Produção Científica

**Artigo em Revista**

Representation of discontinuous seismic velocity fields by sigmoidal functions for ray tracing and travel time modellingWave-modelling methods based on asymptotic ray theory have a lower computational cost than full wave-equation methods but require a smooth velocity field, though discontinuities may be handled by imposing interface conditions between adjacent blocks. We propose to approximate discontinuous velocity fields with model parametrizations based on smooth, rapidly varying functions known as sigmoidal functions. We have implemented the proposed technique on Cartesian grids using the wavelet theory formalism. Numerical experiments with 2-D and 3-D initial-value and two-point ray tracing in heterogeneous media show that the ray paths and traveltimes produced with the sigmoidal representation are consistent with the results produced by conventional ray tracing in block structures, broadening the scope of classical algorithms based on smooth velocity fields. |

**Artigo em Revista**

Oil Spill Detection and Mapping: A 50-Year Bibliometric AnalysisOil spill detection and mapping (OSPM) is an extremely relevant issue from a scientific point of view due to the environmental impact on coastal and marine ecosystems. In this study, we present a new approach to assess scientific literature for the past 50 years. In this sense, our study aims to perform a bibliometric and network analysis using a literature review on the application of OSPM to assess researchers and trends in this field of science. In methodological terms we used the Scopus base to search for articles in the literature, then we used bibliometric tools to access information and reveal quantifying patterns in this field of literature. Our results suggest that the detection of oil in the sea has undergone a great evolution in the last decades and there is a strong relationship between the technological evolution aimed at detection with the improvement of remote sensing data acquisition methods. The most relevant contributions in this field of science involved countries such as China, the United States, and Canada. We revealed aspects of great importance and interest in OSPM literature using a bibliometric and network approach to give a clear overview of this fieldâ€™s research trends. |

**Artigo em Revista**

A wavefield domain dynamic approach: Application in reverse time migrationThis paper proposes a novel technique to handle the wavefield domain involved in the procedures of seismic modeling, reverse-time migration (RTM), and full-waveform inversion (FWI). This method considers that the size of the wavefield domain varies with time, in other words, that it expands concomitantly to the propagation. However, in the geophysical literature, this dynamism has always been neglected as the wavefield domain is constantly considered to be fixed, thus, representing what we call a static approach (SA). This assumption may incur unnecessary use of available computational resources, thereby compromising application performance. Herein, we create a so-called dynamic approach (DA), capable of obtaining truly significant gains in terms of memory consumption and computational time. This new methodology is based on the application of an empirical filter that delimits the wavefront. This filter functions as a window and it is applied at each timestep until the wavefront reaches the model's boundaries, selecting the area where the seismic wavefield exists. This approach tries to approximate the computational domain to the propagation domain in order to obtain valuable computational gains, by eliminating unnecessary work, thus reducing the amount of work needed to perform forward and backward propagation. We compare both approaches using the Pluto model. The seismic data generated from the Pluto model is very large and it was not possible to use the static approach with it relying only on the random-access memory (RAM) of the used hardware. In order to perform the conventional RTM, we implement and compare the effective boundary technique for wavefield reconstruction with the RTM using the proposed dynamic approach. With the dynamic approach, it was possible to perform RTM of a 2D seismic data obtained from the Pluto model using only the RAM of the computational nodes and without the need of reconstruction techniques. |

**Artigo em Revista**

Prestack seismic data reconstruction and denoising by orientation-dependent tensor decompositionMultidimensional seismic data reconstruction and denoising can be achieved by assuming noiseless and complete data as low-rank matrices or tensors in the frequency-space domain. We propose a simple and effective approach to interpolate prestack seismic data that explores the low-rank property of multidimensional signals. The orientation-dependent tensor decomposition represents an alternative to multilinear algebraic schemes. Our method does not need to perform any explicit matricization, only requiring to calculate the so-called covariance matrix for one of the spatial dimensions. The elements of such a matrix are the inner products between the lower-dimensional tensors in a convenient direction. The eigenvalue decomposition of the covariance matrix provides the eigenvectors for the reduced-rank approximation of the data tensor. This approximation is used for recovery and denoising, iteratively replacing the missing values. We present synthetic and field data examples to illustrate the method's effectiveness for denoising and interpolating 4D and 5D seismic data with randomly missing traces. |

**Artigo em Revista**

Modelling of mechanical wave propagationThe numerical modeling of mechanical waves is currently a fundamental tool for the study and investigation of their propagation in media with heterogeneous physical properties and/or complex geometry, as, in these cases, analytical methods are usually not applicable. These techniques are used in geophysics (geophysical interpretation, subsoil imaging, development of new methods of exploration), seismology (study of earthquakes, regional and global seismology, accurate calculation of synthetic seismograms), in the development of new methods for ultrasonic diagnostics in materials science (non-destructive methods) and medicine (acoustic tomography). In this paper we present a review of numerical methods that have been developed and are currently used. In particular we review the key concepts and pioneering ideas behind finite-difference methods, pseudospectral methods, finite-volume methods, Galerkin continuous and discontinuous finite-element methods (classical or based on spectral interpolation), and still others such as physics-compatible, and multiscale methods. We focus on their formulations in time domain along with the main temporal discretization schemes. We present the theory and implementation for some of these methods. Moreover, their computational characteristics are evaluated in order to aid the choice of the method for each practical situation. |

**Artigo em Revista**

Multiphase flow mobility impact on oil reservoir recovery: An open-source simulationThis work uses Computational Fluid Dynamics (CFD) to simulate the two-phase flow (oil and water) through a reservoir represented by a sandbox model. We investigated the influence in the flows of water having higher and lower mobilities than oil. To accomplish this, we also developed a dedicated solver, with the appropriated equations and representative models implemented in the open-source CFD OpenFOAM platform. In this solver, the black-oil model represented the oil. The results show that the Buckleyâ€“Leverett water-flood equation is a good approach for the three-dimensional flow. We observe that the water wall front is mixed to some extent with the oil and evolves obeying an exponential law. Water with mobility lower than oil is not common. However, in this case, the oil recovery is improved and the amount of injected water is reduced. The results comparing different mobilities show that a careful economic assessment should be performed before the field development. We have shown that the low water mobility can increase, as in this studied example, the water front saturation from 0.57 to 0.73, giving a substantial improvement in the oil recovery. The reservoir simulation can provide all process information needed to perform an economical assessment in an oil field exploration. |

**Artigo em Revista**

Geometrical influence of the source/drains configuration on the flow interactions in a sandbox model: A three-dimensional OpenFOAM simulationThis paper explores the interaction of diÂ®erent Â°ow paths in a porous medium by observing the eÂ®ect of having more than one drain in a simple model domain with a single source. The work is based on three-dimensional numerical simulations of the Â°ow of injected water in a sandbox domain with porous volume completely Â¯lled by water and oil. The calculation uses the OpenFOAM library to solve Darcy's equations for the dynamics of a two-phase Â°ow: water as the wetting, oil as the nonwetting Â°uid. We observe the interactions of Â°ows in diÂ®erent paths under changes of number of drains and their relative positions. |

**Artigo em Revista**

Can euler deconvolution outline threeâ€dimensional magnetic sources?Severe limitations of the standard Euler deconvolution (ED) to outline source shapes have been pointed out. However, ED has been widely employed on field data to outline interfaces, as faults and thrust zones. We investigate the limitations of the 3D EDâ€derived estimates of source dip and volume with the use of reducedâ€toâ€theâ€pole synthetic and field anomalies. The synthetic anomalies are generated by two types of source bodies: 1) uniformly magnetized prisms, presenting either smooth or rough interfaces, and 2) bodies presenting smooth delimiting interfaces but strong internal variation of magnetization intensity. The dip of the first type of body might be estimated from the ED solution cluster if the ratio between the depth to the top and vertical extent is relatively high (>1/4). For the second type of body, besides dip, the source volume can be approximately delimited from the solution cluster envelope, regardless of the referred ratio. We apply ED to two field anomalies which are caused by a curvedâ€shape thrust zone and by a banded iron formation. These anomalies are chosen because they share characteristics with the two types of synthetic bodies. For the thrust zone, the obtained ED solutions show spatial distribution allowing to estimate a source dip that is consistent with the surface geology data, even if the above mentioned ratio is much less than 1/4. Thus, there are other factors, like a heterogeneous magnetization, which might be controlling the vertical spreading of the ED solutions in the thrust zone. On the other hand, for the ironâ€ore formation, the solution cluster spreads out occupying a volume, in accordance with the results obtained with the synthetic sources having internal variation of magnetization intensity. As conclusion, although EDâ€derived solutions cannot offer accurate estimates of source shapes, they might provide a sufficient degree of reliability in the initial estimates of the source dip and volume, which may be useful in a later phase of more accurate modeling. |

**Artigo em Revista**

Enhancing stratigraphic, structural and dissolution features in GPR images of carbonate karst through data processingObtaining highâ€quality ground penetrating radar (GPR) images in karst is difficult because materials resulting from the weathering of carbonate rocks might be electrically conductive. As a consequence, penetration depth and signal resolution might be greatly reduced due to attenuation. In addition, fractures and faults might cause a significant amount of electromagnetic wave scattering. We present a 2D data processing flow which allows improving the quality of GPR images in carbonate karst. The processing flow is composed of the following steps: obtaining a zeroâ€offset section by removing the direct wave, lowâ€frequency noise removal, geometrical spreading and exponential gain compensation, spectral balancing, Kirchhoff migration, bandpass filtering, amplitudeâ€volume enhancement, and topographic correction. For a 200â€MHz dataset, we present in detail each step of the processing flow, exemplifying how to parameterize every step. Spectral balancing is of key importance because it can approximately replenish the highâ€frequency content lost due to propagation effects. In this step, we recommend to shift the centroid frequency as much as possible to highâ€frequency values, even exceeding the nominal value of the antenna center frequency, but still looking for a bandâ€limited spectrum as the goal. Despite the difficulty of migrating GPR data, we show that migration (even assuming a constant velocity) might enhance the lateral continuity of the reflection events and allows identification of discontinuities such as faults and fractures. If imaged in a better way, these structures can have special importance as they are often the boundaries of dissolution features. Obtaining images based on amplitudeâ€volume enhancement techniques allows to better visualize karst voids and deepâ€rooted discontinuities because these features are often associated with lowâ€amplitude zones, which are highlighted in such images. Due to this processing flow, stratigraphic, structural and dissolution features can be enhanced, allowing the interpreter to establish spatial and genetic associations among these elements to obtain a better understanding of the karst formation process. |

**Artigo em Revista**

Estimation of the seismic wavelet through homomorphic deconvolution and well log data: application on well-to-seismic tie procedureWavelet estimation and well-tie procedures are important tasks in seismic processing and interpretation. Deconvolutional statistical methods to estimate the proper wavelet, in general, are based on the assumptions of the classical convolutional model, which implies a random process reflectivity and a minimum-phase wavelet. The homomorphic deconvolution, however, does not take these premises into account. In this work, we propose an approach to estimate the seismic wavelet using the advantages of the homomorphic deconvolution and the deterministic estimation of the wavelet, which uses both seismic and well log data. The feasibility of this approach is verified on well-to-seismic tie from a real data set from Viking Graben Field, North Sea, Norway. The results show that the wavelet estimated through this methodology produced a higher quality well tie when compared to methods of estimation of the wavelet that consider the classical assumptions of the convolutional model. |