1. A method of processing seismic data to obtain a processed seismic data volume comprising: a) obtaining seismic data covering a predetermined volume of earth, which seismic data comprises a quantity assigned to a voxel in a seismic data volume that represents the pre-determined volume of earth; b) determining for each voxel in the seismic data volume a local orientation of the seismic data, which local orientation is the orientation of a plane tangent to the seismic data in that voxel; c) determining for each voxel whether there is an edge in its neighborhood; and d) carrying out a smoothing operation on each voxel in the seismic data volume, in a direction such that the direction of the smoothing operation is the local orientation of the data and wherein the smoothing operation does not go over the edge, to obtain a processed seismic data volume in which the quantity assigned to each voxel in the processed data volume is a result obtained by carrying out the smoothing operation in that voxel in the seismic data volume.