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Rio de Janeiro, Brazil

Martins S.S.,National Observatory | Martins S.S.,University of Alberta | Travassos J.M.,Federal University of Rio de Janeiro
Geophysics | Year: 2014

Most of the data acquisition in ground-penetrating radar is done along fixed-offset profiles, in which velocity is known only at isolated points in the survey area, at the locations of variable offset gathers such as a common midpoint. We have constructed sparse, heavily aliased, variable offset gathers from several fixed-offset, collinear, profiles. We interpolated those gathers to produce properly sampled counterparts, thus pushing data beyond aliasing. The interpolation methodology estimated nonstationary, adaptive, filter coefficients at all trace locations, including at the missing traces' corresponding positions, filled with zeroed traces. This is followed by an inversion problem that uses the previously estimated filter coefficients to insert the new, interpolated, traces between the original ones. We extended this two-step strategy to data interpolation by employing a device in which we used filter coefficients from a denser variable offset gather to interpolate themissing traces on a few independently constructed gathers. We applied the methodology on synthetic and real data sets, the latter acquired in the interior of the Antarctic continent. The variable-offset interpolated data opened the door to prestack processing, making feasible the production of a prestack time migrated section and a 2D velocity model for the entire profile. Notwithstanding, we have used a data set obtained in Antarctica; there is no reason the same methodology could not be used somewhere else. © 2015 Society of Exploration Geophysicists.

In this study we examine glaciogenic rhythmites from the Late Palaeozoic Itararé Group, Paraná Basin, Brazil. We conduct spectral analysis on lithological cycle ('couplet') thickness series, and declination of maximum axis of anisotropy of magnetic susceptibility ellipsoidal tensor (K1) data. We tested the efficiency of K1 as a palaeoclimatic proxy. To constrain the timescale of harmonic features in the data, we analysed the couplet thickness spectra, converting the spectra to the time domain using an astronomical calibration based on Milankovitch frequency ratios. Comparison of the two rhythmites provides insights into their sedimentation rate evolution and cyclicity. Millennial-scale mechanisms of climatic origin influenced the deposition of both rhythmites, generating the lithological couplets, and are consistent with millennial-scale variations recognized as triggers for large-scale climatic changes during the Late Pleistocene. The common harmonic features in the couplet thickness and K1 spectra support the view that the azimuth of the K1 axis in sedimentary fabric is a useful palaeoclimatic proxy, reflecting sedimentation processes that were directly influenced by flow-induced, sediment transport, which is linked to external climate factors. © The Geological Society of London 2013.

Franco D.R.,National Observatory | Hinnov L.A.,Johns Hopkins University | Ernesto M.,University of Sao Paulo
Geochemistry, Geophysics, Geosystems | Year: 2011

We investigate the depositional time scale of lithological couplets (fine sandstone/siltstone-siltstone/mudstone) from two distinctive outcrops of Permo-Carboniferous glacial rhythmites in the Itararé Group (Paraná Basin, Brazil). Resolving the fundamental issue of time scale for these rhythmites is important in light of recent evidence for paleosecular variation measured in these sequences. Spectral analysis and tuning of high-resolution gray scale scans of sediment core microstratigraphy, which comprises pervasive laminations, reveal a comparable spectral content at both localities, with a frequency suite interpreted as that of short-term climate variability of Recent and modern times. This evidence for decadal- to centennial-scale deposition of these lithological couplets is discussed in light of the 'varvic' character, i.e., annual time scale that was previously assumed for the rhythmites. Copyright 2011 by the American Geophysical Union.

Mandea M.,CNRS Paris Institute of Global Physics | Holme R.,University of Liverpool | Pais A.,University of Coimbra | Pinheiro K.,National Observatory | And 2 more authors.
Space Science Reviews | Year: 2010

The secular variation of the core field is generally characterized by smooth variations, sometimes interrupted by abrupt changes, named geomagnetic jerks. The origin of these events, observed and investigated for over three decades, is still not fully understood. Many fundamental features of geomagnetic jerks have been the subject of debate, including their origin internal or external to the Earth, their occurrence dates, their duration and their global or regional character. Specific tools have been developed to detect them in geomagnetic field or secular variation time series. Recently, their investigation has been advanced by the availability of a decade of high-quality satellite measurements. Moreover, advances in the modelling of the core field and its variations have brought new perspectives on the fluid motion at the top of the core, and opened new avenues in our search for the origin of geomagnetic jerks. Correlations have been proposed between geomagnetic jerks and some other geophysical observables, indicating the substantial interest in this topic in our scientific community. This paper summarizes the recent advances in our understanding and interpretation of geomagnetic jerks. © 2010 Springer Science+Business Media B.V.

Franco D.R.,National Observatory | Hinnov L.A.,Johns Hopkins University | Ernesto M.,University of Sao Paulo
Geology | Year: 2012

Two late Paleozoic glacial rhythmite successions from the Itararé Group (Paraná Basin, Brazil) were examined for paleoclimate variations. Paleomagnetic (characteristic remanent magnetization, ChRM) and magnetic susceptibility (Kz) measurements taken from the rhythmites are interpreted as paleoclimatic proxies. Ratios of low-frequency components in the Kz variations suggest Milankovitch periodicities; this leads to recognition of other, millennial-scale variations reminiscent of abrupt climate changes during late Quaternary time, and are suggestive of Bond cycles and the 2.4 k.y. solar cycle. We infer from these patterns that millennial-scale climate change is not restricted to the Quaternary Period, and that millennial forcing mechanisms may have been prevalent throughout geologic time. © 2012 Geological Society of America.

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