Array observation of background atmospheric waves in the seismic band from 1 mHz to 0.5 Hz

Abstract

SUMMARY In an eventual aim to detect background long-period acoustic waves in the atmosphere that have to be excited incessantly by the same sources as for background free oscillations of the solid Earth, we installed a cross array of barometers in a 10-km-wide university forest in central Honshu. The array has 28 microbarometers employing quartz crystal resonator technology with a station spacing of about 500 m. We analysed 1-s continuous sampling records in a time period from 2002 March to 2003 November to obtain 1-D frequency–wavenumber spectra and 2-D frequency–slowness spectra to detect three kinds of waves in the seismic band from 1 mHz to 0.5 Hz. We measured the dispersion curves of these waves with an assumption of stochastic stationary plane waves. The waves we detected are (1) background internal gravity waves travelling from northwest at frequencies from 1 to 5 mHz with a phase velocity of about 50 m s−1, (2) background acoustic waves traveling from northwest at frequencies from 0.01 to 0.1 Hz with a frequency-dependent phase velocity of about 400 m s−1 at 0.1 Hz and about 800 m s−1 at 0.01 Hz and (3) microbaroms travelling from southeast at frequencies from 0.1 to 0.5 Hz with a phase velocity of about 350 m s−1. Internal gravity waves (1) dominate over the random component of atmospheric turbulence in the relevant frequency range. At higher frequencies the random component with coherent length shorter than 500 m is dominant. Background acoustic waves (2) show clear annual variation with maxima in winter. The annual variation and their incident azimuths suggest their possible origins in mountain regions. Microbaroms (3) are most likely to be excited by standing ocean waves in nearby coastal regions. The power spectra of these acoustic waves were compared to the seismic spectra obtained in the same university forest. Their spectral shapes are mutually similar and change commonly with variable weather conditions, indicating that the microbaroms and microseisms have their common origins.