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Summary Remigration trajectories describe the position of an image point in the image domain for different source-receiver offsets as a function of the migration velocity. They can be used for prestack time-migration velocity analysis by means of determining kinematic migration parameters, which in turn, allow to locally correct the velocity model. The main advantage of this technique is that it takes the reflection-point displacement in the midpoint direction into account, thus allowing for a moveout correction for a single reflection point at all offsets of a common image gather (CIG). We have tested the feasibility of the method on synthetic data from three simple models and the Marmousoft data. Our tests show that the proposed tool increases the velocity-model resolution and provides a plausible time-migrated image, even in regions with strong velocity variations. The most effort was spent on the event picking, which is critical to the method. Introduction Remigration, also known as residual migration or velocity continuation, can be seen as a process to construct a seismic image for a refined velocity model from another one already available from a previous migration for a different velocity model (Hubral et al., 1996; Tygel et al., 1996). Velocity continuation can be also used on migrated diffractions (Sava et al., 2005; Fomel et al., 2007; Novais et al., 2008) for MVA. Based on velocity continuation, Coimbra et al. (2013b) recently introduced a process of extracting velocity updates for depth migration from the moveout of incorrectly migrated diffraction events by tracing remigration trajectories to their focus point in poststack migrated images, and Coimbra et al. (2013a) extended their work to the prestack case. This technique makes use of local-slope information extracted from the data with the help of trail stacks. Coimbra et al. (2014) modified this remigration trajectory MVA method to make it suitable for an application to time-migration of reflection events in prestack data, presenting an improved derivation of the time-remigration trajectories. In this work, we present an improved derivation of the method’s theory as compared to Coimbra et al. (2014), detail the model-building algorithm, and report on numerical tests of the method applied to synthetic data from three gradient models and to the Marmousoft data. These additional tests confirm the potential of the method to produce plausible velocity model updates in regions with strong velocity variations.
Summary We develop a new tool for initial seismic migration-velocity model building based on a recent gravity inversion method. This method consists of an iterative algorithm that provides a 3D density-contrast distribution on a grid of prisms, the starting point being a user-specified prismatic element called "seed". By means of this technique of planting anomalous densities, we are able to interpret multiple bodies with different density contrasts. Therefore, the method does not require the solution of a large equation system, which greatly reduces the computational demand. Once the geometry of the anomalous-density body is known, we can extract the skeleton of the inverted body and fill each prism with a velocity consistent with the presumed geology. Starting at this velocity model, the next step is to perform a migration velocity analysis (MVA). The result of MVA can then, in turn, be used to improve on the geometry for the gravity inversion. This joint processing and interpretation can be considered as an alternative way to improve the knowledge of complex structures. For example, the image quality of salt structures and sub-salt sediments obtained by reflection seismic is almost always limited by the effects of wavefield transmission, scattering and absorption. Simple synthetic examples show the capacity of the proposed velocity-model-building algorithm to generate initial velocity models for depth-migration velocity analysis, including those for specific geological targets. Introduction Time and depth migration are to fundamental processes in seismic imaging that are regularly applied to seismic data. For their success, high-quality velocity models are indispensable. However, automatic and/or efficient velocity-model construction tools are still a challenge. Most present-day model-building techniques are iterative procedures that improve a starting model based on intermediate results. Examples for new algorithms for time-migration velocity analysis based on prestack time migration (PSTM), which bypass the conventional CMP-based velocity analysis, are presented in Fomel (2003), Schleicher et al. (2008), Schleicher and Costa (2009), Coimbra et al. (2013a,b, 2014) and Santos et al. (2014a). While time migration is more robust and tolerant to velocity errors, methods acting in the depth-domain are more precise. Prestack depth migration (PSDM) techniques are capable of imaging more complex structures including lateral velocity variation and dipping reflectors (Liu and Bleistein, 1995; Liu, 1997). This is so because depth migration is highly sensitive to the velocity model (Zhu et al., 1998). Its strong dependence on a precise velocity model makes PSDM an interesting tool for velocity analysis (Abbad et al., 2009; Mulder and ten Kroode, 2002; Al-Yahya, 1989). On the other hand, a more accurate velocity model is required for its application, which almost always increases the computational cost. This, in turn, leads many researchers to search for alternative methods.
Abstract The main purpose of this study was to carry out the physical and mechanical characterization of dolomitic limestone from "Camadas de Coimbra" Formation with different degrees of weathering. The study begins with a characterization of the geotechnical properties of the intact stones used in this research. Seven tests (sound velocity, porosity, density, uniaxial compressive strength, point load strength, Schmidt rebound hardness and slake-durability) were carried out. The characterization permitted to understand and evaluate the variation of the mechanical and physical properties throughout the weathering processes. The porosity tends to growth according to the rock weathering degree. Its increase corresponds to a decrease of the ultrasonic velocity and rock density. Considering the totality of the specimens tested, moderate correlation coefficient was found between the longitudinal waves propagation and the point load strength values (r = 0.62). For the longitudinal and transversal waves propagation the correlation coefficient was equal to 0.75. 1 Introduction The main purpose of this work is to contribute to the geotechnical knowledge of several weathering degrees of dolomitic limestone belonging to the "Camadas de Coimbra" Formation. The characterisation of the physical and mechanical properties was carried out. The study area is located in the city of Coimbra (Central Portugal). Log samples were collected in a prospection campaign performed at Polo I of the University of Coimbra. The rock characterisation was carried out through seven laboratory tests: sound velocity, porosity, density, uniaxial compressive strength, point load strength, Schmidt rebound hardness and slake-durability tests. Correlations were established based on the test results, in order to define and predict the geotechnical behaviour of dolomitic limestone with different degrees of weathering.
This paper presents an equipment to perform shear tests under low constant normal stresses. This equipment maintains a normal stress during the whole joint sliding by means of a lever arm and dead weights. For common joint samples (around 100 to 150 cm), the applied normal stresses can range from 20 to 200 kPa. The results of some sets of joint tests are presented. The joints came from road cuts in the central part of Portugal. One of the sets is from quartzite discontinuities and the remaining are from schistous discontinuities. A description of the geological characteristics of the joints and the analysis of the results of the tests are presented. 1. GENERAL REMARKS Rock slope stability analyses require the knowledge of the shear strength of the most relevant discontinuities. The determination of the strength parameters should be evaluated by shear tests. These tests should reproduce as close as possible the features that occur in the field. In this case, normal stresses are low, typically smaller than 0.2 MPa, and should be kept constant during the sliding. In the proposed paper, a new equipment to perform shear tests under low constant normal stresses is presented. This equipment maintains a normal stress during the whole joint sliding by means of a lever arm and dead weights. For common joint samples (around 100 to 150 cm), the stresses used can range from 20 to 200 kPa. These values of normal stress cover the range of stresses that can commonly be found in road cuts. The height of this kind of slopes, which are particularly frequent in mountain roads, can go from 10m to around 40 m. This paper presents the results of some sets of joints. The joints came from road cut slopes in the central part of Portugal. One of the discontinuity sets is from jointed quartzite discontinuities and the remaining sets are from schistous discontinuities. A description of the geological characteristics of the joints is presented and an analysis of the results of the tests is also put forward. 2. GEOLOGIC FRAMEWORK The shear tests were performed on joints prepared from rock material collected in 4 rock slopes from road cuts in the central part of Portugal. The roads are the IP3 (Itinerario Principal n3) and the EN 234 (Estrada Nacional n° 234), near Coimbra. In both cases, they are located in the Buçaco Ordovician-Silurian Syncline and in the Schist-Greywacke pre-Ordovician Complex (Beiras Group). Slope 1 is located at km 55 of the IP3. It develops along 150 m and is placed close to the central zone of the Buçaco Palaeozoic Syncline. This slope is composed mainly by impure quartzites, though schists and carbonaceous schists are also present. The latter display a dark colour and are very weathered (W5). The impure quartzites are set in persistent benches with a weathering degree ranging from W2 to W4. They present purplish, greyish and whitish colours and are composed primarily by quartz (more than 75%) and white mica and ferrous oxides.
Oliveira Tavares, A.M. (Departamento de Ciencias da Terra, Universidade de Coimbra) | Quinta Ferreira, M.O. (Departamento de Ciencias da Terra, Universidade de Coimbra) | Almeida Saraiva, A.L. (Departamento de Ciencias da Terra, Universidade de Coimbra)
ABSTRACT: Coimbra sanitary landfill location is analysed under several aspects such as site topography, geology, hydrology and human occupation in the area. Site geological structure was studied in order to be related with problems of leachate migration coming from waste materials. This structure has high horizontal permeability and a low vertical permeability raising several problems. At landfill base leachates flow towards the topographic surface and partially dry leaving the soil contaminated. Through more permeable levels, leachates are drained towards water lines running to farming lands and high density population areas. Impact on geological environment is preliminary evaluated. Landfill dumping technique is analysed and some considerations and solutions are pointed towards a minimum environmental impact in the surrounding area. RÉSUMÉ: La decharge de matière solide situee à Coimbra est analysee à travers plusieures aspects: topographie du site, geologie, hydrologie et occupation humaine du locale. La structure geologique du site a ete etudie pour avoir une relation avec les problèmes de migration des produits lixiviables provenant des dechets. Cette structure possède une haute permeabilite horizontale et une baisse permeabilite verticale qui origine des problèmes divers. Le flux des produits lixiviables à travers la surface topographique contamine le sol. Quand les niveaux sont plus permeables les lixiviables sont draines par des cours d'eaux qui coulent vers les locales où se trouvent une grande densite de population et de fèrmes. L'evaluation de l'impacte dans l'environnement geologique est preliminaire et sa future evolution est aussi anticipee. La technique de decharge des dechets est analysee et quelques considerations et solutions sont proposees pour reduire vers un minimum, l'impacte environnementale à l'entour du site. ZUSAMMENFASSUNG: Die Muelldeponie von Coimbra wurde topographisch, geologisch, hydrologisch und hinsichtlich menschlicher Besiedlung in der Umgebung untersucht. Die geologischen Verhaltnisse des betreffenden Gelandes wurden mit dem Ziel untersucht, Beziehungen zwischen der bewegung der den Muell durchflieβenden Sickerwasser und der geologie aufzustellen. Das Gelande ist durch eine hohe horizontale, aber eine niedrige vertikale Durchlassigkelt gekennzeichnet, was einige Fragen aufwirft. An der Basis der Muelldeponie flieβen die Sickerwasser auf der Oberflache, wobei ihre partielle Verdunstung Bodekontamination verursacht. Die Entwasserung der Schichten mit höherer Permeabilitat erfolgt durch Wasserrinnen, die dicht besiedelte und landwirtlich nutzbare Flache durchqueren. Einfluβ auf das geologische Milieu wurde diskutiert und Prognosen ueber die weitere Entwicklung aufgestellt. Möglichkeiten der Abdichtung wurden analysiert und Einige Betrachtungen sowie Lösungen vorgerschlagen, mit dem Ziel, der Einfluβ der Schadstoffe auf das Minimum zu reduzieren. 1 INTRODUCTION At present one of the more stringent problems of our societies is the final destination of the enormous amount of generated waste materials. Several solutions can be used but all of them have some drawbacks that must be reduced as much as possible. The sanitary landfill option must be well integrated with the geological environment, as it can profit from the natural characteristics of local geological formations, also needing to be adequately designed, in order to incorporate the more suitable technical solutions. Coimbra sanitary landfill characteristics are presented and analysed in an attempt to define adequate procedures in the sequence of landfill operation and to focus attention to some important problems that should be solved in order to reduce environmental damages in the surrounding area. 2 LOCATION Coimbra sanitary landfill is located SW of the town, close to the limits between Coimbra and Condeixa-a-Nova municipalities (Figure 1). The site name is Bruno and belongs to Antanhol parish. By road the landfill is 11 Km away from Coimbras town center.