The seismic method in exploration geophysics consists of creating a mechanical disturbance at or close to the surface of the earth, and observing its effects at a number of chosen locations along the surface. The purpose of seismic data acquisition is to record these effects in such a manner that their relation with the initial disturbance can be interpreted as a guide to the earth's subsurface structure (Nettleton, 1940).The validity of data interpretation depends upon the fidelity of recording. A better seismic interpretation can result from correctly collected data using instrumentation which faithfully records the seismic signal. Subsequent computer processing cannot reconstruct information which is not contained in the recorded field data. Hence, the quality of field data recording must be at an optimum level, otherwise the result will be an inferior interpretation (Donnell,1957).A reflection seismic data acquisition system was assembled and put into operation. The basic instrument was a Texas Instruments DFS IV, obtained from marine vessel M/V Banksia, and commissioned for land application.The system was tested and evaluated. The instrument analog filter phase distortion was studied in detail. The study indicated that phase distortion can be a major cause of seismic misties. Without a knowledge of the particular recording instrumentation filter transfer function, data processing bureaux may not compensate for phase distortion effects adequately (Gray et al., 1968).Once testing was completed satisfactorily, the operational system was applied to several practical field situations of commercial standard. A series of noise studies was performed to evaluate not only source generated noise, but also to study the effect of different types of energy sources on seismic data. In addition, two multi-fold seismic lines wore recorded, both of which were considered superior ++ / to those previously produced by the industry, at each location (Jacia, pers. comm., 1984).Finally, a single fold three-dimensional areal seismic survey was performed over the Woodada gas field. The results of this survey will be released after processing has been completed by Allied Geophysical Laboratories (University of Houston), and are not contained in this thesis.Future areas for field application are discussed. Recommendations are made for further research work in the area of phase distortion; the examination of different energy sources; a review of receiver properties and horizontally travelling seismic waves; a bore-hole seismic study and finally, a fourth-dimensional recording technique involving the performance of an offset VSP survey at the same time as an areal 3-D seismic survey.Volume 1 describes the establishment of the acquisition system and its subsequent field application.Volume 2 contains the Appendix of instrument tests and their analysis.

Ce travail constitue une étude préliminaire au développement d’un nouvel outil d’observation de la thermocline saisonnière de la Mer d’Iroise, la sismique réflexion. Une application inédite de cette technique à l’imagerie de structures océanographiques très peu profondes (z < 50 m) est réalisée. Dans un premier temps des règles de construction d’un dispositif sismique dédié à l’observation de telles cibles ont été définies à partir d’une réflexion théorique sur la mesure de sismique. Une évaluation de systèmes existants a ensuite été réalisée au travers de l’étude de 4 profils sismiques des campagnes GO, Carambar et Sigolo retraités spécifiquement, présentant des signaux aux mêmes profondeurs que la thermocline de l’Iroise. Ces dispositifs conventionnels apparaissent inadaptés à l’observation d’une cible océanographique superficielle, du fait i – d’un fort effet de filtre d’antenne lié à de grands offsets et longueurs de trace, et ii – de sources qui ne combinent pas puissance et fine résolution verticale. A partir de données océanographiques nouvelles acquises en Mer d’Iroise lors de la campagne Fromvar (été 2010), l’acquisition sismique a été modélisée, et les paramètres du dispositif idéal ajusté. Un système courts – offsets comprenant 4 flûtes de 6 traces de 1.8 m et un Sparker fournissant un signal de 400 Hz avec un niveau d’émission de 210 dB re 1 μPa @1m, a été proposé. Testé lors de la campagne ASPEX 2012, il a fournit une image de résolution latérale inédite de la thermocline saisonnière à 30 m de profondeur et de ses variations de fine échelle horizontale (quelques centaines de m à quelques km) causées par des ondes internes, au large de la Bretagne sud. / This work is a preliminary study for the development of a new tool, marine seismic reflection, for the observation of the seasonal thermocline of the Iroise Sea. It is the first application of this technique to the observation of shallow oceanographic structures (z < 50 m). In order to define general requirements for the design of a seismic acquisition system suited to the imaging of such targets, the theory of the seismic measurement was first investigated. Four multi-channel seismic reflection profiles from the GO, Carambar and Sigolo cruises were then reprocessed and analyzed in order to assess the potential of conventional seismic systems. They were found ill suited to the imaging of shallow oceanographic structures, because of a high antenna filter induced by large offsets and seismic trace lengths, and sources that do not reconcile with the high level of emission and fine vertical resolution. New oceanographic data acquired in the Iroise Sea during the Fromvar 2010 cruise allowed simulation of the seismic acquisition, and the definition of optimal acquisition parameters for the imaging of the seasonal thermocline. Sea trials of this specifically designed system were performed during the ASPEX survey, conducted in early summer 2012. The seismic device featured: four seismic streamers, each consisting of 6 traces of 1.80 m.; a 1000 J SIG Sparker source, providing a 400 Hz signal with a level of emission of 210 dB re 1 μPa @1m. This survey captured the 15 m thick, 30 m deep seasonal thermocline in unprecedented detail, showing images of vertical displacements of horizontal wavelength of a several hundreds m to a few km , most probably induced by internal waves.

Neste trabalho avaliou-se a potencialidade do uso da sísmica rasa de reflexão multicomponente para investigação geológica-geotécnica. Foram abordados vários aspectos relacionados à aquisição dos dados sísmicos de reflexão multicomponente, com o objetivo de entender as vantagens e limitações do método para aplicação em investigações de subsuperfície rasa. Os ensaios de campo foram realizados em duas áreas, ambas em terrenos da Bacia Sedimentar de São Paulo, em área urbana da cidade de São Paulo. Para a interpretação dos dados sísmicos multicomponente foram investigados procedimentos para a inversão não-linear dos tempos de trânsito e das amplitudes. O testes realizados orientaram a escolha da aproximação não-hiperbólica mais apropriada para calculo dos tempos de trânsito visando à análise de velocidades do pacote acima do refletor. O estudo numérico desenvolvido para a inversão das amplitudes mostrou a viabilidade da estimativa das velocidades e densidades, acima e abaixo do refletor, empregando-se as equações de Zoeppritz para as ondas refletidas PP, PSv, SvP e SvSv, antes e depois do ângulo crítico. Dada a complexidade da inversão nao-linear das amplitudes, se fez necessário elaborar uma estratégia estocástica de otimização e desenvolver uma nova abordagem para análise da função objetivo multi-dimensional, garantindo confiabilidade ao resultado da inversão não-linear. Os resultados deste trabalho mostraram o potencial da sísmica de reflexão rasa multicomponente para caracterização geológica-geotécnica, possibilitando um melhor entendimento das camadas superficiais. / This thesis aims to evaluate the useful of the multicomponent seismic methods for shallow investigations, mainly its potential for the geotechnical and geological characterization of the nearsurface. Several aspects regarding the acquisition and processing data of multicomponent seismic data are discussed. They were based on data set acquired in the urban area of Sao Paulo city, Brazil. Two different areas were investigated. Both located in sedimentary terrains belonging to the Sao Paulo Sedimentary Basin. We present a non-linear travel time and seismic amplitude inversion scheme to quantitative interpretation of multicomponent seismic data. Several tests were performed to guide the choice of non-hyperbolic equation more suitable for travel time inversion aiming the velocity analysis above the reflector. A numerical experiment developed to solve the nonlinear inversion of seismic amplitudes showed the feasibility to estimate seismic interval velocities and layer densities above and below the reflector using the exact Zoeppritz equations for PP, PSv, SvP e SvSv reflected waves, before and after critical angle. Due to the apparent complexity of the nonlinear seismic amplitude inversion, it was necessary elaborate the strategy for stochastic optimization and develop a new approach to analyze the multi-dimensional objective function, with different implications for the accuracy and efficiency of the non-linear inversion. The study show the benefits of using the multicomponent seismic method for shallow geological-geotechnical characterization, improving the nearsurface understanding, once allows an integrated analyzes of a more complete record of the wave field.

Neste trabalho avaliou-se a potencialidade do uso da sísmica rasa de reflexão multicomponente para investigação geológica-geotécnica. Foram abordados vários aspectos relacionados à aquisição dos dados sísmicos de reflexão multicomponente, com o objetivo de entender as vantagens e limitações do método para aplicação em investigações de subsuperfície rasa. Os ensaios de campo foram realizados em duas áreas, ambas em terrenos da Bacia Sedimentar de São Paulo, em área urbana da cidade de São Paulo. Para a interpretação dos dados sísmicos multicomponente foram investigados procedimentos para a inversão não-linear dos tempos de trânsito e das amplitudes. O testes realizados orientaram a escolha da aproximação não-hiperbólica mais apropriada para calculo dos tempos de trânsito visando à análise de velocidades do pacote acima do refletor. O estudo numérico desenvolvido para a inversão das amplitudes mostrou a viabilidade da estimativa das velocidades e densidades, acima e abaixo do refletor, empregando-se as equações de Zoeppritz para as ondas refletidas PP, PSv, SvP e SvSv, antes e depois do ângulo crítico. Dada a complexidade da inversão nao-linear das amplitudes, se fez necessário elaborar uma estratégia estocástica de otimização e desenvolver uma nova abordagem para análise da função objetivo multi-dimensional, garantindo confiabilidade ao resultado da inversão não-linear. Os resultados deste trabalho mostraram o potencial da sísmica de reflexão rasa multicomponente para caracterização geológica-geotécnica, possibilitando um melhor entendimento das camadas superficiais. / This thesis aims to evaluate the useful of the multicomponent seismic methods for shallow investigations, mainly its potential for the geotechnical and geological characterization of the nearsurface. Several aspects regarding the acquisition and processing data of multicomponent seismic data are discussed. They were based on data set acquired in the urban area of Sao Paulo city, Brazil. Two different areas were investigated. Both located in sedimentary terrains belonging to the Sao Paulo Sedimentary Basin. We present a non-linear travel time and seismic amplitude inversion scheme to quantitative interpretation of multicomponent seismic data. Several tests were performed to guide the choice of non-hyperbolic equation more suitable for travel time inversion aiming the velocity analysis above the reflector. A numerical experiment developed to solve the nonlinear inversion of seismic amplitudes showed the feasibility to estimate seismic interval velocities and layer densities above and below the reflector using the exact Zoeppritz equations for PP, PSv, SvP e SvSv reflected waves, before and after critical angle. Due to the apparent complexity of the nonlinear seismic amplitude inversion, it was necessary elaborate the strategy for stochastic optimization and develop a new approach to analyze the multi-dimensional objective function, with different implications for the accuracy and efficiency of the non-linear inversion. The study show the benefits of using the multicomponent seismic method for shallow geological-geotechnical characterization, improving the nearsurface understanding, once allows an integrated analyzes of a more complete record of the wave field.