Gravity Inversion of Blocky Basement Relief Using L0 Norm Constraint with Exponential Density Contrast Variation

Abstract

Depicting the basement relief of a sedimentary basin with gravity data is vital to predicting the hydrocarbon potential of a sedimentary basin and guiding exploration work. We have developed a gravity inversion method to estimate the depth to a blocky basement of a sedimentary basin. The basement rocks are assumed to be homogeneous and have uniform density, while the density of the sediment over the basement increases exponentially with depth. The density contrast between the sediment and the basement at the surface varies horizontally. The decay factor of density contrast is also nonuniform. The sediment above the basement is divided into vertically juxtaposed prisms, and the depth of the bottom of each prism represents the depth to the basement and is the parameter to be estimated. The L0 norm is introduced to limit the gradient of the parameter vector to obtain the model constraint function. We then establish the objective function for inversion by combining the gravity data misfit function, the known depth constraint function, and the model constraint function. The inversion is performed by minimizing the objective function using the nonlinear conjugate gradient algorithm. The inversion method is evaluated using a 2D and a 3D sedimentary basin model. The results show that our proposed method is capable of delineating the blocky basement relief of a sedimentary basin, and the result is sharper than that obtained using the L1 norm constraint. The method is applied to real data from the western part of the Zhu 1 depression in the Pearl River Mouth Basin, northern South China Sea. The solution reveals a strongly faulted basement, which is in accordance with the known tectonic information indicating the basin is a fully developed graben.