A method of generating a semblance panel comprises summing two or more gathers of traces. The dip of the reflector is taken into account in the summation process, and this prevents semblance peaks from becoming smeared during the summation process and so allows a greater number of gathers to be used to generate the semblance panel. In an embodiment of the invention the dip is determined from the seismic data. Alternatively, the reflector dip used in the summation process may be obtained from pre-existing data acquired at the survey location, or the reflector dip may be already known. The invention can be applied to seismic data containing events from more than one reflector.