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Slides for the oral presentation "Adaptive learning gravity inversion for 3D salt body imaging" presented at the SEG International Exposition andSeventy-Eighth Annual Meeting in Las Vegas, Nevada, USA. Abstract We have developed an iterative scheme for inverting gravity data produced by salt bodies with density contrasts relative to the sediments varying from positive to negative, crossing, in this way, the nil zone. Our inversion method estimates a 3D density-contrast distribution, through a piecewise constant function defined on a user-specified grid of cells. It consists of two nested iterative loops. The outer loop uses an adaptive learning strategy that starts with a coarse grid of cells, a set of first-guess geometric elements (axes and points) and the corresponding assigned density contrasts. From the second iteration on, this strategy refines the grid and automatically creates a new set of geometric elements (points only) and associated density contrasts. Each geometric element operates as the first-guess skeletal outline of a section of the salt body to be imaged. The inner loop estimates the 3D density-contrast distribution for the grid of cells and for the set of geometric elements defined in the outer loop. The outer loop allows for easy incorporation of prior geologic information about the lithologic units and automatic evolution of the prior information. The inner loop forces the estimated density contrast of each cell to be close either to a null or to a non-null pre-specified value. The iteration stops when the geometries of the estimated salt bodies are invariant along successive iterations. We apply our method to synthetic gravity data produced by a homogeneoussalt body embedded in heterogeneous sediments. We tested two geologic hypotheses about the real gravity data from Galveston Island salt dome, USA. In the first, the estimated salt body attains a maximum bottom depth of 5 km, whereas in the second hypothesis, it is shallower and discloses an overhang. Both solutions fit the data and are feasible geologically, so both hypotheses are acceptable.

Slides for the oral presentation "3D depth-to-basement and density contrast estimates using gravity and borehole data" presented at the SEG International Exposition and Seventy-Ninth Annual Meeting in Houston, Texas, USA. ABSTRACT We have developed a gravity-inversion method for simultaneously estimating the 3D basement relief of a sedimentary basin and the parameters defining a presumed parabolic decay of the density contrast with depth in a sedimentary pack, assuming prior knowledge about the basement depth at a few points. The sedimentary pack is approximated by a grid of 3D vertical prisms juxtaposed in both horizontal directions of a right-handed coordinate system. The prisms’ thicknesses represent the depths to the basement and are the parameters to be estimated from the gravity data. To estimate the parameters defining the parabolic decay of the density contrast with depth and to produce stable depth-to-basement estimates, we imposed smoothness on the basement depths and proximity between estimated and known depths at boreholes. We applied our method to synthetic data from a simulated complex 3D basement relief with two sedimentary sections having distinct parabolic laws describing the density-contrast variation with depth. The results provide good estimates of the true parameters of the parabolic law of density-contrast decay with depth and of the basement relief. Inverting the gravity data from the onshore and part of the shallow offshore Almada Basin on Brazil’s northeastern coast shows good correlation with known structural features.

Slides for the oral presentation "Reconstruction of geologic bodies in depth associated with a sedimentary basin using gravity and magnetic data " presented at EGM 2010 International Workshop Capri, Italy. Abstract: We present a comprehensive review of the most common gravity and magnetic interpretation methods to in depth retrieval of the geometry of geologic bodies associated with a sedimentary basin. We identify three types of bodies: 1) the sedimentary basement relief, 2) salt bodies and 3) mafic intrusions in a sedimentary section. In reconstructing basement topography through gravity and/or magnetic data we identify three groups of methods: the automatic, the spectral and the nonspectral methods. The reconstruction of salt bodies from gravity data usually uses interactive forward modelling but recently gravity inversion methods have been developed to interpret this kind of geologic environment. Finally, the problem of reconstructing intrusive bodies using magnetic and/or gravity data employs three strategies to interpret mafic or ultramafic intrusions in a sedimentary section: the automatic methods, interactive forward modelling and the inversion methods.

We have provided supplementary material to our article titled “Estimating the nature and the horizontal and vertical positions of 3D magnetic sources using Euler deconvolution” and whose authors are Felipe F. Melo, Valeria C. F. Barbosa, Leonardo Uieda, Vanderlei C. Oliveira Jr, and João B. C. Silva. In this material, we presented an extra synthetic test to illustrate the steps of our methodology didactically. Additionally, we illustrated with synthetic test the ability of our method to correctly estimate the source positions when it is applied to a reduced area that does not cover the anomalies entirely.

Slides for the oral presentation " Interactive 2D magnetic inversion: a tool for aiding forward modeling and testing geological hypotheses" presented at the SEG International Exposition and Seventy-Sixth Annual Meeting in NewOrleans, Louisiana, USA. Abstract We present a method for inverting magnetic data with interfering anomalies produced by multiple complex 2D magnetic sources having arbitrary shapes and known magnetization vectors. Ourmethod is stable and can recover a complex 2D magnetization distribution, leading to a reliable delineation of sectionally homogeneous sources with complex shapes. Our method, although similar to interactive forward modeling, is unique in that it automatically fits the observations and only requires that the interpreter know the outlines of the sources expressed by simple geometric elements such as points and line segments. Each geometric element operates as a skeletal outline of a particular homogeneous section of the magnetic source to be reconstructed. Also, the interpreter can define the geometric elements interactively without worrying about data fitting because data are fit automatically. The examples with synthetic data illustrate the goodperformance of the method in mapping the complex geometry of magnetic sources. The solution sensitivity to uncertainties in the a priori information shows that to produce good results, the uncertainty on the magnetization intensity of each homogeneous extent of the source should be smaller than 40%.Awrong magnetization vector direction can be detected easily because it often leads to poor data fitting and to estimated sources with abrupt borders. The method is also applied to two sets of real data from the Northwest Ore Body at Iron Mountain Mine, Missouri, and the Hatton-Rockall Basin in the northeast Atlantic Ocean. The estimated magnetization distribution in all tests demonstrates a good correlation of estimated magnetic sources with corresponding known geologic features.

Slides for the oral presentation "3D gravity inversion incorporating prior information through an adaptive learning procedure" presented at the SEG International Exposition and Seventy-Seventh Annual Meeting in San Antonio, Texas, USA. Abstract We have developed a gravity inversion method to estimate a 3D density-contrast distribution producing strongly interfering gravity anomalies. The interpretation model consists of a grid of 3D vertical, juxtaposed prisms in the horizontal and vertical directions. Iteratively, our approach estimates the 3D density-contrast distribution that fits the observed anomaly within the measurement errors and favors compact gravity sources closest to prespecified geometric elements such as axes and points. This method retrieves the geometry of multiple gravity sources whose density contrasts positive and negative values are prescribed by the interpreter through the geometric element. At the first iteration, we set an initial interpretation model and specify the first-guess geometric elements and their target density contrasts. Each geometric element operates as the first-guess skeletal outline of the entire homogeneous gravity source or any of its homogeneous parts to be reconstructed. From the second iteration on, our method automatically redefines a new set of geometric elements, the associated target density contrasts, and a new interpretation model whose number of prisms increases with the iteration. The iteration stops when the geometries of the estimated sources are invariant along successive iterations. Tests on synthetic data from geometrically complex bodies and on field data collected over a mafic-ultramafic body and a volcanogenic sedimentary sequence located in the Tocantins Province, Brazil, confirmed the potential of our method in producing a sharp image of multiple and adjacent bodies.

Slides for the oral presentation "Deep-pass — An aeromagnetic data filter to enhance deep features in marginal basins" presented at the SEG International Exposition and Eighty-Second Annual Meeting in Las Vegas, Nevada. ABSTRACTAn aeromagnetic data filter was tested as a tool in enhancing the magnetic responses of deep sources associated with marginal basins. Our approach consists in amplifying the responses of magnetic sources which extend through the continental slope and continental rise. This amplification compensates for the magnetic response attenuation over the continental slope as a result of (1) the seaward increase of the distance between the source and the magnetometer and (2) the seaward decrease in the volume of sources within the continental crust. To amplify the source response, we multiply the total-field anomaly at a given location by the exponential of the bathymetric depth at the same point. Tests on synthetic and real data showed that the filter was able to recover the locations of linear magnetic responses not clearly seen in the total-field-anomaly map. When compared with the analytic signal amplitude and the total horizontal derivative filters, the proposed filter proved to be complementary in enhancing lineaments with different orientations. When applied in conjunction with these filters, the proposed filter produced a synergistic effect, revealing lineaments that were not present in individual applications of the filters. The application of the filter to an offshore Brazilian margin comprising the Camamu-Almada and Jequitinhonha marginal basins highlighted northwest–southeast and northeast–southwest lineaments not clearly seen in the total-field data. The northeast–southwest lineaments were interpreted as related to fracture zones that may cut across the continental margin, controlling basement structures and fluid migration paths.