Lightning strokes create powerful electromagnetic pulses that routinely cause very low frequency (VLF) waves to propagate across hemispheres along geomagnetic field lines. VLF antenna receivers can be used to detect these whistler waves generated by these lightning strokes. The particular time/frequency dependence of the received whistler wave enables the estimation of electron density in the plasmasphere region of the magnetosphere. Therefore the identification and characterisation of whistlers are important tasks to monitor the plasmasphere in real time and to build large databases of events to be used for statistical studies. The current state of the art in detecting whistler is the Automatic Whistler Detection (AWD) method developed by Lichtenberger (2009) [1]. This method is based on image correlation in 2 dimensions and requires significant computing hardware situated at the VLF receiver antennas (e.g. in Antarctica). The aim of this work is to develop a machine learning based model capable of automatically detecting whistlers in the data provided by the VLF receivers. The approach is to use a combination of image classification and localisation on the spectrogram data generated by the VLF receivers to identify and localise each whistler. The data at hand has around 2300 events identified by AWD at SANAE and Marion and will be used as training, validation, and testing data. Three detector designs have been proposed. The first one using a similar method to AWD, the second using image classification on regions of interest extracted from a spectrogram, and the last one using YOLO, the current state of the art in object detection. It has been shown that these detectors can achieve a misdetection and false alarm rate, respectively, of less than 15% on Marion's dataset. It is important to note that the ground truth (initial whistler label) for data used in this study was generated using AWD. Moreover, SANAE IV data was small and did not provide much content in the study.

The ever-increasing demand for wireless communication has led to an incentive to increase the data rate and reduce the size of communication devices, be it antennas or other components of RF front-ends. The emphasis is primarily on increasing data rate, which leads to the use of higher frequency bands and wider bandwidths in modern communication technology research and innovations. However, increasing frequency in many technology areas cannot necessarily be beneficial because of physical constraints. For example, communication under seawater or other RF harsh environment requires very-low-frequency (VLF) or ultra-low-frequency (ULF) signals to penetrate lossy media that block high-frequency signals. Furthermore, recent advances in neuroscience have demonstrated the potential of VLF and ULF electromagnetic (EM) waves for studying brain function and treating neurological conditions. The main challenge is that most VLF and ULF generators are large and power-hungry, making them impractical to use in many applications. As a result, recent approaches using permanent magnets have started to provide groundbreaking solutions that can revolutionize VLF/ULF communication. This work presents a new method for generating low-frequency EM waves for navigation and communication in challenging environments, such as underwater and underground, as well as magnetic stimulation of brain neurons. The key concept is to disturb the magnetic energy stored around a permanent magnet in a time-variant fashion. The magnetic reluctance of the medium around the permanent magnet is modulated to alter the magnetic flux intensity and direction (disturb the stored energy) in order to achieve this goal. The nonlinear properties of the surrounding magnetic material are a critical phenomenon for efficient and effective modulation. Since the proposed method of generating the EM field is not based on a second-order system (resonant structure), the bandwidth of any modulation schema is not limited to the overall system quality factor. A transmitter is prototyped as a proof of concept, and the generated field is measured. Compared to the rotating magnet, the prototyped transmitter can modulate up to 50% of the permanent magnet's stored energy with much lower power consumption. The magnetic equivalent circuit (MEC) approach is also used to analyze the transmitter. Finally, the transmitter is optimized, and the measurement results show a 7 dB improvement in efficiency compared to the primary structure. As a result of promising performance, we propose that this method be used to improve the performance of transcranial magnetic stimulation (TMS) devices. Furthermore, the comparison simulated results back up the validity of the proposed technique, revealing that focality and penetration depth are improved while utilizing much less power than traditional TMS devices. / Doctor of Philosophy / The growing demand for wireless communication has created an incentive to increase the data rate while decreasing the size of communication devices, whether they are antennas or other radio frequency (RF) components between the antenna and at least one mixing stage of a receiver and/or the power amplifier of the transmitter. The emphasis is primarily on increasing data rate, which leads to the use of higher frequency bands and wider bandwidths in modern communication technology research and innovations. However, increasing frequency in many technology areas may not be beneficial because of physical constraints. For example, communication under seawater or underground requires very-low-frequency (VLF) or ultra-low-frequency (ULF) signals to penetrate lossy media that block high-frequency signals. Furthermore, recent advances in neuroscience have demonstrated the potential of VLF and ULF electromagnetic (EM) waves for studying brain function and treating neurological conditions. The main challenge is that most VLF and ULF generators are large and power-hungry, making them unsuitable for many applications. As a result, recent approaches using permanent magnets have started to provide groundbreaking solutions that can revolutionize VLF/ULF communication. This work presents a new method for generating low-frequency EM waves for navigation and communication in challenging environments, such as underwater and underground, as well as magnetic stimulation of brain neurons. The key idea is to disturb the magnetic energy stored around a permanent magnet in a time-variant fashion. The magnetic reluctance of the medium around the permanent magnet is modulated to change the magnetic flux intensity and direction (disturb the stored energy) in order to achieve this goal. The nonlinear properties of the surrounding magnetic material are a critical factor in achieving efficient and effective modulation. Since the proposed method of generating the EM field does not rely on a second-order system (resonant structure), the bandwidth of any modulation schema is not constrained by the overall system quality factor. As a proof of concept, a transmitter is prototyped, and the generated field is measured. Compared to the rotating magnet, the prototyped transmitter can modulate up to 50% of the permanent magnet's stored energy with much lower power consumption. The magnetic equivalent circuit (MEC) approach is also used to analyze the transmitter. Finally, the transmitter is optimized, and the measurement results show a 7 dB improvement in efficiency compared to the primary structure. As a result of promising performance, we propose that this method be used to improve the performance of transcranial magnetic stimulation (TMS) devices. Furthermore, the comparison simulated results support the validity of the proposed technique, revealing that focality and penetration depth are improved while using much less power than traditional TMS devices.

Made available in DSpace on 2016-03-15T19:37:41Z (GMT). No. of bitstreams: 1 Jorge Enrifque Samanes Cardenas.pdf: 3561395 bytes, checksum: ba28ce6fbd3bd0e1573255ccacb30d17 (MD5) Previous issue date: 2012-02-17 / Fundo Mackenzie de Pesquisa / Deep amplitude minima are observed in the daily records of Very Low Frequency (VLF, between 3 - 30 kHz) wave amplitudes. Amplitude minima are attributed to modal interference and modal conversion of waves propagating in the Earth - Ionosphere waveguide. The time of amplitude minima is related with the localization of the Terminator line and, therefore, is denominated of Terminator Time or TT time. In this work, we developed a methodology based on the monitoring of TT time to infer the distance of modal interference D and its temporal variation in relation to atmospheric and natural phenomena. For this purpose, we used data provided by the South America VLF Network (SAVNET) to the period 2007 - 2011. The results showed that the methodology is a promising tool to study the dynamics of the lower ionosphere, by means of the daily values of the undisturbed nighttime ionospheric height(hN). We identified short-term and long-term time variations of hN, where the long-term variation is possibly related with the solar activity cycle. We propose the use of this method to search for seismic- electromagnetic effects, and that it should be applied to medium and short propagation paths. Using the Long Wave Propagation Capability (LWPC) code, we simulated TT times and the results show good agreement with the observations, so a value for nighttime height hN = 88 km is suggested. This work will contribute to a greater viability of the use of the VLF technique for the study of the lower ionosphere and the forcings acting on this region of the atmosphere. / Pronunciados mínimos de amplitude são observados nos registros diários de sinais de muito baixa frequência ou VLF (do inglês Very Low Frequency, entre 3 - 30 kHz). A presença destes mínimos é atribuída a interferência e conversão modal de ondas se propagando no guia de onda Terra - Ionosfera. Os tempos de ocorrência destes mínimos são relacionados com a localização do Terminadouro e, por isso, são denominados de tempos de Terminadouro ou Tempos TT. Neste trabalho, desenvolvemos uma metodologia baseada no monitoramento dos tempos TT para inferir a distância de interferência modal D, e sua variabilidade temporal em relação a fenômenos atmosféricos e naturais. Para isso, utilizamos dados fornecidos pela rede SAVNET (South America VLF Network) durante o período 2007 - 2011. Os resultados mostraram que a metodologia é relevante para estudar a dinâmica da baixa ionosfera, através da obtenção de valores diários da altura noturna hN. Identificamos variabilidades de curto e longo período para hN, sendo estas últimas relacionadas ao ciclo de atividade solar. O método deverá ser aplicado à procura de efeitos sismo-eletromagnéticos utilizando caminhos com comprimento médios ou curtos. Foram realizadas simulações dos tempos TT, utilizando o código Long Wave Propagation Capability (LWPC), que mostraram boa concordância com as observações, sugerindo um valor de hN = 88 km para a altura noturna da baixa ionosfera. Os resultados apresentados neste trabalho contribuem para uma maior viabilização do uso da técnica de VLF, para o estudo da baixa ionosfera e os forçantes que agem sobre esta região da atmosfera terrestre.

The objective of this thesis is to use radio emissions from lightning, known as `radio atmospherics' or `sferics', to study the temporal and spatial variation of the lower ionosphere, a layer of ionized atmosphere beginning at $\sim$70 km altitude (D-region). Very Low Frequency (VLF, 3$-$30kHz) radio waves are a useful diagnostic for lower ionospheric monitoring due to their reflection from this region and global propagation. Traditionally, the lower ionosphere has been sensed using single-frequency VLF transmitters allowing for analysis of a single propagation path, as there are only a small number of transmitters. A lightning stroke, however, releases an intense amount of impulsive broadband VLF radio energy in the form of a sferic, which propagates through the Earth-ionosphere waveguide. Lightning is globally distributed and very frequent, so a sferic is therefore also a useful diagnostic of the D-region. This is true both for ambient or quiet conditions, and for ionospheric perturbations such as solar flare x-ray bursts. Lightning strokes effectively act as separate VLF transmitting sources. As such, they uniquely provide the ability to add a spatial component to ionospheric remote sensing, in addition to their broadband signature which cannot be achieved with man-made transmitters. We describe the methods of processing in detail. As an example, we analyze a solar flare during which time there is a significant change in magnitude and frequency content of sferics. This disturbance varies with distance from the source, as well as time. We describe the methods of processing in detail, and show results at Palmer Station, Antarctica for both a quiet and active solar day.

abstract: Electric field imaging allows for a low cost, compact, non-invasive, non-ionizing alternative to other methods of imaging. It has many promising industrial applications including security, safely imaging power lines at construction sites, finding sources of electromagnetic interference, geo-prospecting, and medical imaging. The work presented in this dissertation concerns low frequency electric field imaging: the physics, hardware, and various methods of achieving it. Electric fields have historically been notoriously difficult to work with due to how intrinsically noisy the data is in electric field sensors. As a first contribution, an in-depth study demonstrates just how prevalent electric field noise is. In field tests, various cables were placed underneath power lines. Despite being shielded, the 60 Hz power line signal readily penetrated several types of cables. The challenges of high noise levels were largely addressed by connecting the output of an electric field sensor to a lock-in amplifier. Using the more accurate means of collecting electric field data, D-dot sensors were arrayed in a compact grid to resolve electric field images as a second contribution. This imager has successfully captured electric field images of live concealed wires and electromagnetic interference. An active method was developed as a third contribution. In this method, distortions created by objects when placed in a known electric field are read. This expands the domain of what can be imaged because the object does not need to be a time-varying electric field source. Images of dielectrics (e.g. bodies of water) and DC wires were captured using this new method. The final contribution uses a collection of one-dimensional electric field images, i.e. projections, to reconstruct a two-dimensional image. This was achieved using algorithms based in computed tomography such as filtered backprojection. An algebraic approach was also used to enforce sparsity regularization with the L1 norm, further improving the quality of some images. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

Made available in DSpace on 2016-03-15T19:38:53Z (GMT). No. of bitstreams: 1 ALBERTO FARES AKEL JUNIOR.pdf: 5112998 bytes, checksum: f18fc33d2f9508c3ec265c0efa016b43 (MD5) Previous issue date: 2015-08-21 / Fundação de Amparo a Pesquisa do Estado de São Paulo / We study the behavior of the Earth-ionosphere waveguide through the modeling of the propagation of very low frequency radio waves (VLF). We use the computational model LWPC (Long Wave Propagation Capability) to estimate changes in amplitude and phase of the VLF signals detected by the SAVNET network (South America VLF NETwork), and thus try to understand the behavior of the lower ionosphere under different ionization conditions. The research was divided into two parts. The first part investigates the behavior of the VLF signals in quiescent regimes of ionization. Amplitude and phase simulations for the were carried out, modifying adapting polynomials for the β and h parameters (or Wait s parameters) as a function of the zenithal angle. The second part of this research, uses these polynomials in the study of the lower ionosphere under transient ionization regimes in two distinct conditions: first during of solar flares and second during solar eclipse. For the simulations under solar flare conditions, we calculate the changes in β and ℎ′ parameters during the 25/03/2008 solar explosion. With these values, we calculate the electronic density profile through an exponential model and we find that the electronic density at 75 km is ∼ 104 cm−3, that is twenty times higher than during quiescent conditions. To evaluate our parameter estimates, we calculate the variation of the Wait s parameters for the case of twelve solar events of different classes. We note that the variations Δℎ′ found in this work are larger than that in Muraoka, Murata e Sato (1977) because they consider the variations in the conductivity gradient. For the solar eclipse simulations on 11/07/2011, we investigate its effect on the VLF phase. For this, we use the obscuration coefficient to estimate the guide height variation along the whole path during the eclipse. The simulations reproduce the phase behavior during the eclipse. However, a delay of about twenty four minutes was observed between the simulated and observed measurements. The observed delay is a direct consequence of own estimates of the perturbed ionospheric height and it causal relation with the obscuration during the eclipse. lower ionosphere, VLF, modeling, ionospheric disturbances, solar flares, solar eclipse. / Neste trabalho realizamos o estudo do comportamento do guia de ondas terra-ionosfera através da modelagem da propagação ondas de rádio de frequência muito baixa (VLF). Para isto, utilizamos o modelo computacional LWPC (Long Wave Propagation Capability) para estimar as variações de amplitude e fase de sinais de VLF detectados nos trajetos da rede SAVNET (South America VLF NETwork) e assim compreender o comportamento da baixa ionosfera em diferentes regimes de ionização. A pesquisa foi dividida em duas partes. A primeira parte, investigou o comportamento do sinal VLF em regimes quiescente de ionização, assim realizou-se simulações de amplitude e fase adaptando polinômios que definem os parâmetros β e ℎ′ (ou parâmetros de Wait) em função do ângulo zenital solar. Na segunda parte desta pesquisa, aplicou-se os polinômios no estudo da baixa ionosfera sob regimes transientes de ionização em duas condições distintas. A primeira para o caso de explosões solares e a segunda um para eclipse solar. Nas simulações relativas a explosões solares, calculamos as variações dos parâmetros β e ℎ′ durante o evento do dia 25/03/2008. Com esses valores, calculamos o perfil de densidade eletrônica, através de um modelo exponencial e observamos que a densidade eletrônica em 75 km é ∼ 104 cm−3, ou seja, vinte vezes maiores que antes da explosão. Para avaliar nossas estimativas, calculamos a variação dos parâmetros de Wait para doze eventos de diferentes classes. Observamos que as variações Δℎ′ neste trabalho são sempre maiores do que as descritas em Muraoka, Murata e Sato (1977), devido elas considerarem as variações no gradiente de condutividade. Nas simulações relativa ao eclipse solar do dia 11/07/2011, investigamos seu efeito na fase observada. Para esse estudo, utilizou-se o coeficiente de obscurecimento para realizar as simulações, desta forma foi possível estimar a variação da altura do guia ao longo de todo o trajeto durante o eclipse. As simulações reproduziram o comportamento da fase durante o eclipse. Entretanto, foi observado um atraso entre as medidas calculadas e observadas de aproximadamente ∼ vinte e quatro minutos. O atraso observado é diretamente decorrente da estimativa da altura de referência da ionosfera pertubada e de sua relação causal com o obscurecimento durante o eclipse.

High voltage testing of the stator winding insulation is one of the most recognized methods used to determine the state of degradation in the insulation. HV tests performed at 0.1 Hz do have potential advantages compared to more traditional 50 Hz tests. This thesis therefore aims to perform and compare tan delta, capacitance and partial discharge measurements on stator windings when using a 0.1 Hz voltage source and a more traditional 50 Hz voltage supply. Several associated test parameters with considerable influence on the test results were varied during the tests. An associated data analysis followed that was focused on the differences and similarities of the analyzed parameters and the results due to the differences in frequency. The results show that there are substantial levels of noise present in the partial discharge measurements when utilizing the VLF voltage source. There are also more numerous partial discharges for VLF measurements than for regular power frequency measurements if the same amount of voltage cycles is considered. The generated patterns show similarities with those generated at 50 Hz, but a larger sample base is probably needed for more thorough conclusions. The tan delta/capacitance part of the test do indicate potential advantages compared to power frequency measurements regarding the sensitivity in the measurements.

The increased need for electric power combined with an aging underground cable infrastructure in a deregulated market environment have forced utilities to refocus their attention on reliability while at the same time reducing maintenance costs as much as possible. This has created a significant need for diagnostic methods and technologies to assess the condition of the underground cable systems. However, while several cable diagnostic technologies are available, they have not all yet been fully accepted in the United States. This is because the different technologies lead to different conclusions for the same cable system, and thus utilities do not completely trust the conclusions. A better understanding of the diagnostic technologies and their correct application is therefore required. The most widely used diagnostic technologies in the United States include dissipation factor (Tan-delta and partial discharge measurements; these tests are therefore, the main focus of this thesis; in particular, when applied to underground extruded cable systems. The purpose of this research is to advance the field of characterization of power cable defects by addressing a number of theoretical and practical diagnostic measurements and their interpretation issues. The discussion is based on data from laboratory experiments and field tests. This thesis consists of two major parts. The first part is devoted to the characterization by Tan-delta measurements in which the major contribution is a new approach for condition assessment using this technology. The second part is devoted to the work on characterization by partial discharge measurements, and the major contribution is a novel approach that is able to analyze, evaluate, and reduce the number of partial discharge diagnostic features.

Abstract Brain activity fluctuations occur in multiple spatial and temporal scales. Functional magnetic resonance imaging (fMRI) has shown that infra slow fluctuations (ISF) of blood oxygen level-dependent signal (BOLD) are organized into well-defined areas called resting state networks (RSN). ISFs have also been detected in full-band EEG (fbEEG) data and in recent years, many have combined these two modalities to enable more accurate measurements of brain fluctuations. In simultaneous EEG-fMRI measurements the ISFs of BOLD signal have been found to be correlated with amplitude envelopes of faster electrophysiological data, suggesting the same underlying neuronal dynamics. Also direct correlations have been found in task related studies but not previously in resting state studies. Understanding the relation between EEG and BOLD signal in resting state might prove beneficial in the research of baseline activity fluctuations of the brain. Functional connectivity (FC) of the RSNs has been found to vary in different tasks and in some diseases, but also in resting state in healthy people. Despite numerous studies, no clear cause for these variations has yet been found. To research these open questions we performed simultaneous fbEEG-fMRI studies. The measurements from both modalities were analyzed with independent component analysis to improve the comparability of these results. Correlation analysis revealed that the EEG ISFs correlate with BOLD signal both temporally and spatially. These correlations showed spatiotemporal variability that was related to the strength of RSN functional connectivity. These results indicate that the ISFs of EEG and BOLD reflect a common source of fluctuations. The understanding of the correlations between ISFs in EEG and fMRI BOLD signals gives basic information of brain dynamics and of the variables that affect it. A better understanding of the background of brain activity helps in the development of more effective treatments for various neurological diseases as the knowledge of the mechanisms behind them grows. The ability to measure RSN activity with EEG more accurately can help in the development of new methods for early diagnosis of diseases. / Tiivistelmä Aivojen toiminta vaihtelee monissa avaruudellisissa ja ajallisissa mittakaavoissa. Toiminnallisissa magneettikuvauksissa (TMK) on havaittu, että veren happipitoisuudesta riippuvan (engl. BOLD) signaalin erittäin hitaat vaihtelut ovat järjestäytyneet hyvin määriteltyihin alueisiin, joita kutsutaan lepotilahermoverkostoiksi. Erittäin hitaita vaihteluita on havaittu myös täysikaistaisesta aivosähkökäyrästä (fbEEG). Viime vuosina nämä kaksi menetelmää on usein yhdistetty tarkemman mittaustuloksen aikaansaamiseksi. Samanaikaisissa EEG-TMK-mittauksissa BOLD signaalin erittäin hitaiden vaihteluiden on huomattu korreloivan nopeampien elektrofysiologisten värähtelyjen amplitudien verhokäyrien kanssa, mikä viittaa samaan perustana olevaan neuraaliseen dynamiikkaan. Myös suoria korrelaatioita on löydetty tehtäviin liittyvissä tutkimuksissa, mutta ei aiemmin lepotilatutkimuksissa. Lepotilan EEG:n ja BOLD-signaalin suhteen ymmärrys voi osoittautua hyödylliseksi aivojen perustilan aktiivisuuden vaihteluiden tutkimisessa. Hermoverkostojen toiminnallisen liittyvyyden on todettu huojuvan tietyissä tehtävissä ja joissain sairauksissa, mutta myös lepotilassa terveillä henkilöillä. Runsaasta tutkimuksesta huolimatta ei liittyvyyden huojunnalle ole vielä löytynyt selkeää aiheuttajaa. Näiden avoimien kysymysten tutkimiseksi suoritimme yhdenaikaisia fbEEG-TMK-mittauksia. Kummankin modaliteetin mittaustuloksia analysoitiin itsenäisten komponenttien analyysillä tulosten vertailtavuuden parantamiseksi. Korrelaatioanalyysit osoittivat, että EEG:n erittäin hitaat vaihtelut korreloivat ajallisesti ja avaruudellisesti TMK:n BOLD-signaalin kanssa. Näissä korrelaatioissa esiintyi sekä paikkaan että aikaan liittyvää huojuntaa, joka oli yhteydessä lepotilahermoverkostojen toiminnallisen liittyvyyden vahvuuteen. Nämä tulokset viittaavat siihen, että samat tekijät tuottavat EEG:n ja TMK:n BOLD-signaalien hitaat vaihtelut. EEG:n ja TMK:n signaalien erittäin hitaiden vaihteluiden välisen korrelaation ymmärtäminen antaa perustason tietoa aivojen toiminnan dynamiikasta sekä siihen vaikuttavista tekijöistä. Parempi ymmärrys aivotoiminnan taustoista auttaa kehittämään tehokkaampia hoitoja neurologisiin sairauksiin, kun tieto mekanismeista niiden takana tarkentuu. Mahdollisuus mitata lepotilahermoverkostojen toimintaa EEG:llä aiempaa tarkemmin voi auttaa kehittämään uusia menetelmiä sairauksien varhaiseen diagnostiikkaan.

Abstract Polymetallic (Pb-Zn-Cu-Ag-Au-Sb-As-Bi-Cd-Te-Se-Ge…) quartz veins occur in an area from Eidsvoll in southeastern Norway to west of LakeVänern in southwestern Sweden. They most likely formed during the waning stage of the c. 1 Ga Sveconorwegian orogeny. In Värmskog parish, Värmland county, several mineralized quartz veins of different types are known. Of these, three of the larger vein deposits, Vegerbol, Karlsbol and Södra Gärdsjön, have been investigated. Despite a history of mining activities (mostly for silver) and exploration from the mid-1800s up until the present day, details about their ore mineralogy have remained essentially unknown. In order to classify and characterize them better, the present study was undertaken. The main ore minerals in the veins are galena, sphalerite, chalcopyrite, tetrahedrite sensu lato and pyrite. Additionally, this investigation showed the occurrence of an array of silver-bearing phases such as argentian tetrahedrite to freibergite, native silver, polybasite, pyrargyrite, jalpaite, aguilarite, cervelleite, hessite and matildite. These are the main and accessory silver carriers in the studied deposits. Gold (argentiferous) was also found, as well as the nickel-cobalt sulphide siegenite.    The silver contents of the tetrahedrites may vary within one deposit, ranging from tetrahedrite sensu stricto to freibergite. A weak positive correlation occurs between silver and iron in the tetrahedrites. Cadmium was found both in the tetrahedrites and in the sphalerites. Many sphalerites, however, are very pure ZnS. The complex accessory ore mineralogy, including native gold as well as tellurium and selenium-bearing phases (Vegerbol), makes these vein deposits comparable to mineralized veins to the west and southwest of the Värmskog area.    Combining mineralogical and textural observations with previous studies, the vein mineralizations most likely formed during a change in tectonic regime, in recurrently active brittle structures, at temperatures ranging from c. 350 down to below 100°C. The occurrence of pyrite and hematite and the lack of pyrrhotite and magnetite points to an oxidizing ore-forming environment with relatively high sulphur activity.    Finally, in order to assess the possibility of connecting two of the mined vein systems, VLF (Very Low Frequency) electromagnetic measurements were performed across the strike of a possible continuation between the Vegerbol and Karlsbol deposits. The VLF survey showed a significant anomaly where such a continuation was to be expected, thus indicating an overall strike length of at least 1 km for that vein system. / Sammanfattning Polymetalliska (Pb-Zn-Cu-Ag-Au-Sb-As-Bi-Cd-Te-Se-Ge…) kvartsgångar förekom­mer inom ett område från Eidsvoll, Norge i norr, till sydväst om Vänern i Sverige. De tros ha bildats under den svekonorvegiska orogenesens (ca 1 Ga) slutskede. I Värmskogs socken, Värmland, förekommer mineraliserade kvartsgångar av flera olika typer. Bland dessa har tre undersökts med avseende på malmmineralogi. Dessa kallas för Vegerbol, Karlsbol och Södra Gärdsjön, och de har brutits i huvudsak på silver. Trots att brytning och prospekteringsarbeten pågått från 1800-talet fram till i dag har det saknats detaljerade mineralogiska beskrivningar över fyndigheterna. Denna studie har genomförts för att bättre kunna karakterisera och klassificera dessa mineralis­eringar. Huvudmalmmineral vid dessa förekomster är blyglans, zinkblände, koppar­kis, pyrit och för Södra Gärdsjön även tetraedrit. Denna undersökning har påvisat förekomsten av silverförande faser som silverhaltig tetraedrit till freibergit, gediget silver, polybasit, jalpait, aguilarit, cervelleit, hessit och matildit, vilka utgör både huvudsakliga och accessoriska silverbärare i de undersökta förekomsterna. Guld (silverhaltigt) har också hittats, samt ett Ni-Co-förande mineral, siegenit.    Silverinnehållet i tetraedriterna varierar inom en och samma förekomst, några analyser har givit freibergitsammansättningar. En svag positiv korrelation finns mellan silver och järnhalt i tetraedrit. Kadmium förekommer både i tetraedrit och i zinkblände. Många zinkbländen består dock av rent ZnS. Förekomsten av relativt komplexa accessoriska mineral inklusive guld och Se-Te-faser (Vegerbol) visar att dessa gångar är jämförbara med liknande förekomster väster och sydväst om Värmskogs­området.    Genom att sammanföra data från denna undersökning med sådana från tidigare studier gjorda på gångarna kan man anta att de bildats vid vid en förändring av den tektoniska miljön, i upprepat aktiva spröda strukturer och vid temperaturer mellan ungefär 350 till under 100 °C. Förekomsten av pyrit och hematit samt avsaknad av magnetkis och magnetit tyder på att malmbildningen skett under oxiderade förhållan­den och relativt hög svavelaktivitet.      Elektromagnetiska mätningar i VLF-bandet (Very Low Frequency) utfördes tvärs över den förmodade strykningsriktningen för en möjlig fortsättning av gången mellan Karlsbol och Vegerbol. Mätningarna gav en tydlig anomali där en sådan fortsättning kan förväntas vilket skulle kunna betyda att det finns en sammanhängande mineralisering med ca 1 kilometers längd.