Factors Affecting the Detection of a Soil Moisture

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 http: //www. civenv. unimelb. edu. au/~jwalker/data/gsm/hydrograce. html Western Pacific AGU Geophysics Meeting, Hawaii August 2004 Jeffrey Walker

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. 5 m 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) FG 5 -L A 10 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) SG G CG-3 M 0. 01 µ Gal 3 µ Gal Adam Smith

Site locations http: //www. civenv. unimelb. edu. au/~jwalker/data/oznet 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 Stabilisation ~25 µ Gal – Drift – Post-transport stabilisation 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 ~100 µ Gal corrected Adam Smith

Factors affecting relative gravity readings • Mechanical • Geodynamical – Solid earth tides – Ocean loading 20 min 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/ 3 mm 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 Adam Smith

Thank You! Acknowledgements: This research was funded by an Australian Research Council Discovery Grant DP 0343778