3D INVERSION OF MARINE MAGNETOTELLURIC DATA FROM SANTOS BASIN

Abstract

A large pioneer marine magnetotelluric (MMT) campaign were made at the Santos Basin in Brazil in 2007 aiming to asses the outcome of the method in a wide range of water depths in region where pre-salt sediments are present. A total of 90 sites are displaced along three profiles of about 250 km of total. Good data quality from 10−1 to 104 s were recorded. Dimensionality analysis using different tools indicates the strong influence of 3D structures in the MMT data. 3D inversion was carried out using a code with minimization performed by a minimum structure non-linear conjugate gradient inversion scheme (ModEM). The grid discretization was defined by 95 x 78 x 100 cells and 1579 km x 1546 km x 696 km of size in x, y and z directions respectively. Homogeneous half-spaces with different background resistivities were tested as initial models as well as different values of the initial regularization parameter for the inversion. The 10 Ω.m homogeneous half-space and the value of 10 for the starting regularization parameter were selected as the optimal choices. The model assessment was made based on the normalized root mean square (nRMS) values and a comparison between the values of resistivity resulted from inversion and an available induction log data. A good fit were found between them. Finally, we correlated the inversion result with a seismic based interpretation. The basement is very clear in the northwest portion of the central profile and a vertical relative resistive feature in the southeast portion can be indicative of the salt diapirism.