HR: 1340h
AN: S33B-1100    [Abstracts]
TI: Continuous Observation of Travel Time of Seismic Waves Using ACROSS Vibrator and Seismic Array
AU: * Saiga, A
EM: saiga@eps.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 464-8602 Japan
AU: Yamaoka, K
EM: yamaoka@eri.u-tokyo.ac.jp
AF: University of Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan
AU: Kunitomo, T
EM: kunitomo@tono.jnc.go.jp
AF: Japan Nuclear Cycle Development Institute, Yamanouchi1-63, Akeyo, Mizunami, 509-5132 Japan
AU: Watanabe, T
EM: watanabe@seis.nagoya-u.ac.jp
AF: Nagoya University, Furocho, Chikusa, Nagoya, 464-8602 Japan
AB: ACROSS (Accurately Controlled Routinely Operated Signal System) monitors the temporal variation in seismic velocity with high resolution using a precisely controlled sinusoidal signal. Experiments using an ACROSS vibrator and a seismic array were carried out for 1 month in a vault at Mizunami, Gifu Prefecture, Japan. Two ACROSS vibrators were continuously operated with frequency modulation centered at 17.52 and 25.53 Hz, respectively and with modulation amplitude of 2.5 Hz and modulation period of 20 seconds. We deployed a seismic array of 15 seismometers in a cross-shaped vault at an interval of 8 m as crossed array (64_~64m). The distance from the ACROSS vibrator is 2.4 km. Transfer function between the ACROSS vibrator and each seismometer was obtained by applying a deconvolution of the observed data with the theoretical force generated from the source in the frequency domain. We determined propagation direction and apparent velocity of dominant phases using the semblance method. The P and S waves arriving at 0.65 and 1.25 seconds were identified with apparent velocities of 4.0 km/s and 2.2 km/s, respectively. They were interpreted as refracted P and S waves traveled along the velocity boundary between a sedimentally layer and a basement layer. Temporal variation in the travel time of the refracted P and S waves was calculated using cross-spectrum density method. Travel time change was derived from phase along in the frequency domain (Ikuta et. al., 2002). Uncertainties of travel time change of the P and S waves were evaluated within 0.1 ms and 0.05 ms for 1 month observation. The temporal variations showed a similar pattern to the variation of atmospheric temperature. The travel time delayed when the atmospheric temperature decreased. The daily variation had a value of about 0.5 ms at the maximum. Phase variation measured on the source foundation also showed the similar pattern, with smaller value about 0.1 ms at the maximum. The apparent velocity did not show such a daily variation. The result implies that the daily variation, possibly related to the variation of atmospheric temperature, was caused not only around the source but in its propagation. The sensitivity analysis derived from scattering theory suggests that the refracted wave can be affected by the velocity change near the surface. The result indicates that the change near the surface can cause the temporal variation in travel time.
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting