judah levine, nist, mar-2006: 1 using g to monitor the snow pack judah levine john wahr department...
TRANSCRIPT
Judah Levine, NIST, Mar-2006: 1
Using g to monitor the snow Using g to monitor the snow packpack
Judah LevineJudah LevineJohn WahrJohn Wahr
Department of PhysicsDepartment of PhysicsUniversity of ColoradoUniversity of Colorado
[email protected]@Colorado.edu303 492 7785303 492 7785
Judah Levine, NIST, Mar-2006: 2
The experimentThe experiment
Monitor changes in gravity in the Monitor changes in gravity in the mine using a superconducting mine using a superconducting gravity metergravity meter
Remove deterministic signalsRemove deterministic signals– Earth tides, barometric pressure, …Earth tides, barometric pressure, …
Estimate contributions of mining Estimate contributions of mining operationsoperations
Use residuals to monitor changes in Use residuals to monitor changes in the mass of surface water and snow the mass of surface water and snow
Judah Levine, NIST, Mar-2006: 3
Characteristics of the instrumentCharacteristics of the instrument
Smallest possible drift and long-Smallest possible drift and long-period noiseperiod noise– Mechanical gravity meters not good Mechanical gravity meters not good
enoughenough Very large dynamic rangeVery large dynamic range
– System response remains linear even System response remains linear even for very large signals (e.g., seismic for very large signals (e.g., seismic events)events)
Judah Levine, NIST, Mar-2006: 4
Commercial Instrument (GWR Instruments)Commercial Instrument (GWR Instruments)
A superconducting ball is levitated in A superconducting ball is levitated in an inhmogeneous magnetic field. an inhmogeneous magnetic field.
Additional small electrostatic forces Additional small electrostatic forces keep the ball centered as g keep the ball centered as g changes. The meter outputs changes. The meter outputs voltage.voltage.
NOAA is presently operating a meter NOAA is presently operating a meter in Boulder.in Boulder.
Judah Levine, NIST, Mar-2006: 5
Judah Levine, NIST, Mar-2006: 6
1gal=10-6 cm/s2
Judah Levine, NIST, Mar-2006: 7
Analysis of tidal dataAnalysis of tidal data
Signal/Noise ratio for earth tides is about 80 db Band width= 1 cycle/month
0.02 gal @ 1 month0.6 gal @ 1 day
Barometric pressure admittance ~ 0.42 gal/mbar
Judah Levine, NIST, Mar-2006: 8
Gravity residualsGravity residuals
Change in mass above or below Change in mass above or below instrumentinstrument
Data has no vertical resolutionData has no vertical resolution Horizontal response determined by Horizontal response determined by
Green’s functionGreen’s function
Judah Levine, NIST, Mar-2006: 9
Mass sensitivity assuming
flat topography
Gravity signal at 1500 m depth,
from 3 cm of water spread over a disc.
Judah Levine, NIST, Mar-2006: 10
So, probably sensitive to mass averaged So, probably sensitive to mass averaged over over
a disc of radius 3-5 km; an area of a disc of radius 3-5 km; an area of ~~80 km80 km22 ..
More sensitive to mass at center of disc More sensitive to mass at center of disc than at edges.than at edges.
1 1 µgal accuracy translates to a water µgal accuracy translates to a water thickness accuracy of ~3 cm.thickness accuracy of ~3 cm.– Probably do betterProbably do better
Judah Levine, NIST, Mar-2006: 11
ApplicationsApplications
Monitor variation of winter snowpack Monitor variation of winter snowpack – Limited by background noise, model Limited by background noise, model
accuracyaccuracy
Monitor melting of snow during the springMonitor melting of snow during the spring– How much water is retained in the soilHow much water is retained in the soil– would complement other datawould complement other data
Monitor ground water during and after Monitor ground water during and after summer rainstormssummer rainstorms
Judah Levine, NIST, Mar-2006: 12
Why do this in a mine?Why do this in a mine?
Gravity measurements at the surface are Gravity measurements at the surface are sensitive only to local water mass.sensitive only to local water mass.– Snow/water at the same level make no Snow/water at the same level make no
contributioncontribution
Wind and cultural noise on the surface Wind and cultural noise on the surface
Judah Levine, NIST, Mar-2006: 13
Complicating factorsComplicating factors
How noisy is the mine at long periods?How noisy is the mine at long periods?– Short period noise not important unless instrument Short period noise not important unless instrument
saturatessaturates Removal of rock mass will cause a gravity Removal of rock mass will cause a gravity
signal. How well can we model it?signal. How well can we model it? Vertical displacements of the meter will cause Vertical displacements of the meter will cause
gravity signals. Can we monitor vertical gravity signals. Can we monitor vertical displacements, or do we have to live with displacements, or do we have to live with them?them?– Free-air gradient ~ cmFree-air gradient ~ cm
The atmosphere causes a gravity signal. We The atmosphere causes a gravity signal. We need barometric pressure data to remove it.need barometric pressure data to remove it.– Resolution ~ 1 millibarResolution ~ 1 millibar
Judah Levine, NIST, Mar-2006: 14
Possible Instrumentation Possible Instrumentation
Superconducting gravity meter. Superconducting gravity meter.
Cost: $450,000 new. Or, NOAA instrument Cost: $450,000 new. Or, NOAA instrument might be available for no cost in short term, might be available for no cost in short term, though would eventually require $50,000 to though would eventually require $50,000 to restore computer & data acquisition system. restore computer & data acquisition system.
GPS receiver at the surface. Cost: $8000 GPS receiver at the surface. Cost: $8000 each.each.
Snotel station at the surface, to monitor Snotel station at the surface, to monitor snowpack at a single location. Cost: $18,000.snowpack at a single location. Cost: $18,000.
Barometer(s) at the surface. Cost: Barometer(s) at the surface. Cost: ~~$4000(?) $4000(?) each.each.