Academic literature on the topic 'Seismic techniques'

Thesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2003.<br>Includes bibliographical references (p. 59-62).<br>Recent earthquakes, starting with the 1971 San Fernando Earthquake in California, left major destructions, damaged the infrastructure, and raised questions about the vulnerability and design practice of structures, especially concrete structures. Design codes have being updated to include seismic previsions but structures build before 1971 have to be retrofitted. The focus of this paper is concrete structures. Surveys done after earthquakes have shown that the major problem with concrete structures is columns. Pre- 1971 detailing left column with lack of confinement as well as lap-slice in plastic hinge regions creating potential failures in flexure strength and/or ductility, and in shear. Other critical structural elements include, but are not limited to, gravity design frames, footings, shear walls, connections, and beams. There are two major categories of retrofit options for concrete structure; local and global methods. Local methods focus at the element level on a particular member that is deficient and in improving it to perform better. Those methods include adding concrete, steel, or composite to the outside of the member. All three methods are effective but each present some disadvantages: concrete is labor intensive, steel requires heavy construction equipments, and composites have high initial cost. Global methods concentrate at the structure level and retrofit to obtain a better overall behavior of the entire structure. The different global techniques are addition of shear walls or steel bracings, and base isolation. All three methods are effective. Shear walls are usually an expensive solution but they are flexible in their distribution allowing them to be hidden in the architecture. Steel bracings allow for openings but their connections to the existing structure can be problematic. Finally, base isolation is an option that is becoming increasingly popular and that provides good behavior in earthquake for low to mid high structures. The different systems presented all have some advantages and disadvantages and the option chosen for the retrofit depends on the existing structure requirement. The different system presented can be combined to provide more efficient and more flexible retrofit schemes.<br>by Charlotte A.C. Bouvier.<br>M.Eng.

A visualização de horizontes sísmicos constitui uma importante área de conhecimento amplamente aplicada na prospecção de hidrocarbonetos pela indústria do petróleo. Diferentes técnicas são atualmente empregadas na apresentação destas superfícies, sendo usualmente utilizadas as soluções baseadas na geração de malhas poligonais, que se beneficiam da otimização das placas gráficas atuais no desenho de triiângulos. Este trabalho faz uma avaliação do uso da renderização baseada em pontos, no lugar de polígonos, para a visualização de horizontes sísmicos. Para isso as técnicas de cada etapa do processo são avaliadas, levando-se em conta a natureza específica dos dados de interpretação de horizontes em volumes sísmicos e o resultado final esperado para a visualização deste tipo de dados. O algoritmo utilizado baseia-se no método conhecido como Surface Splatting para a renderização dos pontos originais, sendo estudados a estruturação apropriada para os dados a serem visualizados, a técnica para obtenção de normais, a abordagem adequada para o cálculo da iluminação e mecanismos adicionais necessários ao processo. Resultados da aplicação do método em dados reais são, ao final do trabalho, analisados e comparados à renderização tradicional para os horizontes avaliados.<br>Seismic horizon visualization stands as an important knowledge area used to support exploration on the oil industry. Different techniques currently employed to render this kind of surfaces are usually based on polygonal meshes generation, which benefits from graphics boards optimization on drawing triangles. This work is an evaluation of Point Based rendering techniques to replace polygonal approaches in seismic horizons visualization. To do so, this study revisits each stage of the seismic visualization process. The algorithm adopted here is based on the Surface Splatting with the EWA filter. This work also presents a study on normal evaluation and data structures to store points and normal. Special care is taken in shading techniques. The implementation yielded results that are used to support the evaluation of the Point Based Techniques on real 3D Seismic data. Traditional triangle based rendering is also presented to compare results.

An investigation is presented of the behavior of time-reversal focusing in soils. Initial numerical models demonstrate time-reversal focusing to be effective in elastic media, including when a large number of scattering objects were present in the medium. When scattering objects are present, time-reversal focusing demonstrates superior focusing ability when compared to other excitation methods such as uniform excitation or time-delay focusing. Multiple experimental investigations of experimental time-reversal focusing performed in sand evaluate time-reversal focusing effectiveness when multiple near-surface scattering objects are present in the medium. Experimental results demonstrate that time-reversal focusing is effective in the experimental context as well as the numerical models. Further experiments examine time-reversal focusing in more extreme cases where the entire ballistic wave is blocked, and the only energy reaching the focus point is reflected from scattering objects in the medium. A comparison to other focusing methods demonstrates that under these conditions, most focusing attempts with traditional methods will fail completely while time-reversal focusing does not. Additional configurations of time-reversal focusing examine its effectiveness when scattering is caused by an asymmetrical surface layers. The impact of an asymmetrical or non-uniform excitation array is also examined for time-reversal focusing in the presence of scattering objects. An investigation of the effects of scattering object geometry on focusing resolution in time-reversal focusing is also presented. Scattering object field density is found to have a strong, but diminishing effect on focusing resolution as the scattering object field density increased. Loss of surface wave energy available for focusing due to mode-conversion is found to be correlated with the density of the scattering object field. The impact of the weak non-linear nature of the soil on time-reversal focusing is examined through a study of time-reversal focusing behavior for a variety of amplitudes that generate different levels of non-linearity in the soil. This study of nonlinearity is coupled with a study of the impact of noise on time-reversal focusing. It appears that both non-linearity and noise have an impact on time-reversal focusing effectiveness. Further, the loss from these mechanisms seems to be interrelated. Noise seems to enhance non-linear loss in the soil.

Multiple methods exist for modeling with synthetic seismograms, each with its own characteristic application; local and detailed; global and asymptotic; body and/or surface waves. Events such as the nuclear tests in the Tarim Basin in China, recorded at regional distances require more than one such characteristic. A successful model would need detail close in and a global result. The ability to join two methods can therefore be very powerful. Within this text the exploration is of finite difference and discrete wavenumber integration methods. The basis of the conversion between methods is the idea in Huygen's principle of representing a wave front as multiple sources, then propagated as an alternate method. Modeling detail locally, finite difference eventually becomes computationally intensive or undetailed. Representation theory replaces finite difference with discrete wavenumber integration propagating to the receiver at a regional distance. The requirement for multiple sources means that efficiency and optimization of methods are paramount.