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We present a new, stable method for interpreting the basement relief of a sedimentary basin which delineates sharp discontinuities in the basement relief and incorporates any law known a priori for the spatial variation of the density contrast. The subsurface region containing the basin is discretized into a grid of juxtaposed elementary prisms whose density contrasts are the parameters to be estimated. Any vertical line must intersect the basement relief only once, and the mass deficiency must be concentrated near the earth’s surface, subject to the observed gravity anomaly being fitted within the experimental errors. In addition, upper and lower bounds on the density contrast of each prism are introduced a priori (one of the bounds being zero), and the method assigns to each elementary prism a density contrast which is close to either bound. The basement relief is therefore delineated by the contact between the prisms with null and nonnull estimated density contrasts, the latter occupying the upper part of the discretized region. The method is stabilized by introducing constraints favoring solutions having the attributes (shared by most sedimentary basins) of being an isolated compact source with lateral borders dipping either vertically or toward the basin center and having horizontal dimensions much greater than its largest vertical dimension. Arbitrary laws of spatial variations of the density contrast, if known a priori, may be incorporated into the problem by assigning suitable values to the nonnull bound of each prism. The proposed method differs from previous stable methods by using no smoothness constraint on the interface to be estimated. As a result, it may be applied not only to intracratonic sag basins where the basement relief is essentially smooth but also to rift basins whose basements present discontinuities caused by faults. The method’s utility in mapping such basements was demonstrated in tests using synthetic data produced by simulated rift basins. The method mapped with good precision a sequence of step faults which are close to each other and present small vertical slips, a feature particularly difficult to detect from gravity data only. The method was also able to map isolated discontinuities with large vertical throw. The method was applied to the gravity data from Reco⁁ncavo basin, Brazil. The results showed close agreement with known geological structures of the basin. It also demonstrated the method’s ability to map a sequence of alternating terraces and structural lows that could not be detected just by inspecting the gravity anomaly. To demostrate the method’s flexibility in incorporating any a priori knowledge about the density contrast variation, it was applied to the Bouguer anomaly over the San Jacinto Graben, California. Two different exponential laws for the decrease of density contrast with depth were used, leading to estimated maximum depths between 2.2 and 2.4 km.

Euler deconvolution has been widely used in automatic aeromagnetic interpretations because it requires no prior knowledge of the source magnetization direction and assumes no particular interpretation model, provided the structural index defining the anomaly falloff rate related to the nature of the magnetic source, is determined in advance. Estimating the correct structural index and electing optimum criteria for selecting candidate solutions are two fundamental requirements for a successful application of this method. We present a new criterion for determining the structural index. This criterion is based on the correlation between the total‐field anomaly and the estimates of an unknown base level. These estimates are obtained for each position of a moving data window along the observed profile and for several tentative values for the structural index. The tentative value for the structural index producing the smallest correlation is the best estimate of the correct structural index. We also propose a new criterion to select the best solutions from a set of previously computed candidate solutions, each one associated with a particular position of the moving data window. A current criterion is to select only those candidates producing a standard deviation for the vertical position of the source smaller than a threshold value. We propose that in addition to this criterion, only those candidates producing the best fit to the known quantities (combinations of anomaly and its gradients) be selected. The proposed modifications to Euler deconvolution can be implemented easily in an automated algorithm for locating the source position. The above results are grounded on a theoretical uniqueness and stability analysis, also presented in this paper, for the joint estimation of the source position, the base level, and the structural index in Euler deconvolution. This analysis also reveals that the vertical position and the structural index of the source cannot be estimated simultaneously because they are linearly dependent; the horizontal position and the structural index, on the other hand, are linearly independent. For a known structural index, estimates of both horizontal and vertical positions are unique and stable regardless of the value of the structural index. If this value is not too small, estimates of the base level for the total field are stable as well. The proposed modifications to Euler deconvolution were tested both on synthetic and real magnetic data. In the case of synthetic data, the proposed criterion always detected the correct structural index and good estimates of the source position were obtained, suggesting the present theoretical analysis may lead to a substantial enhancement in practical applications of Euler deconvolution. In the case of practical data (vertical component anomaly over an iron deposit in the Kursk district, Russia), the estimated structural index (corresponding to a vertical prism) was in accordance with the known geology of the deposit, and the estimates of the depth and horizontal position of the source compared favorably with results reported in the literature.

A integracao de 37.435 dados gravimetricos (11.814 em terra e 25.621 no mar), provenientes de varios levantamentos independentes, permitiu a confeccao de um mapa de anomalias Bouguer do Nordeste Setentrional do Brasil, com densidade media de cerca de 1 estacao gravimetrica por cada 13 km2. Este mapa esta limitado pelas longitudes 33° 30' W e 43° W e pelas latitudes 2° 55' S e 7° 30' S, cobrindo uma area com cerca de 1.000 km na direcao leste-oeste por 500 km na direcao norte-sul. O mapa abrange a porcao da Provincia Borborema que esta a norte do lineamento Patos, bem como a parte leste da Bacia do Parnaiba e a margem continental adjacente. A anomalia mais expressiva do mapa Bouguer e um forte gradiente, margeando toda a costa, originado pela subida da Moho na transicao da crosta continental para a crosta oceânica, implantada na separacao America do Sul - Africa. As anomalias de medio a curto comprimento de onda sao causadas por heterogeneidades intracrustais, representadas por blocos de idade precambriana, bem como granitoides brasilianos e bacias sedimentares mesozoicas. A utilizacao da hipotese de isostasia, associada a um modelo flexural, permitiu estimar, em carater preliminar, o relevo da interface crosta-manto, com delimitacao da zona de afinamento crustal associada a implantacao do trend Cariri-Potiguar, definido pelos riftes intracontinentais mesozoicos. Alem disso, foi identificada uma area de relativo espessamento crustal, na borda leste da Bacia do Parnaiba, que foi interpretada como estando associada a uma antiga colisao de blocos, durante o Ciclo Brasiliano. O mapa de anomalias isostaticas residuais, que ressalta a contribuicao ao campo gravimetrico das bacias interiores, dos macicos granitoides e das faixas de supracrustais proterozoicas, permite identificar importantes variacoes laterais de densidade, que podem estar associadas com cisalhamentos de escala litosferica e limites dos terrenos que compoem o arcabouco estrutural da Provincia Borborema.

We present a gravity interpretation method for estimating the relief of an arbitrary interface separating two homogeneous media. The upper medium is discretized into rectangular, juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated from the gravity anomaly. The density contrast of each prism is assumed to be constant and known. To stabilize the inversion, we introduce two kinds of constraints on the depths. The first one requires proximity between the observed and computed depths at isolated points such as those obtained from boreholes (absolute equality constraint). The second one requires that groups of depths approximately follow an established linear relationship among the depths (relative equality constraint). Both kinds of constraints are imposed in the least‐squares sense. We illustrate the method performance by applying it to a synthetic anomaly produced by a simulated basement relief consisting of four narrow and adjacent structural lows. Only two structural lows produced isolated gravity lows. Nonetheless, the whole basement topography was successfully reconstructed with an average error of 4% of the maximum relief amplitude. In this example, the relative constraints established that the thicknesses of adjacent prisms should be as close as possible to each other (overall relief smoothness). As absolute constraints we used point information about the basement depth at five points. In addition to the overall relief smoothness, other relevant geologic information, such as localized relief smoothness, occurring at structural terraces, for example, can be incorporated by assigning different weights to the relative equality constraints. The method was applied to the gravity anomaly of Recôncavo Basin, Brazil, leading to a sharper definition of the basement features relative to previous gravity interpretations of this area.

Current inversion methods estimating a physical property on a grid use two kinds of constraints: (1) absolute constraints requiring that the solution vector be close to a prespecified vector as, for example, proximity to the null vector in the ridge regression, and (2) relative constraints requiring that the elements of the solution vector be close to each other as, for example, smoothness imposed by methods requiring that spatial derivatives of the physical property be continuous. Using just absolute constraints may produce a biased solution conflicting with some true geological attributes of the source. On the other hand, using just relative constraints may be insufficient to stabilize the inversion. We present a stable inversion method incorporating absolute constraints only at points where the physical property is known as, for example, outcroppings and boreholes. Elsewhere, relative constraints are introduced according to a linear relationship for the spatial distribution of the physical property. The proposed method is analogous to an interpolation method applied to the physical property distribution: the interpolating function must satisfy some property, like continuity, and pass through the data. The proposed method is versatile in incorporating not only factual, but also indirect information such as continuity, symmetry, and trends. In addition, it may be applied both to linear and nonlinear problems. The relevant results obtained from applying the method to synthetic potential‐field data were: (1) ability to produce reliable mappings of the basement relief of a sedimentary basin (from gravity data) and of an areal distribution of magnetization, using only smoothness constraints and restricted surface and borehole information; (2) operational simplicity because of the solution insensitivity to the damping parameter. The method is also applied to the Bouguer anomaly over San Jacinto Graben, California, producing results consistent with previous interpretations.

Magnetic interpretations are usually carried out either by assuming induced magnetization and estimating the model geometry, or by presuming a known source spatial orientation to estimate the total magnetization. We present a 3-D magnetic interpretation method that estimates simultaneously the total magnetization direction and the spatial orientation of the source. It is based on the approximation of the anomaly by the series derived from expanding the magnetic potential into multipoles and retaining source moments up to second order. The moments and linear combinations of moments appearing in the series are then inverted from the magnetic anomaly. The total magnetization is assumed constant in direction but not in magnitude. It is also presumed implicitly that the anomalous distribution of magnetization intensity has nonzero values in a finite‐volume region, is far from the observation points, and presents three othogonal planes of symmetry intersecting at the center of the dipole moment. The method is essentially linear and requires no a priori explicit assumption of a fixed geometry for the sources. The method is particularly suited to interpret compact, isolated or disjoint, but spatially correlated sources. If the source satisfies all assumptions presumed by the method, it is possible to obtain accurate, stable estimates of the total dipole moment vector, the position of the center of dipole moment, and the directions of all three principal axes of symmetry. If the source is not far from the observation plane and/or if the total magnetization direction is not constant, it is still possible to obtain accurate and stable estimates of the direction of the mean total magnetization and the projection, on the observation plane, of the center of dipole moment. The method is applied to magnetic data from the Gulf of Guinea Seamount. The estimated magnetic palaeopole is at 50°48′S and 74°54′E which is in good agreement with estimates published by other authors.

We present a 3-D gravity interpretation method based on the inversion of source moments employing the series derived from the gravity anomaly expansion in multipoles and retaining moments up to second order only. It presents the advantages of being essentially linear and allowing a straightforward computer implementation. The method requires neither an explicit assumption about the source geometry nor a priori knowledge about the density contrast distribution, which may even be nonuniform. The method assumes implicitly: (1) that the source fits in the interior of an imaginary sphere whose center coincides with the source center of mass and whose radius is smaller than the depth to the source center of mass relative to the measuring plane, and (2) that the spatial density distribution presents three ortogonal planes of symmetry intersecting at the source center of mass. The first assumption can be met by upward continuing the observed anomaly. When both assumptions are met, the method produces reliable and stable estimates of the total anomalous mass, the coordinates of the center of mass, the three principal axes directions of the anomalous body, and the relative importance among the axes. The method is particularly suited for interpreting compact, isolated or disjoint, but spatially correlated sources. The method is sensitive to an incomplete removal of the regional field (presumably superimposed on the residual anomaly of interest) because the unremoved part of the regional field may strongly affect the estimates of the residual source moments.

A generic gravity source moment is an integral, over the source volume, of the product of the density distribution by a polynomia in the Cartesian coordinates of a point belonging to this volume. We obtained a formal expression for a generic moment in terms of integrals involving the gravity anomaly and the gravity potential. By analyzing the conditions under which this expression is valid, we conclude that, without usinga priori information regarding the sources, it is possible to determine, from the gravity anomaly, any moment or linear combination of moments whose associated polynomial has null Laplacian and depends only on the coordinates defining the measurement plane. Additionally, no moment whose associated polynomial has a nonnull laplacian can be determined without usinga priori information of the source.

To understand the depositional process of a sand dune located on Sylt Island, northern region of Germany, a Ground Penetrating Radar (GPR) survey was carried out. The survey is composed by eighty profiles in SE-NW and NE-SW directions. The data allowed the identification of different generations of dunes and bounding surfaces, especially reactivation surfaces associated to changes in wind patterns and can be used as markers to conduct future dense 3D GPR survey.

In order to enhance our knowledge about the tectonic processes responsible for the geological evolution of the Borborema Province and adjacent regions, the National Institute of Space Research (INPE) and the National Observatory (ON) are conducting Electromagnetic Induction studies in northeastern Brazil, which are being supported by the National Institute of Science & Technology for Tectonic Studies (INCTET) project during the past three years. Such studies contemplate the deep geophysical probing of this region with Magnetotelluric surveys (MT) and Transient Electromagnetic soundings (TEM) along several geoelectric transects and an array of Geomagnetic Depth Soundings (GDS), which complement other geophysical methods. The focus of this study is directed to the identification of geoelectric variations in electric conductivity at distinct lithospheric depths and associations of geoelectric strikes and anisotropy with structural grain and stress patterns, particularly in the long ubiquitous structural features that cross the province and that constitute major geoelectric heterogeneities, at the near surface crust and also in the lower crust and upper mantle. The preliminary results indicate a lithosphere with a very variable electrical resistivity. The upper-mid crust shows up as a very resistive layer, overlying a less resistive lower crust and upper mantle. In some areas the mantle is very resistive, characteristic of a cratonic origin. Exceptions to a resistive crust are found under the Serido belt, Jatoba rift basin and the Araripe intracratonic basin, marked by an underlying conductive crust. Laterally, the deeper portion of the crust and upper mantle is highly segmented in blocks with alternating juxtaposed higher and lower resistivities. This pattern is suggestive of highly deformed regions by transform dominated tectonic regimes, with the presence of contrasting mechanically strong cratonic-like keels against weaker zones likely subjected to delamination processes that might have produced magmatic related lithospheric re-fertilization by metasomatism. Introduction The National Institute of Science & Technology for Tectonic Studies congregates many geoscientists from several Brazilian research institutions with the purpose of studying the deep crust and upper mantle structures. It is multidisciplinary in its approach, merging different techniques and methodologies, which allow the study of the Earth’s interior based on different properties of its constituting rocks. The focus of this research is the study of the crust and upper mantle of the Borborema Province and adjoining Sao Francisco Craton and Parnaiba Basin. The Borborema Province is a complex association of crustal blocks with differing ages, origin and geologic evolution, and which were amalgamated during the last orogenic event that took place in what is now the Brazilian territory, namely the late Neoproterozoic-early Phanerozoic Brasiliano orogeny, composing the Gondwana supercontinent. Therefore, Electromagnetic Induction studies are being developed with the goal of making a major contribution to a better comprehension of the actual articulation of the amalgamated crustal blocks and the role of large transcurrent lineaments in bringing these blocks together, through an integrated multidisciplinary geophysical study.

Na porção central do Domínio da Zona Transversal (Província Borborema, Nordeste do Brasil), foram desenvolvidos estudos de cartografia geológica em campo e interpretação de produtos aerogamaespectrométricos, aeromagnéticos e imagens de satélite (Landsat 7 ETM+), no intuito de avaliar a importância (em especial, a continuidade em profundidade) de zonas de cisalhamento, candidatas a representarem suturas e limites de terrenos. A análise conjunta desses produtos permitiu ressaltar a existência de três estruturas de maior profundidade, as quais representam limites de blocos crustais, dentre elas as zonas de cisalhamento Serra do Caboclo e Congo-Cruzeiro do Nordeste. Uma anomalia profunda, localizada entre aquelas duas estruturas, pode representar a continuidade de uma delas, horizontalmente deslocada, ou uma terceira zona, não aflorante mas a ela(s) conectada. A interpretação favorecida propõe que este setor da Zona Transversal seria formado por zonas de cisalhamento com direção nordeste-sudoeste, mais profundas (topos das anomalias atingindo 6 a 16 km de profundidade), seccionado por estruturas mais rasas (topos das anomalias atingindo no máximo 2 a 6 km de profundidade), com direção este-oeste. Neste contexto, ressalta a importância e continuidade em profundidade da zona de cisalhamento Serra do Caboclo, inferida como a sutura entre os terrenos Piancó-Alto Brígida e Alto Pajeú, em conformidade com os dados geológicos de campo. In the central portion of the Transversal Zone (Borborema Province, northeastern Brazil), field geological mapping combined with interpretation of airborne gammaspectrometric, magnetic and satellite imagery (Landsat 7 ETM+) data were performed in order to evaluate the importance (especially, continuation at depth) of shear zones as potential candidates to represent terrain boundaries and sutures. This integrated analysis emphasized three deeper structures which separate crustal blocks, including as such the Serra do Caboclo and the Congo-Cruzeiro do Nordeste shear zones. A deep anomaly, located between those two structures, may represent the continuation, horizontally displaced, of one of them, or a third, non outcropping but interconnected shear zone. The favored interpretation considers this sector of the Transversal Zone as formed by deep-rooted, northeast-southwest trending shear zones (with tops of anomalies reaching 6 to 16 km depths), cut by shallower structures (tops of the anomalies reaching, at maximum, 2 to 6 km depth), with east-west trends. In this context, the importance and continuity at depth of the Serra do Caboclo shear zone is highlighted, and as such inferred to be the suture between Piancó-Alto Brígida and Alto Pajeú terranes, in accordance with field geological data.

Os estudos da atenuacao das ondas sismicas e uma ferramenta importante na discriminacao de reservatorios. Os processos associados sao sintetizados numa descricao quantitativa chamada fator de qualidade Q. Neste trabalho fazemos uma comparacao entre diferentes metodos para determinar Q, sendo dividido em duas partes. Na primeira fizemos um teste de performance de tres metodos (deslocamento de pico, de centroide e razao espectral) em um modelo de tres camadas. Na segunda parte invertemos os valores de Q utilizando os metodos de deslocamento de centroide e de pico. O deslocamento de centroides mostrou-se o mais robusto dos metodos em ambas as partes.

Aflorando em falesias costeiras no extremo oeste da Bacia Potiguar (Plataforma de Aracati), na regiao de Icapui-CE, a Formacao Barreiras exibe estruturas que caracterizam uma deformacao fragil de grande magnitude. Um levantamento em detalhe da geometria das falhas e dobras que afetam a Formacao Barreiras conduziu ao reconhecimento de estruturas distensionais (em Ponta Grossa) e contracionais (em Vila Nova), associadas a um sistema de transcorrencias. Tais dados permitem caracterizar um campo de tensoes neocenozoico, que gerou falhas e dobras e reativou estruturas mais antigas, presentes na secao neocretacea subjacente. A fim de investigar a continuidade e a geometria dessas feicoes em profundidade, foi realizado um levantamento com GPR que consistiu no registro de varias linhas 2D na borda de algumas falesias na localidade de Ponta Grossa (figura 1), onde as estruturas neotectonicas frageis sao mais expressivas. O imageamento com GPR, alem de reproduzir as feicoes impressas na porcao aflorante das falesias, tambem revelou a geometria da deformacao fragil em profundidade, contribuindo sobremaneira para o entendimento do contexto neotectonico da area.

A sand dune located near São Bento do Norte and Caiçara do Norte towns (northern coastal region of Rio Grande do Norte state, Northeastern Brazil) is rapidly migrating toward urban areas. To understand the depositional processes (evolution of the dune), a Ground Penetrating Radar (GPR) survey was carried out. The survey is composed by sixteen profiles in E-W direction, which is parallel to the dominant wind direction, and five profiles in N-S direction. In both directions, it was used antennas of 200 MHz and 80 MHz. Additionally, a topographic survey, excavations of two trenches, and grain size analysis were also carried out. The GPR imaged the internal sedimentary structures of the coastal deposits so that it was possible to reconstruct the depositional history of the area. Initially, marine sediments were deposited during the final period of a marine regression of Holocene age. After the exposure of the area, a first phase of aeolian deposition occurred over the marine sediments. Later, a depositional hiatus took place, probably associated with partial erosion of the dune. Finally, a second phase of aeolian deposition occurred, covering all previous strata. Based on the geometry of foresets and bounding surfaces, it was interpreted that, in the second phase of aeolian deposition, the dune changed its morphology from barchan to dome. The morphology change appears to have occurred when the migration front of the dune faced the obstacle constituted by a ramp along in the depositional surface dipping against the wind direction. According to the similarities of the foreset dips in the first and second phases of aeolian deposition, the prevailing dominant winds were very similar to present day dominant winds. So, it is likely that the rapidly migration process of the dune toward urban areas is an entirely natural process. Uma duna localizada próxima às cidades de São Bento do Norte e Caiçara do Norte (litoral nordeste do Rio Grande do Norte, nordeste brasileiro) está migrando rapidamente sobre áreas urbanas. Para entender o processo deposicional (evolução da duna), um levantamento com Radar Penetrante no Solo (GPR) foi realizado. O levantamento é composto por dezesseis perfis na direção E-W, paralelos à direção dos ventos dominantes, e cinco perfis na direção N-S (utilizando antenas de 200 MHz e 80 MHz). Adicionalmente, foram efetuados escavações de duas trincheiras, um levantamento topográfico e análises granulométricas. O GPR imageou as estruturas sedimentares dos depósitos costeiros de modo que foi possível reconstruir a história deposicional da área. Inicialmente, sedimentos marinhos foram depositados durante o período final de uma regressão marinha ocorrida no Holoceno. Após a exposição da área, uma primeira fase de deposição eólica ocorreu sobre os sedimentos marinhos. Mais tarde, ocorreu um hiato deposicional, provavelmente associado com a erosão parcial da duna. Finalmente, ocorreu uma segunda fase de deposição eólica, cobrindo todos os estratos anteriores. Com base na geometria dos foresets e nas superfícies limitantes, foi interpretado que, dentro da segunda fase de deposição eólica, a duna mudou sua morfologia de barcana para dômica. A mudança na morfologia parece ter ocorrido quando a frente de migração da duna encontrou um obstáculo constituído por uma rampa ao longo da superfície deposicional. Devido às semelhanças entre os mergulhos dos foresets dentro da primeira e da segunda fase de deposição eólica, os antigos ventos dominantes foram muito similares aos atuais. Portanto, é provável que o rápido processo de migração da duna em direção a áreas urbanas seja um processo totalmente natural.

The best locations for water wells in crystalline terrains of semiarid lands are narrow, densely fractured and altered belts, normally associated with fault zones and folding axes. A simple electrical procedure, based on vertical soundings and multiple resistivity profiling, has been successfully applied for the structural mapping of these two-dimensional aquifers in northeast Brazil. The apparent resistivity sections constructed from partial electrical sounding measurements taken at a closely spaced interval along a traverse were interpreted using a finite-difference algorithm. This scheme allowed detailed description of the lateral and vertical extent of fractured aquifers and some of their hydrogeophysical characteristics. The results of a selected case study from Bahia state compare satisfactorily with available geophysical logs and production data of existing wells.

Electromagnetic induction studies of magnetotelluric (MT) and Geomagnetic Depth Soundings (GDS) have been conducted in the northeast of Brazil during the last two years, supported by the Millenium Program, the Litoborborema Project and INPE. Two MT profiles composed of 52 individual soundings were carried out in the Borborema Province along a 700 km long SE-NW transect, and more than 30 (GDS) were deployed in the northeast region, focusing on the geoelectric properties of the lithosphere. Preliminary results are discussed in this paper.

A elaboracao dos mapas gravimetricos da Provincia Borborema representa um avanco importante para a compreensao de varios problemas tectonicos que desafiam a comunidade geocientifica. Na anomalia ar livre, a interpretacao do “efeito de borda” resultante da quebra da plataforma, permite fazer inferencias sobre a segmentacao da margem e a arquitetura da transicao litosferica continente-oceano (Oliveira, 2006).

Aproximadamente 50% do substrato do NE brasileiro e composto por rochas cristalinas, inseridas em ambiente de clima semi-arido. Neste contexto, o exito em encontrar agua em subsuperficie e proporcional a capacidade de existirem, e identificar, descontinuidades nas rochas capazes de servirem como meio aquifero. Metodos geofisicos aeroportados tem a capacidade de imagear amplas areas e, com o detalhe dos metodos terrestres, delimitar as zonas hidrogeologicamente importantes. A complexidade do embasamento cristalino pode entao ser desvendada a partir da correlacao entre a geofisica, a geologia estrutural enfocando a neotectonica e os modelos disponiveis de fluxo e armazenamento de agua subterrânea em terrenos cristalinos.

Borborema Province (BP) has a very complex history, but up to now, few geophysical data have been used to constrain its evolution history. This paper presents the first integrated gravity/magnetic/geological data interpretation of the tectonic framework of the province. BP is a Proterozoic fold belt extended between the Sao Luis - West Africa and Sao Francisco - Congo/Kasai cratons. Currently, as a result of an increase in geochronologic data, there is growing evidence that its evolution has occurred by a mechanism of terrane collage. This evolution model suggests that large crustal blocks were put together during Meso and Late-Proterozoic events, resulting in crust growing by the arrival of pieces of crust or by the addition of new crusts. Interpretation of regional magnetic and gravity maps revealed the boundaries between crust blocks or compartiments inside BP. As result of the integrated interpretation, it is proposed that BP is formed by the union of at least 11 crust blocks. Boundaries between these blocks form long lineaments in NE-SE direction, in some cases showing vergences to E-W direction. The major block to the south of BP is the Sao Francisco Craton. The boundary between BP and the craton is still a controversial question, but the geophysical data suggest that N-S lineaments, which occur inside the craton, prolong below Riacho do Pontal and Sergipano hrust fold belts, which are part of BP. In this way, the plate associated to the craton lies beyond its geologically mapped north limit.

Ao se propagar no meio geologico, o pulso do GPR sofre atenuacao por efeitos combinados de absorcao e dispersao. Esses efeitos de propagacao provocam a degradacao da qualidade da imagem e ocorrem segundo mecanismos que dependem da frequencia de excitacao da onda e das caracteristicas eletricas do meio. Para conseguir um bom imageamento deve-se corrigir os efeitos de propagacao, o que passa, geralmente, pela necessidade de conhecimento dos atributos do meio, notadamente o fator Q (quality factor), o que nem sempre e possivel na pratica dos levantamentos de GPR. Neste trabalho e apresentada uma abordagem pratica de correcao dos efeitos de propagacao, baseada no processamento sismico e adaptada para o GPR. Os principais recursos utilizados sao o ganho esferico e exponencial (SEC) e o balanceamento espectral. Sera mostrada em dados reais a expressiva melhoria da qualidade do imageamento, obtida a partir da tecnica descrita neste trabalho.

This paper presents the use of GPR (Ground Penetrating Radar) as an investigation tool of the spatial distribution of faults and fractures in a reservoir analogue exposed at surface. In this multidisciplinary study of a faulted reservoir analogue, the observed fault geometry and any associated fracture system has been sampled by different methodologies, at different scales, to verify the hypothesis of scale invariance. The aim of this survey is to establish the spatial continuity and distribution of these faults and fracture at different scales, from outcrop down to a few tens of meters. The survey of this analogue reservoir was carried out in The Tucano Basin, where good oil reservoir analogues are exposed. Our strategy is: (1) to test the image of macroscopic scale faults using GPR; (2) compare surface structural data (in several scales) with the GPR data. The final objective is to test predictive methods for the characterization of the spatial distribution of faults and fractures on reservoirs.

Na area da ETO / ETE (estacao de tratamento de oleo/efluentes) do campo de Fazenda Belem – Ce, e comum o surgimento de feicoes erosivas em superficie, de forma circular ou alongada que provocam perda de volumes expressivos de solo. Estas feicoes, a depender do seu porte e da profundidade, podem comprometer as estruturas superficiais, tais como edificacoes, paredes de diques, tanques, postes, etc. A partir da necessidade de conhecer em detalhe a morfologia e a area de ocorrencia dessas feicoes, foi realizado um levantamento 3D com GPR (ground penetrating radar) objetivando investigar a presenca de estruturas de dissolucao em uma area de tancagem da ETO/ETE. Os resultados mostram a presenca de ravinas alongadas, associadas a fraturas, e permite orientar as acoes mitigadoras que devem ser adotadas para preservar a integridade estrutural dos tanques da ETO/ETE.

A analise da polaridade do sinal do GPR pode ser muito util na interpretacao dos dados por trazer embutida em si informacoes sobre os atributos eletromagneticos do meio. Entretanto existem poucos registros na literatura sobre o assunto e nao existe nem mesmo uma padronizacao estabelecida, como na sismica, por exemplo, havendo somente convencoes de fabricantes baseadas em criterios e definicoes proprias que, na ausencia de um padrao geral, servem como guia para orientar a interpretacao dos dados de GPR, utilizando criterios de contraste de impedância dieletrica entre os diferentes meios atravessados pela onda eletromagnetica. Este trabalho apresenta algumas consideracoes sobre este assunto com base na compilacao de uma nota tecnica de um fabricante, na qual e mostrado o comportamento da polaridade, associando-a a eventos especificos e sempre presentes nas seccoes de GPR, tais como: onda direta pelo ar, onda direta pelo solo, e onda refletida. A utilizacao de atributos de polaridade do pulso na interpretacao de dados de GPR sera exemplificada atraves de duas situacoes estudadas pelos autores. Na primeira mostrar-se-a a utilizacao dos conceitos de polaridade na deteccao de dutos metalicos enterrados, enquanto o segundo caso exemplifica o uso de informacoes de polaridade na localizacao de estruturas de dissolucao (cavernas) em substrato calcario.

O trabalho aqui apresentado visa indicar o GPR (Ground Penetrating Radar) como metodo de prospeccao de feicoes geologicas em alguns terrenos cristalinos capeados por coberturas sedimentares. Em diversas porcoes da regiao NE do Brasil, a exemplo de Equador-RN, as caracteristicas geologicas e climaticas favorecem o uso do GPR. Estas caracteristicas se refletem basicamente sob a forma de um capeamento sedimentar pouco espesso e relativamente resistivo, o qual nao interfere efetivamente na propagacao das ondas eletromagneticas geradas pelo equipamento. Com base no truncamento de refletores subhorizontais referentes ao bandamento local, podese identificar diversos sistemas de fraturas. Estes fraturamentos diferem entre si basicamente com respeito ao mergulho (desde baixo ângulo a subverticalizado) e ao tipo de conectividade. A consistencia dos resultados com GPR e atestada pelos dados dos levantamentos de eletroresistividade e VLF elaborados no mesmo local.

Recentemente, varios estudos de cunho neotectonico estao sendo realizados no Estado do Rio Grande do Norte, com o objetivo de melhor compreender a deformacao atual da crosta, caracterizando as estruturas produzidas e os regimes tectonicos envolvidos. As ferramentas utilizadas nesses estudos sao variadas, destacando-se as analises de mecanismos focais, de rede hidrografica, de geomorfologia, de fotosensores e levantamentos geologico-estruturais.

Euler deconvolution has been widely used in automatic aeromagnetic interpretations because it requires no knowledge of the source magnetization direction and assumes no particular interpretation model. This rapid inversion method produces stable estimates for the horizontal and vertical positions and respectively) of point sources. Another main advantage of this method is that it does not demand a detailed geologic knowledge of the area. However the two crucial weakenesses of this method are: i) the need of knowing the nature of the magnetic point or line source, that is, the knowledge of the structural index and ii) the presence of an undesirable “spray” of solutions which is obtained when the method is applied in a redundant way by moving the discrete inverse operator around the anomalous area. We present two new criteria for solving these difficulties. The first criterion is aimed at determining It is based on the correlation between the total-field anomaly and the estimates of a baselevel (b) computed at the same locations of the observations and using different This correlation attains a minimum when the correct is assumed. The second criterion selects, from the “spray” of solutions, only the best solutions. We propose that, besides selecting the solutions producing a standard deviation for smaller than a threshold value, only those solutions producing the “best fit” to the known quantities be selected. This additional criterion substantially reduces the “spray” of solutions.

We present a gravity interpretation method for estimating the relief of an arbitrary interface separating two homogeneous media. The upper medium is discretized into rectangular, juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated from the gravity anomaly. To stabilize the inversion, we introduce two kinds of constraints on the depths. The first one requires proximity between the observed and computed depths at isolated points such as those obtained from boreholes (absolute equality constraint). The second one requires that groups of depths approximately follow an established linear relationship among them (relative equality constraint). Both kinds of constraints are imposed in the least-squares sense. This stabilization procedure does not bias the solution either toward an initial approximation, or toward the null solution. The method is applied to synthetic data, producing a good reconstruction of a complex basement consisting of several structural high and lows. It is also applied to real data of Reconcavo Basin, Brazil, leading to a sharper definition of the basement structures as compared with previous gravity interpretations of the area.

Utilizando-se dados topograficos e gravimetricos, pode-se estimar o relevo da interface crostamanto atraves de, pelo menos, duas abordagens independentes. A primeira abordagem (amplamente utilizada) emprega a hipotese de que a regiao em estudo encontra- se isostaticamente compensada (e.g. Banks et aI., 1977). Por outro lado, a segunda abordagem (menos utilizada) efetua previamente a separacao regional-residual do campo gravimetrico total e estima o relevo da interface crosta-manto atraves da inversao da coinponente regional do campo (e.g. Beltrao, 1989). Por ser urn problema mal-posto (cuja solucao e instavel), esta inversao requer a introducao de vinculos para fomecer solucőes estaveis. Um dos vinculos mais utilizados e o da suavidade do relevo da interface crostamanto.

O Nordeste Setentrional Brasileiro esta inserido geologicamente na Provincia Borborema. As rochas precambrian as desta regiao apresentam uma complexa evolucao geodinâmica, dominada por trends estruturais com direcao principal NE, alem de um importante sistema de zonas de cisalhamento E-W e NE-SW (Almeida e Hasui, 1984). As rochas sedirnentares das bacias costeiras e interiores do Nordeste Setentrional, bem como os sedimentos da Bacia do Parnaiba, representam as coberturas fanerozoicas nao deformadas na Provincia Borborema.

We present theoretical uniqueness, stability, and parameter bias analyses for the simultaneous estimation of: the structural index the horizontal and vertical positions and respectively) of a point magnetic source, and a baselevel b, using Euler’s differential equation. We found that and are independent of the other parameters and can be estimated from the knowledge of the total-field anomaly and its gradients only. On the other hand, and cannot be simultaneously estimated because they are linearly dependent. As a result, to estimate must be either known a priori or independently determined (and vice versa). For a known estimates of and are unique and stable regardless of the value of However, if the vertical (or horizontal) gradient of the observed total field assumes negligible values, the estimation of (or becomes unstable. In contrast, as far as is not too close to zero, estimates of are stable. For equal to zero, estimates of are undetermined.

Gravity inversion results aiming to estimate the crystalline basement relief of the onshore Potiguar Basin are presented. It is assumed that the density contrast between the sediments and the crystalline basement is constant and known. The sediments are approximated with vertical rectangular prisms whose thicknesses, representing the depths to crystalline basement, are the parameters to be estimated from the gravity inversion. In all cases, the inversion process was stabilized with smoothness constraint on the spatial variation of the basement relief. Besides the gravity data, we used information about the basement relief obtained from boreholes and from seismic interpretation. These data were combined in order to compose three different scenarios simulating different phases during the exploratory development of the basin. In the first scenario we used the gravity data only. In the second scenario we worked with two combinations, first we combined the gravity data with information from boreholes, and second, with the basement relief reference model that resulted from the seismic interpretation. Finally, in the third scenario, we used the gravity data, the information from the boreholes and the reference model obtained from the seismic interpretation. In all scenarios, we obtained reliable estimates for the basement relief. Concerning the discrepancies among the results, the estimates obtained with additional constraints (from boreholes and/or seismic interpretation) increased the overall resolution of the basement relief and the reliability of the absolute depth values. However, it is important to stress that the first scenario already yielded a reliable estimate to the basin relief. If one takes into account the huge difference both in financial cost and environmental impact associated with gravity and seismic surveys, respectively, as well as with borehole drilling, this result highlights the great importance of properly using the gravity data in the initial exploratory phase of the basin.Keywords: gravity inversion, basement relief, Potiguar Basin, constrained inversion. RESUMO. Este trabalho apresenta aplicações de inversão gravimétrica para a estimativa do relevo do embasamento cristalino da Bacia Potiguar emersa. Admite-se que o contraste de densidade entre os sedimentos e o embasamento da bacia é constante e conhecido. O pacote sedimentar é aproximado por um conjunto de prismas lateralmente justapostos cujas espessuras, ou profundidades até o embasamento, são os parâmetros a serem estimados na inversão. Em todos os casos, o processo de inversão foi estabilizado com o uso do vínculo de suavidade na variação espacial do relevo do embasamento. Além das medidas gravimétricas, foram utilizados dados de poços e/ou oriundos da interpretação sísmica, em diferentes combinações, de modo a simular diferentes etapas, ou cenários, de exploração da bacia ao longo do tempo. No primeiro cenário foram utilizados somente dados gravimétricos. No segundo cenário, simulamos duas situações, em ambas adicionando outras informações aos dados gravimétricos: na primeira delas, adicionamos somente vínculos de poços e, na segunda, apenas um modelo sísmico de referencia para o embasamento. Por fim, no último cenário foram agregados aos dados gravimétricos os dados de poços e o modelo sísmico. As estimativas obtidas para o relevo do embasamento revelaram nitidamente o arcabouço da bacia em todos os cenários. Em relação às variações das profundidades entre os cenários, as inclusões dos vínculos dos poços e do modelo sísmico aprimoraram a resolução global dos resultados, como esperado. Contudo, é relevante destacar que a inversão no primeiro cenário já forneceu um relevo bastante aproximado do arcabouço conhecido da bacia. Se levarmos em conta as grandes diferenças de custo financeiro e de impacto ambiental, respectivamente associados com os levantamentos gravimétrico e sísmico, bem como com a perfuração de poços, esse resultado evidencia a grande importância de uma adequada utilização dos dados gravimétricos nas fases exploratórias iniciais de uma bacia.Palavras-chave: inversão gravimétrica, relevo do embasamento, Bacia Potiguar, inversão vinculada.

The present article is a synthesis of studies developed by fundação Norte Rio Grandense de Pesquisa e Cultura -FUNPEC and Universidade Federal do Rio Grande do Norte - UFRN for the Companhia de Águas e Esgoto do Rio Grande do Norte - CAERN, in the lower course of Doce River, RN. This paper also presents alternatives to increase the water supply from wells in the Barreiras Aquifer located to the north of Natal district, RN, considering the degraded water quality due to nitrate contamination from the right margin of Doce River. The methodology was based on the combined use of hydrogeological and geophysical techniques (gravity andelectrical resistivity). The regional structure was previously characterized by morphotectonic analysis and gravimetric and geoelectric data interpretation. According to these data, two preferential fault directions (NW and NE) directly influence local geometry of the aquifer causing abrupt variations of the saturated thickness. The potential of the Barreiras Aquifer increases gradually on the left bank of Doce River from east to west, due to larger spacing between hydrogeological equipotential surface and larger saturated thickness. These observations provided the basis to locate a new water collection area, where new wells are going to be drilled with 300 m3 /h estimated total discharge. RESUMO: O presente artigo representa uma síntese de estudos desenvolvidos pela Fundação Norte-Rio-Grandense de Pesquisa e Cultura-FUNPEC e Universidade Federal do Rio Grande do Norte-UFRN para a Companhia de Àguas e Esgotos do Rio Grande do Norte-CAERN, na área do baixo curso do Rio Doce-RN. O mesmo objetivou apontar alternativas para o incremento do abastecimento d’água proveniente de poços no Aquífero Barreiras na zona norte do município de Natal-RN, tendo em vista o agravamento da contaminação de captações da margem direita do Rio Doce por nitrato. A metodologia adotada foi fundamentada no emprego conjunto de técnicas de hidrogeologia e geofísica (gravimetria e eletrorresistividade). A estruturação regional, caracterizada previamente a partir de análise morfotectônica e interpretação de dados gravimétricos e geoelétricos, foi evidenciada nas direções preferenciais NW e NE, influenciando diretamente na geometria local do aquífero na forma de variações bruscas de sua espessura saturada decorrente dos falhamentos. A potencialidade do Aquífero Barreiras apresenta um acréscimo gradativo na margem esquerda do Rio Doce de leste para oeste, decorrente de maiores espaçamentos entre equipotenciais hidrogeológicas e espessuras saturadas maiores. Estas observações fundamentaram a locação de uma subárea a leste da captação Rio Doce da CAERN como bastante promissora para a execução de uma segunda bateria de poços, com descarga total estimada da ordem de 300 m3 /h.Palavras-chave: Aquífero Barreiras; Rio Doce-RN, Geofísica; potencialidade hidrogeológica

Apparent susceptibility and magnetization mappings are traditionally obtained with the ridge regression, or generalized inverse (GI) estimators. These estimators constrain the solutions to have minimum Euclidian norms, which is equivalent to require that all parameters composing the solutions be as close as possible to zero. As a result, the ridge and GI solutions are almost always in conflict with the actual geological information. We present a stable inversion method to estimate the spatial distribution of magnetization which does not bias the solution against the known geological information. In addition, at points where there is no geological information, the proposed method assumes continuity of the spatial variation of magnetization. The continuity constraint obeys a general linear relationship stablished by the interpreter; smoothness is an important particular case. The utility of the particular estimator imposing smoothness on the spatial variation of magnetization is illustrated and compared with the ridge and GI estimators in a simulated magnetization mapping of a complex geologic area. We found that this particular estimator is more versatile, effective, and operationally simpler than either the ridge or the GI estimator.

The interpretation of magnetic anomalies is not a trivial task not only because of the difficulties in allowing for a possible existence of remanent magnetization, but also due to the nonlinear dependence of the magnetic anomaly upon the source geometry. For the sake of simplicity, the magnetization is usually assumed to be induced only while trying to estimate the interpretation model geometry. Alternatively, the spatial orientation of the source may be assumed known in order to estimate the total magnetization. We present a method to simultaneously estimate the total magnetization direction and the spatial orientation of the source. The method is based on the approximation of the anomalous field by the series derived from expansion of the magnetic potential in multipoles, keeping moments up to 2nd order. Because the parameters to be inverted are the source moments, the method has the advantages of being linear and appropriate to interpret anomalies produced by a broad class of sources. This class comprises the finite-volume bodies, homogeneously magnetized in direction, but not necessarily in modulus, and whose spatial distribution of magnetization modulus presents three orthogonal symmetry planes.

Gravity inversion is an ill-posed problem because it presents infinite solutions. One efective way to transform it into an well-posed problem is to constrain the solutions with a priori information (about the anomalous density distribution) introduced by a stabilizing functional. In this paper we present a stabilizing functional which incorporates information about symmetry and either overall or directional smoothness in the spatial density distribution. In the case of a linear inverse problem where the parameters to be determined are the densities of several retangular blocks, the proposed functional is the sum of the squared differences between previously established pairs of parameters. The estimator associated with this functional may contribute to stabilize the inversion process if it incorporates information about the density variation in a direction in which the gravity observations have poor resolution particularly in the vertical direction. Let us assume that by inspecting a given gravity profile, we can limit a region R containing the cross-section l? of the anomalous source. To estimate, from the gravity profile, the distribution p(z, z) of density contrasts, we discretize R into M blocks (Figure 1) and estimate them by solving the equation

O levantarnento gravirnetrico do Nordeste Setentrional do Brasil vem sendo executado pelos Deptos. de Fisica e Geologia/UFRN e lAG/USP, totalizando atualmente cerca de 1500 estacőes. Estes dados foram integrados aos de outros levantamentos pre-existentes (CPRM/Petrobras na B. Potiguar; Umarizal/RN e ON/CNPq) para composiao do mapa de anomalias Bouguer entre os limites 4° - 70S e 35° - 390W, com densidade media de 0,02 estacőes/km2 (Fig. 01).