HR: 0830h
AN: C11C-0835    [PDF]
TI: New Techniques: Muon Glaciology and Ultrasonic Logging
AU: * Chirkin, D
EM: chirkin@physics.berkeley.edu
AF: Physics Department, University of California, Berkeley, CA 94720 United States
AU: Allen, J
EM: jdallen@uclink.berkeley.edu
AF: Physics Department, University of California, Berkeley, CA 94720 United States
AU: Bay, R C
EM: bay@cletus.physics.berkeley.edu
AF: Physics Department, University of California, Berkeley, CA 94720 United States
AU: Bramall, N
EM: bramall@socrates.berkeley.edu
AF: Physics Department, University of California, Berkeley, CA 94720 United States
AU: Price, P B
EM: bprice@uclink.berkeley.edu
AF: Physics Department, University of California, Berkeley, CA 94720 United States
AB: The strain rate of cold glacial ice depends mainly on the stress tensor, temperature, grain size, and crystal habit. Lab measurements cannot be made at both the low stresses and low temperatures relevant to flow of cold glacial ice. Field studies with inclinometers measure only the horizontal components of flow. We have developed a new method for measuring the 3D strain-rate field at $-40^{\circ}$ to $-15^{\circ}$C, using the AMANDA neutrino-detecting array frozen into deep ice at South Pole. Each strain detector consists of a photomultiplier tube (PMT) in its pressure vessel. AMANDA has $\sim\!600$ PMTs at depths 1500 to 2300 m in a $\sim$0.02 km$^3$ volume. The coordinates of each PMT relative to a coordinate system moving down slope at 9 m yr$^{-1}$ can be measured with s.d. $<1$ m in 1 day by mapping trajectories of down-going cosmic-ray muons that pass through the array. The PMTs record the arrival times of the Cherenkov light emitted along the muon trajectory. Use of maximum likelihood for 10$^5$ muon tracks allows PMT positions to be determined; their positions are then updated at six-month intervals. We will report results of strain-rate measurements in three dimensions, made in 2000, 2001, and 2002 at T $\approx -30^\circ$C. Applying the same technique to the future 1 km$^3$ IceCube array, by averaging over subsets of the $5000$ detectors, values of the strain-rate tensor as small as $3\times 10^{-5}$ yr$^{-1}$ can be measured as a function of temperature and lateral position. The vertical strain rate due to snow accumulation, estimated to be $\sim 3\times 10^{-5}$ yr$^{-1}$, can be measured and will serve as a check on the method. The second new method is designed to measure mean grain size in the ice surrounding a borehole. We will adapt an all-digital logging tool originally developed by Advanced Logic Technology (Luxembourg) for geophysics prospecting in rock boreholes. A $1.3$ MHz transducer emits acoustic pulses horizontally into the ice in increments of 5$^\circ$ in azimuth and records the wave train back-scattered from grain boundaries. For fine-grained glacial ice, individual grains will not be resolved; instead, the amplitude of back-scattering as a function of time provides a measure of mean grain size. Data rate is $\sim$1 meter depth/minute. In addition, the logger can map borehole structure with 0.1 mm resolution. At depths down to $\sim$400 m, it measures air bubble concentrations, which can be related to local temperature at the time of firn close-off.
DE: 0915 Downhole methods
DE: 1694 Instruments and techniques
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 1894 Instruments and techniques
SC: Cryosphere [C]
MN: 2003 Fall Meeting