HR: 17:00h
AN: G32C-05    [PDF]
TI: On Boreholes and PBO Borehole Strain
AU: * Gladwin, M T
EM: mike.gladwin@csiro.au
AF: C.S.I.R.O.Exploratioin and Mining, Pinjarra Hills, Brisbane, Qld 4069 Australia
AU: Mee, M W
EM: marie.mee@csiro.au
AF: C.S.I.R.O.Exploratioin and Mining, Pinjarra Hills, Brisbane, Qld 4069 Australia
AB: Borehole tensor strainmeters (GTSM) installed in Australia and California have established a baseline of data spanning more than twenty years. The current baseline of data allows characterisation of a moderate number of instruments in a range of very different environments in a way which defines reasonable performance expectations for the upcoming PBO deployments. A generic understanding of effects which result from the process of installation of the instrument in a stressed rock mass emerges. This indicates that, provided due allowance is made for experimentally determined borehole recovery effects, the contribution of borehole strain meters more than adequately fills the observational gap between high stability/long term geodetic measurements of strain and strain rates and high resolution/high frequency seismic observations of earth deformation processes. The various strain relief processes associated with the installation procedures and borehole recovery effects associated with pre-existing stress fields will be documented. Procedures for calibration of the total borehole inclusion and for progressive removal of effects due to rock anisotropy and visco-elastic creep of the grout and rock close to the borehole from far field tectonic effects will be defined and illustrated with examples. Observed deviations from these processes will be shown to be small and consistent with otherwise observed or implied fault motions. Full details of these borehole induced processes are, however, difficult to determine in the early years following installation, particularly if there is significant tectonic activity at the time. Once quantified for each site, the effects can be robustly removed from data streams.
DE: 1242 Seismic deformations (7205)
DE: 1294 Instruments and techniques
DE: 7230 Seismicity and seismotectonics
DE: 8150 Plate boundary--general (3040)
SC: Geodesy [G]
MN: 2003 Fall Meeting