Two tactics for accurate frequency-domain CSEM modelling

Controlled-source electromagnetic (CSEM) modelling in 3D is usually computationally expensive, due to large number of grids to discretize the physical domain to mimic real acquisition extending up to tens of kilometers. In order to reduce the number of computing grids, the nonuniform grid has been widely adopted to smoothly increase the gridsize away from the source, which however significantly degrades the modelling accuracy. To better compromise between modelling accuracy and computational cost, we develop an optimized stretching strategy for CSEM modelling on nonuniform grid, with special treatment of medium homogenization in horizontal and vertical directions. The modelling domain has been divided into three parts: the source vicinity, the saturation zone with equal grid spacing and the exterior damping zone. The grids in the vicinity of the sources and the exterior damping zone are stretched following the power law, according to different growth rate. The horizontal conductivity and vertical resistivity are averaged following an equidistribution principle. The modelling accuracy has been validated using a multigrid frequency-domain finite-difference solver.