Sergio Fontes
Observatório Nacional, Geophysics, Faculty Member
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A geoelectric image of a portion of the Sao Fransisco sedimentary basin (Minas Gerais) was obtained based on magnetotelluric (MT) data from 43 MT soundings (400 Hz - 0.001 Hz). The soundings were carried out along 10 seismic lines.... more
A geoelectric image of a portion of the Sao Fransisco sedimentary basin (Minas Gerais) was obtained based on magnetotelluric (MT) data from 43 MT soundings (400 Hz - 0.001 Hz). The soundings were carried out along 10 seismic lines. covering an area of 11.000 km˛ approximately. The distances between soundings were irregular and varied from 2.5 km to 20 km. Static shift corrections were applied to the data using the median resistivity of the first conductor. One dimensional (1D) Occam geoelectrical sections were obtained for all profiles. A sequence of conductors and resistors was found in the sedimentary package. followed by a geoelectrical basement. The MT sections show good agreement with both borehole information and seismic sections.
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A magnetotelluric (MT) survey was conducted on the eastern margin of the Reconcavo basin, state of Bahia (Brazil). Eleven stations were recorded along a pre-existent seismic line covering 500Hz-0.01 Hz frequency range. The profile cuts an... more
A magnetotelluric (MT) survey was conducted on the eastern margin of the Reconcavo basin, state of Bahia (Brazil). Eleven stations were recorded along a pre-existent seismic line covering 500Hz-0.01 Hz frequency range. The profile cuts an area where the presence of conglomerates prevents better seismic and gravity resolution. By giving information on the conductivity values this MT survey could provide constrai11ts thatwould help reveal the true geological structure of the area. Pseudosections from apparent resistivity and phase values along with 1-0 Niblett- Bostick transformations clearly depict the major geological feature in the region- the Salvador Fault. Noise interference from cathodic protection on a nearby pipeline and clear surface distortions were observed. Therefore, further conclusions should not be made UDtil the data is refined.
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Para solicitação de resumo, entrar em contato com editor-chefe (editor@sbgf.org.br).
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The paper describes the characteristics of the equatorial electrojet at Huancayo (HUA, 12.1oS, 75.3oW, inclination 1.5oN, declination 1.0oE) in western side of South America, where the geomagnetic field is aligned almost along the... more
The paper describes the characteristics of the equatorial electrojet at Huancayo (HUA, 12.1oS, 75.3oW, inclination 1.5oN, declination 1.0oE) in western side of South America, where the geomagnetic field is aligned almost along the geographic meridian, and at Itinga (ITI, 4.3oS, 47.oW, inclination 1.4oN, declination 19.3oW) in eastern part of South America, where the geomagnetic field is aligned about 19o west of the geographic meridian; although the mean intensity of the magnetic field in the two regions are almost of the same order. Further comparisons are made of the current at Itinga and at Tatuoca (TTB, 1.2oS, 48.5oW, inclination 7.8o N, declination 18.7oW), a low latitude station in the same longitude sector. The daily range of horizontal component of the geomagnetic field, H, is shown to be almost 16% higher at HUA compared to that at ITI. The daily variation of the eastward field, Y, showed a strong minimum of -40 nT around 13-14 hr LT at ITI whereas very low values were obse...
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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... more
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.
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Three-dimensional (3-D) forward modeling results of a magnetotelluric (MT) survey carried out around the Serra da Cangaiha impact crater located in northeastern region of Parnaiba basin Brazil are presented. The 13 km impact structure has... more
Three-dimensional (3-D) forward modeling results of a magnetotelluric (MT) survey carried out around the Serra da Cangaiha impact crater located in northeastern region of Parnaiba basin Brazil are presented. The 13 km impact structure has been investigated because of its explicit 3-D shape and scientific interest. We recorded MT data in the period range between 0.001s and 1095s at 25 sites. The sites are aligned across the crater structure along three radial profiles (NE - SW, NW -SE and ENE - WSW). The MT results at some sites are strongly affected by 3-D effects. A 3-D finite-difference forward modeling algorithm was applied in order to explain the observed data. The central part of the model covers an area of approximately 169 km . The model of the study area, constructed using a 3-D mesh of 22 x 19 x 25 (29325 cells) was based on the a priori information of the geology of the region, and the results of the two-dimensional (2-D) inversion results. Eleven frequencies were used for the 3-D modelling from 0.01 to 1000 Hz. Induction vectors that show the conductivity distribution around the crater region were calculated for the 25 MT sites The real induction arrows at short periods point towards the center of the crater and indicate the existence of a 3-D circular structure. The resistivity structure obtained suggests perturbations of the upper crust due to the meteorite impact; the base of the uplifted structure extending to 3.0 km depth was well delineated. The observed 3-D effects are explained as a shallow resistive ring that represents a local 3-D body embedded in a regional layered earth. The results confirm the importance of 3-D forward modeling in impact crater studies. The study provided insight into the post-impact electrical signature of the upper crust.
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SUMMARY The study region in southeastern Brazil is composed of two domains: the Cretaceous onshore Espírito Santo Basin and the Neoproterozoic Araçuaí Orogen. In this region, associations among geological evolution, observed structures... more
SUMMARY The study region in southeastern Brazil is composed of two domains: the Cretaceous onshore Espírito Santo Basin and the Neoproterozoic Araçuaí Orogen. In this region, associations among geological evolution, observed structures and geophysical data are limited. Therefore, this study aimed to further investigate this region using magnetotelluric (MT) data obtained from 22 W-E spanning stations over the Espírito Santo State. Because the dimensionality analysis revealed a substantial 3-D characteristic of the data, a 3-D MT inversion using the ModEM code was performed to generate a regional geoelectric model, and potential and well data were used as qualitative support for the interpretation. The resistivity profile obtained from the model revealed a resistive pattern for the Precambrian terrain and a conductive pattern for the sedimentary rocks of the onshore Espírito Santo Basin. The basement of the basin was delimited in the resistivity model, revealing consistency with the ...
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Research Interests: Geology and Kimberlite
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The utility of the joint transient electromagnetic (TEM) and audiofrequency magneto-telluric (AMT) methods in stratigraphic and hydrogeological mapping has been evaluated in the southeastern part of the Parnaiba basin. The field... more
The utility of the joint transient electromagnetic (TEM) and audiofrequency magneto-telluric (AMT) methods in stratigraphic and hydrogeological mapping has been evaluated in the southeastern part of the Parnaiba basin. The field experiments employed central-induction and coincident-loop TEM and tensorial AMT depth sounding techniques at several observational stations along a 400 Km traverse. The processed data are in excellent agreement with borehole data and known geology of the area. The results show that all the main stratigraphic formations can be distinguished on the basis of their resistivity characteristics and also allow proposing a zone for the Transbrasiliano lineament. A preliminary groundwater resource evaluation study of the Piaui State transect showed that the prolific aquifers in the basin are easily detectable using the TEM/AMT methods due to the favourable juxtaposition of electrically-contrasting permeable and relatively impermeable geological formations forming confined aquifers of regional extent. It is contended that these regional groundwater reservoir resources may be developed by machine-drilled boreholes to meet the water needs of the region but further detailed geophysical evaluation will be required for a cost-effective borehole siting program. Suggestions are also offered to guide any future detailed exploration for the more localized and shallower reservoir occurrences and resource development possibilities in the basement areas in the southeastern margin of the basin.
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An attempt has been made to develop a simple framework for identifying and classifying magnetotelluric (MT) static shift in a sedimentary basin. environment. The concept of regional resistivity behavioural patterns is proposed and used to... more
An attempt has been made to develop a simple framework for identifying and classifying magnetotelluric (MT) static shift in a sedimentary basin. environment. The concept of regional resistivity behavioural patterns is proposed and used to identify distorted soundings and the pattern zonations effectively map geological targets. The nature ofthe causative bodies is well defined and their depth. locations can be sought using the constraints provided by transient electromagnetic (TEM) data. In general, most of the static shift problems found in the Parnaiba basin can be explained in the context of intrusive volcanism and small outcrops or patches of conductive materials. Unlike conventional practice, we use a TEM transmitter loop of the same size as the MT electric dipoles thus yield. ing a better representation of the subsurface resistivity distribution in the vicinity of the MT observational station. The TEM and MT data at stations where the ground is almost homogeneous are in excellent agree ment suggesting that any of the MT polarizations can be used to determine the true subsurface resistivity structure. For areas of complex geology in the study region, we suggest that more accurate structural information can be gleaned from the combined central-loop and coincident- loop TEM data as against the popular approach that uses just the central-loop data in the joint analysis of MT and TEM data. For such areas, it was found that of the two sets of unrotated MT apparent resistivity curves measured at each location, the higher one at the short periods overlapping with the TEM data is in accord with the TEM data and this provides a criterion for selecting the appropriate MT data set for 1-D interpretations.
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Research Interests: Geology and Kimberlite
Jequitinhonha basin, one of the marginal sedimentary basins of Northeastern Brazil, presents a complex tectonic framework and petroleum system among marginal basins. Both seismic and structural interpretations were used to carry out an... more
Jequitinhonha basin, one of the marginal sedimentary basins of Northeastern Brazil, presents a complex tectonic framework and petroleum system among marginal basins. Both seismic and structural interpretations were used to carry out an integrated study of the basin. Seismic attributes highlighted significant geological features as salt domes and volcanic rocks cross cutting the main geological formations. Seismic lines marked the surfaces of the Paleocene, Upper Cretaceous, Albian, Top Salt and Base Salt, and top of the volcanics, thus contributing to the knowledge of the tectonostratigraphic evolution of the basin. Three main basement controls were identified using potential field gravimetric and magnetic data. 2-D integrated model was created constraining the basin features using seismic interpretation for a seismic line in the dip direction and potential field data. The model inferred a granulitic lower crust, and horst and grabens at the top of the basement. Moho is ~ 30 km deep beneath the continent and upwelling up to 16 km beneath the ocean. COB was also marked through integrated 2D modeling by the presence of rocks with oceanic and continental affinities.
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Research Interests: Geology and Impact Crater
This work describes a successful application of the controlled source audio-magnetotelluric method (CSAMT) in kimberlite exploration. We conducted the EM survey at a known outcropping pipe in Minas Gerais State, Brazil. A total of 54... more
This work describes a successful application of the controlled source audio-magnetotelluric method (CSAMT) in kimberlite exploration. We conducted the EM survey at a known outcropping pipe in Minas Gerais State, Brazil. A total of 54 CSAMT_STRATAGEM soundings were acquired along two north-south and one west-east profiles. Data were processed and rotated to obtain the TE and TM modes. 2D smooth inversions of both TE and TM data at each profile defined the existence of a lowresistivity body associated to the main pipe. Our results drove the subsequent drilling program by the mining company.
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<p>Long period magnetotelluric (LMT) stations were deployed on an array format covering much of the Parnaíba basin and western edge of the Borborema Province in NE Brazil. A grid with 56 LMT stations... more
<p>Long period magnetotelluric (LMT) stations were deployed on an array format covering much of the Parnaíba basin and western edge of the Borborema Province in NE Brazil. A grid with 56 LMT stations (from 10 s to over 10<sup>4 </sup>s) with 70 km spacing were acquired during periods of acquisition varying from 2-3 weeks up to 6 months through two field campaigns between November 2018 and July 2019. This study is the first of this kind undertaken in Brazil, much in line with the American EarthScope, the Chinese SiinoProbe and AUSLAMP - Australia array initiatives suggesting the way forward for a comprehensive understanding of large 3D electrical structures of the continental crust and the lithospheric mantle in the entire country.  The results already published show that the resolution of the models obtained is comparable to the inversions of seismic tomography, the sensitivity of the MT method being superior in sensing the melting fraction, temperature and water content in the mantle. As an example, the 70 km spacing between EarthScope stations proved adequate to delineate the main structural features of the middle crust to the upper mantle of the United States. The Parnaíba Basin is a cratonic basin that has been formed by sedimentary mega sequences deposited along the Phanerozoic, after the formation of the Gondwana Supercontinent, with sedimentation expanding over the Borborema Province. The basin's evolution was conditioned by subsidence processes along unstable crustal areas at the end of the Brazilian cycle and demarcated by grabens, fault zones and magmatism. The shallow sediments of the basin (around 2.5 to 3.4 km maximum) however widely spread 60,000 km<sup>2</sup> cover prominent crustal features including the location of the trans-Brazilian lineament, limits of the Borborema Province, Amazon craton and other important shear and suture zones that may be important to better understand the evolutionary geodynamics of the supercontinents. The time series of electric and magnetic fields recorded in the field have been passed through extensive quality control analysis and then were robustly processed in the frequency domain generating good quality impedance tensor and tipper transfer functions. Currently we are concentrating our efforts on testing several parameters (e.g. mesh design, starting resistivity, damping and covariance analysis) involved in the 3D inversion nonlinear conjugate gradient ModEM code. Following the best practical procedures suggested by previous MT studies to avoid bad impact on the inversion results, misfits have been achieved nRMS ~ 2.2 in joint inversion of impedance tensor and tipper and nRMS < 2 for separate inversion of these transfer functions. Notwithstanding, preliminary resistivity models present good agreement with previous geophysical studies in the area and portray remarkable large-scale middle crust and deep conductive structures inside the covered area. Since there are several geodynamic processes associated with this area and electrical signatures may still exist the MT data set can be an important key to understand the evolution of the pan-African cycle and unravel new findings related to the formation of the Parnaiba basin.</p>
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Um conjunto de metodos geoeletromagneticos foi empregado na parte sudeste da bacia do Parnaiba para a investigacao de aquiferos profundos. Diversas sondagens eletrica vertical (SEV), eletromagnetica transiente (TEM) e magnetotelurica (MT)... more
Um conjunto de metodos geoeletromagneticos foi empregado na parte sudeste da bacia do Parnaiba para a investigacao de aquiferos profundos. Diversas sondagens eletrica vertical (SEV), eletromagnetica transiente (TEM) e magnetotelurica (MT) foram realizadas ao longo de um perfil de 180 km, que atravessa importantes feicoes estruturais e se estende ate a borda cristalina sudeste da bacia. Os dados geoeletromagneticos foram invertidos ‘conjuntamente’ utilizando esquema de inversao 1-D, apresentando a distribuicao da resistividade eletrica em profundidade para cada sondagem. O uso integrado de TEM/MT permitiu corrigir os dados MT de “static shift” e melhor definir as estruturas rasas e profundas. Resultados da inversao 2-D dos dados MT permitiu mapear a distribuicao lateral dos aquiferos e identificar as principais feicoes estruturais que influenciam o regime hidrodinâmico da bacia, apresentando boa concordância com a interpretacao dos dados aeromagneticos. Os resultados obtidos evidenciam a importância da utilizacao de metodos geoeletromagneticos integrados como meio de se obter informacao fundamental para estudos hidrogeologicos em bacias sedimentares.
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Research Interests: Geology and Geophysics
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Magnetotelluric (MT) data spanning from 0.001 to 50 000 s were acquired across the Parnaíba cratonic basin along an east–west profile approximately 1430 km long. Our preferred 3D resistivity inversion model revealed the presence of three... more
Magnetotelluric (MT) data spanning from 0.001 to 50 000 s were acquired across the Parnaíba cratonic basin along an east–west profile approximately 1430 km long. Our preferred 3D resistivity inversion model revealed the presence of three distinct lithospheric blocks bounded by major electrical discontinuities. The western block is characterized by mainly resistive crust and upper mantle, comprising the Amazonian Craton and the western border of the Parnaíba Basin. The central block reveals a broad highly conductive anomaly extending to the upper mantle and is entirely within the basin limits. The eastern block extends from the eastern portion of the Parnaíba Basin towards the Borborema Province, and discloses two resistive profound vertical anomalies cutting through the crust and upper mantle, intercalated by a conductive zone. We associate this large conductive zone (>100 km long) with the signature of the Transbrazilian Lineament Shear Zone which appears to have served as palae...
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We evaluate the use of the magnetotelluric (MT) method to locate crystalline basement and overlying carbonate reservoir rocks underneath a thick overburden in the São Francisco basin in Brazil. Mapping the complex basement and the... more
We evaluate the use of the magnetotelluric (MT) method to locate crystalline basement and overlying carbonate reservoir rocks underneath a thick overburden in the São Francisco basin in Brazil. Mapping the complex basement and the carbonate reservoir using seismic reflection is a major problem in hydrocarbon exploration in this intracratonic basin, and it is expected that MT will provide useful complementary information. In the present study, we analysed 31 MT soundings along four survey lines in the central region of the basin. The MT soundings covered a period range of 0.001–100 s, probing the subsurface resistivity structure down to a maximum depth of about 15 km. The MT data were inverted using a regularized two-dimensional (2D) inversion algorithm with a variety of a priori data for comparison. For model appraisal, we analysed well log (gamma ray, deep resistivity and neutron porosity) data as well as seismic, gravity and magnetic profiles coincident with one MT line passing through the well. We found that shallow geological boundaries separating zones of strong resistivity contrasts also coincide with seismic boundaries in the inversion models with or without a priori data. Using gravity data, it was also possible to define the compartmentalized basement in this sector of São Francisco Basin, not clear in the seismic section. However, only by integrating all available information were we able to map the Lagoa do Jacaré and Sete Lagoas carbonate member-formations of the Bambuí Group, which are considered to host both the source and reservoir rocks identified from past exploratory history of this basin. We also imaged a basement structural high with thinned or disrupted conductive cover rocks over a known zone of hydrocarbon microseepage and a buried conductive (source rock?) channel at its NW margin, the trace of which coincides with the present-day River Paracatu along which gas bubbles have been observed. This suggests that MT may be fruitfully integrated with gravity, magnetic and seismic data to study the structural controls on hydrocarbon occurrence in this basin.
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We have applied a crossgradient joint inversion and geospectral visualization method to marine seismic reflection, magnetotelluric, gravity, and magnetic data sets acquired along a 162 km profile across a segment of Santos Basin oil... more
We have applied a crossgradient joint inversion and geospectral visualization method to marine seismic reflection, magnetotelluric, gravity, and magnetic data sets acquired along a 162 km profile across a segment of Santos Basin oil province in the continental margin of southeast Brazil. The main exploration targets are the top of the fractured Precambrian crystalline basement and any concealed basement grabens, the overlying presalt and salt/carbonate deposits, and the postsalt cover deposits. The results of joint inversion clearly mapped the various units and are a significant improvement over previous models derived from separate 2D seismic reflection processing and 2D magnetotelluric imaging. Additionally, multispectral fusion of these models resulted in a single image that permits highly constrained geologic interpretations enabling a better understanding of basin architecture. We suggest that joint inversion and image fusion is the way forward for effective geophysical integra...
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ABSTRACT. This article presents a geological-geophysical model along the 320 km NW-SE profile at the central region of the Paraná basin, based primarily on a 2D gravity model that complements the magnetotelluric (MT) field campaigns... more
ABSTRACT. This article presents a geological-geophysical model along the 320 km NW-SE profile at the central region of the Paraná basin, based primarily on a 2D gravity model that complements the magnetotelluric (MT) field campaigns conducted by the National Petroleum Agency. Studies of large structures within the basement underneath a sedimentary basin are of vital importance for the understanding of its tectonic-stratigraphic evolution. The Paraná basin is a basin with a depocenter, reaching approximately 7000m in estimated depth. Two large regional structures are remarkable in the study area: the Ponta Grossa Arch, a NW-SE direction feature; and the a NW-SE gravimetric high anomaly. The region also includes great lava flows represented by basalt rocks of the Serra Geral Formation. Intrusion of dikes and sills of diabase, dating mainly from the Cretaceous that equally strikes the region. A regional model was created by a 40 km wavelength filtering process, used to estimate the lim...
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The Brazilian Seismographic Network (RSBR) began operating in 2011 with the joint effort of four different institutions: Universities of São Paulo (USP), Brasília (UnB), Rio Grande do Norte (UFRN), and the National Observatory (ON).... more
The Brazilian Seismographic Network (RSBR) began operating in 2011 with the joint effort of four different institutions: Universities of São Paulo (USP), Brasília (UnB), Rio Grande do Norte (UFRN), and the National Observatory (ON). Initially funded by Petrobras (Brazilian State Oil Company) in a large-scale infrastructure project that started in 2009, RSBR is now sponsored by Brazilian Geological Survey (CPRM). Station installation began in 2011 in southeast (SE) Brazil and finished in 2014 in the Amazon. The network is composed of 84 stations (as of December 2017) operated by those four institutions. CPRM now coordinates the general maintenance and compilation of the Brazilian Seismic Bulletin. RSBR has lowered the detection threshold down to m b 3.5 for earthquakes in Brazil as a whole, and in some areas, such as northeast (NE) and SE Brazil, detection thresholds are about m b 3.0. Besides monitoring Brazilian and South American seismicity, RSBR serves as a backbone for temporary deployments to study crustal and upper-mantle structure.
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The Paraná basin is a major Phanerozoic feature in Brazil and adjacent countries. Data from 261 MT stations distributed along 3 lines (P1, P2 and P3) have been inverted in 3D to reveal the resistivity structure from surface to 50km depth.... more
The Paraná basin is a major Phanerozoic feature in Brazil and adjacent countries. Data from 261 MT stations distributed along 3 lines (P1, P2 and P3) have been inverted in 3D to reveal the resistivity structure from surface to 50km depth. The average station spacing is 1.8 km and the frequency bandwidth is 1000 to 0.001 Hz. 2D inversion was also done for comparison with an earlier roughly parallel regional line P0 and found to be consistent with our new 3D model. The goodness of our 3D model was first evaluated by comparison with data from 3 deep exploration wells within our study area. The 3D model correctly identified resistive (>100 Ωm) basalt cover, underlying conductive (~10 Ωm) sediments (up to 5-6 km thick) and resistive (>1000 Ωm) crystalline basement. The MT results are consistent with other geophysical anomalies previously observed in the study area; the segmented midcrustal resistive layer found in a zone of depressed Moho is interpreted as evidence of magmatic unde...