21 research outputs found
Time-lapse inversion of Controlled Source Electromagnetics using vertical sources and receivers
No full textKnowledge of spatial and temporal distribution of fluids in the subsurface is crucial
in a wide range of applications. During the production of crude oil typically high saline
produced formation water is injected into the reservoir layer, aiming to push the
oil towards production wells. While oil is commonly seen as an electrical insulator,
the injected saline brines are characterised by low electrical resistivity. Thus, electromagnetic
(EM) methods and especially Controlled Source Electromagnetics (CSEM)
attracted an increasing interest to monitor these resistivity changes inside the reservoir
over time.
This thesis mainly reports on numerical aspects of modelling and inversion of land
based CSEM with particular focus towards hydrocarbon monitoring applications.
Most of the presented developments were inspired by a superordinate research project
including CSEM field surveys across an actively producing onshore oil field in
Northern Germany.
In producing oil fields there exists a large number of steel-cased wells. Such existing
oil field infrastructure and especially the presence of metal casings significantly alters
the propagation of EM fields in the subsurface. Their spatially unfavourable dimensions
effectively prohibits a straightforward implementation into the modelling grid.
Thus I developed a new modelling approach allowing consideration of such thin but
vertically extended highly conductive structures including their mutual interaction.
The developed methodology had been implemented into existing modelling and inversion
codes. Using the new approach to investigate the influence of metal casings
on CSEM data shows that they act as additional inductively coupled vertical electric
dipole sources at depth and thereby increase resolution capabilities at depth. The
presence of metal casings can thus be exploited by optimising the source receiver
layout in such a way that the strength of these additional vertical dipole sources is
maximised.
An additional working package of the superordinate project was the measurement of
vertical electric fields in a shallow observation well. However, measurements of vertical
electric fields requires long measurement dipoles to achieve satisfactory signalto-
noise ratios. Such extended dipoles span several modelling cells and are therefore
in conflict with assumptions usually made for modelling, that receivers can be represented
as point dipoles. I therefore expanded the modelling and inversion codes
to consider the physical receiver dimensions. The new algorithm implicitly considers
imperfect alignment of the receiver with the corresponding field component. Without
the consideration of this effect inversion of vertical electric field measurements is
likely to cause erroneous results.
Finally I discuss different aspects of time-lapse inversion required to track changes in
fluid saturation over time. The cascaded inversion scheme is applied to synthetic timelapse
data for a simplified oilfield undergoing brine flushing. The influence of various
inversion parameters in particular different regularisation techniques are examined.
Surface based sources and receivers typically provide low sensitivity towards deep
targets in highly conductive backgrounds. Despite that using additional constraints,
in particular a model weighting scheme together with energised steel casings allowed
to track resistivity changes inside the reservoir based on synthetic time-lapse data.Wissen über räumliche und zeitliche Verteilung von Fluiden im Untergrund is unerlässlich
für eine Reihe von Anwendungen. Typischerweise wird während der Förderung
von Rohöl salinares produziertes Formationswasser in die ölführende Formation
injiziert um das Öl-Wasser-Gemisch in Richtung der Förderbohrungen zu spülen.
Während Öl als elektrischer Isolator gilt, zeichnen sich die injizierten salinaren Fluide
durch eine hohe elektrische Leitfähigkeit aus. Daher erfahren elektromagnetische
Methoden und insbesondere Controlled Source Electromagentics (CSEM) zunehmendes
Interesse diese Änderung des elektrischen Widerstands mit der Zeit zu
überwachen.
Diese Arbeit beschäftigt sich im wesentlichen mit numerischen Aspekten der Modellierung
und Inversion von CSEM an Land mit speziellem Fokus auf der Überwachung
der Kohlenwasserstoff Produktion. Die meisten der gezeigten Entwicklungen
sind entwickelt im Zuge eines übergeordneten Forschungsprojektes inklusive
CSEM Feldmessungen in einem produzierenden Ölfeld in Norddeutschland.
Produzierende Ölfelder sind gekennzeichnet durch eine große Anzahl von Stahl verrohrten
Bohrungen. Die Anwesenheit von Stahlinfrastruktur insbesondere von Stahlschutzrohren
beeinflusst die Ausbreitung von elektromagnetischen Feldern im Untergrund.
Deren unvorteilhafte Geometrie erlaubt keine direkte Berücksichtigung in
dem Modellierungsgitter. Daher habe ich einen neuen Modellierungsanzatz entwickelt
der es erlaubt solch dünne aber vertikal ausgedehnte hochgradig leitfähige Strukturen
inklusive deren gegenseitige Wechselwirkung zu berücksichtigen. Die entwickelte
Methode wurde in bestehende Modellierungs- und Inversionssoftware implementiert.
Mithilfe dieses neuen Ansatzes konnte der Einfluss von Stahlverrohrungen auf
CSEM Daten untersucht werden. Stahlverrohrungen wirken wie zusätzliche induktiv
angeregte vertikale elektrische Dipolquellen im Untergrund und helfen daher die Auflösung
in der Tiefe zu erhöhen. Die Anwesenheit von Stahlverrohrungen kann daher
ausgenutzt werden in dem man die Quell-Empfänger-Geometrie in einer Art und
Weise optimiert, die die Stärke dieser zusätzlichen vertikalen Dipolquellen maximiert.
Ein weiteres Arbeitspaket des übergeordneten Forschungsprojektes bestand in der
Messung von vertikalen elektrischen Feldern in flachen Beobachtungsbohrungen. Messungen
des vertikalen elektrischen Feldes erfordert lange Messdipole um ein ausreichendes
Signal-Rausch-Verhältnisses zu gewährleisten. Solch ausgedehnte Dipole
überspannen mehrere Zellen des Modellierungsgitters und verletzen die übliche Annahme,
wonach die Länge der Empfänger vernachlässigbar ist. Daher habe ich die
bestehenden Modellierungs- und Inversionsprogramme erweitert um die physischen
Dimensionen von elektrischen Feld Empfängern zu berücksichtigen. Der implementierte
Algorithmus berücksichtigt implizit Abweichungen der Orientierung des Messdipols
von der Richtung der zu messenden Feldkomponente. Ohne dieser Berücksichtigung
führt eine Inversion von vertikalen elektrischen Feld Daten zu fehlerhaften
Ergebnissen.
Schließlich werden unterschiedliche Aspekte von time-lapse Inversion diskutiert, welche
notwendig ist um Änderungen der Fluidzusammensetzung abzubilden und zu verfolgen.
Eine kaskadiertes Inversionsschema wurde auf synthetische time-lapse Daten
eines vereinfachten Ölfeldes angewendet. Untersucht wurde der Einfluss verschiedener
Parameter insbesondere verschiedener Regularisierungstechniken. Sender und Empfänger
an der Erdoberfläche sind typischerweise wenig sensitiv zu tiefen Strukturen
in leitfähiger Umgebung. Anhand von synthetische Daten konnte gezeigt werden,
dass das benutzen zusätzlicher Nebenbedingungen wie einer Modellgewichtung und
dem ausnutzen von vorhandenen Stahlverrohrungen es dennoch erlaubt Änderungen
innerhalb des Ölreservoirs zu lokalisieren
Elongated horizontal and vertical receivers in three‐dimensional electromagnetic modelling and inversion
No full textTime-lapse CSEM inversion using focusing regularization techniques for reservoir monitoring
No full text
Koillismaalla SEEMS DEEP -projektissa mitatun magnetotelluurisen aineiston 3D-inversio
No full textThe Koillismaa Layered Igneous Complex (KLIC) is an Early Palaeoproterozoic layered intrusion located in northeastern Finland. It is subdivided into two exposed parts, the Western Intrusion and the Näränkävaara Intrusion. Since the 1950s, the complex has been extensively studied due to a prominent gravity and magnetic anomaly, referred to as the “hidden dyke”, located between the exposed parts. As part of the SEEMS DEEP project (Seismic and Electromagnetic Methods for Deep Mineral Exploration), this thesis presents a three-dimensional (3D) magnetotelluric (MT) inversion model constructed using the MT data acquired in the SEEMS DEEP project and additional regional MT data. The model was developed to better constrain the resistivity structure of the KLIC and to assess the potential presence of Ni-Cu-PGE (platinum group elements) mineralisation that can be associated with highly conductive massive sulphides. The model was compared with the 3D seismic model from SEEMS DEEP, as well as the gravity model and magnetic data obtained in previous studies, to assess the suitability of MT for imaging layered ultramafic intrusions. The MT study revealed no significant conductive structures in the study area, suggesting a low probability of potential Ni-Cu-PGE deposits. Instead, two major high-resistivity features and five low-resistivity structures were identified. The low-resistivity zones (100–3,000 Ω m) partially correlate with magnetic anomalies and, to a lesser extent, with seismic reflectivity patterns. These may represent mafic or ultramafic dykes, fractured zones, or banded iron formations (BIFs) related to the magnetic anomalies. The high-resistivity structures (>30,000 Ω m) likely correspond to Archean basement rocks surrounding the intrusion, along with the rocks that are part of the intrusion. While some boundaries within the ultramafic intrusion are clearly visible in the seismic model, they are not resolved in the MT inversion model. These results suggest that MT may be limited in its effectiveness for investigating layered ultramafic intrusions due to insufficient spatial resolution.Koillismaan varhaispaleoproterotsooiset ultramafiset kerrosintruusiot muodostavat "Koillismaan kompleksin", joka sijaitsee Koillis-Suomessa. Kompleksi jakautuu kahteen maanpinnalla havaittaviin osiin: Länsi-intruusioon ja Näränkävaaran intruusioon. Aluetta on tutkittu laajasti 1950-luvulta lähtien merkittävän painovoima- ja magneettianomalian vuoksi. Tämä piilossa oleva anomalia sijaitsee Länsi-intruusion ja Näränkävaaran intruusion välissä. Osana SEEMS DEEP -projektia, tämä tutkielma esittelee kolmiulotteisen (3D) magnetotelluurisen (MT) inversiomallin, joka on rakennettu SEEMS DEEP -projektissa hankittujen MT-datan ja alueella sijaitsevan aikaisemman MT-datan perusteella. MTdatasta luotiin resistiivisyysmalli Koillismaan alueelta, millä pyrittiin arvioimaan kompleksin rakennetta sekä mahdollisten Ni-Cu-PGE mineralisaatioiden esiintymistä, joita voidaan yhdistää erittäin johtaviin massiivisiin sulfideihin. Mallia verrattiin SEEMS DEEP -projektin 3D-seismiseen malliin sekä aikaisemmissa tutkimuksissa saatuihin painovoima- ja magneettiaineistoihin arvioitaessa MT-menetelmän soveltuvuutta ultramafisten kerrosintruusioiden kuvantamiseen. MT-tutkimus ei paljastanut merkittäviä johtavia rakenteita tutkimusalueella, mikä viittaa pieneen todennäköisyyteen potentiaalisille Ni-Cu-PGE-malmiesiintymille. Sen sijaan tunnistettiin kaksi suurta korkean resistiivisyyden kohdetta ja viisi matalan resistiivisyyden rakennetta. Matalan resistiivisyyden alueet (100–3,000 Ω m) korreloivat osittain magneettisten anomalioiden kanssa ja jossain määrin myös seismisten heijastuksien kanssa. Ne voivat edustaa mafisia tai ultramafisia juonia, ruhjevyöhykkeitä tai magneettisiin anomalioihin liittyviä rautamuodostumia. Korkean resistiivisyyden rakenteet (>30,000 Ω m) todennäköisesti vastaavat Arkeeisia peruskallion kiviä, jotka ympäröivät intruusiota, sekä itse intruusion osia. Vaikka ultramafisen kerrosintruusion tietyt rajat ovat selvästi näkyvissä seismisessä mallissa, ne eivät erotu MT-inversiomallissa. Nämä tulokset viittaavat siihen, että MT-menetelmä voi olla rajallinen ultramafisten kerrosintruusioiden tutkimuksessa johtuen sen riittämättömästä resoluutiosta
Class Attendance and Students’ Evaluations of Teaching: Do No-Shows Bias Course Ratings and Rankings?
No full textBackground: Many university departments use students’ evaluations of teaching (SET) to compare and rank courses. However, absenteeism from class is often nonrandom and, therefore, SET for different courses might not be comparable. Objective: The present study aims to answer two questions. Are SET positively biased due to absenteeism? Do procedures, which adjust for absenteeism, change course rankings? Research Design: The author discusses the problem from a missing data perspective and present empirical results from regression models to determine which factors are simultaneously associated with students’ class attendance and course ratings. In order to determine the extent of these biases, the author then corrects average ratings for students’ absenteeism and inspect changes in course rankings resulting from this adjustment. Subjects: The author analyzes SET data on the individual level. One or more course ratings are available for each student. Measures: Individual course ratings and absenteeism served as the key outcomes. Results: Absenteeism decreases with rising teaching quality. Furthermore, both factors are systematically related to student and course attributes. Weighting students’ ratings by actual absenteeism leads to mostly small changes in ranks, which follow a power law. Only a few, average courses are disproportionally influenced by the adjustment. Weighting by predicted absenteeism leads to very small changes in ranks. Again, average courses are more strongly affected than courses of very high or low in quality. Conclusions: No-shows bias course ratings and rankings. SET are more appropriate to identify high- and low-quality courses than to determine the exact ranks of average courses
A borehole trajectory inversion scheme to adjust the measurement geometry for 3D travel-time tomography on glaciers
No full textISSN:1869-9510ISSN:1869-9529ISSN:1869-952
A borehole trajectory inversion scheme to adjust the measurement geometry for 3D travel time tomography on glaciers
No full textA reassessment of the role of Hippias of Elis as a proto-doxographical source
No full textIn 1944, Bruno Snell inaugurated a new field of research we may call Protodoxographical Studies by arguing that there should be a common source for parallel passages with doxographic features in Plato and Aristotle, which he identified as the sophist Hippias of Elis. Later scholars – in particular, Joachim Classen, Jaap Mansfeld, and Andreas Patzer – replicated his method for other parallel passages, and thereby expanded the range of possible content from Hippias. This generalization, however, has not come without methodological losses, in particular a gradual dismissal of parallel confirmatory anchors for the suggested passages: for Patzer, for example, the mere occurrence of parallel texts with a structure similar to the one originally identified by Snell would be taken as an unequivocal sign of the presence of Hippias. One of the effects of this precedent was the propagation of a hypertrophied image of Hippias as philosopher, historian of Philosophy, and inaugurator of the doxographic genre, anticipating the Peripatetic doxographical tradition, and setting him up as a kind of grey eminence behind Plato and Aristotle’s appreciation of the so-called “Presocratic Philosophy”. Although Mansfeld elaborated his contributions on Snell’s theory more carefully, even producing a major advance with identification of a Gorgian protodoxographical matrix parallel to the Hippian one, the lack of continuity in the protodoxographical studies (that remain practically frozen since 1986), resulted in a biased reception in which Mansfeld even started to function as a confirmer of Patzer’s excesses. Against this backdrop, this thesis intends to return to Snell and his successors in order to retrace the evolution of the protodoxographical theory, critically re-evaluate the evidence they produced, and add evidence that has been neglected, including recent studies on Hippias and other implicated characters. From this analysis, a more adequate profile for Hippias as an author, and a more adequate characterization of the structure and purpose of his Collection will emerge, as well as a more accurate characterization and consequent appreciation of the proto-doxographic contribution of Gorgias, and the identification of a possible new proto-doxographic source in the naturalistic physician Hippo.Em 1944, Bruno Snell inaugurou um novo campo de pesquisas – que podemos chamar de Estudos Protodoxográficos – ao defender que haveria uma fonte comum para passagens paralelas com características doxográficas em Platão e Aristóteles, que ele identificou como sendo o sofista Hípias de Élis. Estudiosos posteriores – em particular Joachim Classen, Jaap Mansfeld e Andreas Patzer – replicaram seu método para outras passagens paralelas e, com isso, acabaram expandindo o rol de possíveis conteúdos oriundos de Hípias. Esta generalização, contudo, não virá sem perdas de caráter metodológico, em particular uma gradual dispensa de âncoras paralelas de confirmação para as passagens aventadas: para Patzer, por exemplo, a simples ocorrência de textos paralelos com estrutura similar à que fora identificada originalmente por Snell será tomada como signo inequívoco da presença de Hípias. Um dos efeitos desse procedimento, será a propagação de uma imagem hipertrofiada de Hípias como filósofo, historiador da filosofia e inaugurador do discurso doxográfico, antecipando a tradição doxográfica peripatética e configurando-se como uma espécie de eminência parda por trás da apreciação platônico-aristotélica da chamada “Filosofia Pré-Socrática”. Embora Mansfeld tenha elaborado suas contribuições sobre a teoria de Snell de forma mais cuidadosa, produzindo inclusive um importante avanço, ao identificar uma matriz protodoxográfica gorgiana, paralela à hipiana, a falta de continuidade nos estudos protodoxográficos (que praticamente ficaram congelados no ano de 1986), resultou em uma recepção enviesada, na qual Mansfeld passou a funcionar até mesmo como confirmador dos excessos de Patzer. Diante desse cenário, esta tese pretende retornar a Snell e seus sucessores, a fim de retraçar a evolução da teoria protodoxográfica, reavaliar criticamente as evidências apresentadas e acrescentar evidências que foram negligenciadas, incluindo estudos recentes sobre Hípias e outras personagens implicadas. Desta análise, emergirão um perfil mais adequado para Hípias enquanto autor, e uma caracterização mais adequada da estrutura e da finalidade de sua Coletânea, além de uma caracterização mais precisa e consequente valorização da contribuição protodoxográfica de Górgias, e a identificação de uma possível nova fonte protodoxográfica no médico naturalista Hípon.CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superio
Towards the imaging of deep land ore deposits with ERT-IP method – first results from a demonstration survey in Finland
No full textIn this study, we conducted a comprehensive geophysical survey near Kuusamo (Finland) to assess the potential of electrical resistivity methods in delineating mineral deposits at depths greater than 1 km. Preliminary investigations, including magnetic and gravity methods as well as drilling, revealed significant anomalous structures in the survey area. We employed multiple electrical and electromagnetic methods at the site, comprising controlled-source electromagnetic (CSEM), magnetotelluric (MT), electrical resistivity tomography (ERT), and induced polarization (IP). To obtain the geophysical data in very large-scale area, we used a total of 25 transmitter dipoles with 1km long using three distinct transmitter systems and recorded data at 119 receiver stations. In this paper, we present the acquisition and preliminary results from ERT-IP. Analysis of the resistivity and IP responses revealed notable IP signals at depths exceeding 1.5 km. Meanwhile, the resistivity data indicated generally very high values, around 10,000 ohm-m, with complex variations observed near the surface
