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.