Hydrostatic sensors detect "earth tides"

LEAD, S.D. -- A series of sensors at the Sanford Underground Laboratory at Homestake will measure ground movement so precisely they will detect tiny changes in rock caused by the moon's gravity -- so called "earth tides." More importantly, the arrays also will measure ground movement caused by pumping water out of the former gold mine.

The South Dakota Science and Technology Authority is dewatering Homestake to convert it into an underground laboratory.  The mine is 8,000 feet deep, and it is honeycombed with 370 miles of tunnels or "drifts."  As technicians, engineers and scientists move into Homestake's lower levels to enlarge caverns or carve out new ones for delicate experiments, fast and precise measurements of ground movement will be critically important.

The first two arrays of the Homestake Hydrostatic Water Level System (Homestake HLS) were installed on the 2,000-foot level during the week of January 5, 2009. Two more arrays will be installed later this year, when access is opened to lower levels of Homestake.

Hydrostatic systems operate on the principal that water seeks its own level.  Arrays of long, water-filled tubes connected to pools are monitored by electronic sensors that stream data over telephone and fiberoptic lines. The arrays can detect changes in elevation of millionths of a meter. The Homestake HLS is a collaboration led by Dr. Larry Stetler, a geological engineer from South Dakota School of Mines, and Dr. James Volk, a high-energy physicist from Fermi National Accelerator Laboratory near Chicago.  The two researchers met in Lead, S.D., in April 2008 during a workshop on the initial suite of experiments proposed for the underground lab.

The Homestake HLS will ultimately consist of four series of six Tevatron-style sensors connected by water-filled tubing. Each array will wind through 1,000 feet of tunnel. The array are expected to provide detailed data on ground motion due to dewatering, seismic events, excavations and earth tides. Comparison of data among sensors in the array provide tilt and a direction of motion.

Sensors used at Homestake were designed and built at Fermilab (http://www.fnal.gov/). They are used in various collider experiments. For example, 204 units were installed in the Tevatron accelerator facility at Fermilab. Other units monitor the MINOS experiment at the Soudan Laboratory in Minnesota. The Tevatron-style sensors also provided data for ground motion studies for the proposed International Linear Collider.

Two HLS arrays are currently operating on the 2000-foot level. They were installed with the assistance of Jim Volk, who made the trip to Homestake from Chicago. The arrays consist of 12 sensors, 3,200 feet of ½-inch tubing and 3,600 feet of phone cable.  The "A array" is located about 2,500 feet from the Ross shaft and the "B array" is near the Ross shaft.  Each sensor sits on a 12-inch diameter concrete pillar that is anchored to solid bedrock.  Each hole was excavated using a compressed-air blowgun.  Three holes were drilled to between 12 inches and 18 inches deep, then a half-inch rebar was grouted in.  The concrete pillar was poured around the rebar.  Each sensor was mounted on an aluminum plate with adjusting screws for fine calibration. Each array of 6 HLS sensors includes relative humidity, temperature and barometric pressure sensors.
These arrays are being used to establish baseline data and to monitor ground response to dewatering. The arrays also will measure tidal effects.  The researchers anticipate that dewatering from "stopes" (mined out caverns) and renewed excavations at lower levels will induce ground motion on scales different than ground motion caused by dewatering tunnels and shafts.  These data will provide insight to the hydrologic character and properties of the Precambrian rock at Homestake. 
Two additional sensor arrays will be installed  -- one on the 4,550-foot level and one on the 4,850-foot level -- after access is available later in 2009.  All data will ultimately be fed out of the mine using the new fiber optic backbone installed by the South Dakota Science and Technology Authority personnel at the Sanford Underground Laboratory at Homestake.
The January installation was successful due to the assistance of South Dakota School of Mines and Technology graduate student Jason Van Beek and SDSTA personnel, who excavated holes and poured concrete bases.  Special thanks go to Tom Trancynger, underground  for assistance in locating the sites and installing the sensors.
The HLS experiment has been funded through a South Dakota School of Mines and Technology Nelson Research Grant and form the National Science Foundation's Small Grants for Exploratory Research program, which awarded a grant to Dr. Stetler. The HLS experiment also received donations of equipment and electronics from Fermilab and support and construction assistance from Sanford Underground Laboratory.
While the SDSTA is building the Sanford Underground Laboratory at Homestake, the National Science Foundation is considering an even bigger proposal to convert the Sanford Lab into a national Deep Underground Science and Engineering Laboratory, which could host a wide variety of experiments from the surface to 8,000 feet underground or deeper. In addition, the Department of Energy is considering an experiment involving a beam of neutrinos fired from Fermilab to a detector deep underground at Homestake. Understanding ground motion at the site will be especially useful for future particle beam experiments.