jeffrey walker factors affecting the detection of a soil moisture signal in field relative gravity...
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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
MeasurementsMeasurements
1Adam Smith, 1Jeffrey Walker, 1Andrew Western, 1Kevin Ellett, 1Rodger Grayson, and 2Matthew 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 AGUGeophysics Meeting, Hawaii
August 2004
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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
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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
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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 CG-3MG
0.01 µ Gal 3 µ Gal 3 µ Gal
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Adam Smith
Site locations
http://www.civenv.unimelb.edu.au/~jwalker/data/oznethttp://www.civenv.unimelb.edu.au/~jwalker/data/oznet
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Adam Smith
Typical soil moisture site
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Adam Smith
Factors affecting relative gravity readings
• Mechanical– Drift
~40 µ Gal/day
392 µ Gal/day linear drift already removed
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Adam Smith
Factors affecting relative gravity readings
• Mechanical– Drift– Post-transport stabilisation
Sta
bilis
atio
n ~
25 µ
Gal
1.5 hr
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Adam Smith
Factors affecting relative gravity readings• Mechanical
– Drift– Post-transport stabilisation– Internal temperature
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Adam Smith
Factors affecting relative gravity readings
• Mechanical• Geodynamical
– Solid earth tides
uncorrected
corrected~100 µ Gal
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Adam Smith
Factors affecting relative gravity readings• Mechanical
• Geodynamical– Solid earth tides– Ocean loading
20min moving average
drift removed
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Adam Smith
Factors affecting relative gravity readings• Mechanical• Geodynamical
– Solid earth tides– Ocean loading – Earthquakes
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Adam Smith
Factors affecting relative gravity readings• Mechanical• Geodynamical• Environmental
– Meteorological: atmospheric pressure
~ 0.3 µ Gal / mbar
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Adam Smith
Factors affecting relative gravity readings• Mechanical• Geodynamical• Environmental
– Meteorological: atmospheric pressure
air temperature
wind speed
radiant heating
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Adam Smith
Factors affecting relative gravity readings• Mechanical• Geodynamical• Environmental
– Hydrological: streamflow
groundwater and soil moisture
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Adam Smith
Factors affecting relative gravity readings• Mechanical• Geodynamical• Environmental• Anthropogenic
– Non-systematic mass distribution– Vibrations– Repositioning of gravimeter (1µ Gal/ 3mm elevation)
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Adam Smith
Anthropogenic factors
post transport stabilisation
linear drift
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Adam Smith
Anthropogenic factors
car moved right beside meter
linear drift
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Adam Smith
Anthropogenic factors
car moved away
linear drift
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Adam Smith
Anthropogenic factors
car engine started and left running
linear drift
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Adam Smith
Anthropogenic factors
car parked at twice typical distance
linear drift
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Adam Smith
Anthropogenic factors
a/c, radio and engine turned off
linear drift
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Adam Smith
Anthropogenic factors
enclosure gate opened
linear drift
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Adam Smith
Anthropogenic factors
enclosure gate closed
linear drift
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Adam Smith
Anthropogenic factors
stopped, relevelled and restarted meter
linear drift
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Adam Smith
Anthropogenic factors
tractor drove by & 19 cattle walked up
linear drift
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Adam Smith
Anthropogenic factors
removed and repositioned meter
linear drift
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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
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Adam Smith
Conclusions: significant factors
• Gravimeter drift– Tie to bedrock & repeat sites during survey day
Superconducting Gravimeter
Bedrock Site
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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
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Adam Smith
Conclusions: significant factors
• Gravimeter drift• Post-transport stabilisation of gravimeter• Earth tides & ocean loading
– Difference field gravity measurements from superconducting gravimeter measurements
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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
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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
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Thank You!Thank You!
Acknowledgements:
This research was funded by an Australian Research Council Discovery Grant DP0343778