Stable determination of polyhedral interfaces from boundary data for the   Helmholtz equation
  We study an inverse boundary value problem for the Helmholtz equation using the Dirichlet-to-Neumann map as the data. We consider piecewise constant wavespeeds on an unknown tetrahedral partition and prove a Lipschitz stability estimate in terms of the Hausdorff distance between partitions. 
Elena Beretta, Maarten V. de Hoop, Elisa Francini and Sergio Vessella
math.AP