Abstract
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.
Published Version
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