitude variation with time

17. … and the famous variation of gravity due to polar motion

18. Canadian SG (Cantley, 1989) • delicate electronics in humidity controlled rack • gravimeter and levellers surrounded by styrofoam insulation (here partially removed) to protect from room temperature changes (± 3º C) • yes there is air conditioning Reinstallation 1995

19. 1990-96, start of GGP

20. 2 months later... ... at Strasbourg!

21. some of the rationale …

22. initial studies

23. GGP today GGP is now an Inter - Commission Project of IAG (like WEGENER) reports to: Commission 3 – Earth Rotation and Geodynamics Commission 2 – The Gravity Field (Inter Commission Project 3.1) until IUGG 2007: Chair: D. Crossley Secretary: J. Hinderer activities: Meetings: 1 per year (next - IUGG Perugia) Workshops: 1 every year or two (Hsinchu) Newsletters: as needed

24. GGP mission • maintain standards for SG instrument siting and data recording • provide means for data exchange and accessibility • foster discussion of scientific issues Scientific goals have not changed • studies of solid earth and ocean tides and tidal loading • atmospheric pressure changes to gravity • earthquakes and normal modes • geodynamics processes, e.g. sea level changes • hydrology at various length and time scales • seasonal variations, and long-term tectonics

28. Current European SGs, and possible network extensions N Germany Pecny

29. Months of data at ICET for GGP stations

30. SG station recording history

31. SG station reporting

32. newsletters …

33. GGP & GGOS (Global Geodetic Observing System) • Provide access to GGP database – expand GGP mailing list to GGOS representatives (Newsletters etc.) • Undertake a project within GGP to record and report on all GPS measurements at the stations – these are necessary anyway to account for height variations that contribute to gravity variations • Undertake a project within GGP to record and report all Absolute Gravity measurements made at the GGP sites – these would be benchmark measurements (one point with error bar and supplementary information) • Assist in the coordination of future Absolute Gravimeter Intercomparisons at a site (or sites), where there is an SG • Be receptive to joint initiatives in geodesy or tectonics where the use of an SG would significantly improve the interpretation of measurements from other instruments.

34. Earthquake studies • An SG (or accelerometer) has two responses to an earthquake: • normal modes • for Mw > 6.0 can be seen globally • static displacement • can be seen only close to source PhD thesis M. Van Camp

35. Bad Homburg – lower sphere 36 days following Sumatra (12/26/04) tides nominal tides removed

36. note upper and lower spheres are almost identical spectrum of 36 days

37. static displacements, Bolivia 1994, Mw=8.4, very localized

38. Static earthquake displacements from satellites - Alaska 1964 • Mikhailov et al., 2004. Can tectonic processes be recovered from new satellite gravity data? EPSL,228, 281-297.

39. … but if you are careful (and lucky) in Japan Mw 8.0 Tokachi-oki earthquake on Sept. 2003 off the coast of Japan

40. Using 0S0 for GSN calibration • Davis, Ishii, and Masters (2004)* used 95 stations from the global seismic network (GSN) to measure amplitude of 0S0 • they assumed we know f=0.8146 mHz, Q=5400  measure initial amplitude A0 excited by earthquake • they used two techniques and found a range of values for the initial amplitude, depending on instrument, and commented that • “Superconducting gravimeters also recorded 0S0 very well … A sampling of these data indicate the GSN mean is about 4% larger than measurements at several superconducting gravimeters thought to be calibrated to better than 0.5% (Widmer-Schnidrig, personal communication). Resolving these and other inconsistencies poses an interesting challenge to the GSN station operators …” *An assessment of the accuracy of GSN sensor response information, Seis. Res. Lett, 76, 678-683

41. Seismic amplitude vs latitude

42. Gravity vs latitude

43. 0S0 from Sumatra-Andamen 2004 done with student Yan Xu red circles= 13 SG stations green circles= 13 GSN stations epicenter Mw=9.3

44. Seismometer amplitude response 0S0

45. Method assume a damped cosine with amplitude Following Nowroozi (1968), the amplitude of the spectral peak from a data set between times t1 and t2 can be expressed as: we estimate A12 for each 72 hr window starting 2 hr after earthquake and displaced 1 hr until last window reaches end of day 36 (31 January 2005)

46. Examples of amplitudes and Q from SGs MC

47. preliminary result of comparison of 13 SG data sets with 13 STS1 and STS2 data sets  SG amplitude is more consistent (less scatter), but about 10% higher than Davis et al. seismometer amplitude histogram SG amplitude histogram

48. Fig. 7 Parallel measurements with FG5-220 (IfE) and FG5-221 (FGI) at station Metsähovi in Finland. Comparison SG and AG SG Medicina AG vs AG Metsahovi

49. Spectral comparison AG-SG differences at > 6 mo depending on SG drift various AG spectra meet at ~ 3 day composite SG Van Camp et al. (JGR, 2005)

50. Instructive comparison of AG, SG, and hydrology over 8 years