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Using g to monitor the snow pack
Judah Levine
John Wahr
Department of Physics
University of Colorado
[email protected]
303 492 7785
Judah Levine, NIST, Mar-2006: 1
The experiment
Monitor changes in gravity in the mine
using a superconducting gravity meter
 Remove deterministic signals

– Earth tides, barometric pressure, …
Estimate contributions of mining
operations
 Use residuals to monitor changes in the
mass of surface water and snow
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Characteristics of the instrument

Smallest possible drift and long-period
noise
– Mechanical gravity meters not good
enough

Very large dynamic range
– System response remains linear even for
very large signals (e.g., seismic events)
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Commercial Instrument (GWR Instruments)
A superconducting ball is levitated in an
inhmogeneous magnetic field.
Additional small electrostatic forces keep
the ball centered as g changes. The
meter outputs voltage.
NOAA is presently operating a meter in
Boulder.
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1gal=10-6 cm/s2
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Analysis of tidal data
Signal/Noise ratio for earth tides is about 80 db
Band width= 1 cycle/month
0.02 gal @ 1 month
0.6 gal @ 1 day
Barometric pressure admittance ~ 0.42 gal/mbar
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Gravity residuals
Change in mass above or below
instrument
 Data has no vertical resolution
 Horizontal response determined by
Green’s function

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Mass sensitivity assuming
flat topography
Gravity signal at 1500 m depth,
from 3 cm of water spread over a disc.
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So, probably sensitive to mass averaged over
a disc of radius 3-5 km; an area of ~80 km2 .
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More sensitive to mass at center of disc than at
edges.
1 µgal accuracy translates to a water thickness
accuracy of ~3 cm.
– Probably do better
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Applications

Monitor variation of winter snowpack
– Limited by background noise, model accuracy

Monitor melting of snow during the spring
– How much water is retained in the soil
– would complement other data

Monitor ground water during and after summer
rainstorms
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Why do this in a mine?

Gravity measurements at the surface are sensitive
only to local water mass.
– Snow/water at the same level make no contribution

Wind and cultural noise on the surface
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Complicating factors

How noisy is the mine at long periods?
– Short period noise not important unless instrument
saturates


Removal of rock mass will cause a gravity signal.
How well can we model it?
Vertical displacements of the meter will cause
gravity signals. Can we monitor vertical
displacements, or do we have to live with them?
– Free-air gradient ~ cm

The atmosphere causes a gravity signal. We need
barometric pressure data to remove it.
– Resolution ~ 1 millibar
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Possible Instrumentation
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Superconducting gravity meter.
Cost: $450,000 new. Or, NOAA instrument might
be available for no cost in short term, though would
eventually require $50,000 to restore computer &
data acquisition system.
GPS receiver at the surface. Cost: $8000 each.
Snotel station at the surface, to monitor snowpack at
a single location. Cost: $18,000.
Barometer(s) at the surface. Cost: ~$4000(?) each.
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