  In the reconstruction process of sound waves in a 3D stratified waveguide, a
key technique is to effectively reduce the huge computational demand. In this
work, we propose an efficient and simple multilevel reconstruction method to
help locate the accurate position of a point source in a stratified ocean. The
proposed method can be viewed as a direct sampling method since no solutions of
optimizations or linear systems are involved. The novel method exhibits several
strengths: fast convergence, robustness against noise, advantages in
computational complexity and applicability for a very small number of
receivers.
