Ícaro Vitorello

Geomagnetically induced currents in an electric power transmission system at low latitudes in Brazil: A case study

[1]xa0Geomagnetically induced currents (GICs) are a ground end manifestation of space weather processes. During large geomagnetic storms, GICs flow between the grounding points of power transformers and along electric power transmission lines connecting the transformers. In high-latitude regions, damages to power transformers are reported where storm time geomagnetic variations are very rapid and large (>1000 nT), and hence the GICs as large as or even greater than 100 A end up flowing through the windings of power transformers. At low latitudes, geomagnetic variations are less severe, and hence much smaller GIC values are generally reported there. However, the flow of GICs and their effects on power transformers are complex processes, and careful evaluation is needed even in such low-latitude regions as, for example, Brazil. We report here a study on GIC measurements in Brazil conducted under a cooperative project between FURNAS (the Brazilian electric power company) and the National Institute for Space Research. During a large geomagnetic storm, which took place on 7–10 November 2004, the GIC amplitudes, measured on the basis of geomagnetic variations in 500 kV power transmission lines in the S–E region of Brazil, were found to be around 15 A.

Variability of Laboratory Measured Soil Lines of Soils from Southeastern Brazil

Abstract Linear relationships (soil lines) between the conventional red (R) and near-infrared (NIR), and between unconventional NIR reflectance bands were investigated for the effects of spectral positioning and widths of approximately simulated bands of some broad- and narrow-band sensors, and for the influence of the chemical constituents and moisture in the samples. The investigation was based on laboratory spectra of air-dried samples from soil profiles from southeastern Brazil, an experiment for the chemical removal of organic matter, and on a wetting process of the soil samples. The relationships were evaluated for a general soil line and for the individual lines of some soil types, under air-dried condition, through the determination of the line parameters, dispersion statistics, and principal components analysis (PCA). Soil lines obtained with broad or narrow NIR bands positioned at shorter wavelengths, and consequently at smaller distance differences between the pair of bands, presented better fitted lines than the ones obtained with NIR bands located at longer wavelengths and at larger distances. Compared with the broad bands, spectrally better positioned narrow NIR/R bands produced closely similar results because of the propitious effect of the larger bandwidth. However, unconventional NIR/NIR bands presented better fitted lines than the R/NIR bands. The inherent interplay between organic matter and iron oxides is accountable for the variability in the linear parameters of the individual lines. The general line for the samples considered is composed of nonparallel segments of the different soil types, with a tendency for an increased obliquity in the low albedo soils. Soil types with very low albedo present line slope values below 1, poor correlations, but small root mean square (RMS) values, whereas soil types with moderate and high albedo present line slope values above 1, strong correlations, and large RMS values. The former is rich in organic matter and iron oxides whereas the latter has more transparent components, such as quartz, or less opaque substances. However, in soil types with a lesser content of organic matter, the iron spectral features are enhanced and variations in the shape of the spectra are produced because of the larger reflectance increase in the R band than in the NIR interval. The maximum spectral contrast resulting from this compositional interplay is assessed when the NIR band is positioned at longer wavelengths. Similarly, soil wetting introduces nonlinear spectral changes that also affect the characteristics of the soil lines. The analysis of field measured reflectance spectra derived from an experiment designed to monitor the growth of a bean crop indicates that the shift of the NIR band towards shorter wavelengths produces not only better fitted soil lines but also stronger vegetation signals. The enhancement of the vegetation signal obtained with the displacement of the NIR band results from the steeper descending slope towards longer wavelengths of the NIR plateau observed in the vegetation spectra. The results place constraints on an unsupported choice of bands to derive vegetation indices, especially those of the advanced hyperspectral imaging sensors. The most recommended pair of NIR-R bands for crop vegetation indices is the one from LANDSAT-TM, closely followed by JERS-OPS and SPOT-HRV, or from narrow NIR-R bands positioned within these ranges at spectral intervals not affected by atmospheric attenuation.

Relationships of spectral reflectance and color among surface and subsurface horizons of tropical soil profiles

Abstract The reflectance spectra (400–2500 nm) and color attributes of samples from the major horizons of six soil classes with broad distribution in Brazil were analyzed for intrinsic relationships (principal components analysis) as well as for correlations with chemical components. The objective was to detect the spectral changes with depth that are related to the soil chemical properties, thus improving the pedological characterization of the soil profile and vertically extending the links between topsoil properties and remote sensing data. The albedo (first principal component) differentiated the studied soil profiles in all horizons and was inversely related to FeA (total iron), Ti02, and Al203 contents. The spectral variability within each soil profile was associated with changes in the shape (slope) of the spectra (second principal component). Such changes produced ratio values between longer and shorter wavelengths that tended to decrease with depth because of the increasing quantities of clay components, and the interplay between iron and carbon, respectively. Thus, in each profile, the albedo was generally preserved, but the shape of the spectra was not, except for soils with very low albedo. Hematite-rich soils were distinguished from goethite-rich soils by the redness index, which was estimated from the three color parameters: dominant wavelength, purity, and luminance. The results suggested that the use of spectroradiometers to measure reflectance of samples from several horizons can expedite soil surveys, and also optimize laboratory analysis for soil chemical evaluation. Also, they can be important to characterize the relative distribution of some soil constituents in large unvegetated areas.

Effects of Band Positioning and Bandwidth on NDVI Measurements of Tropical Savannas

Abstract The consequences of shifting the spectral location of narrow and broad red (R) and near-infrared (NIR) bands on measurements of the normalized difference vegetation index (NDVI) of Brazilian savannas were investigated for different plant communities and seasonal phenology. The basic data came from an Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) image obtained during the regional dry season and two Landsat 5-Thematic Mapper (TM) images from the dry and rainy seasons. Principal components analysis (PCA) was applied to a set of 64 AVIRIS bands from the interval of 500–1100 nm in order to select the R–NIR band pair of greatest spectral contrast and respective endmember spectra for the R and NIR band variations. For the endmember spectra, modeled AVIRIS-derived NDVI values, obtained by changing the spectral positioning and bandwidths of R–NIR band pairs, were compared with NDVI calculated from simulated R–NIR band pairs of orbital sensors. Furthermore, the suitability of different positions and bandwidths was also examined employing, as criterion, the calculated NDVI contrast between the endmember spectra. The results revealed that the NDVI of the green vegetation was essentially affected by the proximity of the R and NIR bands to the spectral interval of the red edge (690–750 nm). The strongest effects were observed for the R bands that partly extended longwards the 690 nm wavelength. On the other hand, the NDVI of the nonphotosynthetic vegetation varied up to 100% within the studied interval, especially as a function of the R–NIR spectral distance. The largest NDVI contrast between green and dry vegetation was obtained with the convergent displacement of the R and NIR bands, respectively, towards the 690 nm and 750 nm wavelengths, as predicted from PCA. Results from simulated orbital sensors demonstrated that bandwidths have no significant influence on the NDVI provided the R and NIR bands are not extended into the red edge domain. The variable NDVI contrasts along traverses presented the smallest values when associated with the conspicuous green vegetation, and the largest values when associated with the dried-out grasslands or exposed soils.

Electromagnetic constraints for subduction zones beneath the northwest Borborema province: Evidence for Neoproterozoic island arc-continent collision in northeast Brazil

The Borborema province in northeast Brazil occupies a crucial position in the complex Neoproterozoic West Gondwana reconstruction puzzle. However, correlation attempts between northeast Brazil and West Africa have been hampered because key links in the internal structure of the Borborema province have yet to be identifi ed. To aid such an correlation, a magnetotelluric study was undertaken along two subparallel profi les to image the deep electrical structure in the northwestern part of the province. Despite the occurrence of recurrent tectonothermal episodes that affected the region in the past, two-dimensional models show that a large-scale signature of the assembled terrane during the Neoproterozoic accretion and collision is plausibly preserved in the area. Two resistive features dipping from the upper crust into the upper mantle in downward convergence (opposite directions) are defi ned beneath one of the profi les that are interpreted to be related to remnants of former subduction slabs, since the observed high-resistivity zone is consistent with a dehydrated oceanic lithosphere depleted of sediments. On the basis of geological and geochemical information, a model of collision of an intraoceanic magmatic arc coalesced into an earlier passive margin is proposed for the Neoproterozoic tectonic evolution of the province, involving processes of reversal of subduction polarity and oceanic slab breakoff.

2-D geoelectrical model for the Parnaiba Basin conductivity anomaly of northeast Brazil and tectonic implications

Abstract A magnetometer array study in the north-northeast of Brazil has revealed a roughly NE–SW-trending conductive structure in the southeastern part of the intracratonic Parnaiba Basin. The magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation. The 2-D numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body. The anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data, the graben structure in the basement is corroborated by the aeromagnetic data. The formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the Brasiliano orogeny or with the Jurassic–Cretaceous magmatic events. The diabase dikes intruded in the basin in association with the Jurassic–Cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting Paleozoic sediments. The thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon-saturated sediments.

Spectroradiometric discrimination of laterites with principal components analysis and additive modeling

Abstract Bidirectional reflectance factor determinations in the visible and near-infrared of mineralized laterite samples collected at the surface of the alkaline / ultramafic rocks-carbonatite complex of Maicuru, northern Brazil, were evaluated for iron, phosphate, and titanium content. Nineteen spectra were subjected to principal components analysis in order to get a better insight into the factors affecting spectral discrimination. The spatial distribution in a PC1 vs. PC2 plot is related to the overall reflectance (albedo) along the PO axis and to the spectral inversion around 1100 nm along the PC2 axis. The latter can be characterized by any spectral ratio of reflectance values taken before and after the inversion wavelength. Two major groups were thus identified: titanium-rich (anatase, ilmenite) laterites and aluminum phosphate laterites. The former presented spectra with low to intermediate albedos and lower near-infrared / visible and very near-infrared reflectance ratio values. The phosphate-rich samples presented intermediate to high albedos and higher reflectance ratios. Titaniferous laterites have a greater content of opaques than the phosphates which are usually richer in iron oxides. The presence of titanium lowers the albedo whereas iron oxides absorb only in the visible and very near-infrared. Binary mixtures of Fe 2 0 3 and carbon provided spectral variations analogous to the ones observed in the titaniferous laterites. Such modeling helped in evaluating the effects of opaques. As a practical result, a band radiometer could conceivably identify the titanium mineralized areas in the field.

Magnetotelluric soundings across the Taubaté Basin, Southeast Brazil

Thirteen magnetotelluric (MT) soundings were carried out in Neoproterozoic crystalline terrains and Tertiary sediments of the Taubaté basin, southeastern Brazil. The soundings were deployed in a cross-strike profile bisecting the basin along one of its thickest sub-basins and extending over mountain plateaus to the southeast and the northwest. Occurrences of numerous alkaline plugs and aligned tectonic grabens in the region are records of intense Mesozoic-Cenozoic tectonic activity. MT analysis techniques were used to evaluate data quality, infer strike direction, and correct for near-surface distortion. As the studied area is located only some tens of kilometers away from the Atlantic Ocean and in one of the most densely populated regions of the country, the data are severely distorted by industrial interference and the coast effect. Because of such effects, the data are modelled using a 2D inversion scheme within periods shorter than 0.1 s for the Taubaté Basin sites and shorter than 1–10 s for the off-basin sites, the latter depending on the distance of the site from the coast. The main result observed in the modelling is the identification of a conducting zone below 10 km depths beneath the region. There is poor resolution in the data of structures below this conductor, which is also not imaged beneath the basin. Studies carried out in different tectonic regions of the world have also reported conductive layers at about the same depth but in the studied area it is impossible to reach any conclusion about the total conductance of the layer with the available MT data.

Disturbances on magnetotelluric data due to DC electrified railway: A case study from southeastern Brazil

Magnetotelluric (MT) soundings were carried out in the period range of 20 to 6000 s along profiles roughly orthogonal to the Campos do Jordão Railway (CJRW), in the Brazilian southeastern region. The profiles were located over two adjacent regions with contrasting conductivity, the conductive sedimentary region of the Taubaté Basin and the resistive crystalline region of the Serra da Mantiqueira. The railway operates with DC current that produces an intense electromagnetic noise but only during diurnal periods, being turned off at night. The objective of this study is to characterize the CJRW noise in order to verify its effect on MT parameters. It was inferred that the entire length of the Taubaté Basin is probably affected by the noise, whereas in the crystalline terrains the noise reaches distances in the range of 76 to 126 km. The electric channels show a strong dependence on geology which is suggestive of the potential application of the CJRW as a controlled source in geophysical studies. The data were processed with modern techniques presently available to the scientific community. Under the conditions of the present study, it was observed that the robust Single Station technique is as efficient as the robust Remote Reference to remove the kind of noise generated by the CJRW, an intense perturbation that affects only some well-defined portions of the time series. Finally, the analyses reaffirm the necessity of a careful choice of the station to be used as reference in the Remote Reference technique.

Effects of the equatorial electrojet on magnetotelluric surveys: Field results from northwest Brazil

The distortion effect that the nonuniform equatorial electrojet (EEJ) currents might have on the plane-wave assumption in magnetotelluric (MT) appli- cations is investigated under experimental conditions. A broad-band (0.0006-2048 s) MT survey, consisting of 8 stations aligned at right angles to the magnetic equator and probing diverse geologic terrains, was car- ried out during the daytime, under the presence of the EEJ nonuniform source, and at nighttime, when the EEJ flux almost vanishes. A comparison between the daytime and nighttime soundings did not show any sig- nificant divergence, contrary to expectation from some theoretical calculations. Thus, the results assure that the traditional plane-wave formula employed in MT can provide reliable subsurface conductivity structures even under the influence of the EEJ, at least up to periods of 2048 s. An additional modelling exercise conside- ring the two models most commonly used to simulate the EEJ sources was also performed. It indicates that the line-current model generates departures from the plane-wave results not observed in.;practice and that the Gaussian model only agrees with the experimental data, within their error bars, if the geoelectric section approaches its most conductive limit defined from a sta- tic shift evaluation.