Magnetotelluric sounding of the Earth's crust in a region with strong industrial electromagnetic noise

V této práci se zabývám zpracováním magnetotelurických dat ze západočeské seismoak- tivní oblasti. Magnetotelurická metoda může být využita k detekci vodivých zón spjatými s fluidy, které mohou hrát významnou roli ve vzniku západočeských zemětřesení. Tato me- toda je závislá na přírodním elektromagnetickém poli Země, které je na zemském povrchu v porovnání s civilizačními zdroji poměrně slabé. V západních Čechách je úroveň civili- začního šumu velice vysoká, a proto jsem se pokusil v datech z let 2001 a 2002 pomocí různých metod potlačit šumovou složku. Konkrétně se jednalo o přístup pro automatickou detekci a odstranění výskoků v magnetotelurických datech za použití vzdálené referenční stanice a o vizuální inspekci dat ve frekvenční oblasti. Vzhledem k abnormálně silnému civilizačnímu šumu ani jedna z metod nedokázala data nijak významně zlepšit, stejně jako neuspěly standardní postupy zpracování. Vysokofrekvenční data byla naštěstí šumem re- lativně méně poškozená, a proto jsem se pokusil o inverzi magnetotelurických dat z roku 2001. Deset stanic tvoří profil ve směru SZ-JV a protíná seismicky aktivní oblast západ- ních Čech v okolí obcí Počátky a Nový Kostel. V důsledku kontaminace šumem nelze data modelovat do hloubek větších než asi 4 km, hlubší struktury jsou již skryty v šumu. Z výsledku...In this thesis, I am dealing with a re-processing of magnetotelluric data from the West Bohemia seismo-active region. By revealing zones of enhanced electrical conductivity in the Earth, the magnetotelluric method is potentially perspective for detecting fluids which may play a major role in the generation of local earthquakes. The magnetotelluric method employs the natural electromagnetic field of the Earth, which is, on the Earth's surface, relatively weak as compared to various cultural sources. In West Bohemia, the level of the cultural electromagnetic noise is very high. Therefore, I have tested various approaches suggested for reducing the cultural noise in the data. In particular, I used a data variance technique for automatically despiking magnetotelluric data by employing simultaneous records from a remote reference site. I also employed a noise separation technique for magnetotelluric single site data by applying a frequency domain data selection scheme. Due to an extremely high level of the cultural noise in the study area, both those approaches failed as standard pre-processing methods. High frequency data show a relatively lower level of the corruption by noise, and a subset of data acquired in 2001 could be modelled via magnetotelluric inversion procedures. Ten magnetotelluric sites...Institute of Hydrogeology, Engineering Geology and Applied GeophysicsÚstav hydrogeologie, inž. geologie a užité geofyzikyPřírodovědecká fakultaFaculty of Scienc

Stochastic simulations and modelling in the magnetotelluric method

In the thesis I deal with the development of a stochastic inversion procedure for the magnetotelluric method in 1D/2D isotropic and anisotropic cases, and its application to both synthetic and real data. The magnetotelluric method is a geoelectric inductive technique that utilizes variations of naturally occurring electromagnetic fields as a source of the electromagnetic induction for estimating the Earth's subsurface resistivity to depths of several tens of kilometres. The purpose of the inversion procedure is to estimate a real distribution of the electrical resistivity in the Earth's subsurface from surface measurements. Common inversion procedures in magnetotellurics perform a model optimization by minimizing the misfit between the data and the model response. Stochastic methods are based on the exploration of the model parameter space, and they pick models according to their probability, which makes them effective for the solution of high-dimensional problems which do not show a single pronounced minimum of the target function. The effective ways of mapping the parameter space are sampling algorithms based on Monte Carlo simulations which allow to sort models according to their probability. Results of these methods are obtained in the form of a fully probabilistic description of the..

Effects of cultural noise in natural magnetotelluric signals

Magnetotelluric method is an electromagnetic induction method which studies the dis- tribution of the electrical conductivity in the Earth by analyzing natural variations of the Earth's electromagnetic field of the external origin. A wide frequency range of the natural sources of the magnetotelluric field makes it possible to study the Earth's conductivity from near-surface structures down to depths of the upper mantle. Dependence on weak natural fields, however, causes the method to fail in case of data contaminated by electro- magnetic disturbances of cultural origin. This thesis summarizes basic principles of the statistical magnetotelluric data processing in the frequency domain, and shows the recent progress of the processing due to the application of robust statistical methods as well as due to employing reference data from remote stations. In some cases, the measurements are disturbed excessively, and even advanced statistics method fail in processing the data. In some of these cases a more thorough analysis of the noise field in terms of its directional and source characteristics may be useful. We present an example of simultaneous recor- dings of two magnetotelluric stations from the West Bohemian seismo-active region and show that the noise field is dominated by two types of strong electric..

Stochastic simulations and modelling in the magnetotelluric method

In the thesis I deal with the development of a stochastic inversion procedure for the magnetotelluric method in 1D/2D isotropic and anisotropic cases, and its application to both synthetic and real data. The magnetotelluric method is a geoelectric inductive technique that utilizes variations of naturally occurring electromagnetic fields as a source of the electromagnetic induction for estimating the Earth's subsurface resistivity to depths of several tens of kilometres. The purpose of the inversion procedure is to estimate a real distribution of the electrical resistivity in the Earth's subsurface from surface measurements. Common inversion procedures in magnetotellurics perform a model optimization by minimizing the misfit between the data and the model response. Stochastic methods are based on the exploration of the model parameter space, and they pick models according to their probability, which makes them effective for the solution of high-dimensional problems which do not show a single pronounced minimum of the target function. The effective ways of mapping the parameter space are sampling algorithms based on Monte Carlo simulations which allow to sort models according to their probability. Results of these methods are obtained in the form of a fully probabilistic description of the..

Magnetotelluric sounding of the Earth's crust in a region with strong industrial electromagnetic noise

In this thesis, I am dealing with a re-processing of magnetotelluric data from the West Bohemia seismo-active region. By revealing zones of enhanced electrical conductivity in the Earth, the magnetotelluric method is potentially perspective for detecting fluids which may play a major role in the generation of local earthquakes. The magnetotelluric method employs the natural electromagnetic field of the Earth, which is, on the Earth's surface, relatively weak as compared to various cultural sources. In West Bohemia, the level of the cultural electromagnetic noise is very high. Therefore, I have tested various approaches suggested for reducing the cultural noise in the data. In particular, I used a data variance technique for automatically despiking magnetotelluric data by employing simultaneous records from a remote reference site. I also employed a noise separation technique for magnetotelluric single site data by applying a frequency domain data selection scheme. Due to an extremely high level of the cultural noise in the study area, both those approaches failed as standard pre-processing methods. High frequency data show a relatively lower level of the corruption by noise, and a subset of data acquired in 2001 could be modelled via magnetotelluric inversion procedures. Ten magnetotelluric sites..

Assessing the Volume of Defensive Structures for Architectural Energetics Analysis Using 3D Electrical Resistivity Tomography

Architectural energetics is a methodology that translates architectural objects into a quantitative time-labor equivalent, from which information about past societies, labor organizations, or political relations can be inferred. Preceding such study, the volume of every architectural structure must be determined. This is usually done by in situ measurements and computing of volume by mathematical formulae or using UAV-based photogrammetry processed into digital surface model. However, both of these methods are impracticable in the case of buried or semi-buried monuments where the only remaining option is direct excavation. Hence, we introduce a new method for the determination of volumetric information based on the electrical resistivity tomography (ERT) geophysical method. We conducted our study at defensive lines (ramparts/ditches) within two hillforts of different ages, constructed from different building materials, in the Czech Republic. ERT surveys performed in 3D can differentiate ramparts/ditches in detail from the surrounding environment based on resistivity. Compared to previous excavations, the 3D inversion results show that ERT can obtain meaningful volumes based on the chosen resistivity threshold. The best results were achieved on homogeneous semi-buried ramparts and the ditch. ERT can be performed at a fraction of the cost of direct excavation. This method also leaves an intact site for future generations

Assessing the Volume of Defensive Structures for Architectural Energetics Analysis Using 3D Electrical Resistivity Tomography

Architectural energetics is a methodology that translates architectural objects into a quantitative time-labor equivalent, from which information about past societies, labor organizations, or political relations can be inferred. Preceding such study, the volume of every architectural structure must be determined. This is usually done by in situ measurements and computing of volume by mathematical formulae or using UAV-based photogrammetry processed into digital surface model. However, both of these methods are impracticable in the case of buried or semi-buried monuments where the only remaining option is direct excavation. Hence, we introduce a new method for the determination of volumetric information based on the electrical resistivity tomography (ERT) geophysical method. We conducted our study at defensive lines (ramparts/ditches) within two hillforts of different ages, constructed from different building materials, in the Czech Republic. ERT surveys performed in 3D can differentiate ramparts/ditches in detail from the surrounding environment based on resistivity. Compared to previous excavations, the 3D inversion results show that ERT can obtain meaningful volumes based on the chosen resistivity threshold. The best results were achieved on homogeneous semi-buried ramparts and the ditch. ERT can be performed at a fraction of the cost of direct excavation. This method also leaves an intact site for future generations

Hypogenic Versus Epigenic Origin of Deep Underwater Caves Illustrated by the Hranice Abyss (Czech Republic)—The World\u27s Deepest Freshwater Cave

Extremely deep freshwater filled cave systems are common in karst systems globally. The origin and evolution of such caves are usually attributed to hypogenic (bottom-up) processes, in which acidic groundwater dissolves limestone from below. However, these deep cave systems can form by epigenic (top-down) processes, with meteoric waters descending from the surface underground. The Hranice Abyss (Czech Republic), with a reached depth of 473.5 m, is the deepest mapped extent of such a system globally, although its maximum depth is unknown. Multiple geophysical data sets (gravity, electrical resistivity tomography, audiomagnetotellurics, and seismic refraction and reflection) are used to investigate the extent and formation of the Hranice Abyss. The geophysical results suggest the Hranice Abyss extends to depths of ~1 km. Further, we identify structures within the karst, including buried cockpit karst towers with several NW-SE-oriented valleys. The new geophysical results from the Hranice Abyss, considered in combination with geological constraints of the region (tectonic evolution and morphology of karst structure), suggest an epigenic formation process, rather than the traditionally invoked hypogenic origin. Formation by epigenic rather than hypogenic processes has implications for local and regional karst history associated with areas hosting deep karst systems

3D imaging of the subsurface electrical resistivity structure in West Bohemia/Upper Palatinate covering mofettes and Quaternary volcanic structures by using Magnetotellurics

The region of West Bohemia and Upper Palatinate belongs to the West Bohemian Massif. The study area is situated at the junction of three different Variscan tectonic units and hosts the ENE-WSW trending Ohře Rift as well as many different fault systems. The entire region is characterized by ongoing magmatic processes in the intra-continental lithospheric mantle expressed by a series of phenomena, including e.g. the occurrence of repeated earthquake swarms and massive degassing of mantle derived CO2 in form of mineral springs and mofettes. Ongoing active tectonics is mainly manifested by Cenozoic volcanism represented by different Quaternary volcanic structures. All these phenomena make the Ohře Rift a unique target area for European intra-continental geo-scientific research. With magnetotelluric (MT) measurements we image the subsurface distribution of the electrical resistivity and map possible fluid pathways. Two-dimensional (2D) inversion results by Muñoz et al. (2018) reveal a conductive channel in the vicinity of the earthquake swarm region that extends from the lower crust to the surface forming a pathway for fluids into the region of the mofettes. A second conductive channel is present in the south of their model; however, their 2D inversions allow ambiguous interpretations of this feature. Therefore, we conducted a large 3D MT field experiment extending the study area towards the south. The 3D inversion result matches well with the known geology imaging different fluid/magma reservoirs at crust-mantle depth and mapping possible fluid pathways from the reservoirs to the surface feeding known mofettes and spas. A comparison of 3D and 2D inversion results suggests that the 2D inversion results are considerably characterized by 3D and off-profile structures. In this context, the new results advocate for the swarm earthquakes being located in the resistive host rock surrounding the conductive channels; a finding in line with observations e.g. at the San Andreas Fault, California

Contrasting tectonic styles of the western and eastern parts of the Western Carpathian Klippen Belt in Slovakia based on magnetotelluric sounding of deep tectonic structures

To characterize the deep structure of the Klippen Belt, we made magnetotelluric measurements in profiles across the western and eastern segments in the territory of Slovakia, from which we created 3D models. The models revealed significant differences in tectonic structure between these segments. In the western segment, the Klippen Belt is located in the southern reversing wing of the original subduction flower structure (retroarc thrusting) with an overthrust to the south onto the Inner Western Carpathian units. This structure was later modified by significant transpressional movements. In the eastern segment, the Klippen Belt is primarily an organic part of the accretionary wedge of the Outer Western Carpathians and it is overthrusted onto the Flysch Belt. This was followed by modification of the structures, mostly in a transpressional regime, including local reversing overthrusts and the development of a steep fault boundary, mainly along the southern margin, against the Inner Carpathian Paleogene succession. These differences between the structure of the western and eastern Klippen Belt segments indicate the contrast between the interaction of the western and eastern parts of the Inner Western Carpathians with the European Platform. In the western part, oblique collision and sinistral transpression dominate. In the eastern part, by contrast, subduction and orthogonal collision dominated over later transpressional modifications