2nd International Congress of the Brazilian Geophysical Society

A sequence of seismic events occurred near Taperuaba, district of Sobral-CE, following the main event of April 19 that reached magnitude 4.8 mb, the second largest ever recorded in this state. A cooperation effort among the Seismological Observatory, the Special Secretary of Civil Defense and the Municipality of Sabral was done to study these events within the period April 20 to 24. Macrosseismic survey was also realized and the maximum intensity found was VI (Modified Mercalli Scale). The data recorded in four local stations were analyzed and appointed a low activity confined in a narrow zone with NW-SE linear trend, and depths between 7 and 11 km. A composite focal mechanism, using the main event and its aftershocks, indicates a dextral strike-slip motion with small component of normal faulting. The NW-SE P axis is inconsistent with the roughly E-W stress field orientation for the northeastern region and may indicate local process related to the Potiguar Basin.

A seismic zone in Goias state was described by several authors, trying to relate it with geological features, although unsuccessfully due to the lack of informations. We use here the first event with mechanism solution occurred in Porangatu in March 1991. The focal mechanism showed a 30° dextral slip fault with P axis orientation ENE -WSW. This orientation and the strike-slip motion accords to the Transbrasiliano lineaments associated to collisional process occurred during Late Proterozoic. Association with gravimetric and magnetic survey is, also analyzed.

A magnitude sísmica e um parâmetro útil nos estudos de sismicidade, que permite avaliar o nível e as características da atividade sísmica associados aos processos geodinâmicos de uma regiăo. Os estudos de sismicidade requerem o conhecimento completo da série histórica de sismos ocorridos numa regiăo. No caso da America do Sul, o catálogo é incompleto, principalmente para as tręs primeiras decadas deste seculo, pois contem somente as sismos de maior magnitude que eram detectados pela rede internacional de sismógrafos, composta de cinco estaçőes neste continente (Gutenberg & Richter, 1949 e Presgrave el; at, 1985). Com a finalidade de reduzir a falta de dados de magnitude dos católogos sismológicos suI-americanos, foram calculadas, neste trabalho, as magnitudes Ms (onda; superficiais) e mb (ondas intemas), utilizando as leituras de amplitude e período de sismogramas e de boletins sismológicos correspondente ao intervalo 1923-1932, de sismos que ocorreram na regiăo delimitada pelos paralelos 30°N e 70°S e pelos meridianas 0° a 120°W..

El factor de calidad de la atenuación Q está estrechamente relacionado con propiedades geológicas y geofísicas de la corteza tales como vulcanismo, edad de la actividad tectónica, variación de flujo de calor, conductividad eléctrica y velocidad de las ondas sísmicas. Los diversos estudios del factor de calidad de la atenuación obtenidos a partir del decaimiento de la coda sísmica (Qc), muestran que valores altos de Qc corresponden a zonas tectónicamente estables, mientras que valores bajos de Qc están relacionados con zonas tectónicamente activas (Singh y Herrmann, 1983). Esto muestra que Qc es un parámetro que refleja la actividad tectónica de una región.

A Convençăo das Naçőes Unidas sobre o Direito do Mar (CNUDM), resultado de trabalho profícuo e extenso da comunidade internacional, e que contou com participaçăo efetiva e atuante do Brasil, foi aberta ŕ assinatura em 10 de dezembro de 1982, em Montego Bay, Jamaica. Nosso país assinou a CNUDM naquela mesma data, junto com outros 118 países. Mais recentemente, em 22 de dezembro de 1988, o Brasil depositou seu instrumento de ratificaçăo, e hoje a CNUDM já conta com um total de 45 ratificaçőes das 60 necessárias ŕ sua entrada em vigor (Tabela I).

Integration of surface seismic and borehole sonic data processing and display techniques provides a means for a detailed description of rock properties such as dipping boundaries, fractures, and permeability. The seismic attributes analysis and the color-coded displays which are adapted for investigation of full wave sonic (FWS) responses of formations reveal additional FWS characteristics helpful in describing the formation near the borehole. In this application the seismic attributes have distinct geophysical meanings Which are specifically pertinent to FWS logging and are called Instantaneous Waveform Characteristics, or IWC. The color-coded displays are called colorgrams. The IWC displayed in the colorgrams are also quantified, plotted as log curves, and interrelated to give useful geophysical parameters such as the quality factor (Q) of the formation. The application of the IWC analytical method is demonstrated with colorgrams from four different wells. All these examples illustrate how the IWC method, enhanced with colorgrams, broadens our analytical understanding of FWS well log data and assists in geological interpretation. It provides valuable information useful for the integration of borehole sonic and seismic data as well as for the development of cross-borehole tomography.

O levantamento geofísico do Prospecto LEPLAC-I situa-se entre as latitudes 21°S e 31°S e as longitudes 33°W e 42°W (Fig. I) e abrange, principalmente, as regiőes da elevar;ao continental e da planície abissal da margem continental sudeste brasileira. Nesta etapa do trabalho procurou-se mapear as principais feiçőes regionais e dimensionar a espessura do pacote sedimentar. A modelagem gravimétrica de algumas linhas visou esclarecer a problemática do limite crosta continental/oceânica nesta regiăo.

Os primeiros estudos da plataforma continental brasileira visando a pesquisa de hidrocarbonetos ocorreram em 1957. Porém, a exploraçăo na porçăo marítima das bacias costeiras somente pode ser iniciada na década de 60 com a chegada de equípamentos adequados. Desde entăo, grande quantidade de dados exploratórios (sismica de reflexăo, gravimetria, magnetometria, amostras de calha, testemunhos, perfis elétricos de poços) tęm sido coletada, hoje integrando o acervo do Departamento de Exploraçăo da Petrobras (DEPEX).

A few deep seismic reflection profiles, using conventional acquisition equipment, were shot in Campos Basin in the last decade. These lines (Seismic Survey 203) were processed to 10 s TWT (two-way travel time) using standard processing techniques. Early in 1990, Petrobrás approved an extensive program of deep seismic reflection lines offshore the Brazilian Atlantic margin (Seismic Survey 239). The lines of this program were shot in the first semester of 1990 using special equipment onboard the seismic ship GECO ALPHA. In the Southeastern region of Brazil (from Espírito Santo to Rio Grande do Sul), the total coverage amounts to 4884 km. Briefly and in a preliminary form, some of the main features to be investigated by this survey will be discussed. Processing, which includes migration to 18 s TWT, was initiated in the second semester of 1990 and will be completed by 1991.