HR: 1330h
AN: V52B-0440    [PDF]
TI: Long Valley Deep Hole Geophysical Observatory --- Strain Instrumentation and Installation.
AU: * Sacks, S I
EM: sacks@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Linde, A
EM: linde@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Malin, P
EM: malin@duke.edu
AF: Duke University, Box 90235, 109 Old Chemistry, Durham, NC 27708 United States
AU: Roeloffs, E A
EM: evelynr@usgs.gov
AF: US Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, BC WA 986 Canada
AU: Hill, D P
EM: hill@usgs.gov
AF: US Geological Survey, 3456 Middlefield Road, Menlo Park, CA 94025 United States
AU: Ellsworth, W L
EM: ellsworth@usgs.gov
AF: US Geological Survey, 3456 Middlefield Road, Menlo Park, CA 94025 United States
AB: The Long Valley Exploratory Well, drilled in the middle of the resurgent dome in the Long Valley caldera, was started in 1989 and after rather checkered progress eventually reached a depth of about 9,831 feet. The hole is cased to a depth of 7178 feet with bare rock below that. At 8,500 feet there is an open fracture system with substantial permeability. One of the goals of the instrument installation is to enable monitoring of this deep aquifer. The most satisfactory rock away from obvious large fractures was at about 7,400 feet, and this was the installation depth. The instrumentation package consisted of a bottom hole seismometer at a depth of about 8500 feet, and a coupled instrument string that was cemented to the rock at a depth of 7400 feet. The instrument string, 73 feet long, had an inflatable packer with an extension at the bottom, coupled to a seismometer with a cement exit port above it, a 22 foot long spacing tube connected to a 20 foot long sensing volume strainmeter assembly. The strainmeter unit is essentially an annulus with the cementing pipe passing through it. In addition, two seismometer cables, two water bypass tubes and a packer inflation tube, pass through the strainmeter, which is actually two concentric strainmeters. The outer unit is a dilatometer and the inner unit is a vertical component strainmeter. Before installation, the strainmeters and the 8000 foot long stainless steel coupling tubes were filled with filtered and degassed water. The instrument string and attached bottom hole seismometer were then lowered down the hole attached to drill pipe. Two optical fiber vertical strainmeters (one interferometer and one time-of-flight loop) consisting of three fibers were attached to the drill pipe as it was installed. After the drill pipe reached target depth, it was secured to the well head. The packer, at the bottom of the instrument package, was inflated, thus providing a sealed bottom for the cement. Cement was then pumped down the drill pipe, through the strainmeter assembly and out the tube about 25 feet below the bottom of the strain sensing assembly. About 450 feet of the hole was cemented, the cement going into the casing. The coupling tubes from the strainmeters were connected to a surface mounted sensing head that had hydraulic amplification and electronic transducers. Pressure changes in the lower aquifer cause flow through two 1/4 inch diameter tubes into the annulus outside the mounting and cementing pipe. An opening sleeve in the installed pipe will allow the resulting water level changes to be monitored in a protected environment. All installed instrumentation seems to be functioning satisfactorily.
DE: 8135 Hydrothermal systems (8424)
DE: 8424 Hydrothermal systems (8135)
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting