Seismic noise in Fennoscandia, with emphasis on high frequencies

Abstract

As part of a siting survey for the NORESS small-aperture regional array, the seismic noise field for frequencies up to about 50 Hz was studied for various sites in Fennoscandia, with emphasis on the NORSAR siting area. Power spectral bias sources were studied in order to ensure a sufficient resolution and dynamic range. It was found that the 1-Hz noise level at NORSAR usually ranges from 1 to 10 nm²/Hz. Above 2 to 3 Hz, the noise power falls off at a rate of about f⁻⁵ to levels of about 10⁻⁵ nm²/Hz at 10 Hz and 10⁻⁸ nm²/Hz at 40 Hz. The slope is much steeper for lower frequencies, where levels between 10⁴ and 10⁸ nm²/Hz were observed at 0.2 to 0.3 Hz. The noise field shows variability both in time and space for frequencies below 2 to 3 Hz, and stability in time for higher frequencies, where essentially only cultural noise sources contribute to the variation. Windgenerated noise is usually negligible around 10 Hz, but contributes 5 to 15 dB at 20 to 30 Hz. Diurnal variations are about 2 dB at 4 Hz and negligible at low frequencies. Only small variations in noise levels are found throughout Fennoscandia, with slightly lower values for Finland and slightly higher for northern Norway, as compared to southeastern Norway. The spatial correlation properties of the noise show variability for frequencies below 2 to 3 Hz and stability for higher frequencies: in the 1- to 3-Hz band, the correlation coefficients drop to zero at interstation distances ranging from 800 to 1500 m, and in the 3- to 5-Hz range from 400 to 500 m. After reaching zero, the correlation values usually approach a level of about −0.2 before increasing again. A frequency dependence was also found for the propagational characteristics, with westerly noise directions dominating for frequencies of 1 Hz and below, and easterly directions in the 2- to 8-Hz range. The degree of randomness in the noise increases with increasing frequency, leaving only a small propagational part at 8 Hz. There is clear evidence for a difference in noise-generating processes below and above 2 to 3 Hz due to differences in spectral slope, in spectral time and space variability, in correlation characteristics, and in propagational characteristics.