By studying the frequency domain of seismic signals generated by earthquakes, the source parameters can be recovered, i.e., the seismic moment (M0) and the stress drop (Δσ). This method is an advantage especially since if the source parameters are calculated from the time domain a full waveform inversion is needed, therefore this procedure facilitates the computation. Besides, the moment magnitude (Mw) can be calculated from the seismic moment and, in turn, the local magnitude (ML) can be obtained by using an algorithm that matches different ranges of moment magnitude with their corresponding local magnitude. In the present thesis, small to moderate earthquakes in Northern Sweden have been used to develop a code that calculates the source parameters through the fitting of five different spectral models and, this way, discerns which model obtains the best determination of the parameters. These models have been chosen in a way that we can also extract information about the attenuation. The different models are; the Brune spectral model, Boatwright spectral model, Boatwright spectral model with a fixed fall-off rate, a general form of the spectral model with quality factor equal to 1000 and a general form of the spectral model with quality factor equal to 600. Among these models, the Boatwright model with fixed fall-off rate equal to 2, has been found to give the best fit to the data used in this thesis. This might be due to the regional conditions which are the low attenuation in the crust of northern Fennoscandia and the short hypocentral distances of the studied earthquakes. The earthquakes studied in the present thesis have shown a range of magnitudes from ML 4.2 to -0.2 with radius of an assumed circular fault ranging from 269 m to 66 m.

In this thesis a ground-penetration radar survey was carried out in four areas, Borgmästarängen, Draken, Urmakaren and Koppardosan in Sigtuna. The aim of this paper is to detect remnants of the Viking Age harbor at those chosen sites. Former research investigations have discovered findings which are related to shipbuilding like clench nails, rivets and boat nails; however, the question remains whether Sigtuna ever had a central harbor, and researchers suggested instead that every town plot had its own mooring. This paper addresses this question by deploying geophysical surveys to increase the understanding of the town’s organization. Furthermore this paper will discuss how the detected structures can be understood in comparison to earlier Viking Age and early Medieval towns, in this case Birka, Hedeby and Schleswig where central harbors have been discovered and investigated.

The main cause of failure in embankment dams - which represent 75% of all dams in the world (ICOLD, 2018) - is the internal erosion produced by excessive seepage not accounted for in their designs. This erosion can cause that a small anomalous structure, not likely to be considered as risky, turns rapidly into a significant structural damage if not recognized on time. This creates a necessity for methods that can detect these anomalies in a non-intrusive, cost-effective and sensitive way. The purpose of this work is to analyse the strength of three geophysical methods (ERT and Seismic Refraction and Reflection) in detecting and accurately localizing anomalous structures inside an embankment dam. This study has been successfully approached in two different ways: by synthetic modelling and by an experimental field work at an embankment dam in northern Sweden. The results show that these methods are capable of detecting different structures in the interior of the dam in an accurate and rapid manner. / Den främsta orsaken till brister i jordfyllningsdammar, som utgör 75% av alla dammar i världen (ICOLD, 2018) och är föremål för detta arbete, är inre erosion som orsakas av extrem läckage som togs inte med i beräkningar i deras konstruktioner. Denna erosion kan resultera i att en liten anomal struktur, som sannolikt inte anses vara riskabel, snabbt blir en betydande strukturell skada om den inte är uppmärksammas i tid. Därför krävs metoder som kan upptäcka dessa anomalier på ett icke-påträngande, kostnadseffektivt och känsligt sätt. Syftet med detta arbete är att analysera möjligheterna med tre geofysiska metoder (ERT, Seismisk Refraktion och Seismisk Reflektion) för att upptäcka och exakt lokalisera anomala strukturer inuti en jordfyllningsdamm. Denna studie har utförts på två fronter: genom (i) syntetisk modellering och (ii) ett experimentellt fältarbete vid en damm i norra Sverige. Resultaten visar att dessa metoder kan detektera olika strukturer inuti dammen på ett pålitligt och snabbt sätt.

Precursors related to seismicity patterns are probably the most promising phenomena for short-term earthquake forecasting, although it remains unclear if such forecasting is possible. Foreshock activity has often been recorded but its possible use as indicator of coming larger events is still debated due to the limited number of unambiguously observed foreshocks. Seismicity data which is inadequate in volume or character might be one of the reasons foreshocks cannot easily be identified. One method used to investigate the possible presence of generic seismicity behavior preceding larger events is the aggregation of seismicity series. Sequences preceding mainshocks chosen from empirical data are superimposed, revealing an increasing average seismicity rate prior to the mainshocks. Such an increase could result from the tendency of seismicity to cluster in space and time, thus the observed patterns could be of limited predictive value. Randomized tests using the empirical catalogues imply that the observed increasing rate is statistically significant compared to an increase due to simple clustering, indicating the existence of genuine foreshocks, somehow mechanically related to their mainshocks. If network sensitivity increases, the identification of foreshocks as such may improve. The possibility of improved identification of foreshock sequences is tested using synthetic data, produced with specific assumptions about the earthquake process. Complications related to background activity and aftershock production are investigated numerically, in generalized cases and in data-based scenarios. Catalogues including smaller, and thereby more, earthquakes can probably contribute to better understanding the earthquake processes and to the future of earthquake forecasting. An important aspect in such seismicity studies is the correct estimation of the empirical catalogue properties, including the magnitude of completeness (Mc) and the b-value. The potential influence of errors in the reported magnitudes in an earthquake catalogue on the estimation of Mc and b-value is investigated using synthetic magnitude catalogues, contaminated with Gaussian error. The effectiveness of different algorithms for Mc and b-value estimation are discussed. The sample size and the error level seem to affect the estimation of b-value, with implications for the reliability of the assessment of the future rate of large events and thus of seismic hazard. / Οι προσεισμοί αποτελούν τα πλέον υποσχόμενα πρόδρομα φαινόμενα για τη βραχυπρόθεσμη πρόγνωση των σεισμών, παρόλο που παραμένει άγνωστο το αν μια τέτοια πρόγνωση είναι εφικτή. Η χρήση της προσεισμικής δραστηριότητας ως ένδειξη ενός επερχόμενου μεγάλου σεισμού είναι αμφιλεγόμενη, κυρίως λόγω του περιορισμένου πλήθους των προσεισμών, γεγονός που πιθανά οφείλεται στην ανεπαρκή καταγραφή σεισμικών δεδομένων. Η άθροιση σεισμικών σειρών είναι μια μέθοδος που εφαρμόζεται προκειμένου να μελετηθεί η πιθανή παρουσία ενός γενικευμένου μοτίβου σεισμικότητας πριν από ισχυρούς σεισμούς. Η υπέρθεση σεισμικών ακολουθιών που προηγήθηκαν των κυρίων σεισμών αναδεικνύει μια αυξανόμενη μέση δραστηριότητα πριν από τους κύριους σεισμούς. Μια τέτοια συμπεριφορά θα μπορούσε να προκύψει και από την εγγενή τάση των σεισμών να ομαδοποιούνται χωρικά και χρονικά, με αποτέλεσμα τα παρατηρούμενα μοτίβα να έχουν περιορισμένη προγνωστική αξία. Τυχαιοποιημένοι έλεγχοι των πραγματικών δεδομένων υποδηλώνουν ότι ο παρατηρούμενος αυξανόμενος ρυθμός είναι στατιστικά σημαντικός σε σύγκριση με τη μεταβολή που οφείλεται στη γένεση απλών συστάδων σεισμών, αναδεικνύοντας την ύπαρξη προσεισμών αιτιολογικά συσχετιζόμενων με τους κύριους σεισμούς. Μια ενδεχόμενη αύξηση της ευαισθησίας των σεισμικών δικτύων πιθανά να συμβάλει στην αποτελεσματικότερη αναγνώριση των προσεισμών. Η πιθανότητα μιας τέτοιας βελτίωσης ελέγχεται με τη χρήση συνθετικών δεδομένων τα οποία προκύπτουν υπό προϋποθέσεις ως προς τη σεισμική διαδικασία. Οι επιπλοκές που μπορεί να προκύψουν από την παρουσία σεισμικότητας υποβάθρου και των μετασεισμικών ακολουθιών διερευνώνται αριθμητικά, με γενικευμένες περιπτώσεις και σενάρια που βασίζονται σε πραγματικά δεδομένα. Οι κατάλογοι που περιλαμβάνουν μικρότερους και επομένως περισσότερους σεισμούς μπορούν πιθανώς να συμβάλουν στην καλύτερη κατανόηση των σεισμικών διεργασιών και στη μελλοντική πρόγνωση των σεισμών. Σημαντική πτυχή σε τέτοιες μελέτες αποτελεί η σωστή εκτίμηση των ιδιοτήτων των σεισμικών καταλόγων, όπως είναι το μέγεθος πληρότητας και η παράμετρος b. Η επίδραση των σφαλμάτων των μεγεθών που υπάρχουν στους σεισμικούς καταλόγους στην εκτίμηση των προαναφερθέντων ιδιοτήτων ερευνάται χρησιμοποιώντας συνθετικά μεγέθη στα οποία ενυπάρχουν κανονικώς κατανεμημένα σφάλματα. Κατά τη διερεύνηση της αποτελεσματικότητας των διαφόρων μεθόδων που χρησιμοποιούνται για την εκτίμηση του μεγέθους πληρότητας προκύπτει ότι το μέγεθος του δείγματος και του σφάλματος των μεγεθών μπορούν να επηρεάσουν την εκτίμηση της παραμέτρου b, με επιπτώσεις στην εκτίμηση του ρυθμού των μελλοντικών ισχυρών σεισμών και την αξιολόγηση του σεισμικού κινδύνου.

This dissertation is a comprehensive summary of four papers on the development and application of new strategies for locating tremor and relocating events in earthquake catalogs. In the first paper, two new strategies for relocating events in a catalog are introduced. The seismicity pattern of an earthquake catalog is often used to delineate seismically active faults. However, the delineation is often hindered by the diffuseness of earthquake locations in the catalog. To reduce the diffuseness and simplify the seismicity pattern, a relocation and a collapsing method are developed and applied. The relocation method uses the catalog event density as an a priori constraint for relocations in a Bayesian inversion. The catalog event density is expressed in terms of the combined probability distribution of all events in the catalog. The collapsing method uses the same catalog density as an attractor for focusing the seismicity in an iterative scheme. These two strategies are applied to an aftershock sequence after a pair of earthquakes which occurred in southwest Iceland, 2008. The seismicity pattern is simplified by application of the methods and the faults of the mainshocks are delineated by the reworked catalog. In the second paper, the spatial distribution of seismicity of the Hengill region, southwest Iceland is analyzed. The relocation and collapsing methods developed in the first paper and a non-linear relocation strategy using empirical traveltime tables are used to process a catalog collected by the Icelandic Meteorological Office. The reworked catalog reproduces details of the spatial distribution of seismicity that independently emerges from relative relocations of a small subset of the catalog events. The processed catalog is then used to estimate the depth to the brittle-ductile transition. The estimates show that in general the northern part of the area, dominated by volcanic processes, has a shallower depth than the southern part, where tectonic deformation predominates. In the third and the fourth papers, two back-projection methods using inter-station cross correlations are proposed for locating tremor sources. For the first method, double correlations, defined as the cross correlations of correlations from two station pairs sharing a common reference station, are back projected. For the second method, the products of correlation envelopes from a group of stations sharing a common reference station are back projected. Back projecting these combinations of correlations, instead of single correlations, suppresses random noise and reduces the strong geometrical signature caused by the station configuration. These two methods are tested with volcanic tremor at Katla volcano, Iceland. The inferred source locations agree with surface observations related to volcanic events which occurred during the tremor period.

Purpose: One of the major soil problems that the construction industry may face is the presence of a certain type of highly sensitive marine clay, known as quick clay. The quick clay possesses properties that allow the clay to move from being relatively stable to on mechanical impact losing virtually all shear strength and act as a viscous mass. The majority of the Swedish, Norwegian and Canadian landslides, caused by quick clay, have caused significant consequences due to lack of bearing capacity of quick clay. In order to prevent these landslides knowledge about where there is quick clay is required. A large-scale mapping work has therefore been ongoing for a long time. The government has for some time been researching how to streamline mapping work and the result has shown a connection between quick clay attendance and increased resistivity. The purpose of this study is to develop a theoretical framework for areas with higher risk for presence of quick clay based on linking the resistivity to geotechnical properties. Method: The study has mainly applied quantitative data collection methods in the form of data analyzes combined with literature studies. Measurement data used in the study comprise of data collected through geotechnical and geophysical surveys. Literature studies have mainly been supported by reports from previous research in the field. Result: The result of the study points to a variety of geological factors that can give rise to increased risk of quick clay existence such as good permeability, artesian groundwater and uneven mountain subsidence etc. The study also shows a possible link between resistivity and sensitivity. Consequences: The study demonstrates the possibility of linking resistivity to the geotechnical property sensitivity to facilitate and streamline the mapping of quick clay through airborne resistivity measuring. Limitations: The study is limited to studying two research areas in south western Sweden, Strömstad and Lödöse. The study intends to examine the evidence from previous surveys and no further supplementary studies are conducted. The work is also delimited to study the geotechnical and geophysical properties of quick clay (in terms of resistivity). Chemical aspects are not taken into account. / Syfte: Ett av de större markproblem bygg- och anläggningsbranschen kan tvingas stå inför är förekomsten av en viss typ av högsensitiv havsavsatt lera, kallad kvicklera. Kvickleran besitter egenskaper som innebär att leran kan gå från relativt stabil till att vid mekanisk påverkan förlora praktiskt taget all sin skjuvhållfasthet och uppträder som en trögflytande massa. Majoriteten av de svenska, norska och kanadensiska lerskred som medfört betydande konsekvenser har varit ett resultat av kvicklerans bristande bärighet. För att förebygga dessa kvickleraskred krävs vetskap om var kvicklera förekommer. Ett omfattande kartläggningsarbete har därför pågått under en lång tid. Staten har under en tid bedrivit forskning för att effektivisera kartläggningsarbetet och resultatet har visat på ett samband mellan kvickleraförekomst och förhöjd resistivitet. Syftet med detta examensarbete är att ta fram ett teoretiskt ramverk för områden med större risk för förekomst av kvicklera baserat på att koppla samband mellan resistivitet och geotekniska egenskaper i form av vattenkvot, sensitivitet och densitet. Metod: I studien har i huvudsak kvantitativa datainsamlingsmetoder i form av dataanalyser tillämpats i kombination med litteraturstudier. Mätdatat som studien omfattar är insamlad via geotekniska och geofysiska undersökningar, litteraturstudierna har främst tagit stöd i rapporter från tidigare forskning inom området. Resultat: Resultatet pekar på en rad olika geologiska faktorer som kan ge upphov till ökad risk för kvickleraförekomst, exempelvis god permeabilitet, artesiskt grundvatten samt ojämn bergunderyta etc. Studien visar även på en möjlig koppling mellan resistivitet och sensitivitet. Konsekvenser: Studien visar på möjligheten att koppla resistivitet till den geotekniska egenskapen sensitivitet för att underlätta och effektivisera kartläggningen av kvicklera genom flygburna resistivitetsmätningar. Begränsningar: Uppsatsen begränsas till att studera två undersökningsområden i sydvästra Sverige, Strömstad och Lödöse. Studien avser granska underlag från tidigare utförda undersökningar och inga ytterligare kompletterande undersökningar har utförts. Arbetet avgränsas till att studera kvicklerans geotekniska egenskaper samt dess resistivitetsegenskaper. Arbetet tar således inte hänsyn till kemiska aspekter.

Eskers are long winding ridges that originate from gravel that has travelled in meltwater streams in glaciers. At the study site, Virttaankangas plane in southwest Finland, there are esker structures covered by sediments. One reason why it is important to study eskers is because they are used for purifying drinking water. The data used in the study were collected during a seismic survey in July 2014. During the survey a controlled source created seismic waves that travelled down through the earth and then reflected back up again. By detecting the travel time of the waves and estimating the velocity of the geologic layers, the depth to the reflecting structures could be calculated. There are two types of waves that travel through the body of the earth, pressure waves (P-waves) and shear waves (S-waves). In a previous study (Maries et al., 2017) P-wave data from the same survey have been analyzed, so this work focuses on S-wave data but also compares the result from the two. Some structures related to eskers were identifiable, such as fractures in the bedrock from the pressure of the main esker core. By comparing S- and P-wave results it was possible to see hints of the arched esker cores and esker fan lobes. Overall the result confirmed the model that was achieved of the profile in the previous study. The location of the bedrock both matched with the previous study, and added information about its orientation. An additional goal was to demonstrate the insensitivity of S-waves to water content by showing that if there was a water table reflection in the P-wave data, this reflection was missing in the S-wave data. The results showed water table reflections in the P-wave data, but there were no distinguishable water table reflections with appropriate velocity for S-waves in the S-wave data. / Rullstensåsar definieras som långa åsar med storlekssorterat grus som avlagrats av smältvattenströmmar i glaciärer. Vid undersökningsplatsen, Virttaankangasheden i sydvästra Finland, finns rullstensåsstrukturer som är begravda under sediment. En anledning till varför det är viktigt att undersöka rullstensåsar är att de används för filtrering vid framställning av dricksvatten. De data som användes i denna studie inhämtades under en seismisk undersökning i juli 2014. Undersökningen gick till på så vis att en kontrollerad källa skapade seismiska vågor som färdades ner i jorden för att sedan reflekteras tillbaka upp mot ytan. Genom att notera tiden det tog för vågorna att färdas, samt uppskatta hastigheten i de geologiska lagren, kunde djupet till de reflekterande strukturerna beräknas. Det finns två sorters vågor som kan färdas genom jorden, tryckvågor (P-vågor) och skjuvvågor (S-vågor). I en tidigare studie (Maries et al., 2017) analyserades P-vågsdata från samma seismiska undersökning, så detta arbete fokuserar på S-vågsdata men jämför också resultaten av båda två. Vissa strukturer kopplade till rullstensåsar kunde identifieras, så som sprickor i begrgrunden från trycket av den största rullstensåsen. Genom att jämföra resultat från S- och P-vågor kunde man se reflektioner från rullstensåsar och sediment. Sammantaget bekräftade resultatet den modell över profilen som framtagits i den tidigare studien. Berggrundens läge stämde överens med den förra studien och tillförde ny information om dess orientering. Utöver detta försökte man också demonstrera S-vågors okänslighet för vatten genom att visa att om det fanns reflektioner från grundvattenytan i P-vågsdatan så skulle de reflektionerna inte synas i S-vågsdatan. I P-vågsdatan visade det sig att det fanns grundvattenreflektioner, men det gick inte att urskilja några liknande reflektioner i S-vågsdatan.

Volcanic rifting environments, such as in Iceland, are challenging for conventional seismic reflection methods using active surface seismic sources. This study demonstrates the potential of a novel technique, called Virtual Reflection Seismic Profiling (VRSP) for imaging reflections in geothermal regions, like Hverahlíð, where a dense seismic array recorded a number of local microearthquakes for cross-correlation. Uppsala University, in collaboration with Reykjavik Energy, recorded seismicity in Hverahlíð using both seismometers and geophones. Acquired data were processed using the VRSP method, which applies seismic interferometry only to selected events, in this thesis local microearthquakes. Cross-correlation of the signal from a microearthquake recorded at one of the stations, which acts as a virtual source, with a ghost reflection recorded by the remaining receivers, produces a virtual shot gather. Stacking each station’s result, for all available events, and following a conventional multichannel processing sequence resulted in two stacked seismic images. Potential reflections observed in the obtained sections could be associated with major feed zones identified in the area by the borehole measurements. Eight dynamite explosions were processed with a conventional seismic reflection method, as a complementary source to the microearthquakes. In the produced stacked seismic section two potential reflections could be observed. Results from both passive and active datasets were 3D visualised to verify whether the reflections correspond to each other between sections. Two horizons were traced throughout all three stacked sections. One more interface appeared on two images obtained from processing the passive data. This study shows promising results for using natural sources to image the subsurface in a challenging environment.

The stability of rock masses is influenced by the occurrence of rock joints. Therefore, the shear strength of rock joints must be considered in dimensioning of underground constructions. One way to predict the shear strength is through usage of failure criteria, which are validated from results of direct shear tests under controlled laboratory conditions. Consequently, the quality of the results from the tests are crucial to the accuracy with which the criteria will be able to predict the shear strength. Since rock joints are unique by nature usage of replicas (man-made copies of rock joints) is of importance in parameter studies. The overall objective of this work is to facilitate the development of improved criteria for predictions of the shear strength of rock joints. To support this objective, two sources of uncertainty have been investigated, namely the geometry of replicas and the influence of the normal stiffness of test systems. Two quality assurance parameters for evaluation of geometrical differences between replicas and rock joints based on scanning data have been derived. The first parameter describes the morphological deviations. The second parameter describes the deviations in orientation with respect to the shear plane. The effective normal stiffness approach, which compensates for the influence of the normal stiffness of the test system indirect shear testing, has been developed, validated, and applied. With help of the quality assurance parameters it is demonstrated that it is possible to reproduce replicas within narrow tolerances. Application of the effective normal stiffness approach basically eliminates the normal load error. In all, the results support generation of improved quality of test data and consequently, the development of shear strength criteria with improved accuracy will also be facilitated. / Bergmassors stabilitet påverkas av bergssprickor. Bergssprickors skjuvhållfasthet behöver därför beaktas vid fastställandet av vilka laster berganläggningar skall dimensioneras mot. Skjuvhållfastheten predikteras bland annat med hjälp av brottkriterier, vilka valideras med hjälp av resultaten från skjuvtester i kontrollerad laboratoriemiljö. Kvaliteten på resultaten från testerna är därför av avgörande betydelse för med vilken noggrannhet kriterierna kommer att kunna prediktera skjuvhållfastheten. Det övergripande målet med detta arbete är att underlätta utvecklingen av förbättrade kriterier för prediktioner av bergssprickors skjuvhållfasthet. Som ett bidrag till att uppnå detta mål har två osäkerhetsfaktorer undersökts, nämligen geometrin av replikor (kopior) av bergssprickor och inverkan av testsystems normalstyvhet. Två kvalitetssäkringsparametrar för utvärdering av de geometriska skillnaderna mellan replikor och bergprov baserade på skanningdata har tagits fram. Den första parametern beskriver de morfologiska avvikelserna. Den andra parametern beskriver avvikelserna i orientering med avseende på skjuvplanet. Ett tillvägagångssätt med en effektiv systemnormalstyvhet, vilken kompenserar för inverkan av testsystemets normalstyvhet, har utvecklats, validerats och tillämpats. Med hjälp av kvalitetssäkringsparametrarna påvisas att det ar möjligt att reproducera replikor inom snäva toleranser. Genom tillämpning av tillvägagångssättet med en effektiv normalstyvhet kan felet i normallast i princip elimineras. Sammantaget stödjer resultaten framtagning av testdata med förbättrad kvalitet och därigenom underlättas även utvecklingen av skjuvhållfasthetskriterier med förbättrad noggrannhet. / <p>QC 210518</p>

Controlled Source Audio Magnetotellurics (CSAMT) is a geophysical method for characterizing the resistivity of the subsurface with the help of electromagnetic waves. The method is used for various purposes, such as geothermal- and hydrocarbon exploration, mineral prospecting and for investigation of groundwater resources. Electromagnetic fields are created by running an alternating current in a grounded electric dipole and by varying the frequency, different depths can be targeted. Orthogonal components of the electromagnetic fields are measured at receiver stations a few kilometers away from the source. From these field components, so called magnetotellurics transfer functions are estimated, which can be used to invert for the resistivity of the subsurface. The data used in this project is from a survey conducted in 2014 and 2016 in Kiruna by Uppsala University and the mining company LKAB. Measurements were made at 31 stations along two orthogonal profiles. The data have been processed earlier, but due to noise, especially in the lower frequencies, a significant part of the data set could not be inverted. The aim of this project was to improve the results by analyzing the data and testing different methods to remove noise. First, robust regression was used to account for possible non-Gaussian noise in the estimation of the magnetotelluric transfer functions. Except for one station on profile 1, the robust method did not improve the results, which suggests that the noise is mostly Gaussian. Then modified versions of least squares, each affected by a different bias, were used to estimate the transfer functions. Where there is more noise, the estimates should differ more due to their different biases. The estimates differed most for low frequencies and especially on the part of profile 2 that was measured in 2014. It was investigated whether the railway network could explain part of the low frequency noise. Measures were taken to reduce spectral leakage from the railway signal at 16 ⅔ Hz to the closest transmitter frequencies 14 Hz and 20 Hz, but no clear improvement was seen and more detailed studies need to be conducted to determine this matter. Finally, a method based on comparing the ratio of short-term and long-term averages was tested to remove transients in the measured time series of the electromagnetic field components. This proved difficult to implement due to the variability of the time series’ behavior between different stations, frequencies and field components. However, the method showed some potential for stations 9 and 10 on profile 1, and could probably be developed further to remove transients more efficiently and thus improve the data. / Magnetotellurik med kontrollerad källa (förkortat CSAMT på engelska) är en metod där elektromagnetiska fält används för att undersöka markens resistivitet. Resisitivitet är ett mått på hur bra eller dåligt marken leder elektriska strömmar. Metoden används till exempel för att mäta djupet till berggrunden, som oftast har högre resistivitet (sämre ledningsförmåga) än marken ovanför. Man kan också hitta metaller, så som guld och koppar, vilka har väldigt låg resistivitet (bra ledningsförmåga). Elektromagnetiska vågor skapas genom att man låter en växelström gå igenom en lång ledning. Vågorna färdas först genom luften och sen ner i marken. Hur djupt ner de når beror på växelströmmens frekvens; med låga frekvenser når vågorna djupare ner i marken än med höga. Under markytan inducerar de elektromagnetiska vågorna elektriska strömmar, så kallade telluriska strömmar (dvs. jordströmmar). Strömmarna blir svagare ju längre de färdas och hur snabbt de avtar i styrka beror på jordens resistivitet. Strömmarna skapar också nya elektriska och magnetiska fält som färdas tillbaka mot ytan. Vid markytan mäter man fältens styrka för olika frekveser, vilket då ger information om resistiviteten på olika djup. Från mätningarna tar man ofta fram så kallade magnetotelluriska överföringsfunktioner. Dessa överföringsfunktioner gör det lättare att tolka datan och ta reda på resistiviteten hos marken. I detta projekt har CSAMT-data använts från en undersökning i Kiruna som genomfördes av Uppsala Universitet och gruvföretaget LKAB. Datan har bearbetats tidigare, men på grund av mycket brus i mätningarna blev inte resultatet så bra som väntat. Brus kan komma från allt som genererar elektromagnetiska fält, till exempel elledningar, tågledningar eller naturliga variationer i jordens egna magnetfält. Målet med projektet var att förbättra resultatet genom att analysera datan och testa olika metoder för att ta bort brus. Den vanligaste metoden för att beräkna överföringsfunktionerna antar att det magnetiska fältet är fritt från brus. Detta är inte nödvändigtvis sant och kan leda till bias, alltså ett snedvridet resultat. Andra sätt att beräkna överföringsfunktionerna på ger olika bias. Det här kan man utnyttja för att se hur mycket brus som finns i datan. Om det inte finns något brus alls så blir alla överföringsfunktioner lika, medan om det finns mycket brus så skiljer de sig mer åt. På detta sätt upptäcktes att det var mer brus för frekvenserna 14 och 20 Hz (där 1 Hz är 1 svängning per sekund). En förklaring till det kan vara att tågledningar, som genererar elektromagnetiska fält med 16.67 Hz, ligger nära i frekvens och stör dessa signaler. För att minska brusets påverkan testades så kallad robust processering. Det innebär att man lägger mindre vikt vid de mätningar som tycks vara mycket annorlunda (alltså innehåller mer brus) från andra mätningar. Tyvärr så hjälpte inte denna strategi nämnvärt för att förbättra resultatet. Till sist tog vi fram en metod för att ta bort transienter, vilket är kortvarigt brus med hög intensitet. Transienter kan till exempel komma från åskblixtar, som ju är kortvariga elektriska urladdningar. Det visade sig dock att detta inte var helt enkelt, då det var svårt att se vad som var brus och vad som bara var naturliga variationer hos de elektromagnetiska fälten. Men i några fall kunde bruset urskiljas och därför verkar det troligt att fortsatt arbete med denna metod skulle kunna ge ännu bättre resultat.