A method of detecting the change in the structure of a turbine rotor (24) comprises injecting an ultrasonic signal into the turbine rotor (24) at a first position using a first ultrasonic transducer (102) to produce a diffuse field ultrasonic signal in the turbine rotor (24). Detecting the diffuse field ultrasonic signal in the turbine rotor (24) at a second position using a second ultrasonic transducer (106). Measuring the time for the diffuse field ultrasonic signal to travel from the first ultrasonic transducer (102) to the second ultrasonic transducer (106). Comparing the measured time for the diffuse field ultrasonic signal to travel from the first ultrasonic transducer (102) to the second ultrasonic transducer (106) with a stored time for the diffuse field ultrasonic signal to travel from the first ultrasonic transducer (102) to the second ultrasonic transducer (106) to determine if there is a change in the structure of the turbine rotor (24). Used to detect contact between the sealing members (74,76,78,80) of the turbine discs (26,28,30).