Estudo da crosta no sudoeste do cráton amazônico utilizando técnicas sismológicas
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Data
2018-11-08
Autores
Morais, Douglas Souza Figueiredo
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
Técnicas sismológicas como Função do Receptor, Análise de Dispersão de Ondas de Superfície, Correlação de ruídos sísmicos e razão espectral H/V são cada vez mais estudadas e utilizadas para o conhecimento geológico da crosta. Sendo possível determinar a composição crustal média e os principais contatos obtidos por fortes contrastes de impedância tais como limite crosta superior e inferior, assim como, limite crosta inferior e manto superior (descontinuidade de Moho). O conhecimento da velocidade de percurso das ondas de um sismo registrados através de uma estação sismográfica fornecem informações, como: o tempo de percurso, relação Vp/Vs e outros. A região de Pontes e Lacerda/MT está localizada no Sudoeste do Cráton Amazônico e o levantamento dos dados se deram em fases distintas com implantação de sismômetros alinhados em direção perpendicular (NE-SW) às principais estruturas da Faixa Móvel Aguapeí (NNW-SSE). Além da coleta de dados dessas estações temporárias foram compilados dados de sismos para a estação fixa PTLB, esta estação faz parte das estações sismológicas coordenadas pela UNB (Universidade Nacional de Brasília) e pela USP (Universidade de São Paulo). A espessura média encontrada na crosta para a região variou pouco para as diferentes técnicas abordadas, sendo que, para a técnica de Função do Receptor (estação fixa PTLB) foi obtido valor de aproximadamente 44 Km de espessura e, para as estações temporárias (S01 ao S10) valor médio de 42 km. Já a seção sísmica gerada pela correlação dos sinais para todas as estações temporárias de período curto, trouxe coerência dos sinais em 4s, 5s e, posterior coerência variando de 9,5 a 11 segundos. Essa primeira interface em 4s pode indicar a zona de cisalhamento Aguapeí em profundidade de 11Km, a de 5s pode ser interpretada como sendo a região SIAL da crosta a 14 Km. A que varia de 9,5 (S01) a 11s (S10) corresponde a uma média de 28Km que pode associar com a descontinuidade de Conrad. Para o modelo de velocidade gerado pela análise da dispersão das ondas superficiais correlacionado aos dados obtidos através da correlação de ruídos e a função do receptor foi possível interpretar quatro interfaces. A técnica da razão espectral H/V que é um método que apresenta menor resolução a grandes profundidades da crosta sendo, portanto, aplicado a profundidades menores, apresentou como resultado valores bastante diferentes para cada estação, sendo que, para a estação mais a SW da área foi obtido valor de 3,6 metros e a estação que apresentou maior espessura, localizada no extremo NE da área foi de 3500 metros, evidenciando fortemente morfologias bastante distintas ao longo do perfil o que está correlacionado a presença de metassedimentos pertencentes à Faixa Móvel Aguapeí.
Seismic techniques such as receiver function, surface wave dispersion, seismic noise correlation and H / V spectral ratio are increasingly studied and used for geological knowledge of the crust. It is possible to determine the average crustal composition and the main contacts obtained by strong impedance contrasts as upper and lower crust boundary, as well as, lower crust limit and upper mantle (Moho discontinuity). The knowledge of the speed of travel of the waves of an earthquake recorded through a seismographic station provides information, such as: the travel time, Vp / Vs ratio and others. The region of Pontes e Lacerda / MT is located in the Southwest of the Amazonian craton and the data were taken in different phases with the implantation of seismometers aligned in a perpendicular direction (NE-SW direction) to the main structures of the Faixa Móvel Aguapeí (direction NNW- SSE). In addition to collecting data from these temporary stations, earthquake data were collected for the PTLB fixed station. The average thickness found in the crust for the region did not vary much for the different techniques, and for the Receiver Function (PTLB fixed station), a value of approximately 44 Km of thickness was obtained and for the temporary stations (S01 to S10) average value of 42 km. The seismic section generated by the correlation of the signals for all short-time temporary stations brought the signals coherence in 4s, 5s and later coherence ranging from 9.5 to 11 seconds. This first interface in 4s can indicate the Aguapeí shear zone in depth of 11Km, that of 5s can be interpreted as being the SIAL region of the crust at 14 Km. A that varies from 9.5 (S01) to 11s (S10) corresponds to an average of 28Km that can associate with the discontinuity of Conrad. For the velocity model generated by the analysis of the dispersion of the surface waves correlated to the data obtained through the noise correlation and the receiver function, it was possible to interpret four interfaces. The H / V spectral ratio technique, which is a method that presents lower resolution at great depths of the crust and is therefore applied at lower depths, presented as result quite different values for each station, being that for the station plus SW of area was obtained a value of 3.6 meters and the station that presented the greatest thickness, located at the extreme NE of the area was of 3,500 meters, evidencing strongly morphologies quite distinct along the profile which is correlated the presence of metassedimentary belonging to the Faixa Móvel Aguapeí.
Seismic techniques such as receiver function, surface wave dispersion, seismic noise correlation and H / V spectral ratio are increasingly studied and used for geological knowledge of the crust. It is possible to determine the average crustal composition and the main contacts obtained by strong impedance contrasts as upper and lower crust boundary, as well as, lower crust limit and upper mantle (Moho discontinuity). The knowledge of the speed of travel of the waves of an earthquake recorded through a seismographic station provides information, such as: the travel time, Vp / Vs ratio and others. The region of Pontes e Lacerda / MT is located in the Southwest of the Amazonian craton and the data were taken in different phases with the implantation of seismometers aligned in a perpendicular direction (NE-SW direction) to the main structures of the Faixa Móvel Aguapeí (direction NNW- SSE). In addition to collecting data from these temporary stations, earthquake data were collected for the PTLB fixed station. The average thickness found in the crust for the region did not vary much for the different techniques, and for the Receiver Function (PTLB fixed station), a value of approximately 44 Km of thickness was obtained and for the temporary stations (S01 to S10) average value of 42 km. The seismic section generated by the correlation of the signals for all short-time temporary stations brought the signals coherence in 4s, 5s and later coherence ranging from 9.5 to 11 seconds. This first interface in 4s can indicate the Aguapeí shear zone in depth of 11Km, that of 5s can be interpreted as being the SIAL region of the crust at 14 Km. A that varies from 9.5 (S01) to 11s (S10) corresponds to an average of 28Km that can associate with the discontinuity of Conrad. For the velocity model generated by the analysis of the dispersion of the surface waves correlated to the data obtained through the noise correlation and the receiver function, it was possible to interpret four interfaces. The H / V spectral ratio technique, which is a method that presents lower resolution at great depths of the crust and is therefore applied at lower depths, presented as result quite different values for each station, being that for the station plus SW of area was obtained a value of 3.6 meters and the station that presented the greatest thickness, located at the extreme NE of the area was of 3,500 meters, evidencing strongly morphologies quite distinct along the profile which is correlated the presence of metassedimentary belonging to the Faixa Móvel Aguapeí.
Descrição
Palavras-chave
Função do receptor, Análise de dispersão de ondas de superfície, Correlação de sismogramas, Razão espectral H/V, Receiver function, Surface waves dispersion analysis, Correlation of seismograms, Spectral ratio H/V