1. A method of evaluating a processing parameter value to determine the optimum setting of that parameter for processing seismic data, comprising the steps of: forming an initial measure of a chosen seismic attribute from a selected portion seismic data of known content; processing the selected portion of the seismic data separately for each of a set of values of the processing parameter; storing the results of each of said separate steps of processing; determining a value of the chosen seismic attribute for each of the stored results; and comparing the determined values of the chosen seismic attribute with the initial measure to determine the optimum value of the parameter for processing seismic data.