“…For instance, some effective methods are developed for the homogeneous background model: the linear sampling or probing methods (LSM) [16], the time-reversal multiple signal classification method (MUSIC) [7,8,12], the direct sampling method (DSM) [25], the multilevel sampling method (MSM) [32], the contrast source inversion method (CSIM) [11], etc. Moreover, several efficient algorithms are applied in various practical situations, e.g., in the stratified ocean waveguide model: the simple method (SM) [30], extended direct sampling methods (EDSMs) [13,14,15,26,27,28,31,33,34], the extended MUSIC method (EMUSIC) [6,24], the topological based imaging functional [3], etc. Furthermore, several efficient approaches are introduced in the two-layered medium model: the extended multilevel sampling method (EMSM) [36], the extended linear sampling method (ELSM) [18], etc.…”