Factors Affecting the Detection of a Soil Moisture Signal in Field Relative Gravity Measurements

1. Factors Affecting the Detection of a Soil Moisture Signal in Field Relative Gravity Measurements 1Adam Smith, 1Jeffrey Walker, 1Andrew Western, 1Kevin Ellett, 1Rodger Grayson, and 2Matthew Rodell Department of Civil and Environmental Engineering, University of Melbourne, Australia 2. Hydrological Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, USA http://www.civenv.unimelb.edu.au/~jwalker/data/gsm/hydrograce.html Western Pacific AGU Geophysics Meeting, Hawaii August 2004

2. Has not yet been demonstrated To give a “low effort” integrated measure of change in terrestrial water storage (deep soil moisture and groundwater) To improve model prediction (via assimilation) of root zone soil moisture To aid the development of methods to utilise GRACE gravity data Measured in Gal; 1 µ Gal ~ 2.5 cm water OR 2%v/v soil moisture over a 2.5m deep layer Why detect soil moisture changes with gravity?

3. Absolute gravimeters measure gravity by dropping a corner cube Cons Expensive (~ US$300,000) Difficult transportation (dedicated van) Long station occupancy (~ 1/2 day) Field meters have low accuracy (~ 10 µ Gal) Why relative gravity measurements? FG5 FG5-L A10 50 µ Gal 2 µ Gal 10 µ Gal

4. Relative gravimeters measure gravity by levitating a sphere in a magnetic field, or spring extension Pros Cheap (relatively!) (~ US$50,000) Easier transportation (though still an issue...) Shorter station occupancy (~1 hour) Field meters have high accuracy (~ 3 µ Gal) Why relative gravity measurements? SG G CG-3M 0.01 µ Gal 3 µ Gal 3 µ Gal

5. Site locations http://www.civenv.unimelb.edu.au/~jwalker/data/oznet

6. Typical soil moisture site

7. Factors affecting relative gravity readings • Mechanical • Drift ~40 µ Gal/day 392 µ Gal/day linear drift already removed

8. Factors affecting relative gravity readings • Mechanical • Drift • Post-transport stabilisation Stabilisation ~25 µ Gal 1.5 hr

9. Factors affecting relative gravity readings • Mechanical • Drift • Post-transport stabilisation • Internal temperature

10. Factors affecting relative gravity readings • Mechanical • Geodynamical • Solid earth tides uncorrected ~100 µ Gal corrected

11. Factors affecting relative gravity readings • Mechanical • Geodynamical • Solid earth tides • Ocean loading 20min moving average drift removed

12. Factors affecting relative gravity readings • Mechanical • Geodynamical • Solid earth tides • Ocean loading • Earthquakes

13. Factors affecting relative gravity readings • Mechanical • Geodynamical • Environmental • Meteorological: atmospheric pressure ~ 0.3 µ Gal / mbar

14. Factors affecting relative gravity readings • Mechanical • Geodynamical • Environmental • Meteorological: atmospheric pressure air temperature wind speed radiant heating

15. Factors affecting relative gravity readings • Mechanical • Geodynamical • Environmental • Hydrological: streamflow groundwater and soil moisture

16. Factors affecting relative gravity readings • Mechanical • Geodynamical • Environmental • Anthropogenic • Non-systematic mass distribution • Vibrations • Repositioning of gravimeter (1µ Gal/ 3mm elevation)

17. Anthropogenic factors post transport stabilisation linear drift

18. Anthropogenic factors car moved right beside meter linear drift

19. Anthropogenic factors car moved away linear drift

20. Anthropogenic factors car engine started and left running linear drift

21. Anthropogenic factors car parked at twice typical distance linear drift

22. Anthropogenic factors a/c, radio and engine turned off linear drift

23. Anthropogenic factors enclosure gate opened linear drift

24. Anthropogenic factors enclosure gate closed linear drift

25. Anthropogenic factors stopped, relevelled and restarted meter linear drift

26. Anthropogenic factors tractor drove by & 19 cattle walked up linear drift

27. Anthropogenic factors removed and repositioned meter linear drift

28. Conclusions: insignificant factors • Gravimeter internal temperature • Earthquakes (at least in Australia) • Air temperature • Wind speed & direction • Non-systematic mass distribution • Low frequency vibrations

29. Conclusions: significant factors • Gravimeter drift • Tie to bedrock & repeat sites during survey day Superconducting Gravimeter Bedrock Site

30. Conclusions: significant factors • Gravimeter drift • Post-transport stabilisation of gravimeter • Take measurement every 2.5 minutes for more than one hour at each site

31. Conclusions: significant factors • Gravimeter drift • Post-transport stabilisation of gravimeter • Earth tides & ocean loading • Difference field gravity measurements from superconducting gravimeter measurements

32. Conclusions: significant factors • Gravimeter drift • Post-transport stabilisation of gravimeter • Earth tides & ocean loading • Atmospheric pressure • Measure with handheld barometer and correct gravity to standard atmosphere

33. Conclusions: significant factors • Gravimeter drift • Post-transport stabilisation of gravimeter • Earth tides & ocean loading • Atmospheric pressure • Levelling • Stable reference point; periodically optically level

34. Thank You! Acknowledgements: This research was funded by an Australian Research Council Discovery Grant DP0343778