HR: 0800h
AN: S41B-0970    [Abstracts]
TI: Master Event Relative Location of Excavation Induced Micro Seismicity at the Underground Research Laboratory.
AU: Reyes-Montes, J M
EM: jmreyes@liverpool.ac.uk
AF: Department of Earth and Ocean Sciences, University of Liverpool.4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: * Rietbrock, A
EM: a.rietbrock@liverpool.ac.uk
AF: Department of Earth and Ocean Sciences, University of Liverpool.4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: Collins, D S
EM: d.collins@liverpool.ac.uk
AF: Lassonde Institute, University of Toronto. 170 College Street, Toronto, M5S 3E3 Canada
AU: Young, R P
EM: paul.young@utoronto.ca
AF: Lassonde Institute, University of Toronto. 170 College Street, Toronto, M5S 3E3 Canada
AB: Microseismicity has been extensively used to non-destructively monitor induced damage accumulation and stability of underground structures. Source location is one of the most fundamental characteristics of seismicity in order to provide this information. However there is an uncertainty in its determination, arising from a combination of three factors: (1) measurement error, (2) velocity model error, and (3) a non linear term resulting from the second and higher order terms in the Taylor polynomial expansion of the travel time equation. Classical location routines in this context cannot resolve events separated less than 0.5 m, in the order of magnitude of the largest fractures observed. Master event relative locations of the events recorded during the excavation of the TSX tunnel at the Underground Research Laboratory enhance this resolution up to one order of magnitude, as shown by the location of synthetic events performed in this context. This approach implicitly removes the uncertainty introduced by the anisotropic P wave velocity field induced by the presence of the tunnel and the damaged rock. This study presents the relocation of 1468 MS events recorded at the TSX during the excavation phase between February and March 1997. For the relocation, events are divided into two main clusters in each excavation section, according to their initial spatial location (performed using Geiger's routine and an average P wave velocity) corresponding to different regions in the tunnel perimeter, the tunnel floor and the roof. Master events are chosen among those with the maximum number of P wave arrival times available, sharpness of onsets, and being centrally located with respect to each cluster of events. A precision higher than 0.05 m was achieved on the location of synthetic events less than 1 m apart from the master in this context. The relocated events locate closer to the excavation perimeter than the absolute locations, a result that corresponds with the observed damage zone. Additionally the relocated events define a clearer structure within each cluster, with planes of alignment parallel to the tunnel perimeter. A series of hits performed in August 2003 using a Proceq Schmidt hammer source in the tunnel perimeter were used as masters for a subset of 323 events. This provides a repeatable, accurately located source of reference events. The relocated events show a similar trend towards shorter distances as in the previous set. Also the structure within the cluster is consistent with the one observed using the induced events as reference.
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting