Diurnal and seasonal variations in the microseismic noise level observed at the NORESS array

Details Diurnal and seasonal variations in the microseismic noise level observed at the NORESS array Diurnal and seasonal variations in the microseismic noise level observed at the NORESS array

Oct 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990pepi...63..252f&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 63, Issue 3-4, p. 252-268.

Physics

7

Scientific paper

The small-aperture seismic array NORESS is designed to detect high-frequency phase arrivals from regional and teleseismic events. To optimize the use of the array for noise reduction by beam-forming and filtering, a study was conducted to find the noise characteristics for frequencies above 1.0 Hz. Regular recording of noise spectra sampled every hour has been performed over a one-year period, thus giving a data base for studying both diurnal and seasonal variations. The main results from this study are:
(a)|The average noise power density spectrum falls off rapidly (~ ƒ;-8 or 24 dB octave-1) between 0.5 and 2 Hz. Above 2 Hz, and up to at least 50 Hz, the spectral fall-off is ~ ƒ;-4 (or 12 dB octave-1).
(b)|Over a one-year period, the standard deviation of the noise level (measured in dB relative to 1 quantum unit) is relatively constant across the entire frequency band considered (0.5-50 Hz), and is typically 3-5 dB. There is, however, a tendency toward increased noise standard deviation both at the low end and at the high end of this band.
(c)|On average, there is a systematic diurnal variation in noise level, particularly on workdays. The typical difference between night-time and day-time noise (measured at 01.00 and 13.00 hours local time, respectively) is 5 dB in the 2-50 Hz band. At frequencies below 1.5-2 Hz, the difference is insignificant. At particular frequencies, e.g. 6 Hz, there are peaks in the noise spectra caused by specific industrial activity, and the corresponding diurnal variation may at the extreme exceed 10 dB.

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