Controlled-source seismic exploration surveys are not always possible in nature-protected areas. As an alternative, the 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 utilize continuous seismic data acquired by the Sercel Unite wireless multichannel recording system along several profiles during XSoDEx (eXperiment of SOdankylä Deep Exploration) multidisciplinary geophysical project. The objective of XSoDEx was to obtain a structural image of the upper crust in the Sodankylä 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. In addition, continuous passive seismic data were acquired in parallel with reflection seismic data acquisition. Due to this, the length of passive data suitable for noise cross-correlation was limited from several hours to a couple of days. Analysis of the passive data demonstrated that dominating sources of ambient noise are non-stationary and have different origins across the XSoDEx study area. As the long data registration period and isotropic azimuthal distribution of noise sources are two major conditions for empirical Green function (EGF) extraction under the diffuse field approximation assumption, it was not possible to apply the conventional techniques of passive seismic interferometry. To find the way to obtain EGFs, we used numerical modelling in order to investigate properties of seismic noise originating from sources with different characteristics and propagating inside synthetic heterogeneous Earth models representing real geological conditions in the XSoDEx study area. The modelling demonstrated that scattering of ballistic waves on irregular shape heterogeneities, such as massive sulfides or mafic intrusions, could produce a diffused wavefield composed mainly of scattered surface waves. In our study, we show that this scattered wavefield can be used to retrieve reliable EGFs from short-term and non-stationary data using special techniques. One of the possible solutions is application of “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 m. The obtained velocity models agree well with geological data and complement the results of reflection seismic data interpretation.

中文翻译：

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通过被动地震干涉测量法获得的芬兰 Sodankylä 地区的近地表结构

在自然保护区内，控制源地震勘探调查并不总是可行的。作为替代方案，可以建议在这些地区应用被动地震技术。在我们的研究中，我们展示了被动地震干涉测量法应用的结果，用于绘制芬兰北部活跃矿产勘探区域的最上层地壳。在 XSoDEx（SOdankylä 深部勘探实验）多学科地球物理项目期间，我们利用 Sercel Unite 无线多通道记录系统沿多个剖面采集的连续地震数据。XSoDEx 的目标是获得芬兰北部 Sodankylä 地区上地壳的结构图像，以便更好地了解深层矿物系统。该项目的重点实验是高分辨率地震反射实验。此外，连续被动地震数据的采集与反射地震数据的采集并行。因此，适用于噪声互相关的无源数据的长度从几小时到几天不等。对被动数据的分析表明，环境噪声的主要来源是不稳定的，并且在 XSoDEx 研究区域中具有不同的来源。由于长数据配准周期和噪声源的各向同性方位角分布是在扩散场近似假设下进行经验格林函数 (EGF) 提取的两个主要条件，因此无法应用被动地震干涉测量的传统技术。为了找到获得 EGF 的方法，我们使用数值建模来研究源自具有不同特征的源并在代表 XSoDEx 研究区真实地质条件的合成异质地球模型内传播的地震噪声的特性。该模型表明，弹道波在不规则形状的异质性（如块状硫化物或镁铁质侵入体）上的散射可以产生主要由散射表面波组成的扩散波场。在我们的研究中，我们表明这种散射波场可用于使用特殊技术从短期和非平稳数据中检索可靠的 EGF。一种可能的解决方案是应用“信噪比叠加”（SNRS）。对 XSoDEx 剖面计算的 EGF 进行了反演，以获得深达 300 m 的 S 波速度模型。