MSc thesis: Estimativa do vetor de magnetização total de corpos aproximadamente esféricos

(2014) Daiana P. Sales


Abstract

We present a new computationally effective method to estimate the magnetization direction from multiple sources, with roughly spherical center and known by the inversion of total field anomaly produced by these format. Multiple sources can have different directions of magnetization and the method can be applied directly to total failure data field, which may be irregularly spaced on corrugated surfaces. The method is based on the solution of a linear inverse problem to estimate the magnetization vector components of the magnetic sources. Tests with synthetic data show a good performance in recovering the direction of true magnetization, respecting the assumptions of our method. The tests that violate the premise of the knowledge of the center of the fountain show two features: i) if there are errors in the horizontal location of the center of the fountain, our method is not able to recover the true direction of magnetization, and ii) if there are errors in location vertical center of the fountain, our method is able to recover the true direction of magnetization. Additional testing shows that synthetic upward continuation can be useful to allow application of our method in the interpretation of non-spherical bodies. The results obtained by applying our method to real data on a region of Goiás Alkaline Province (PAGO), near the city of Diorama, suggest the presence of non-outcropping sources with strong remanent magnetization. The direction of magnetization estimated for this region implies a total field anomaly reduced to predominantly positive pole. This same magnetization direction also produces anomalies prevailing field overall positive in another region of PAGO, called alkaline complex of Montes Claros de Goiás. These results show that non-outcropping sources in the region of Diorama have practically the same magnetization direction of those alkaline complex in Montes Claros de Goiás, suggesting that these sources intruded the crust inside almost the same range of geological time. We present a new method computationally effective for estimating the magnetization direction from multiple sources with roughly spherical center and known by the inversion of total field anomaly produced by these format. The multiple sources may have different directions of magnetization, and the method can be applied directly to total failure data field, which may be spaced irregularly over undulating surfaces. The method is based on solving a linear inverse problem to estimate the magnetization vector components of the mag- netic sources. Tests with synthetic data respecting the assumptions of our method perform well in recovering the true direction of magnetization. The tests that violate the premise of the knowledge of the center of the fountain show two features: i) if there are errors in the horizontal location of the center of the fountain, our method is not able to recover the true direction of magnetization, and ii) if there are errors in location vertical center of the fountain, our method is able to recover the true direction of magnetization. Additional testing shows that synthetic upward continuation can be useful to allow application of our method in the interpretation of non-spherical bodies. The results obtained by applying our method to real data on a region of Goiás Alkaline Province (PAGO), near the city of Diorama, suggest the presence of non-outcropping sources with strong remanent magnetization. The direction of magnetization estimated for this region implies a total field anomaly reduced to predominantly positive pole. With the same magnetization direction also produces anomalies prevailing field overall positive in another region of PAGO, called alkaline complex of Montes Claros de Goiás. These results show that non-outcropping sources in the region of Diorama have practically the same magnetization direction those in the alkaline complex of Montes Claros de Goiás, suggesting that these sources intruded the crust inside almost the same interval of geologic time.