HR: 08:00h
AN: NS21A-01    [Abstracts]
TI: 3D depth-to-basement and density contrast estimates using gravity and borehole data
AU: * Barbosa, V C
EM: valcris@on.br
AF: Observatorio Nacional, Rua Gal Jose Cristino, Rio de Janeiro, RJ 20921 400, Brazil
AU: Martins, C M
EM: medel@on.br
AF: Observatorio Nacional, Rua Gal Jose Cristino, Rio de Janeiro, RJ 20921 400, Brazil
AU: Silva, J B
EM: joaobcs@oi.com.br
AF: Universidade Federal do Para, Rua Augusto Correa, 1, Campos Universitario do Para, Belem, PA 66017-970, Brazil
AB: We present a gravity inversion method for simultaneously estimating the 3D basement relief of a sedimentary basin and the parameters defining the parabolic decay of the density contrast with depth in a sedimentary pack assuming the 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, x and y, 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 produce stable depth-to-basement estimates we impose smoothness on the basement depths through minimization of the spatial derivatives of the parameters in the x and y directions. To estimate the parameters defining the parabolic decay of the density contrast with depth we mapped a functional containing prior information about the basement depths at a few points. We apply 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. Our method retrieves the true parameters of the parabolic law of density contrast decay with depth and produces good estimates of the basement relief if the number and the distribution of boreholes are sufficient. We also applied our method to real gravity data from the onshore and part of the shallow offshore Almada Basin, on Brazil's northeastern coast. The estimated 3D Almada's basement shows geologic structures that cannot be easily inferred just from the inspection of the gravity anomaly. The estimated Almada relief presents steep borders evidencing the presence of gravity faults. Also, we note the existence of three terraces separating two local subbasins. These geologic features are consistent with Almada's geodynamic origin (the Mesozoic breakup of Gondwana and the opening of the South Atlantic Ocean) and they are important in understanding the basin evolution and in detecting structural oil traps.
DE: 0560 Numerical solutions (4255)
DE: 0903 Computational methods: potential fields (1214)
DE: 0920 Gravity methods (1219)
DE: 3260 Inverse theory
DE: 4863 Sedimentation (1861)
SC: Near-Surface Geophysics [NS]
MN: 2009 Joint Assembly