  In this paper we consider the problem of recovering the conformal factor in a
conformal class of Riemannian metrics from the boundary measurement of one wave
field. More precisely, using boundary control operators, we derive an explicit
equation satisfied by the contrast between two conformal factors (or wave
speeds). This equation is Fredholm and generically invertible provided that the
domain of interest is properly illuminated at an initial time. We also show
locally Lipschitz stability estimates.
