  We show that the travel time difference functions, measured on the boundary,
determine a compact Riemannian manifold with smooth boundary up to Riemannian
isometry, if boundary satisfies a certain visibility condition. This
corresponds with the inverse microseismicity problem. The novelty of our paper
is a new type of a proof and a weaker assumption for the boundary than it has
been presented in the literature before. We also construct an explicit smooth
atlas from the travel time difference functions.
