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Previous issue date: 2014-03-06 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico - CNPq / O ru?do s?smico tem sido tradicionalmente considerado como uma
perturba??o n?o desejada do ambiente que ?contamina? a aquisi??o de dados
de terremotos. Mas ao longo da ?ltima d?cada tem sido mostrado que
informa??es coerentes sobre a estrutura do subsolo podem ser extra?das a
partir de correla??es cruzadas do ru?do s?smico de ambiente. Neste contexto,
as regras s?o reversas, sendo os terremotos o que necessitamos excluir dos
dados. Os principais causadores do ru?do s?smico de ambiente s?o os
microssismos oce?nicos e perturba??es atmosf?ricas. A per?odos menores
que 30 s, o espectro do ru?do s?smico de ambiente ? dominado por energia
micross?smica. O microssismo ? o sinal s?smico mais cont?nuo da Terra e
pode ser classificado como prim?rio (observado na faixa 10-20 s) e
secund?rio (observado na faixa 5-10 s). A fun??o de Green do meio de
propaga??o entre dois receptores pode ser reconstru?da atrav?s da
correla??o cruzada do ru?do s?smico de ambiente registrado simultaneamente
nesses dois receptores. A reconstru??o da fun??o de Green ? geralmente
proporcional ? por??o de ondas de superf?cie do campo de onda s?smico, j?
que a energia micross?smica viaja principalmente como ondas de superf?cie.
Neste trabalho, s?o apresentadas 194 fun??es de Green obtidas a partir
de correla??es cruzadas de 1 m?s de registro da componente vertical do
ru?do s?smico de ambiente para diferentes pares de esta??es s?smicas do
Nordeste do Brasil. As correla??es cruzadas di?rias foram empilhadas
utilizando a t?cnica n?o linear tf-PWS que real?a sinais coerentes fracos
atrav?s da redu??o de ru?do incoerente. As correla??es cruzadas mostram
que o sinal emergido ? dominado por ondas Rayleigh nas componentes
verticais e que as velocidades de dispers?o podem ser medidas
confiavelmente para uma faixa de per?odos entre 5 e 20 s. O estudo inclui
tanto esta??es permanentes para monitoramento s?smico, quanto esta??es
tempor?rias de experimentos passivos na regi?o, formando uma rede
combinada de 33 esta??es separadas por dist?ncias entre 60 e 1311 km,
aproximadamente. Estas medidas de velocidades de dispers?o de ondas
Rayleigh em seguida s?o usadas na elabora??o de imagens tomogr?ficas da
Prov?ncia Borborema do Nordeste do Brasil. As tomografias de ru?do s?smico
obtidas aqui permitem mapear satisfatoriamente fei??es estruturais existentes
na regi?o. As imagens tomogr?ficas de per?odos mais curtos (~5 s) mostram a
estrutura crustal rasa e claramente definem as bacias sedimentares marginais
e intracontinentais, bem como as partes de zonas de cisalhamento
importantes que atravessam a Prov?ncia Borborema. As imagens
tomogr?ficas de per?odos mais longos (10 - 20 s) atingem profundidades da
crosta superior e a maior parte das anomalias desaparece. Algumas delas
localizada no interior da Prov?ncia Borborema, no entanto, persistem. A
evolu??o Cenoz?ica da Prov?ncia Borborema foi marcada por epis?dios de
vulcanismo Cenoz?ico e eleva??o, mas nenhuma correla??o ? observada
com estas caracter?sticas Cenoz?icas e as anomalias profundas. As
anomalias n?o se correlacionam com mapas dispon?veis de fluxo de calor
superficial na Prov?nica Borborema, e a origem das anomalias profundas
permanece enigm?tica. / Ambient seismic noise has traditionally been considered as an unwanted
perturbation in seismic data acquisition that "contaminates" the clean
recording of earthquakes. Over the last decade, however, it has been
demonstrated that consistent information about the subsurface structure can
be extracted from cross-correlation of ambient seismic noise. In this context,
the rules are reversed: the ambient seismic noise becomes the desired
seismic signal, while earthquakes become the unwanted perturbation that
needs to be removed. At periods lower than 30 s, the spectrum of ambient
seismic noise is dominated by microseism, which originates from distant
atmospheric perturbations over the oceans. The microsseism is the most
continuous seismic signal and can be classified as primary ? when observed
in the range 10-20 s ? and secondary ? when observed in the range 5-10 s.
The Green?s function of the propagating medium between two receivers
(seismic stations) can be reconstructed by cross-correlating seismic noise
simultaneously recorded at the receivers. The reconstruction of the Green?s
function is generally proportional to the surface-wave portion of the seismic
wavefield, as microsseismic energy travels mostly as surface-waves.
In this work, 194 Green?s functions obtained from stacking of one month
of daily cross-correlations of ambient seismic noise recorded in the vertical
component of several pairs of broadband seismic stations in Northeast Brazil
are presented. The daily cross-correlations were stacked using a timefrequency,
phase-weighted scheme that enhances weak coherent signals by
reducing incoherent noise. The cross-correlations show that, as expected, the
emerged signal is dominated by Rayleigh waves, with dispersion velocities
being reliably measured for periods ranging between 5 and 20 s. Both
permanent stations from a monitoring seismic network and temporary stations
from past passive experiments in the region are considered, resulting in a
combined network of 33 stations separated by distances between 60 and 1311
km, approximately.
The Rayleigh-wave, dispersion velocity measurements are then used to
develop tomographic images of group velocity variation for the Borborema
Province of Northeast Brazil. The tomographic maps allow to satisfactorily
map buried structural features in the region. At short periods (~5 s) the images
reflect shallow crustal structure, clearly delineating intra-continental and
marginal sedimentary basins, as well as portions of important shear zones
traversing the Borborema Province. At longer periods (10 ? 20 s) the images
are sensitive to deeper structure in the upper crust, and most of the shallower
anomalies fade away. Interestingly, some of them do persist. The deep
anomalies do not correlate with either the location of Cenozoic volcanism and
uplift - which marked the evolution of the Borborema Province in the Cenozoic
- or available maps of surface heat-flow, and the origin of the deep anomalies
remains enigmatic.
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufrn.br:123456789/19947 |
| Date | 06 March 2014 |
| Creators | Dias, Rafaela Carreiro |
| Contributors | 70030929482, http://lattes.cnpq.br/0012168139768170, Nascimento, Aderson Farias do, 90376285400, http://lattes.cnpq.br/8600906973888297, Assump??o, Marcelo Sousa de, 80543855872, http://lattes.cnpq.br/2833240474723324, Casas, Jordi Juli? |
| Publisher | Universidade Federal do Rio Grande do Norte, PROGRAMA DE P?S-GRADUA??O EM GEODIN?MICA E GEOF?SICA, UFRN, Brasil |
| Source Sets | IBICT Brazilian ETDs |
| Language | Portuguese |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/masterThesis |
| Source | reponame:Repositório Institucional da UFRN, instname:Universidade Federal do Rio Grande do Norte, instacron:UFRN |
| Rights | info:eu-repo/semantics/openAccess |
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