Spherical Coordinate Algorithms for Gravity Forward Modeling and Iterative Inversion of Variable-Density Interface: Application to Chinese Mainland Lithosphere

Density interface inversion is an important approach for quantitative interpretation of gravity data, used to reveal the morphology of structural interfaces such as the basement, Moho boundary, and the Lithosphere-Asthenosphere Boundary (LAB). However, common density interface forward modeling and inversion algorithms in Cartesian coordinate system are unsuitable for large regional scales, while conventional algorithms in spherical coordinate system are not suitable for ground observation plane and rarely account for the density variations underground. In this paper, we develop algorithms for gravity forward modeling and iterative inversion of variable-density interface in spherical coordinate system. The gravity forward modeling formula of variable-density tesseroid unit in spherical coordinate system is derived based on the Gauss-Legendre quadrature integration algorithm, and the improved adaptive subdivision strategy is utilized to make the forward modeling algorithm suitable for ground observation plane with high precision. Then, the iterative inversion algorithm for variable-density interface in spherical coordinate system is proposed based on our gravity forward modeling algorithm and Bott’s iteration algorithm. We validate the effectiveness and reliability of the proposed algorithms by using a spherical shell theoretical model and a variable-density interface model. Finally, we apply the proposed algorithms to the gravity data on ground observation plane to derive the depth distribution of the LAB beneath Chinese mainland. The inversion result indicates that the lithospheric thickness generally exhibits a trend of being thicker in the west and thinner in the east, with 110°E as the boundary. The thickest region exceeds 250 km, located in the central Tibetan Plateau, while the thinnest area is approximately 60 km, found from the eastern Northeast China to the eastern South China.