An Observability Estimate in L2 (Omega) x H-1 (Omega) for Second-Order Hyperbolic Equations with Variable Coefficients.
I. Lasiecka, R. Triggiani, and P. Yao. Control of Distributed Parameter and Stochastic Systems, volume 141 of IFIP Conference Proceedings, page 71-78. Kluwer, (1998)