Summary Kevitsa disseminated Ni-Cu-PGE deposit in northern Finland is hosted by a major high velo... more Summary Kevitsa disseminated Ni-Cu-PGE deposit in northern Finland is hosted by a major high velocity (6000–7500 m/s) gabbroic intrusion. The deposit is currently being mined using open-pit mining method at already over 100 m depth, and the final pit will extend to about 400–500 m depth in about 20 years, hence justifying a 3D seismic survey for its careful planning. The 3D seismic survey was carried out in 2010, before mining activities commenced, and resulted in a reflection volume rich in reflectivity, however, poor near the surface likely due to the survey setup and extremely high bedrock velocities. 3D first break tomography was performed with the main objective of linking near-surface geological information with the reflection seismic volume allowing to find a major low-velocity linear zone in the bedrock within the first 50 m depth. This was associated with a gently-dipping reflector with the same strike at 150 m depth and extending to about 500 m depth. The low-velocity zone striking in NE-SW, and originating from a gently-dipping fracture system, may contribute towards the formation of wedge blocks within the northern side of the planned open pit, which can be critical for its stability in the near future.
We describe the ongoing development of joint geophysical imaging methodologies for geothermal sit... more We describe the ongoing development of joint geophysical imaging methodologies for geothermal site characterization and demonstrate their potential in two regions: Krafla volcano and associated geothermal fields in Northeastern Iceland, and Coso Hot Springs in California, USA. The Coso field is a high temperature reservoir similar to Krafla in Iceland. Each area is a locus of significant geothermal energy production. The complex geology of these sites also makes them excellent targets for developing and testing of strategies for joint imaging of magnetotelluric (MT) and micro-earthquake (MEQ) data. Our ultimate aim is to construct coupled 3D resistivity and velocity models of these geothermal systems and use them to better understand and exploit them.
The Swedish National Seismic Network (SNSN) underwent a rapid expansion and modernization between... more The Swedish National Seismic Network (SNSN) underwent a rapid expansion and modernization between the years 2000 2010. The number of stations increased from 6 to 65, all broadband or semi-broadband with higher than standard sensitivity and all transmitting data in real-time. This has lead to a significant increase in the number of detected earthquakes, with the magnitude of completeness being approximately ML 0.5 within the network. During the last 15 years some 7,300 earthquakes have been detected and located, which can be compared to the approximately 1,800 earthquakes in the Swedish catalog from 1375 to 1999.
We introduce a method for mapping the noise-source distribution of microseisms which uses informa... more We introduce a method for mapping the noise-source distribution of microseisms which uses information from the full length of noise covariograms. We derive a forward calculation based on the plane-wave assumption in 2D, to formulate an iterative, linearized inversion of covariogram envelopes in the time domain. The inversion exploits the well-known feature of noise cross-correlation, i.e. that an anomaly in the noise field that is oblique to the inter-station direction appears as cross-correlation amplitude at a smaller time lag than the in-line, surface-wave arrival. Therefore, the inversion extracts more information from the covariograms than that contained at the expected surface-wave arrival, and this allows us to work with few stations to find the propagation directions of incoming energy. The inversion is naturally applied to data that retain physical units, i.e. that are not amplitude normalized in any way, therefore, the final inversion results give the absolute strength of ...
Induced seismic events in a mining environment are a potential hazard, but they can be used to ga... more Induced seismic events in a mining environment are a potential hazard, but they can be used to gain information about the rock mass in the mine which otherwise would be very difficult to obtain. In this study we use approximately 1.2 million mining induced seismic events in the Kiirunavaara iron ore mine in northernmost Sweden to image the rock mass using local event travel-time tomography. In addition, relocation of the events significantly improves the possibility to infer structural information and rock damage. The Kiirunavaara mine is one of the largest underground iron ore mines in the world. The ore body is a magnetite sheet of 4 km length, with an average thickness of 80 m, which dips approximately 55◦ to the east. Mining production is now at a depth of 785 – 855 m. During 2015 the seismic system in the mine recorded on average approximately 1,000 local seismic events per day. The events are of various origins such as shear slip on fractures, non-shear events and blasts, with...
SUMMARY Rayleigh-wave phase-velocity tomography of southern Sweden is presented using ambient sei... more SUMMARY Rayleigh-wave phase-velocity tomography of southern Sweden is presented using ambient seismic noise at 36 stations (630 station pairs) of the Swedish National Seismic Network. We analyse 1 yr (2012) of continuous recordings to get the first crustal image based on the ambient-noise method in the area. Time-domain cross-correlations of the vertical component between the stations are computed. Phase-velocity dispersion curves are measured in the frequency domain by matching zero crossings of the real spectra of cross-correlations to the zero crossings of the zeroth-order Bessel function of the first kind. We analyse the effect of uneven source distributions on the phase-velocity dispersion curves and correct for the estimated velocity bias before tomography. To estimate the azimuthal source distribution to determine the bias, we perform inversions of amplitudes of cross-correlation envelopes in a number of period ranges. Then, we invert the measured and bias-corrected dispersio...
Eos, Transactions American Geophysical Union, 2008
Topography can result from a balance, called isostasy, between the overlying weight of the crust ... more Topography can result from a balance, called isostasy, between the overlying weight of the crust and the buoyancy of the mantle beneath it. The principle of isostasy was put forward almost two centuries ago as a method of explaining variations in mountain heights, and it does a good job of explaining most of the first‐order variations in the Earth's topography.For example, when the crust is in local isostatic balance, elevation increases can be compensated for by an increase in crustal thickness, which in mountainous areas is in the form of a crustal root. Called Airy isostasy, compensation through crustal roots involves a correlation between surface topography and crustal thickness. However, topography may not be solely compensated for by crustal thickness, as other mechanisms such as flexural rigidity, low mantle densities, and dynamic topography can also be in effect.
The Pärvie fault is one of the largest known endglacial faults in the world. It is situated in no... more The Pärvie fault is one of the largest known endglacial faults in the world. It is situated in northernmost Sweden and extends for over 160 km in a northeastward direction. The fault exhibits reverse faulting throw of more than 10 m and based on studies of Quaternary deposits, landslides and liquefaction structures it is inferred to have ruptured as a one-step event at the time of deglaciation in the area, approximately 10,000 years ago. An earthquake of this size would have had a magnitude of approximately 8. The mechanisms driving the endglacial faults are still not well understood. However, knowledge of the fault geometry at depth would significantly contribute to the understanding. In a seismological study of the Pärvie fault we have acquired both a 23 km long seismic reflection profile across the center of the fault, and deployed eight temporary seismic stations in the vicinity of the fault. The results of the reflection seismic processing images the fault system from the near ...
Summary Kevitsa disseminated Ni-Cu-PGE deposit in northern Finland is hosted by a major high velo... more Summary Kevitsa disseminated Ni-Cu-PGE deposit in northern Finland is hosted by a major high velocity (6000–7500 m/s) gabbroic intrusion. The deposit is currently being mined using open-pit mining method at already over 100 m depth, and the final pit will extend to about 400–500 m depth in about 20 years, hence justifying a 3D seismic survey for its careful planning. The 3D seismic survey was carried out in 2010, before mining activities commenced, and resulted in a reflection volume rich in reflectivity, however, poor near the surface likely due to the survey setup and extremely high bedrock velocities. 3D first break tomography was performed with the main objective of linking near-surface geological information with the reflection seismic volume allowing to find a major low-velocity linear zone in the bedrock within the first 50 m depth. This was associated with a gently-dipping reflector with the same strike at 150 m depth and extending to about 500 m depth. The low-velocity zone striking in NE-SW, and originating from a gently-dipping fracture system, may contribute towards the formation of wedge blocks within the northern side of the planned open pit, which can be critical for its stability in the near future.
We describe the ongoing development of joint geophysical imaging methodologies for geothermal sit... more We describe the ongoing development of joint geophysical imaging methodologies for geothermal site characterization and demonstrate their potential in two regions: Krafla volcano and associated geothermal fields in Northeastern Iceland, and Coso Hot Springs in California, USA. The Coso field is a high temperature reservoir similar to Krafla in Iceland. Each area is a locus of significant geothermal energy production. The complex geology of these sites also makes them excellent targets for developing and testing of strategies for joint imaging of magnetotelluric (MT) and micro-earthquake (MEQ) data. Our ultimate aim is to construct coupled 3D resistivity and velocity models of these geothermal systems and use them to better understand and exploit them.
The Swedish National Seismic Network (SNSN) underwent a rapid expansion and modernization between... more The Swedish National Seismic Network (SNSN) underwent a rapid expansion and modernization between the years 2000 2010. The number of stations increased from 6 to 65, all broadband or semi-broadband with higher than standard sensitivity and all transmitting data in real-time. This has lead to a significant increase in the number of detected earthquakes, with the magnitude of completeness being approximately ML 0.5 within the network. During the last 15 years some 7,300 earthquakes have been detected and located, which can be compared to the approximately 1,800 earthquakes in the Swedish catalog from 1375 to 1999.
We introduce a method for mapping the noise-source distribution of microseisms which uses informa... more We introduce a method for mapping the noise-source distribution of microseisms which uses information from the full length of noise covariograms. We derive a forward calculation based on the plane-wave assumption in 2D, to formulate an iterative, linearized inversion of covariogram envelopes in the time domain. The inversion exploits the well-known feature of noise cross-correlation, i.e. that an anomaly in the noise field that is oblique to the inter-station direction appears as cross-correlation amplitude at a smaller time lag than the in-line, surface-wave arrival. Therefore, the inversion extracts more information from the covariograms than that contained at the expected surface-wave arrival, and this allows us to work with few stations to find the propagation directions of incoming energy. The inversion is naturally applied to data that retain physical units, i.e. that are not amplitude normalized in any way, therefore, the final inversion results give the absolute strength of ...
Induced seismic events in a mining environment are a potential hazard, but they can be used to ga... more Induced seismic events in a mining environment are a potential hazard, but they can be used to gain information about the rock mass in the mine which otherwise would be very difficult to obtain. In this study we use approximately 1.2 million mining induced seismic events in the Kiirunavaara iron ore mine in northernmost Sweden to image the rock mass using local event travel-time tomography. In addition, relocation of the events significantly improves the possibility to infer structural information and rock damage. The Kiirunavaara mine is one of the largest underground iron ore mines in the world. The ore body is a magnetite sheet of 4 km length, with an average thickness of 80 m, which dips approximately 55◦ to the east. Mining production is now at a depth of 785 – 855 m. During 2015 the seismic system in the mine recorded on average approximately 1,000 local seismic events per day. The events are of various origins such as shear slip on fractures, non-shear events and blasts, with...
SUMMARY Rayleigh-wave phase-velocity tomography of southern Sweden is presented using ambient sei... more SUMMARY Rayleigh-wave phase-velocity tomography of southern Sweden is presented using ambient seismic noise at 36 stations (630 station pairs) of the Swedish National Seismic Network. We analyse 1 yr (2012) of continuous recordings to get the first crustal image based on the ambient-noise method in the area. Time-domain cross-correlations of the vertical component between the stations are computed. Phase-velocity dispersion curves are measured in the frequency domain by matching zero crossings of the real spectra of cross-correlations to the zero crossings of the zeroth-order Bessel function of the first kind. We analyse the effect of uneven source distributions on the phase-velocity dispersion curves and correct for the estimated velocity bias before tomography. To estimate the azimuthal source distribution to determine the bias, we perform inversions of amplitudes of cross-correlation envelopes in a number of period ranges. Then, we invert the measured and bias-corrected dispersio...
Eos, Transactions American Geophysical Union, 2008
Topography can result from a balance, called isostasy, between the overlying weight of the crust ... more Topography can result from a balance, called isostasy, between the overlying weight of the crust and the buoyancy of the mantle beneath it. The principle of isostasy was put forward almost two centuries ago as a method of explaining variations in mountain heights, and it does a good job of explaining most of the first‐order variations in the Earth's topography.For example, when the crust is in local isostatic balance, elevation increases can be compensated for by an increase in crustal thickness, which in mountainous areas is in the form of a crustal root. Called Airy isostasy, compensation through crustal roots involves a correlation between surface topography and crustal thickness. However, topography may not be solely compensated for by crustal thickness, as other mechanisms such as flexural rigidity, low mantle densities, and dynamic topography can also be in effect.
The Pärvie fault is one of the largest known endglacial faults in the world. It is situated in no... more The Pärvie fault is one of the largest known endglacial faults in the world. It is situated in northernmost Sweden and extends for over 160 km in a northeastward direction. The fault exhibits reverse faulting throw of more than 10 m and based on studies of Quaternary deposits, landslides and liquefaction structures it is inferred to have ruptured as a one-step event at the time of deglaciation in the area, approximately 10,000 years ago. An earthquake of this size would have had a magnitude of approximately 8. The mechanisms driving the endglacial faults are still not well understood. However, knowledge of the fault geometry at depth would significantly contribute to the understanding. In a seismological study of the Pärvie fault we have acquired both a 23 km long seismic reflection profile across the center of the fault, and deployed eight temporary seismic stations in the vicinity of the fault. The results of the reflection seismic processing images the fault system from the near ...
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