Jeffrey Walker Factors Affecting the Detection of a Soil Moisture Signal in Field Relative Gravity Measurements 1 Adam Smith, 1 Jeffrey Walker, 1 Andrew Western, 1 Kevin Ellett, 1 Rodger Grayson, and 2 Matthew Rodell 1.Department of Civil and Environmental Engineering, University of Melbourne, Australia 2. Hydrological Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, USA Western Pacific AGU Geophysics Meeting, Hawaii August 2004

Adam Smith Why detect soil moisture changes with gravity? 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

Adam Smith Why relative gravity measurements? 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) FG5A10FG5-L 2 µ Gal 50 µ Gal 10 µ Gal

Adam Smith Why relative gravity measurements? 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) SGCG-3MG 0.01 µ Gal3 µ Gal

Adam Smith Site locations

Adam Smith Typical soil moisture site

Adam Smith Factors affecting relative gravity readings Mechanical –Drift ~40 µ Gal/day 392 µ Gal/day linear drift already removed

Adam Smith Factors affecting relative gravity readings Mechanical –Drift –Post-transport stabilisation Stabilisation ~25 µ Gal 1.5 hr

Adam Smith Factors affecting relative gravity readings Mechanical –Drift –Post-transport stabilisation –Internal temperature

Adam Smith Factors affecting relative gravity readings Mechanical Geodynamical –Solid earth tides uncorrected corrected ~100 µ Gal

Adam Smith Factors affecting relative gravity readings Mechanical Geodynamical –Solid earth tides –Ocean loading 20min moving average drift removed

Adam Smith Factors affecting relative gravity readings Mechanical Geodynamical –Solid earth tides –Ocean loading –Earthquakes

Adam Smith Factors affecting relative gravity readings Mechanical Geodynamical Environmental –Meteorological: atmospheric pressure ~ 0.3 µ Gal / mbar

Adam Smith Factors affecting relative gravity readings Mechanical Geodynamical Environmental –Meteorological: atmospheric pressure air temperature wind speed radiant heating

Adam Smith Factors affecting relative gravity readings Mechanical Geodynamical Environmental –Hydrological: streamflow groundwater and soil moisture

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

Adam Smith Anthropogenic factors post transport stabilisation linear drift

Adam Smith Anthropogenic factors car moved right beside meter linear drift

Adam Smith Anthropogenic factors car moved away linear drift

Adam Smith Anthropogenic factors car engine started and left running linear drift

Adam Smith Anthropogenic factors car parked at twice typical distance linear drift

Adam Smith Anthropogenic factors a/c, radio and engine turned off linear drift

Adam Smith Anthropogenic factors enclosure gate opened linear drift

Adam Smith Anthropogenic factors enclosure gate closed linear drift

Adam Smith Anthropogenic factors stopped, relevelled and restarted meter linear drift

Adam Smith Anthropogenic factors tractor drove by & 19 cattle walked up linear drift

Adam Smith Anthropogenic factors removed and repositioned meter linear drift

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

Adam Smith Conclusions: significant factors Gravimeter drift –Tie to bedrock & repeat sites during survey day Superconducting Gravimeter Bedrock Site

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

Adam Smith Conclusions: significant factors Gravimeter drift Post-transport stabilisation of gravimeter Earth tides & ocean loading –Difference field gravity measurements from superconducting gravimeter measurements

Adam Smith 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

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

Acknowledgements: This research was funded by an Australian Research Council Discovery Grant DP