Abstract: Described herein is a method for deriving at least one notional source waveform for an array comprising at least one marine vibrator, the method comprising: during operation of the at least one marine vibrator, recording data from a first sensor measuring a property related to the wavefield in the fluid surrounding the at least one marine vibrator, during operation of the at least one marine vibrator, recording data from a second sensor coupled to or internal to a radiating surface of the vibrator and measuring a property related to motion of the radiating surface; and jointly inverting the data from the first sensor and the data from the second sensor to recover the at least one notional source waveform for the array. Also described herein is a method for carrying out a seismic survey and a seismic survey apparatus.