Full waveform inversion based on improved MLQN method

Full waveform inversion （FWI） is a challenging data-fitting procedure between model wave field value and theoretical wave field value. The essence of FWI is an optimization problem, and therefore, it is important to study optimization method. The study is based on conventional Memoryless quasi-Newton （MLQN） method. Because the Conjugate Gradient method has ultra linear convergence, the authors propose a method by using Fletcher-Reeves （FR） conjugate gradient information to improve the search direction of the conventional MLQN method. The improved MLQN method not only includes the gradient information and model information, but also contains conjugate gradient information. And it does not increase the amount of calculation during every iterative process. Numerical experiment shows that compared with conventional MLQN method, the improved MLQN method can guarantee the computational efficiency and improve the inversion precision.