HR: 0800h
AN: NG41A-0149    [Abstracts]
TI: Temporal changes in emission characteristic of elastic wave by vibrator source and its correction
AU: * Ikuta, R
EM: ryoya@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi Bunkyo, Tokyo, 113-0032, Japan
AU: Yamaoka, K
EM: kyamaoka@seis.nagoya-u.ac.jp
AF: Research Center for Seismology and Volcanology, Disaster Mitigation, Nagoya University, Furo-cho Chikusaku, Nagoya, 464-8602, Japan
AU: Watanabe, T
EM: watanabe@seis.nagoya-u.ac.jp
AF: Research Center for Seismology and Volcanology, Disaster Mitigation, Nagoya University, Furo-cho Chikusaku, Nagoya, 464-8602, Japan
AB: We are developing an Accurately Controlled Routinely Operated acoustic Signal System (Acoustic-ACROSS) to observe time evolution of crustal structure. Acoustic-ACROSS source continuously radiates accurately controlled elastic signal by rotating eccentric mass. If the emission of the ACROSS signal does not vary with time, we can attribute the time evolutions in the observed signal to the change of propagation properties along the path. But it is pointed that the radiated signal by ACROSS sources changes due to surface conditions such as atmospheric temperature and rainfall. So we should correct these source instabilities to extract the temporal changes of the signal due to the changes of the propagation properties in the deeper part of the earth. We conducted a 15 months long-term monitoring experiment using a couple of ACROSS sources deployed near Nojima fault, Awaji Island. Receiver seismometer was deployed at the bottom of an 800 m deep borehole dug beside the sources. We have proposed an analytical approach to correct the effect of the radiation instability by referring to records of 12 seismometers deployed near the sources (Yamaoka et al. 2001 and Ikuta et al. 2004). We assumed that the records S(t) obtained by the near-source sensors represent signal input. So the wave field Y(t) recorded by the borehole sensor is expressed by a linear combination of S(t): Y(t)=G(t)S(t), in which the combination coefficient G(t) is transfer function between the input and the borehole sensor. This equation can be rewritten in the following form: Y(t)=[G0+dG(t)][S0+dS(t)], in which G0 and S0 denote the time-invariant parts of G(t) and S(t), respectively. We also assumed that the time-variant terms dG(t) and dS(t) were small. Ignoring higher- order fractions, this is rewritten as Y(t)=G0[S0+dS(t)]+dG(t)S0. Subtracting G0dS(t) from the right-hand side makes the observed wave field Y(t) independent from the radiation instability. We obtained G0 by applying least square method to fit Y(t) with S0+dS(t). The residual of this fitting is dG(t)S0. Now the sum of G0S0 and the residual is what we want to know. We corrected observed wave field as Y'(t) =G0S0+dG(t)S0, which does not include the source input variation. However, this method was not effective for a record longer than a few months. This method could not correct the long-term record all together because the G0 was not stable through the term. We adopted another new method for correction. We separated the records into the short-term and the long-term variations by applying a moving average with an interval of 20 days. Next we corrected both variations individually and then added the results again. For the individual corrections, we adopted a new model in which the time- variant part in the records dY(t) is fitted by linear combination of dS(t): dY(t)=G0dS(t)+e. We can obtain G0 by applying least square method to fit dY(t) with dS(t) and the residual e represents dG(t)S0. This is none other than corrected observed variation by the borehole sensor dY'(t). We applied this method to the record in the whole period of the 15 months experiment. We successfully reduced the variation in the borehole records due to radiation instability by this correction method. The variation in the borehole records reduced twice as good as the previous method. After the correction, a seasonal changes in the records disappeared and a variation in amplitude and travel-time of P and S waves corresponding to a distant earthquake remained clearly.
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0910 Data processing
DE: 0994 Instruments and techniques
SC: Nonlinear Geophysics [NG]
MN: 2007 Fall Meeting