Noise level variation at norsar and its effect on detectability

For a 3-year period, noise level measurements of short- and long-period data at NORSAR have been sampled at hourly intervals. Significant seasonal fluctuations in noise level have been found, in particular for long-period data. The noise amplitude distribution is approximately lognormal for band-pass filtered short-period data in the P -wave detection band, while the long-period noise data show a skewness that cannot be represented by a lognormal distribution. Diurnal fluctuations in noise level are quite small, but definitely present both for short and horizontal component long-period data. Cultural sources are found to account for the short-period variability, while the long-period fluctuations are attributed to atmospheric pressure variation. Event detection performance generally follows the noise level trends, with an increase in the number of reported events during summer of about 50 per cent relative to winter.