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

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)

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)

SG CG-3MG

0.01 µ Gal 3 µ Gal 3 µ Gal

Adam Smith

Site locations

http://www.civenv.unimelb.edu.au/~jwalker/data/oznethttp://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– Drift– Post-transport stabilisation

Sta

bilis

atio

n ~

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

Thank You!Thank You!

Acknowledgements:

This research was funded by an Australian Research Council Discovery Grant DP0343778