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Seismic-Reflection and Seismic-Refraction Imaging of the South Portuguese Zone Fold-And-Thrust Belt

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Seismic-Reflection and Seismic-Refraction Imaging of the South Portuguese Zone Fold-And-Thrust Belt Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 362 Seismic-Reflection and Seismic-Refraction Imaging of the South Portuguese Zone Fold-and-Thrust Belt CEDRIC SCHMELZBACH ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 UPPSALA ISBN 978-91-554-7016-6 2007 urn:nbn:se:uu:diva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ist of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals: I Schmelzbach, C., C. Juhlin, R. Carbonell, and J. F. Simancas (2007), Prestack and poststack migration of crooked-line seismic reflection data: A case study from the South Portuguese Zone fold belt, southwestern Iberia, Geophysics, 72(2), B9–B18. II Schmelzbach, C., J. F. Simancas, C. Juhlin, and R. Carbonell (2007), Seismic-reflection imaging over the South Portuguese Zone fold-and-thrust belt, SW Iberia, submitted to Journal of Geophysical Research. III Schmelzbach C., C. A. Zelt, C. Juhlin, and R. Carbonell (2007), P- and SV -velocity structure of the South Portuguese Zone fold- and-thrust belt, SW Iberia, from traveltime tomography, submit- ted to Geophysical Journal International. Reprints were made with permission from the publishers. Additional publications written during my stay at Uppsala University but not included in this thesis: • Schmelzbach C., H. Horstmeyer, and C. Juhlin (2007), Shallow 3D seismic-reflection imaging of fracture zones in crystalline rock, Geophysics, in press. • Schmelzbach C., H. Horstmeyer, and C. Juhlin (2006), High-resolution 3-D seismic imaging of the upper crystalline crust at a nuclear-waste disposal study site on Ävrö Island, southeastern Sweden, 76th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 1396– 1400. • Schmelzbach C., A. G. Green, and H. Horstmeyer (2005), Ultra-shallow seismic reflection imaging in a region characterized by high source-generated noise, Near Surface Geophysics, 3(1), 33–46. • Schmelzbach C., and C. Juhlin, (2004), Oskarshamn site investigations: 3D processing of high-resolution reflection seismic data acquired within and near the array close to KAV04A on Ävrö, 2003, SKB AB (Swedish Nuclear Fuel and Waste Management Co.), P-04-204. Contents 1 Introduction . 9 1.1 Motivation and General Objective . 9 1.2 Outline of the Thesis . 10 2 Background Information on the South Portuguese Zone and the IBER- SEIS Profile . 11 2.1 Geological Overview . 11 2.1.1 Lithostratigraphy of the South Portuguese Zone . 11 2.1.2 South Portuguese Zone Structural Image . 12 2.1.3 Geodynamic Evolution . 13 2.2 Previous Investigations . 14 2.3 The IBERSEIS Profile . 15 3 Physical Background . 19 3.1 Seismic-Wave Propagation . 19 3.1.1 The Wave Equation . 19 3.1.2 Describing Wave Propagation by Rays . 20 3.1.3 Partitioning at an Interface . 21 3.1.4 Seismic Resolution . 21 3.2 Petrophysical Aspects . 22 3.2.1 Mineralogy . 23 3.2.2 Crack-Like Porosity . 23 3.2.3 Fractures and Faults . 25 3.2.4 Stochastic Analysis of Geophysical Log Data . 25 3.2.5 Implications for Seismic Imaging in Crystalline Environ- ments . 27 4 Review of Seismic-Reflection Techniques . 29 4.1 Applications . 29 4.2 Seismic-Reflection Data Acquisition . 30 4.3 Challenges in Hard-Rock Seismic-Reflection Data Processing . 31 4.3.1 Crooked-Line Acquisition Geometries . 31 4.3.2 Effects of the Near-Surface Layer . 33 4.3.3 Dip-Moveout Corrections, Prestack, and Poststack Mi- gration . 35 5 Review of Seismic-Refraction Techniques . 41 5.1 Applications . 41 5.1.1 Algorithms and Inversion Approaches . 41 5.1.2 S-Wave Traveltime Inversion . 42 5.1.3 Seismic-Refraction vs. Seismic-Reflection Acquisition . 42 5.2 First-Arrival Traveltime Tomography . 43 5.2.1 Forward Modeling . 44 5.2.2 Solving the Inverse Step . 44 5.2.3 Assessment of the Solution . 45 6 Summary of Papers . 49 6.1 Paper I: Prestack and Poststack Migration of Crooked-Line Seismic Reflection Data: A Case Study from the South Por- tuguese Zone Fold Belt, Southwestern Iberia . 49 6.1.1 Motivation . 49 6.1.2 Methods . 49 6.1.3 Conclusions . 50 6.2 Paper II: Seismic-Reflection Imaging over the South Portuguese Zone Fold-and-Thrust Belt, SW Iberia . 52 6.2.1 Motivation . 52 6.2.2 Methods . 52 6.2.3 Results . 53 6.2.4 Conclusions . 55 6.3 Paper III: P- and SV-Velocity Structure of the South Portuguese Zone Fold-and-Thrust Belt, SW Iberia, from Traveltime Tomography . 55 6.3.1 Motivation . 55 6.3.2 Methods . 55 6.3.3 Results and Conclusions . 56 7 Conclusions and Outlook . 59 7.1 General Conclusions . 59 7.2 Outlook . 59 7.2.1 Prestack Migration of Crooked-Line Data . 60 7.2.2 Analysis of Converted, Shear, and Surface Waves . 61 7.2.3 Waveform Tomography . 63 8 Summary in Swedish: Avbildning av väck och överkastnings bältet Södra Portugisiska Zonen med seismisk reflektion och seismisk re- fraktion . 67 8.1 Syfte av avhandlingen . 67 8.2 Seismisk reflektions processering av data med krokiga insam- lingslinjer (Artikel I) . 67 8.3 Geologisk tolkning av reflektioner och diffraktionsenergi (Ar- tikelII).......................................... 67 8.4 Hastighets tomografi med användande av första ankomster från P- och SV-vågor (Artikel III) . 68 8.5 Slutsats . 69 Acknowledgments . 71 Bibliography . 73 Abbreviations General terms 1D One-dimensional 2D Two-dimensional 3D Three-dimensional Geophysical terms CMP Common midpoint DMO Dip-moveout NMO Normal moveout P-wave Primary (compressional, longitudinal) wave PSTM Prestack time migration rms root-mean square S/N Signal-to-noise ratio S-wave Shear or secondary (rotational, equivoluminal) wave SV -wave Vertically polarized shear wave Geological terms CIZ Central Iberian Zone IPB Iberian Pyrite Belt Moho Mohoroviciˇ c´ discontinuity OMZ Ossa-Morena Zone SPZ South Portuguese Zone VMS Volcanogenic Massive Sulfide 1. Introduction 1.1 Motivation and General Objective The South Portuguese Zone fold-and-thrust belt (SPZ) represents the southernmost part of the Iberian massif on the South-Western Iberian Peninsula. It is commonly acknowledged that the Iberian massive forms one of the best exposed fragments of the Variscan orogeny in Western Europe (Variscan orogeny: 480 – 290 Ma; see e.g., Matte, 1986). Hence, the South-Western Iberian Peninsula has attracted a broad scientific interest, as the area offers the opportunity to study the Variscan orogeny in space and time, for example, evoking the SW Iberia interdisciplinary scientific program promoted by EUROPROBE (Ribeiro et al., 1996). Magmatic activity was intense within the SPZ during Early Carbonifer- ous times resulting in world-class massive sulphide deposits being developed within the Iberian Pyrite Belt (IPB) unit of the SPZ (Carvalho et al., 1999; Sáez et al., 1999). Detailed knowledge of the Variscan structure with depth is important for any palinspastic reconstructions, and, consequently, is essen- tial for ore exploration in the IPB area. However, current geological models of the SPZ are primarily based on surface geological mapping (e.g., Soriano and Casas, 2002). Only after the acquisition of the IBERSEIS deep seismic- reflection profile (Simancas et al., 2003; Carbonell et al., 2004) has detailed depth information become available. Previous seismic investigations of the Variscan Iberian massif structure in- volved mainly seismic wide-angle and refraction experiments aiming at re- solving the SPZ structure at large scale (e.g., Mueller et al., 1973; Diaz et al., 1993; González et al., 1998). In contrast, the recently recorded IBERSEIS deep-seismic reflection profile, filling a gap in the deep-seismic imaging of the Variscan Belt in Europe, provided, for the first time, a complete and de- tailed crustal-scale image of the Iberian Variscan Belt (Simancas et al., 2003; Carbonell et al., 2004). Although the seismic-reflection processing.
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