Jeffrey Walker Factors Affecting the Factors Affecting the Detection of a Soil Moisture Detection of a Soil Moisture Signal in Field Relative Signal in Field Relative Gravity Measurements 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
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Factors Affecting the Detection of a Soil Moisture Signal in Field Relative Gravity Measurements
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 Department of Civil and Environmental Engineering, University of Melbourne, Australia - PowerPoint PPT Presentation
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Jeffrey Walker
Factors Affecting the Detection of a Soil Factors Affecting the Detection of a Soil Moisture Signal in Field Relative Gravity Moisture Signal in Field Relative 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)
FG5 A10FG5-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)
– 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