A Fourier-domain approach for 3D magnetotelluric forward modeling based on NUFFT method

Abstract The 2D Gauss-FFT method, which uses Fourier domain methods, is commonly applied in the large-scale 3D magnetotelluric (MT) forward modelling. However, this method is restricted to staggered regular grids and the computational cost usually becomes unacceptable as the number of Gaussian points increases. To address these limitations, we developed an efficient MT forward modeling based on nonuniform fast Fourier transform (NUFFT), combined with a quasi-sampling theorem (QST) in the Fourier domain. This method allows for non-uniform grid discretization both horizontally and vertically. In addition, we can achieve arbitrary sampling in the Fourier domain according to the changes of wavenumber spectrum, and try to reduce the number of 1D equations to be solved in the Fourier domain while ensuring the calculation accuracy. The novel approach has the advantages of flexible sampling, high computational efficiency, and minimal Fourier transform truncation effect compared to the existing Fourier transform method. We designed a low-resistivity model and verified the correctness of the algorithm proposed in this paper against the integral equation (IE) method. Its performance is further evaluated by comparing our results with the Gauss-FFT method for both low-resistivity and large-scale high-resistivity models, which demonstrates approximately 85 percent of computational time and 91 percent memory are saved.