Non-contact measurement of gas turbine blade vibrations has made significant progress over recent years; however, there still exist some limitations in the current techniques available. Specifically with blade tip timing (BTT) methods, some of the limitations are: the requirement of a large number of sensors for each engine stage, difficulties in dealing with multiple excitation frequencies, and sensors being located in the gas path. An alternative technique is examined here, utilising the unsteady casing wall pressure, which has the potential to rectify some of these limitations. Analytical simulation of the internal casing wall pressure is derived, for the situation with rotor blades undergoing forced vibration. The amplitude of the blade forced vibrations is then reconstructed from the simulated unsteady casing wall pressure, with results showing the robustness of the method to sensor location, measurement noise and a limited number of sensors.