Influence of the seismic noise characteristics on noise correlations in the Baltic Shield

SUMMARY It has recently been shown that correlations of seismic noise can contain significant information about the Green's function along the station profile. Using an array of 38 temporary broadband stations located in Finland between September 1998 and March 1999, we study the resulting 703 noise correlations to understand how they are influenced by the directivity of the noise field. The latter information is obtained through f-k analysis of data from two permanent seismic arrays in Germany and Norway and from a subset of stations of the array in Finland. Both types of analysis confirm that the characteristics of the seismic noise is strongly frequency dependent. At low frequencies (0.02-0.04 Hz), we observe diffuse noise and/or randomly distributed sources. In contrast, the noise is strongly direction dependent and not fully diffuse in the intermediate period ranges (0.04-0.25 Hz) which correspond to the first and second microseismic peak, created at the Irish and Scottish coast and the western coast of Norway. In this frequency interval the noise is sufficiently close to a plane wave to introduce systematic errors in group velocities for station pairs which are not parallel to direction of the dominant incident noise. Phase velocities calculated by slant stack over many traces are however correct, independently of profile direction. In the high frequency band (0.25-1.0 Hz), the situation is a mix between the low-frequency and the intermediate frequency cases. Average phase velocities and individual group velocities from well-oriented profiles are in excellent agreement with results from Rayleigh wave studies of the area.