Judah Levine, NIST, Mar-2006: 1 Using g to monitor the snow pack Judah Levine John Wahr Department of Physics University of Colorado

Judah Levine, NIST, Mar-2006: 2 The experiment Monitor changes in gravity in the mine using a superconducting gravity meter Monitor changes in gravity in the mine using a superconducting gravity meter Remove deterministic signals Remove deterministic signals –Earth tides, barometric pressure, … Estimate contributions of mining operations Estimate contributions of mining operations Use residuals to monitor changes in the mass of surface water and snow Use residuals to monitor changes in the mass of surface water and snow

Judah Levine, NIST, Mar-2006: 3 Characteristics of the instrument Smallest possible drift and long-period noise Smallest possible drift and long-period noise –Mechanical gravity meters not good enough Very large dynamic range Very large dynamic range –System response remains linear even for very large signals (e.g., seismic events)

Judah Levine, NIST, Mar-2006: 4 Commercial Instrument (GWR Instruments) A superconducting ball is levitated in an inhmogeneous magnetic field. 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.

Judah Levine, NIST, Mar-2006: 5

Judah Levine, NIST, Mar-2006: 6 1  gal=10 -6 cm/s 2

Judah Levine, NIST, Mar-2006: 7 Analysis of tidal data Signal/Noise ratio for earth tides is about 80 db Band width= 1 cycle/month 0.02  1 month 0.6  1 day Barometric pressure admittance ~ 0.42  gal/mbar

Judah Levine, NIST, Mar-2006: 8 Gravity residuals Change in mass above or below instrument Change in mass above or below instrument Data has no vertical resolution Data has no vertical resolution Horizontal response determined by Green’s function Horizontal response determined by Green’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 over So, probably sensitive to mass averaged over a disc of radius 3-5 km; an area of ~80 km 2. More sensitive to mass at center of disc than at edges. More sensitive to mass at center of disc than at edges. 1 µgal accuracy translates to a water thickness accuracy of ~3 cm. 1 µgal accuracy translates to a water thickness accuracy of ~3 cm. –Probably do better

Judah Levine, NIST, Mar-2006: 11 Applications Monitor variation of winter snowpack Monitor variation of winter snowpack –Limited by background noise, model accuracy Monitor melting of snow during the spring 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 Monitor ground water during and after summer rainstorms

Judah Levine, NIST, Mar-2006: 12 Why do this in a mine? Gravity measurements at the surface are sensitive only to local water mass. 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 Wind and cultural noise on the surface

Judah Levine, NIST, Mar-2006: 13 Complicating factors How noisy is the mine at long periods? 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? 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? 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. The atmosphere causes a gravity signal. We need barometric pressure data to remove it. –Resolution ~ 1 millibar

Judah Levine, NIST, Mar-2006: 14 Possible Instrumentation Superconducting gravity meter. 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. 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. GPS receiver at the surface. Cost: $8000 each. Snotel station at the surface, to monitor snowpack at a single location. Cost: $18,000. Snotel station at the surface, to monitor snowpack at a single location. Cost: $18,000. Barometer(s) at the surface. Cost: ~$4000(?) each. Barometer(s) at the surface. Cost: ~$4000(?) each.