Virtual source gathers were generated using the principles of seismic interferometry from 135 hours of ambient noise recorded during a controlled-source survey across the Salton Sea Geothermal Field in southern California. The non-uniform nature of the noise sources violated a primary assumption of the method and generated artifacts in the data. The artifacts generated by the high-energy impulsive sources (e.g. earthquakes, shots) were removable using traditional methods of amplitude normalization prior to cross-correlation. The continuous source artifacts generated by the geothermal wells and highways required an unconventional approach of utilizing only normalized impulsive sources to successfully reduce the artifacts. Virtual source gathers were produced successfully that contained strong surface waves at 0.4-2.5 Hz, an order of magnitude below the corner frequency of the geophones, and modest body waves at 22-30 Hz, which are generally more difficult to obtain due to the need for many large, well-distributed subsurface sources. The virtual source gathers compare well to nearby explosive shots and are more densely spaced, but have a much lower signal-to-noise ratio. Analysis of the surface waves was complicated by strong higher-order modes. Spectral analysis of virtual source gathers required utilization of the geothermal plant energy, which produced usable signal at offsets required for mode separation. The virtual source dispersion curve compared well to a dispersion curve from a nearby explosive shot. P-waves were observed on the virtual source gathers. Creation of a low-quality multichannel reflection stack revealed two weak reflectors in the upper 2 km. / Master of Science

Over 700 km of industry seismic reflection data in the central Appalachians were reprocessed using both conventional and newly developed processing schemes. A new processing sequence, called dip projection, is introduced. The technique projects crooked-line processed CMPs onto a straight line oriented in the general dip direction for the area. The new stacked sections more closely approximate a dip line and hence are more migration-friendly and interpretable than the crooked-line stacks. Methods of determining the lateral continuity of subsurface density contrasts were also applied to gravity data from the study area. Known collectively as potential field attributes, the analytic signal, the tilt angle, and the gradient of the tilt angle (the potential field wavenumber ) proved valuable in the analysis of the gravity data. Comparison of reflection seismic data from the southern and central Appalachians revealed a dichotomy of seismic reflectivity from east to west. A highly reflective crust beneath the Piedmont in both the central and southern Appalachians contrasts with a general lack of reflectivity beneath the Blue Ridge and Valley and Ridge provinces where coherent reflections are restricted to the upper 3-4 seconds of the data. This difference in reflectivity is interpreted as a fundamental difference in the location and orientation of preexisting zones of weakness between the different crustal regions with respect to the tectonic events affecting the Appalachians since the early Paleozoic. The combination of the results of new methods of seismic and potential fields processing with deep well and geologic information allowed the lateral continuity of two major structures in the central Appalachians to be examined. The Blue Ridge in Virginia was found to overly a duplex of Cambrian-Ordovician carbonates formed in response to stresses during the Alleghanian Orogeny. A large thrust sheet of similar carbonate rocks was interpreted beneath the Nittany Anticlinorium in West Virginia. To the south in Virginia, this thrust sheet is replaced by imbrication of the carbonate package. The change in structural style may be related to the existence of a lateral ramp or it may reflect the overall change in structural style from the central to southern Appalachians. / Ph. D.

Modern oil and gas exploration depends on a variety of geophysical prospect tools. One of them is reflection seismology that allows to obtain interwell information of sufficient resolution economically. This exploration method collects reflection seismic data on the surface of an area of prospect interest and then uses them to build seismic images of the subsurface. All imaging approaches can be divided into two groups: wave equation-based methods and integral schemes. Kirchhoff migration, which belongs to the second group, is an indispensable tool in seismic imaging due to its flexibility and relatively low computational cost. Unfortunately, the classic formulation of this method images only a part of the surface data, if so-called multipathing is present in it. That phenomenon occurs in complex geologic settings, such as subsalt areas, when seismic waves travel between a subsurface point and a surface location through more than one path. The quality of imaging with Kirchhoff migration in complex geological areas can be improved if multiple paths of ray propagation are included in the integral. Multiple arrivals can be naturally incorporated into the imaging operator if it is expressed as an integral over subsurface take-off angles. In this form, the migration operator involves escape functions that connect subsurface locations with surface seismic data values through escape traveltime and escape positions. These escape quantities are functions of phase space coordinates that are simply related to the subsurface reflection system. The angle-domain integral operator produces output scattering- and dip-angle image gathers, which represent a convenient domain for subsurface analysis. Escape functions for angle-domain imaging can be simply computed with initial-value ray tracing, a Lagrangian computational technique. However, the computational cost of such a bottom-up approach can be prohibitive in practice. The goal of this work was to construct a computationally efficient phase space imaging framework. I designed several approaches to computing escape functions directly in phase space for mapping surface seismic reflection data to the subsurface angle domain. Escape equations have been introduced previously to describe distribution of escape functions in the phase space. Initially, I employed these equations as a basis for building an Eulerian numerical scheme using finite-difference method in the 2-D case. I show its accuracy constraints and suggest a modification of the algorithm to overcome them. Next, I formulate a semi-Lagrangian approach to computing escape functions in 3-D. The second method relies on the fundamental property of continuity of these functions in the phase space. I define locally constrained escape functions and show that a global escape solution can be reconstructed from local solutions iteratively. I validate the accuracy of the proposed methods by imaging synthetic seismic data in several complex 2-D and 3-D models. I draw conclusions about efficiency by comparing the compute time of the imaging tests with the compute time of a well-optimized conventional initial-value ray tracing. / text

The National Science Foundation has announced a plan to establish a Deep Underground Science and Engineering Laboratory (DUSEL) for interdisciplinary research in physics, geosciences, biosciences and engineering. The proposed laboratory will extend to a depth of about 2200 meters and will consist of research facilities for long term study. To date, eight sites in North America have been proposed to host DUSEL. One of these sites, known as Kimballton, is located near Butt Mountain in Giles County in southwestern Virginia. Two seismic lines were acquired along the top of Butt Mountain in June of 2004 to support the ongoing integrated site characterization effort by the Kimballton Science Team. Both lines, approximately 3 km in length, are standard multifold seismic reflection data aimed at imaging faults, thrust sheets, and repeated sections of Paleozoic rocks in the vicinity of the proposed Kimballton site. Crooked line geometry, irregular geophone spacing, ground roll, and poor impedance contrasts between juxtapositioned rock units were challenges in processing the data. Non-standard processing techniques included the use of travel time tomography to accurately constrain near surface velocities, the use of 2D median filters to remove ground roll, and stacking only offsets exceeding 500 m. Interpretation of seismic data supports a triplicated stratigraphic section caused by the stacking of the the St. Clair and Narrows thrust sheets. The St. Clair and Narrows faults are interpreted as shear zones within ductile units of the Martinsburg Formation. 3D travel time tomography was used to build a near surface velocity model of Lines 1 and 2 for the purposes of imaging near surface structure and constraining the extent of topographic lineaments, which are interpreted as bedrock joint systems. Interpretation of the velocity models suggests that the broadly folded strata of the Butt Mountain synclinorium dip gently to the east along the hinge surface. The surface extrapolation of the Lookout Rock fault and the intersection of topographic lineaments with the seismic lines are expressed as low velocity zones that extend to depths of 150 m. This may be related to accelerated weathering along jointed rock surfaces. Results of this study have already been incorporated into the NSF proposal submitted by the Kimballton Science Team (http://www.phys.vt.edu/~kimballton/s2p/b2.pdf). / Master of Science

On August 31, 1886 a large scale earthquake occurred in Summerville, S.C. causing severe damage in the coastal city of Charleston. Although intensive geological and geophysical studies have been conducted in the area, uncertainty remains about the details of the event. Recently evidence from seismic reflection profiles have shed light on the tectonic environment of the area. The epicentral area of the 1886 event lies within the South Georgia Rift, a Mesozoic rift terrane. Previous studies have revealed clues to the geologic structure and evolution of this feature. SEISDATA4 is the largest seismic reflection profile recorded in the area. By re-processing the line, information about the tectonic structure of the area was revealed. The early Mesozoic extensional basin that hosted the 1886 earthquake and is host to the modern seismicity recorded in the area, extends several kilometers to the south and west of Charleston, along SEISDATA4. Cenozoic and Mesozoic faults were resolved within the basin and along its northwestern boundary that is distinguished by a strong gradient in the magnetic field. However, the question as to which fault was responsible for the rupture of 1886 still remains. The refraction analysis provides better resolution of the lithology in Lower Mesozoic section. The termination of the strong reflection at the base of the Atlantic Coastal Plain occurs in a section of the profile that shows major disruption of the underlying reflections, and suggests that the termination of a lower Mesozoic basalt flow responsible for the reflection may be related to tectonic deformation. / Master of Science

Um das Verständnis der geologischen Entwicklung des größten bekannten Vorkommens von ultra-hochdruck (UHP) Gesteinen auf der Erde, des Dabie Shan im östlichen China, zu erhöhen, wurde eine multidisziplinäre Studie durchgeführt.<br /> Geophysikalische Daten wurden entlang einer ca. 20 km langen seismischen Linie im östlichen Dabie Shan gesammelt. Diese reflektionsseismischen Daten zeigen, dass die Kruste aus drei Lagen besteht. Die Oberkruste besitzt eine durchgehend niedrige Reflektivität und meist subhorizontale Reflektoren bis in eine Tiefe von ca. 15 km. Aufgrund dieser Charakteristika wird diese Zone als UHP-bezogener krustaler Keil interpretiert, der auf nicht UHP Kruste überschoben wurde. Ein abrupter Wechsel in der Geometrie aber auch Intensität der Reflektoren markiert die Grenze zu einer mittel- bis unterkrustalen Zone, die sich bis ca. 33 km Tiefe erstreckt. Diese Zone repräsentiert wahrscheinlich kratonale Yangtze Kruste, die von der triassischen UHP-Orogenese nicht erfasst wurde, aber während der Exhumierung das Liegende relativ zum UHP Keil war. Starke und kontinuierliche Reflektoren im Tiefenintervall von 33-40 km bilden höchstwahrscheinlich die Moho an der Basis der Kruste ab. Relikte einer Krustenwurzel, die sich wahrscheinlich während der Kollisionstektonik gebildet hatte, sind nicht sichtbar. Ein flaches tomographisches Geschwindigkeitsmodell, das auf der Inversion der Ersteinsätze gründet, konnte zusätzlich erstellt werden. Dieses Modell bildet deutlich die unterschiedlichen Lithologien auf beiden Seiten der Tan Lu Störung ab. Sedimente östlich der Störung zeigen Geschwindigkeiten von 3.4 - 5.0 km* s^-1, wohingegen die Gneise im Westen 5.2 - 6.0 km*s^-1 aufweisen. Die Geometrie der Geschwindigkeits-Isolinien kann als Ausdruck der Strukturen der Gesteine angenommen werden. Somit zeigen die Sedimente ein nordwestliches Einfallen zur Störung hin, wohingegen isoklinale Falten in den Gneisen abgebildet werden.<br /> Geländedaten aus der UHP Einheit des Dabie Shan ermöglichen die Definition von Grundgebirgs- und Deckeinheiten, die Teile des ehemaligen passiven Kontinentalrandes des Yangtze Kratons repräsentieren. Eine der Deckeinheiten, die Changpu Einheit, besitzt nach wie vor einen stratigraphischen Kontakt zu den Grundgebirgs-Gneisen. Der anderen Einheit hingegen, der Ganghe Einheit, fehlt ein entsprechendes Grundgebirge. Diese Einheit steht vielmehr über einen Blasto-Mylonit in tektonischem Kontakt zum Grundgebirge der vorherigen. Die Changpu Einheit baut sich aus kalk-arenitischen Metasedimenten auf, die mit Metabasalten assoziiert sind. Die Ganghe Einheit wird von arenitisch-vulkanoklastischen Metasedimenten, die ebenfalls mit metabasaltischen Gesteinen vergesellschaftet sind, dominiert. Das Grundgebirge baut sich aus diversen felsischen Gneisen auf, die von reliktisch eklogitfaziell bis grünschieferfaziell ausgeprägt sind, und in denen, zusätzlich zu Metabasalten, sporadisch mafisch-ultramafische Meta-Plutone auftreten. Mit Ausnahme der Ganghe Einheit, führen die Metabasite Coesit und belegen somit das UHP Ereignis. Die Mineralchemie der analysierten Proben dokumentiert deutliche Variationen in der Zusammensetzung der Hauptminerale, Granat und Omphazit, was entweder unterschiedliche Protolithe oder unterschiedliche Grade von Stoffaustausch mit den Wirtsgesteinen reflektiert. Gehalte von dreiwertigem Eisen in Omphaziten mit geringen Gesamteisengehalten, wurden mittels Titration bestimmt, wobei sich Werte von 30-40 % ergaben. Dennoch wurde ein noch konservativerer Wert von 50% dreiwertigem Eisen in den entsprechenden Berechnungen angenommen, hauptsächlich, um mit anderen Arbeiten vergleichbar zu sein. Texturen und chemische Zonierungen in den Mineralen sind kompatibel mit Gleichgewichtsbedingungen während dem Höhepunkt der Metamorphose, der retrograd überprägt wird. P-T Daten wurden mit deutlicher Betonung auf das Granat-Omphazit-Phengit Barometer, das mit Fe-Mg Austausch-Thermometern kombiniert wurde, berechnet. Höchstdrucke reichen von 42-48 kbar (für die Changpu Einheit) bis ca. 37 kbar (für das Grundgebirge und die Ganghe Einheit). Während der eklogitfaziellen Metamorphose wurden Temperaturen von ca. 750 °C erreicht. Obwohl die maximalen Drucke deutlich schwanken, sind die Temperaturbestimmungen in guter Übereinstimmung. Die Druckschwankungen können zum einen durch deutlich Ca-dominierte Granate (bis zu 50 mol% Grossular in der Changpu Einheit) und/oder zum anderen durch Modifikationen der Mineralzusammensetzungen während der retrograden Metamorphose erklärt werden.<br /> Die präsentierten integrativen geologischen Daten ermöglichen die folgenden Schlussfolgerungen<br /> i) Grundgebirgs- und Deckeinheiten treten im Dabie Shan auf und wurden beide UHP metamorph überprägt<br /> ii) Der Dabie Shan ist das metamorphe Äquivalent des früheren passiven Kontinentalrandes des Yangtze Kratons<br /> iii) felsische Gneise, die eine UHP Metamorphose durchlaufen, sind von Volumenver-änderungen betroffen, die durch großräumige Phasenumwandlungen (Quarz <-> Coesit) hervorgerufen werden, was direkt die tektono-metamorphen Prozesse beeinflusst<br /> iv) Initiale Unterschiede in der Temperatur sind möglicherweise dafür verantwortlich, dass generell Unterkrustengesteine in UHP Fazies fehlen / A multidisciplinary study has been carried out to contribute to the understanding of the geologic evolution of the largest known occurrence of ultra-high-pressure (UHP) rocks on Earth, the Dabie Shan of eastern China. <br /> Geophysical data, collected along a ca. 20 km E-W trending seismic line in the eastern Dabie Shan, indicate that the crust comprises three layers. The upper crust has a homogeneously low reflectivity and exhibits roughly subhorizontal reflectors down to ca. 15 km. It is therefore interpreted to portray a crustal UHP slab thrust over non-UHP crust. An aprubt change in intensity and geometry of observed reflectors marks the boundary of a mid- to lower crustal zone which is present down to ca. 33 km. This crustal zone likely represents cratonal Yangtze crust that was unaffected by the Triassic UHP event and which has acted as the footwall during exhumation of the crustal wedge. Strong and continuous reflectors occurring at ca. 33-40 km depth most likely trace the Moho at the base of the crust. Any trace of a crustal root, that may have formed in response to collision tectonics, is therefore not preserved. A shollow tomographic velocity modell based on inversion of the first arrivals is constructed additionally. This model clearly images the distinct lithologies on both sides of the Tan Lu fault. Sediments to the east exhibit velocities of about 3.4 - 5.0 km* s^-1, whereas the gneisses have 5.2 - 6.0 km*s^-1. Geometry of velocity isolines may trace the structures present in the rocks. Thus the sediments dip shallowly towards the fault, whereas isoclinal folds are imaged to occur in the gneisses.<br /> Field data from the UHP unit of the Dabie Shan enables definition of basement-cover sequences that represent sections of the former passive margin of the Yangtze craton. One of the cover sequences, the Changpu unit, still displays a stratigraphic contact with basement gneisses, while the other, the Ganghe unit, includes no relative basement exposure. The latter unit is in tectonic contact with the basement of the former unit via a greenschist-facies blastomylonite. The Changpu unit is chiefly constituted by calc-arenitic metasediments intercalated with meta-basalts, whereas the Ganghe unit contains arenitic-volcanoclastic metasediments that are likewise associated with meta-basalts. The basement comprises a variety of felsic gneisses, ranging from preserved eclogitic- to greenschist-facies paragenesis, and locally contains mafic-ultramafic meta-plutons in addition to minor basaltic rocks. Metabasites of all lithologies are eclogite-facies or are retrogressed equivalents, which, with the exception of those from the Ganghe unit, bear coesite and thus testify to an UHP metamorphic overprint. Mineral chemistry of the analysed samples reveal large compositional variations among the main minerals, i.e. garnet and omphacite, indicating either distinct protoliths or different degrees of interaction with their host-rocks. Contents of ferric iron in low Fetot omphacites are determined by wet chemical titration and found to be rather high, i.e. 30-40 %. However, a even more conservative estimate of 50% is applied in the corresponding calculations, in order to be comparable with previous studies. Textural constraints and compositional zonation pattern are compatible with equilibrium conditions during peak metamorphism followed by a retrogressive overprint. P-T data are calculated with special focus on the application of the garnet-omphacite-phengite barometer, combined with Fe-Mg exchange thermometers. Maximum pressures range from 42-48 kbar (for the Changpu unit) to ~37 kbar (for the Ganghe unit and basement rocks). Temperatures during the eclogite metamorphism reached ca. 750 °C. Although the sample suite reveals variable peak-pressures, temperatures are in reasonable agreement. Pressure differences are interpreted to be due to strongly Ca-dominated garnet (up to 50 mol % grossular in the Changpu unit) and modification of peak-compositions during retrogressive metamorphism.<br /> The integrated geological data presented in this thesis allow it to be concluded that, <br /> i) basement and cover rocks are present in the Dabie Shan and both experienced UHP conditions<br /> ii) the Dabie Shan is the metamorphic equivalent of the former passive margin of the Yangtze craton <br /> iii) felsic gneisses undergoing UHP metamorphism are affected by volume changes due to phase transitions (qtz <-> coe), which directly influence the tectono-metamorphic processes<br /> iv) initial differences in temperature may account for the general lack of lower crustal rocks in UHP-facies

I propose a computationally efficient method to approximate the inverse of the normal operator arising in the multi-parameter linearized inverse problem for reflection seismology in two and three spatial dimensions. Solving the inverse problem using direct matrix methods like Gaussian elimination is computationally infeasible. In fact, the application of the normal operator requires solving large scale PDE problems. However, under certain conditions, the normal operator is a matrix of pseudodifferential operators. This manuscript shows how to generalize Cramer's rule for matrices to approximate the inverse of a matrix of pseudodifferential operators. Approximating the solution to the normal equations proceeds in two steps: (1) First, a series of applications of the normal operator to specific permutations of the right hand side. This step yields a phase-space scaling of the solution. Phase space scalings are scalings in both physical space and Fourier space. Second, a correction for the phase space scaling. This step requires applying the normal operator once more. The cost of approximating the inverse is a few applications of the normal operator (one for one parameter, two for two parameters, six for three parameters). The approximate inverse is an adequately accurate solution to the linearized inverse problem when it is capable of fitting the data to a prescribed precision. Otherwise, the approximate inverse of the normal operator might be used to precondition Krylov subspace methods in order to refine the data fit. I validate the method on a linearized version of the Marmousi model for constant density acoustics for the one-parameter problem. For the two parameter problem, the inversion of a variable density acoustics layered model corroborates the success of the proposed method. Furthermore, this example details the various steps of the method. I also apply the method to a 1D section of the Marmousi model to test the behavior of the method on complex two-parameter layered models.

The Guinea Plateau is a shallow-marine, flat-lying bathymetric province situated along the equatorial West African margin, offshore Republic of Guinea. The Guinea Plateau and the conjugate Demerara Plateau hold particular geologic significance, as they represent the final point of separation between the African and South American continents during Gondwana break-up. Recent interpretation of both 2-D and 3-D seismic surveys along the Guinean margin have illuminated subsurface features related to Early Cretaceous crustal extension. Seismic structural investigations on these datasets suggest that the majority of extension is accommodated along large-scale listric normal faults located on a relatively narrow (<50 km) continental slope (up to ~39% extension). Minimal faulting reveals that little upper-crustal extension has occurred on the Guinea Plateau. Additionally, multiple 2-D seismic profiles image the transition from continental crust on the plateau and slope, to oceanic crust in the deeper marine basin. This continent-ocean boundary is the most representative boundary when testing the accuracy of plate reconstructions. Mapping of both the continent-ocean boundary and fracture zones across the equatorial Atlantic suggests that the Demerara Plateau and the South American plate are too far south in previous pre-rift reconstructions. A revised model introduces 20 km of Early Cretaceous NNW-oriented contraction across the Amazon Basin; an area of relative weakness where both geologic and geophysical data support such accommodation. Sea-surface flow models, which used this revised reconstruction and interpreted paleo-bathymetric data, predict upwelling throughout the newly formed equatorial seaway, and later along the West African margin during periods of regional organic-rich black shale deposition. With reduced decomposition of organic matter strongly correlated to upwelling, being able to predict these zones is of particular significance to petroleum companies, who have recently started exploring both the equatorial South American and West African coastlines.

No description available.

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.