A 27‐point scheme for a 3D frequency‐domain scalar wave equation based on an average‐derivative method

Based on an average‐derivative method and optimization techniques, a 27‐point scheme for a 3D frequency‐domain scalar wave equation is developed. Compared to the rotated‐coordinate approach, the average‐derivative optimal method is not only concise but also applies to equal and unequal directional sampling intervals. The resulting 27‐point scheme uses a 27‐point operator to approximate spatial derivatives and the mass acceleration term. The coefficients are determined by minimizing phase velocity dispersion errors and the resultant optimal coefficients depend on ratios of directional sampling intervals. Compared to the classical 7‐point scheme, the number of grid points per shortest wavelength is reduced from approximately 13 to approximately 4 by this 27‐point optimal scheme for equal directional sampling intervals and unequal directional sampling intervals as well. Two numerical examples are presented to demonstrate the theoretical analysis. The average‐derivative algorithm is also extended to a 3D frequency‐domain viscous scalar wave equation.