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Rastogi R.G.,Physical Research Laboratory | Trtvedi N.B.,National Institute for Space Research | Fontes S.,Observatorio Nacional ON MCT
Proceedings of the National Academy of Sciences India Section A - Physical Sciences

The paper describes the features of H, Y and Z components of the magnetic field at an intense array of 26 stations within the equatorial electrojet (EEJ) belt in N-NE Brazil, on 16 selected quiet days in December 1990 - January 1991. The magnitude of daily range of H at EEJ centre on Individual days varied from the high value of 240 nT to a low value of 50 nT; the time of daily maximum in H varied from 11 h LT to 15 h LT. The daily variation of △H on an individual day minus the mean daily variation of △h for all the selected quiet days showed a semi-diurnal curve, with varying amplitude and phase on different days. It is suggested that the whole equatorial current system is modulated by the luni-solar semi-diurnal tides on a day-today basis. Source

Solon F.F.,Observatorio Nacional ON MCT | Fontes S.L.,Observatorio Nacional ON MCT | Flexor J.M.,Observatorio Nacional ON MCT | Meju M.,Petronas
SEG Technical Program Expanded Abstracts

The magnetotelluric (MT) method has been widely used in studies of terrestrial and marine Brazilian basins. A major problem in oil exploration is precisely locating basement fracture-zones and heterogeneous carbonate reservoirs underneath thick overburden. Fractures granitic zones, massive carbonates rocks can be detectable by electromagnetic (EM) methods, since it is geologically expected that this rock may form regional unconformity-related conductive horizons. In the present study we analyzed several MT soundings from the central region of São Francisco Basin in northwest portion of the state of Bahia near the region called Remanso do Fogo in Brazil. The main thrust was to obtain depth-converted EM attributes of fractured carbonate rocks and to map the resistivity structure of well-known portions of the basins. It is shown that the stratigraphic groups characterized in seismic reflection images are in agreement with structural features revealed by MT conductivity imaging. © 2011 Society of Exploration Geophysicists. Source

Gallardo L.A.,CICESE | Gallardo L.A.,Center for Exploration Targeting | Fontes S.L.,Observatorio Nacional ON MCT | Meju M.A.,Petronas | And 3 more authors.

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 integration. © 2012 Society of Exploration Geophysicists. Source

Pimentel E.T.,Observatorio Nacional ON MCT | Pimentel E.T.,Federal University of Amazonas | Hamza V.M.,Observatorio Nacional ON MCT
Journal of South American Earth Sciences

The present work deals with determination groundwater flows in the Amazon region, based on analysis of geothermal data acquired in shallow and deep wells. The method employed is based on the model of simultaneous heat transfer by conduction and advection in permeable media. Analysis of temperature data acquired in water wells indicates down flows of groundwaters with velocities in excess of 10 -7m/s at depths less than 300m in the Amazonas basin. Bottom-hole temperature (BHT) data sets have been used in determining characteristics of fluid movements at larger depths in the basins of Acre, Solimões, Amazonas, Marajó and Barreirinhas. The results of model simulations point to down flow of groundwaters with velocities of the order of 10 -8 to 10 -9m/s, at depths of up to 4000m. No evidence has been found for up flow typical of discharge zones. The general conclusion compatible with such results is that large-scale groundwater recharge systems operate at both shallow and deep levels in all sedimentary basins of the Amazon region. However, the basement rock formations of the Amazon region are relatively impermeable and hence extensive down flow systems through the sedimentary strata are possible only in the presence of generalized lateral movement of groundwater in the basal parts of the sedimentary basins. The direction of this lateral flow, inferred from the basement topography and geological characteristics of the region, is from west to east, following roughly the course of surface drainage system of the Amazon River, with eventual discharge into the Atlantic Ocean. The estimated flow rate at the continental margin is 3287m 3/s, with velocities of the order of 218m/year. It is possible that dynamic changes in the fluvial systems in the western parts of South American continent have been responsible for triggering alterations in the groundwater recharge systems and deep seated lateral flows in the Amazon region. © 2012 Elsevier Ltd. Source

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