A method of de-ghosting seismic data acquired in a marine seismic survey comprises estimating .differential..sub.zp (where p is the fluid pressure) from a time-derivative of p and a derivative of p in a horizontal direction such as the x-direction. The time-derivative of p and the derivative of p in the horizontal direction can be readily obtained from the acquired seismic data, and are used to provide an improved estimate of .differential..sub.zp which, in general, cannot be directly determined from the acquired seismic data.Once .differential..sub.zp has been estimated it can be used to de-ghost the acquired seismic data.