Development of passive seismic interferometry to study shallow subsurface structure

Abstract This thesis presents the results of the development passive seismic interferometry technique and its application for studying shallow subsurface structure in the stable intraplate area of the Precambrian Fennoscandian Shield. Knowledge about structure and mechanical properties of this part of the geological medium is important in the most applied tasks, such as mineral exploration, seismic hazard assessment, geotechnical site investigations, geoenergy projects. The thesis briefly describes the theoretical background of passive seismic interferometry, conditions for its application and describes the new algorithm for passive seismic data processing developed as a part of this work. This algorithm allows for improvement in the quality of empirical Green’s functions (EGFs) evaluated from ambient seismic noise wavefield, produced by non-uniformly distributed sources of high frequency (higher than 1 Hz). In comparison with the most conventional algorithms of passive seismic interferometry, the proposed algorithm requires a much shorter duration of passive seismic data acquisition time, because diffusivity of the wavefield in that case is mostly ensured by multiple scattering. The proposed methodology has been tested for several case studies located in different areas of Finland, that not only have different scales and targets, but also different spatial distribution and intensity of ambient seismic noise sources: Suasselkä postglacial fault - natural ambient seismic noise of relatively low- frequency; Kuusamo area - high-frequency noise of low energy; the area near the Pyhäsalmi underground mine - non uniform high-frequency seismic noise of high energy; Sodankylä area - anthropogenic and natural high-frequency seismic noise of low energy. Results of the case studies presented in the thesis have demonstrated that using conventional passive seismic interferometry in applied tasks is not always possible under conditions of the Fennoscandian Shield. The development of the data processing common workflow has enabled the use of high-frequency non-uniform ambient noise wavefields to obtain information about seismic velocities and attenuations in studied shallow subsurface by passive seismic interferometry.Tiivistelmä Tässä geofysiikan alaan kuuluvassa väitöskirjassa on kehitetty passiivista seismistä interferometritekniikkaa ja sen soveltamista maankamaran pintaosan rakenteiden ja mekaanisen koostumuksen tutkimukseen Fennoskandian kilven alueella. Geofysikaalisilla tutkimusmenetelmillä, kuten passiivisella seismisellä interferometritekniikalla, saatava tieto maankamaran pintaosien rakenteista ja koostumuksesta voidaan soveltaa muun muassa malminetsinnässä, seismisten riskien arvioinnissa, geoteknisissä tutkimuksissa ja geoenergiaprojekteissa. Väitöskirjassa kuvataan lyhyesti passiivisen seismisen interferometrian teoreettista taustaa ja sen soveltamisen ehtoja sekä kuvataan osana tätä työtä kehitetty uusi passiivisen seismisen tiedonkäsittelyn algoritmi. Tämän algoritmin avulla voidaan parantaa ympäristön seismisen kohinan aaltokentästä arvioitujen empiiristen Greenin funktioiden (EGF) laatua, jotka on tuotettu epätasaisesti jakautuneista korkeataajuisista lähteistä (korkeampi kuin 1 Hz). Verrattuna tavanomaisimpiin passiivisen seismisen interferometrian algoritmeihin, väitöskirjassa kehitetty algoritmi vaatii olennaisesti lyhyemmän passiivisen seismisen tiedonkeruuajan, koska aaltokentän diffuusio varmistetaan siinä tapauksessa useimmiten moninkertaisella sironnalla. Väitöskirjassa esitettyä menetelmää on testattu useilla eri kohdealueilla Suomessa. Kohteet, joilla menetelmää testattiin, erosivat ominaisuuksiltaan mittakaavansa, kohteiden maankamaran laadun, ympäristön seismisen melun tilajakauman ja intensiteetin suhteen. Suasselän postglasiaalisen siirroksen testialue edustaa matalataajuisen seismisen melun aluetta; Kuusamon alue matalan energian korkeataajuista seismistä melua; Pyhäsalmen kaivoksen lähellä oleva alue korkean energian epätasaista korkeataajuista seismistä melua ja Sodankylän alue ihmisen toiminnan ja luonnollisen ympäröivän lähteen tuottamaa korkeataajuista matalaenergistä seismistä melua. Väitöskirjassa esitetyt tutkimustulokset osoittavat, että ns. konventionaalisen passiivisen seismisen interferometrian käyttö sovellettavissa tehtävissä ei ole aina mahdollista Fennoskandian kilven olosuhteissa. Väitöskirjassa kehitetty yhtenäinen tietojenkäsittelyn työnkulku mahdollistaa maankamara pintaosan tutkimisen passiivisen seismisen interferometrian avulla, kun ympäristön korkeataajuista ja epäyhtenäistä seismisen melun aaltokenttää käytetään seismisten nopeuksien ja vaimennuksien määrittämiseen

Development of events detector for monitoring cryoseisms in upper soils

Abstract In this article, we describe the first results of the development of the seismic events detector with an artificial neural network (ANN) based identification. Such a detector is necessary for studying seismic events induced by soil freezing that can be hazardous for urban and mining infrastructures. We used the data of about 300 such seismic events recorded by seismic station OUL of Northern Finland Seismological Network for testing the detector and neural network learning. We processed about two months of continuous data and found out, that in some cases the number of detected and identified seismic events per day depends on air temperature variation

Application of passive seismic interferometry for mapping mining waste storage facilities:a case study of Pyhäsalmi mine in Finland

Abstract Obtaining knowledge about the structural, geotechnical and hydrogeological conditions of mining waste storage facilities (e.g., tailing storage facilities, TSF) is an important task for mining waste management. From the existing applied geophysics techniques, high-resolution controlled source seismic methods are the most appropriate due to their superior spatial and depth resolution. However, the TSF areas are not often accessible for installation of seismic instruments and application of controlled source. Moreover, TSF areas in active mine sites are characterised by high-level of seismic and acoustic noise, produced by mining operations and other human activities at mine sites. In this study, we show that these problems can be addressed by utilizing passive seismic interferometry, in which cross-correlation of ambient seismic noise is used to obtain the Empirical Green’s function (EGF’s) of the studied medium. We used this method to evaluate seismic velocities in the TSF and in the near-surface bedrock at the site of the Pyhäsalmi underground mine in Finland. We used the ambient seismic noise recorded in two seismic experiments in the area. In the first experiment, the data were acquired in 2013 along a line 10 km long crossing the area of the underground mine. The second experiment was carried out in the TSF area in 2019. We obtained S-wave velocity models of both bedrock and the TSF by applying multichannel analysis of surface waves (MASW) to surface wave parts of Empirical Green’s Functions (EGFs). The analysis of autocorrelation functions of passive seismic data recorded on the TSF surface allowed to us retrieve reflected arrivals from the boundaries inside the TSF. At the same time, we used amplitudes of surface wave parts of EGF’s to obtain information about attenuation of surface waves in TSF. Our results show that passive seismic interferometry and interpretation of passive seismic data by different techniques can be a useful tool for inspection of TSF of active mines

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

Abstract 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

Improving the quality of empirical Green's functions, obtained by cross-correlation of high-frequency ambient seismic noise

Abstract Studying the uppermost structure of the subsurface is a necessary part of solving many practical problems (exploration of minerals, groundwater studies, geoengineering, etc.). The practical application of active seismic methods for these purposes is not always possible for different reasons, such as logistical difficulties, high cost of work, and a high level of seismic and acoustic noise. That is why developing and improving passive seismic methods is one of the important problems in applied geophysics. In our study, we describe a way of improving the quality of empirical Green’s functions (EGFs), evaluated from high-frequency ambient seismic noise, by using the advanced technique of cross-correlation function stacking in the time domain (in this paper we use term “high-frequency” for frequencies higher than 1 Hz). The technique is based on the global optimization algorithm, in which the optimized objective function is a signal-to-noise ratio of an EGF, retrieved at each iteration. In comparison to existing techniques, based, for example, on weight stacking of cross-correlation functions, our technique makes it possible to significantly increase the signal-to-noise ratio and, therefore, the quality of the EGFs. The technique has been tested with the field data acquired in an area with a high level of industrial noise (Pyhäsalmi Mine, Finland) and in an area with a low level of anthropogenic noise (Kuusamo Greenstone Belt, Finland). The results show that the proposed technique can be used for the extraction of EGFs from high-frequency seismic noise in practical problems of mapping of the shallow subsurface, both in areas with high and low levels of high-frequency seismic noise

Analysis of high-frequency ambient seismic noise recorded during XSodEx experiment in Finland

Abstract Development of passive seismic methods is an important task for the solution of many practical problems where the study of shallow structure of subsurface is necessary (mineral exploration, microseismic zonic, groundwater study, etc.). Nowadays there are many techniques for estimating Empirical Greens Functions (EGF) from highfrequency seismic noise generated by industrial objects, transports or other human activity (Afonin et. al., 2016; Cheng et. al., 2015; Le Feuvre et. al., 2015; Nakata et. al., 2011; Shirzad et. al., 2014, etc.). Nevertheless, there is the necessity of using high-frequency ambient noise on quiet areas, for example, greenfield exploration tasks in remote territories. In this case, extraction of EGFs is difficult because of inhomogeneous distribution of sources and strong attenuation of high-frequency noise wavefield. That is why the study of high-frequency ambient noise behaviour in quiet areas is an important task for the development of passive seismic methods. For this purpose, we analyze ambient seismic noise recorded during XSodEx experiment by 24 3-component and 54 1-component DSU-SA MEMS seismic sensors with the autonomous data acquisition units produced by Sercel Ltd. The sensors were installed along about 1 km long line with intersensor distances of 7–15 m. The profile recorded continuous passive seismic data since 21.08.2017 to 23.08.2017 with the sampling rate of 500 sps. The data were processed by several steps including single station data analysis, prefiltering and cross-correlation of night-time records between all possible station pairs. The cross-correlation functions were used for EGFs estimation. The analysis of apparent velocities of cross-correlation functions and EGFs shows that there were several noise sources with frequencies of 1–20 Hz along the profile. In some cases, we extracted symmetric EGF from short time records (several hours), which indicates homogeneous azimuthal distribution of noise sources. Nevertheless, wavefields, generated by these sources distributed to about several dozen meters. In our paper, we concentrate mainly on details of our data processing routine and its influence on the quality of EGF extraction results

Structure of the Suasselkä postglacial fault in northern Finland obtained by analysis of local events and ambient seismic noise

Abstract Understanding the inner structure of seismogenic faults and their ability to reactivate is particularly important in investigating the continental intraplate seismicity regime. In our study we address this problem using analysis of local seismic events and ambient seismic noise recorded by the temporary DAFNE array in the northern Fennoscandian Shield. The main purpose of the DAFNE/FINLAND passive seismic array experiment was to characterize the present-day seismicity of the Suasselkä postglacial fault (SPGF), which was proposed as one potential target for the DAFNE (Drilling Active Faults in Northern Europe) project. The DAFNE/FINLAND array comprised an area of about 20 to 100 km and consisted of eight short-period and four broadband three-component autonomous seismic stations installed in the close vicinity of the fault area. The array recorded continuous seismic data during September 2011–May 2013. Recordings of the array have being analysed in order to identify and locate natural earthquakes from the fault area and to discriminate them from the blasts in the Kittilä gold mine. As a result, we found a number of natural seismic events originating from the fault area, which proves that the fault is still seismically active. In order to study the inner structure of the SPGF we use cross-correlation of ambient seismic noise recorded by the array. Analysis of azimuthal distribution of noise sources demonstrated that during the time interval under consideration the distribution of noise sources is close to the uniform one. The continuous data were processed in several steps including single-station data analysis, instrument response removal and time-domain stacking. The data were used to estimate empirical Green’s functions between pairs of stations in the frequency band of 0.1–1 Hz and to calculate corresponding surface wave dispersion curves. The S-wave velocity models were obtained as a result of dispersion curve inversion. The results suggest that the area of the SPGF corresponds to a narrow region of low S-wave velocities surrounded by rocks with high S-wave velocities. We interpret this low-velocity region as a non-healed mechanically weak fault damage zone (FDZ) formed due to the last major earthquake that occurred after the last glaciation.The DAFNE/FINLAND Working Group consists of the following individuals: Ilmo Kukkonen (University of Helsinki, Department of Physics, Helsinki, Finland), Pekka Heikkinen (University of Helsinki, Institute of Seismology, Helsinki, Finland), Kari Komminaho (University of Helsinki, Institute of Seismology, Helsinki, Finland), Elena Kozlovskaya (Oulu Mining School, University of Oulu/ Geological Survey of Finland, Oulu, Finland), Riitta Hurskainen (Sodankylä Geophysical Observatory, University of Oulu, Oulu, Finland), Tero Raita (Sodankylä Geophysical Observatory, University of Oulu, Oulu, Finland), Hanna Silvennoinen (Sodankylä Geophysical Observatory, University of Oulu, Oulu, Finland)

Leveraging active-source seismic data in mining tailings:refraction and MASW analysis, elastic parameters, and hydrogeological conditions

Abstract We applied active-source seismic method for the interpretation of elastic parameters in tailings facilities which is essential for evaluating stability and seismic response. The methodology uses different analysis methods on the same dataset, i.e., conventional seismic refraction (SR) to determine compressional-wave velocity (Vₚ) and multichannel analysis of surface wave (MASW) to estimate shear-wave velocity (Vₛ). Seismic velocities in conjunction with tailings physics approach revealed interpretable data in terms of elastic parameters and hydrogeological conditions. The results determined the empirical linear relationships between Vₚ and Vₛ that are particular to an unconsolidated media such as tailings and showed that variability of hydrogeological conditions influences the elastic seismic response (Vₚ and Vₛ) and the elastic parameters. The analysis of the elastic parameters identified the state condition of the tailings at the time of the survey. The Bulk modulus K that relates the change in hydrostatic stress to the volumetric strain was predominant between 1.0−2.0 GPa. The Young’s modulus E in the tailings media was in the low range of 0.15−0.23 GPa. Poisson’s ratio values in all sections were in the upper limit in the range of 0.37−0.49, meaning that the tailings media is highly susceptible to transverse deformation under axial compression