[Seisunix] Help with Adding Offset Header in a SU File

Hello, I am a beginner. I need to apply sureduce command to a particular shot which is in su file format. I tried with different rv values but could not see any differences in the image. I checked with the headers of that file and found I have ns and dt headers but not offset. Can anyone please help me how can I add offset header to my su file? And please let me know if there are any other things to fix to get result. Thanks