3rd International Congress of the Brazilian Geophysical Society

O presente trabalho apresenta uma metodologia para integraçăo estatística supervisionada de dados. Partindo-se de um amplo conjunto de dados geofísicos e de sensoriamento remoto săo escolhidos aqueles que apresentam o maior conteúdo de informaçăo geológica, através do cálculo do coeficiente de correlaçăo com dados geológicos de referęncia. Escolhidos os parâmetres descritores parte-se para uma normalizaçăo das amplitudes, o que confere aos dados iniciais uma variância referente a uma base de amplitude fixa.

Dados aerogeofísicos săo tradicionalmente tratados sob a forma de perfis e mapas de isolinhas, porém com o auxílio dos Sistemas Geográficos de Informaçőes (SGI) sua representaçăo sob a forma de imagens raster coloridas vem tornando sua utilizaçăo e compreensăo mais imediatas. Neste artigo săo apresentadas rotinas escritas em linguagem C cornpiláveis em micro-computadores tipo IBM-PC de ambiente DOS e/ou em estaçőes de trabalho de ambiente UNIX, permitindo a interface do program a de geraçăo de dados gridados U.S. Geological Survey (USGS) com sua transformaçăo em imagens raster para uso no SGI Geographical Resources Analysis Support System (GRASS versăo 4.0).

Integration of geophysical data with geological, geothermal, and geohydrological data is essential in basin analysis. Spatial patterns produced by seismic, gravity, and magnetic data give insight into certain aspects of 'basement' configuration and composition, sediment thickness, and structure where direct measurements are not available. When combined with similar-type data derived from wells (and other sources) such as rock porosity, geothermal gradients and temperature, brine composition, etc., valuable information can be gained on the basin development and evolution through time. In turn, this is important in the location of mineral resources and for environmental considerations. An area in south-central Kansas, Midcontinent, USA is used as an example of one quantitative approach to the integration and comparison of different data sets. The area has a thin sedimentary cover over the Precambrian crystalline basement. Published data on aeromagnetics and gravity, geothermal properties, and sediment thickness and structure were used. The variables were digitized and standardized prior to the analysis. A pairwise comparison was made to determine statistically the relation between the data sets, then those data sets considered relevant were weighted and combined into resultant maps.

A paleomagnetic study carried out in the Cretaceous Matasiete and Castillo formations which outcrop in the San Bernardo fold belt (central Patagonia) gave a paleopole position: MCPP= Lat. 66.S, Long, 9·E. The correlation of this PP position with a coeval PP position from the extraandean Patagonia shows a counter-clockwise rotation about a vertical axis of the MCPP site. The rotation is in agreement with structural features present in this area.

New seismic reflection data collected by UTHOPROBE across the Trans-Hudson Orogen provide striking images of juvenile Paleoproterozoic arc rocks interposed between the deformed margins of the Archean Superior and Hearne cratons. Along the western half of this collisional tectonic belt the crustal architecture is roughly symmetric about a crustal-scale culmination within the accreted juvenile collage, which geological and isotopic data suggest may be cored by Archean basement. West of the culmination, highly reflective packages dip consistently westward into the lower crust, beneath the straddling Wathaman batholith and reworked Archean continental margin of the Hearne craton. On the eastern flank, crustal imbrication on a scale imaged in few other orogens is observed within the juvenile Flin Flon Belt, where a package of shallowly east-dipping reflections extends from the surface to 14 s; sheets of middle to lower crustal arc rocks have been stacked below a major detachment that carries the upper crustal remnants of an island arc complex. Surprisingly, the seismic images show the juvenile arc rocks dipping moderately eastward beneath the craton in contrast to existing tectonic models. The east-dipping reflection fabric, marking Hudsonian tectonic overprint, extends across the Superior Boundary Zone to the Pikwitonei Granulite Belt where upper crustal reflections are west-dipping. An east-dipping boundary between these domains, which soles into the mid-crust, may represent a westverging thrust fault along which the crust of the Archean Superior craton was uplifted. A well-defined Moho, laterally continuous for >500 km, shows considerable relief (38-47 Ion depth), including a prominent root beneath the crustal culmination. The Imaged structures are thought to reflect lateto post-collisional defonnation rather than early subduction polarity.

O presente trabalho tem como objetivo estudo integrado, envolvendo especialmente paleomagnetismo e geocronologia, de enxame de diques máficos do Craton do Rio de La Plata, no Oeste do Uruguai. Serăo apresentados resultados geocronológicos (dataçőes Rb/Sr, K-Ar e 40Ar/39Ar) e paleomagnéticos preliminares obtidos e as interpretaçőes tectônicas obtidos e as interpretaçőes tectônicas preliminares decorrentes.

In this paper we apply a maximum entropy (ME) approach to the inversion of linear geophysical data, with discrete data and continuous model parameters. This formulation is based on a probabilistic philosophy and on the concept of entropy, where we do not make use of prior information. It is defined an objective function which cont.ains the entropy of the probability density function of the model parameters. The objective function is then minimized under adequate constraints in order to give the output estimate of the model parameters. The tests with synthetic data in travel time tomography show the promissing application of the ME technique. The results with ME are better, both on qualitative and quantitative senses, when compared to SVD.

New gravity data in southern Brazil are combined with previous data in Brazil and Uruguay, permitting detailed analysis of the basement structure. The Brasiliano cycle tectonic framework suggested by previous geological studies is generally confirmed. Pre-Brasiliano structure (Transamazonian cycle), however, is an important and hitherto unstated source of lateral density contrast responsible for distinctive gravity discontinuities. Individual blocks with the Rio de La Plata craton are recognised and a new block, the Paso del los Toros Block, is defined.

En la región norte de la provincia de Mendoza (Argentina), se ha realizado un estudio de atenuación sísmica utilizando el factor de calidad Q, el cual ha sido estimado a partir del decaimento de las ondas coda (Qc) para una frecuencia de aproximadamente 1 Hz (Qc). Este parámetro, evalua los efectos de la absorción intrinseca y de la dispersión debida a heterogeneidades del medio. Es importante advertir que, valores bajos de Qc significan alta atenuación y valores altos de Qc se refieren a una baja atenuación.

During the last six years the Institute of Geophysics, University of Hamburg (IFG) has been engaged in several wide-angle reflection and refraction seismic (WARRS) studies of the European margin west of Ireland and the British Isles (fig. 1). In particular, the cooperation between the German and the Irish through the agreement between the IFG and DIASI has given us the opportunity to investigate the crustal structure and basin development along a 1000 kill line between the west coast of Ireland and the Iceland Basin. Four 'R/V Valdivia' cruises were required for this study which produced more than 200 Ocean Bottom Seismograph (OBS) positions. Our main aims were to resolve the problems of the oceanisation of the Rockall Trough (Roberts, 1975; Roberts et al., 1981; Joppen & White, 1990), to test the concept of 'underplating' at the continent-ocean transition from the Hatton-Rockall Basin to the oceanic crust of the Iceland Basin (White & McKenzie, 1989; Morgan et al., 1989) and to resolve the velocity structure of the 'dipping reflectors' at the edge of the Hatton-Rockall Basin (White et al., 1987).

The Brazilian Lithosphere Seismic Project is part of a multinational geological and geophysical program to investigate the structure, tectonics, and geologic evolution of the continental lithosphere beneath SE Brazil. The project, which will run for a period of at least 18 months, is a collaborative effort by the Carnegie Institution of Washington and the Institute of Astronomy and Geophysics of the University of Sao Paulo. The broad band seismic studies are aimed at mapping the fine structure of the continental crust and lithospheric mantle beneath Brazil and interpreting it in terms of the origin, evolution, and composition of the Brazilian shield. The first nine stations of a planned 14 station portable array of digital REFTEK dataloggers and the broad band threecomponent Streckeisen STS-2 seismometers have been installed, or are in the process of being installed, in southeast Brazil (see Figure 1). The seismic array traverses the four major tectonic provinces of the region --- the Archean Sao Francisco craton, the surrounding late Proterozoic BrasilianolPan-African mobile belts, the intracratonic Parana Basin, and the coastal Ribeira belt, a complex transcurrent shear system. Stations are configured about 50-100 km apart in an approximately east-west saw-tooth array along 200 S latitude, with an E-W aperture of approximately 800 km. The station configuration is designed to capitalize on the abundance of high-quality seismic sources, particularly Andean, that surround and illuminate the Brazilian shield. Digital data are recorded continuously at 10 slsec and in triggered mode at 50 slsec. Timing is via GPS clocks and is accurate to better than 1 msec absolute. The matched STS-2 three component recording covers the frequency band 0.0083 to 20 Hz. Preliminary results from receiver function analysis indicate that crustal thicknesses beneath the Sao Francisco craton and surrounding Archean crust are about 40 to 42 km. Crustal velocities in these regions increase approximately linearly with depth and there is no evidence of major midcrustal discontinuities. Results from receiver functions and from two station surface wave phase velocity inversion indicate that mantle shear wave velocities are high, about 4.8 to 4.9 krn/sec as in other cratonic areas, in the uppermost mantle beneath the Sao Francisco craton and surrounding area and to at least 130 km depth beneath the Parana Basin.

O Enxame de Diques Rio Ceará Mirim (EDCM) situa-se no nordeste do Nordeste Oriental do Brasil e constitui uma entidade alongada E-W em forma de leque, que se extende por mais de 500km longitudinalmente, e que tem, na sua porçăo ocidental, uma largura de mais de 100km. Os diques do EDCM distribuem-se desde o oeste de Natal (RN) até o centro do Estado do Ceará (figura 1).

This paper describes, in a preliminary form, the new, extensive program of deep seismic reflection lines using special acquisition equipment and processing to 18 s TWT (seconds, two-way travel time) using state-of-the art seismic procedures. This program started in 1989 and aims at comparing crustal structures of different basins along the continental margin of northern and northeastern Brazil, a region with contrasting styles of deformation. The rifting process that resulted in the break-up of Pangea originated both Atlantic-type passive margin basins and sheared passive margin basins, with different crustal geometries and subsidence patterns. The regional seismic lines will also help in the evaluation of geological features that are difficult to characterize using standard petroleum industry seismic surveys. Besides tectonic studies that will focus on the geological interpretation and understanding of basinforming processes, the program will also provide a more reliable visualization of rift-phase structures In the lower part of the sedimentary sequence, both in shallow and deep water regions, and the regional characterization of oceanIc fracture zones, intraplate magmatism and halokinetic/lutokinetic structures.

La Sa. De Pie de Palo pertenece al sistema de Sas. Pampeanas, es un afloramiento aislado con forma de escudo elongado en dirección aproximada N-S, de unos 30 Km de ancho por 78 Km de largo. Se extiende desde los 31° hasta los 31°42' de latitud S, y desde los 67°42' hasta los 68°8' de longitud W (Fig. 1).

Se analiza en este trabajo una nueva sección gravimétrica en 32° Sur (Figura 1) preparada con datos de distintas fuentes: a - Universidad Nacional de San Juan, Universidad Nacional de Rosario e Instituto Geográfico Militar Argentino, para el sector Argentino. b - Para el sector Chileno se extrajeron de Introcaso-Pacino (1988), en el cual para el tramo oceánico, las anomalías de Aire Libre fueron transformadas en anomalías de Bouguer sustituyendo el agua y los sedimentos por masas de igual volumen y densidad 2,8 g/cc.

Using the three interface Coherence Function model of Bechtel et al. (1987), the effective elastic plate thickness, Te of the shield area in southern Brazil-Uruguay has been determined. The Tevalue of 114 km corresponds closely with values obtained for similar tectonic provinces. Intercrustal loads are largely the legacy of the Brasiliano cycle shield geology, whereas the eastwest Moho topography is best explained in terms of both flexural buckling origins of the Chaco-Parana Basin and crustal thinning related to the rifting of the South Atlantic.

Em geral os estudos de temperatura do solo săo feitos com metodologias voltadas para os problemas agricolas e se limitam até um metro de profundidade (de Vries. 1975; Ribeiro e Santos, 1975; Ghuman and Lal, 1982; Santos et al., 1988). Por outro lado os estudos de sondagens convencionais visam a determinaçăo do fluxo geotermal profundo e portanto evitam a camada influenciada pelo aquecimento solar, isto é, as perfilagens se iniciam a partir de 30 a 40 metros de profundidade.

O fenômeno da ressurgęncia costeira que ocorre mais intensamente nos meses de verăo na regiăo de Cabo Frio manifesta-se pela ocorręncia de águas superficiais provenientes das camadas profundas, mais frias e ricas em nutrientes. Este fenômeno constitue o suporte físico de uma cadeia biológica: nutrientes-fitoplancton-hervívoros, etc., de alta produtividade. O mecanismo básico da ressurgęncia foi proposto inicialmente por Thorade (1909), como uma aplicaçăo da teoriade Ekman (1905). Este modelo propőe como agente causal, o esforço produzido pela componente de vento paralela a costa (componente leste do vento para o caso de Cabo Frio). O desenvolvimento do mecanismo de Ekman está associado ŕ ocorręncia de uma corrente superficial paralela ŕ costa a favor do vento.

As camadas mais superficiais do subsolo amazônico săo predominantemente constituidas por latosolo argiloso amarelo. Apesar do seu alto teor de argila o subsolo é poroso, o que permite a rapida percolaçăo o da precipitaçăo pluviométrica na superficie. Por outro lado, em regiőes desmatadas observa-se uma compactaçăo deste latosolo (Silva et al, 1992), aumento do escoamento das águas pluviais e elevaçăo de cerca de 5° Celsius da temperatura média á superficie (Souza et al., 1993). Esse processo de aquecimento, rebaixamento do nivel freático e perda de porosidade do solo e subsolo amazônicos constitui um processo de intemperismo que leva a formaçăo de blocos de rocha lateritica que pode inutiliza-lo para agricultura e outros objetivos econômicos.

A exposiçăo dos seres humanos ŕ radiaçăo ionizante ambiental pode ocorrer por via externa e interna. No primeiro caso, os raios cósmicos e as radiaçőes provenientes de radionuclídeos presentes no solo incidem no corpo de fora para dentro. No segundo caso, os elementos radioativos do solo săo transferidos para o corpo dos seres humanos através da ingestăo de água e alimentos e da respiraçăo do ar, vindo a acumular em órgăos específicos de onde irradiam o corpo de dentro para fora. Dessa forma, a estimativa da dose de radiaçăo do solo é de importância básica na dosimetria do meio ambiente e na proteçăo radiológica.

Há vários anos, métodos geofísicos de prospecçăo mineral vęm sendo utilizados com a finalidade de detectar e monitorar a contaminaçăo provocada por depósitos e lixo industrial e doméstico. Seitz et al. (1972), Stollar et al. (1975), Kelly (1976), Urish (1983), entre outros, utilizaram o método eletrorresistividade, em diversas áreas, para determinar a poluiçăo das águas subterrâneas. Klefstad et al. (1975) abordou as limitaçőes deste método quanto a sua resoluçăo e precisăo.

The evolution of linear processing methods is first described and illustrated with seismic data examples. A brief review of current multiple suppression techniques is presented. The possibility of using non-linear inverse scattering for multiple attenuation is described. The procedure is applied to the suppression of multiples and illustrated with synthetic and real data examples.

A utilizaçăo de dados de altimetria por satélite está trazendo grandes contribuiçőes nos estudos da estrutura e evoluçăo do assoalho oceânico. Os dados da missăo SEASAT, na forma de modelo digital com resoluçăo de 5'x5', permitem analisar feiçőes tectônicas importantes onde năo existem dados de geofisica marinha, ou mesmo integrando- se a outros dados, geralmente disponíveis ao longo de trilhas esparsas. Neste trabalho, os dados gravimétricos e batimétricos do projeto EQUANT II foram utilizados para determinar a resoluçăo efetiva dos dados do SEASAT na forma de modele digital, e para estudar o problema da perda de precisăo dos mesmos em regiőes de pequenas profundidades, como ocorre nas margens continentais.

O Plano de Levantamento da Plataforma Continental Brasileira (Projeto LEPLAC) tem como objetivo estabelecer o limite exterior da Plataforma Continental Brasileira, de acordo com os critérios estabelecidos pela Convençăo das Naçőes Unidas sobre o Direito do Mar (CNUDM), conhecida na literatura de língua inglesa como United Nations Convention on the Law of the Sea (UNCLOS). Denominamos LEPLAC EQUATORIAL o conjunto dos LEPLACs III, V, VI e VIIIA, que cobre a área offshore dos estados do Amapá, Pará, Maranhăo, Piauí, Ceará e Rio Grande do Norte, em uma faixa média de 600 km de largura, com cerca de 15 000 krn de linhas geofísicas (Fig. 1).