“…The method of using linearized approximation is usually not accurate enough and easy to be affected by the background information, while the accurate method is more likely to be trapped in a local minimum and more computationally expensive. In addition, due to the paucity of large angle incident information and the different sensitivity for the reflection coefficients to the bulk density term, usually the estimation of density is difficult (Tarantola, 1986;Beydoun and Mendes, 1989;Crase et al, 1990;Nicolao et al, 1993;Swan, 1993;Chen et al, 2001;Cambois, 2001;Lebrun et al, 2001;Djikpesse et al, 2006;Choi et al, 2008;Virieux and Operto, 2009;Zong et al, 2013).…”