Condition monitoring of blades within gas turbines has been and will continue to be of importance in all areas of their use. Non-intrusive measurement of blade condition is the ambition of most techniques, with many methods proposed, investigated and employed for such measurement, with the current dominant method using proximity probes to measure blade arrival time for subsequent monitoring. It is proposed however that the measurement of the casing vibration, due to the aerodynamic-structural interaction within a gas turbine, could provide a means of blade condition monitoring and modal parameter estimation. An analytical model has been developed of the response of a gas turbine casing to what is believed to be the dominant forces acting on it, viz: (i) the moving pressure waveform around each blade throughout its motion and (ii) the forces (and moments) applied through the stationary stator blades, to understand the form of the casing response. A simulated blade fault has then been added to the model to represent a damaged blade, of arbitrary form, by reducing the stiffness of one blade. The change in the contribution to the casing response measurements due to this simulated fault is explored.