Near surface structure of Sodankylä area in Finland, obtained by advanced method of passive seismic interferometry

Abstract. Controlled-source seismic exploration surveys are not always possible in nature-protected areas. As an alternative, application of passive seismic techniques in such areas can be proposed. In our study, we show results of passive seismic interferometry application for mapping the uppermost crust in the area of active mineral exploration in Northern Finland. We are utilizing continuous seismic data acquired by Sercel Unite Wireless multichannel recording system along several profiles during XSoDEx (eXperiment of SOdankyla Deep Exploration) project. The objective of the project was to obtain a structural image of the upper crust in the Sodankyla area of Northern Finland in order to achieve a better understanding of the mineral system at depth. The key experiment of the project was a high-resolution seismic reflection experiment, and continuous passive seismic data was acquired in parallel with reflection seismic data acquisition. Due to this, the length of passive data suitable for noise cross-correlation was limited to several hours. In addition, analysis of the passive data demonstrated that dominating sources of ambient noise are non-stationary and have different origin across the XSoDEx study area. As the long data registration period and isotropic azimuthal distribution of noise sources are two major conditions for diffuse wavefield necessary for Empirical Green's Functions (EGFs) extraction, the conventional techniques of passive seismic interferometry was not possible to apply. To find the way to obtain EGFs, we used numerical modelling to investigate the properties of seismic noise originating from sources with different characteristics and propagating inside synthetic heterogeneous Earth models that models real geological conditions in the XSodEx study area. The modelling demonstrated that scattering of ballistic waves on irregular shape heterogeneities, such as massive sulphides or mafic intrusions, could produce diffused wavefield composed mainly of scattered surface waves. This scattered wavefield can be used to retrieve reliable Empirical Green Functions (EGFs) from short-term and non-stationary data, using a special technique called signal-to-noise ratio stacking (SNRS). The EGFs calculated for the XSoDEx profiles were inverted in order to obtain S-wave velocity models down to the depth of 300 meters. The obtained velocity models agree well with geological data and complement the results of reflection seismic data interpretation.