“…The sampling method was introduced by Colton and Kirsch [8], where they used far-field observation data to determine the position of a scatterer under the Helmholtz equation. The sampling method consists a variety of sub-methods, including the factorization method [18], the linear sampling method [16,19,32], reverse time migration [5], direct (orthogonality) sampling method [3,6,7,36] and the topological sensitivity method [40,41]. In the direct sampling method, or the orthogonality sampling method, the index function is constructed by pairing the boundary observations with some probing function under an inner product.…”