HR: 0830h
AN: U31B-0003    [PDF]
TI: Observed dispersion curves of long period atmospheric acoustic waves
AU: * Nishida, K
EM: knishida@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Fukao, Y
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Watada, S
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kobayashi, N
AF: EPS, Tokyo Inst. of Tech., 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Tahira, M
AF: Aichi Univ. of Edu., 1 Hirosawa, Igaya-cho, Kariya, 448-8545 Japan
AU: Suda, N
AF: Hiroshima Univ., 1-3-1 Kagami-yama, Higashi-Hiroshima, 739-8526 Japan
AU: Nawa, K
AF: AIST, Tsukuba Central 7, Higashi 1-1-1, Tsukuba, 305-8567 Japan
AB: Recently some groups reported Earth's background free oscillations even on seismically quiet days [e.g. Nawa et al., 1998]. Statistical features of them and annual variations of their amplitudes with a peak in July suggest that atmospheric disturbance is the most probable excitation source [Nishida and Kobayashi, 1999; Nishida et al., 2000]. If the atmospheric excitation mechanism is effective, atmospheric acoustic free oscillations must be also excited persistently. In fact there is evidence of acoustic resonance of seismic free oscillations at around 3.7 and 4.4 mHz. The resonant amplitudes of the seismic records suggest the atmospheric excitation of the acoustic free oscillations but there is no direct observation of them. In an attempt to detect the long-period acoustic waves, we installed a cross array of barometers in a 10 km-wide university forest in central Honshu. The array has 28 micro-barometers employing quartz crystal resonator technology with station spacing of about 500 m. A special care was taken to design a sensor-recording system that runs with a precesion of better than 0.1 ppm in clock timing and sampling timing over a year by a single air battery. We analyzed 1-second continuous sampling records in a time period from March 2002 to March 2003. With an assumption of a stochastic stationary plane wave for the observed acoustic waves, we measured the time delay between every pair of all the stations. We determined the slowness vector of a plane wave as the one with which all the measured time delay are most consistent with each other. The plot of the measured slowness as a function of frequency has given for the first time the dispersion curve of atmospheric acoustic wave at frequencies down to 0.01 Hz. The slowness vector indicates two kinds of acoustic waves. (1) Acoustic waves traveling from the northwest (possibly from the mountaneous region) in a frequency range from 0.01 to 0.1 Hz with a phase velocity of about 400 m/s at 0.1 Hz and about 1500m/s at 0.01 Hz. Our array size is still not large enough to detect the expected acoustic free oscillations at frequencies around 3.7 and 4.4 mHz. (2) Microbaroms traveling from the southeast at frequencies from 0.1 to 0.5 Hz with a phase velocity of about 350 m/s. They are most likely to be excited by standing oceanic waves in the coastal region. The power spectra of these acoustic waves are compared to those of the seismic spectra obtained in the same university forest. Their spectral shapes are mutually coincident and change commonly with variable weather conditions.
DE: 3307 Boundary layer processes
DE: 3399 General or miscellaneous
DE: 7255 Surface waves and free oscillations
SC: U
MN: 2003 Fall Meeting