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. Controlled-source seismic exploration surveys are notalways possible in nature-protected areas. As an alternative, the application ofpassive seismic techniques in such areas can be proposed. In our study, weshow results of passive seismic interferometry application for mapping theuppermost crust in the area of active mineral exploration in northernFinland. We utilize continuous seismic data acquired by the Sercel Unitewireless multichannel recording system along several profiles during XSoDEx(eXperiment of SOdankylä Deep Exploration) multidisciplinary geophysicalproject. The objective of XSoDEx was to obtain a structural image of theupper crust in the Sodankylä area of northern Finland in order toachieve a better understanding of the mineral system at depth. The keyexperiment of the project was a high-resolution seismic reflectionexperiment. In addition, continuous passive seismic data were acquired inparallel with reflection seismic data acquisition. Due to this, the lengthof passive data suitable for noise cross-correlation was limited fromseveral hours to a couple of days. Analysis of the passive data demonstratedthat dominating sources of ambient noise are non-stationary and havedifferent origins across the XSoDEx study area. As the long data registrationperiod and isotropic azimuthal distribution of noise sources are two majorconditions for empirical Green function (EGF) extraction under the diffusefield approximation assumption, it was not possible to apply theconventional techniques of passive seismic interferometry. To find the wayto obtain EGFs, we used numerical modelling in order to investigateproperties of seismic noise originating from sources with differentcharacteristics and propagating inside synthetic heterogeneous Earth modelsrepresenting real geological conditions in the XSoDEx study area. Themodelling demonstrated that scattering of ballistic waves on irregular shapeheterogeneities, such as massive sulfides or mafic intrusions, couldproduce a diffused wavefield composed mainly of scattered surface waves. Inour study, we show that this scattered wavefield can be used to retrievereliable EGFs from short-term and non-stationarydata using special techniques. One of the possible solutions is applicationof “signal-to-noise ratio stacking” (SNRS). The EGFs calculated for theXSoDEx profiles were inverted, in order to obtain S-wave velocity modelsdown to the depth of 300 m. The obtained velocity models agree wellwith geological data and complement the results of reflection seismic datainterpretation.

Near-surface structure of the Sodankylä area in Finland, obtained by passive seismic interferometry