Analysis of Microseismic Noise at LASA, NORSAR and ALPA†

Abstract

Summary An analysis has been made of the properties of both the short-period and long-period microseismic noise in the 0.2- to 40-s period range at LASA, NORSAR and ALPA. The high-resolution frequency–wavenumber spectra are presented for the noise, as well as power spectra and coherence results. These data show that the long-period noise in the 10- to 40-s period range consists of fundamental-mode Rayleigh and Love waves, as well as a non-propagating component. The results of a simultaneous analysis of long-period noise at ALPA and LASA provide for the first time an indication of the location of the noise source. It is shown that this location can be associated with low-pressure atmospheric disturbances. The short-period noise in the 2- to 10-s period range consists primarily of higher-mode Rayleigh waves. The 0.2- to 2-s microseisms consist of a significant non-propagating, as well as a propagating, component. This propagating component of the noise has a frequency-wavenumber spectrum whose structure is highly diffuse in wavenumber space. The signal-to-noise ratio gain of maximum-likelihood processing relative to that obtained with beamforming is computed using a theoretical model for the wavenumber structure of the long-period noise which is based on the measurements. These results show that this gain is not substantial in most cases of practical interest.