“INTERPRETAÇÃO DE DADOS MAGNÉTICOS SOBRE CROSTA OCEÂNICA EM BAIXAS LATITUDES”

Abstract

This study implemented and validated a space-domain inversion methodology for estimating the distribution of apparent magnetic susceptibility in oceanic crust, specifically designed for interpreting magnetic data in low-latitude regions where conventional Fourier-domain techniques, such as reduction to the pole, suffer from numerical instabilities. The interpretive model consists of a layer of horizontally juxtaposed rectangular prisms with purely induced magnetization and isotropic susceptibility. The inverse problem formulation combines first-order Tikhonov reg- ularization with a positivity constraint, with the purpose of imposing physically plausible bounds on the estimated solutions.The methodology was tested on eight synthetic scenarios organized into three progressive groups that assessed: (i) the stabilizing role of the positivity constraint; (ii) the effect of irregular observation distributions on inversion quality; (iii) the influence of secular variations in the main geomagnetic field (IGRF) when combining surveys from different epochs; and (iv) performance under models with realistic geometry and variable crustal thickness. The tests demonstrated that positivity enforcement is the primary stabilizing mechanism of the inversion, producing geologically more coherent solutions at the cost of a slightly higher data misfit.For the real-data application, the methodology was applied to the São Paulo Transform Fault System (SPTFS) using magnetic data from the NCEI surveys (1966 to 1983) and the COLMEIA expedition (2013). Joint inversion of both datasets revealed limitations arising from the spatial heterogeneity of the NCEI surveys, with a residual standard deviation of 2.51 nT. Restricting the inversion to COLMEIA data alone reduced the standard deviation to 0.02 nT. The COLMEIA-only inversion identified zones of elevated magnetic susceptibility within the intra-transform segments, interpreted as possible occurrences of basalt, gabbro, mylonite, and peridotite, showing good correlation with dredge samples and lithological interpretations from previous studies.The results contribute to the understanding of crustal and upper mantle structure in the equatorial Atlantic, with particular emphasis on the geophysical characterization of serpentinization zones associated with transform faults and distinct mantle exhumation mechanisms (transpression and detachment faulting).