Abstract: A seismic survey apparatus comprises circuit arrangements for providing a train of first signals each corresponding to a sound wave transmitted towards the sea bed, and circuit arrangements for providing associated trains of second signals each corresponding to sound waves reflected from the sea bed. Signal storage means are provided for storing the trains of associated second signals. A filter is provided for deriving from the first and associated second signals a control signal indicative of the apparent vertical movement of the surface of the sea bed due to wave motion or swell. The read out from the signal storage means is then controlled in accordance with the control signal to compensate for the apparent vertical movement of the sea bed due to wave motion or swell. Preferably the read out from the signal storage means is controlled by delaying or advancing the read out of the second signal in accordance with the control signal.