1. A method for processing, on a computer, seismic data carrying information about subsurface, the method comprising: receiving the seismic data recorded by detectors arranged on receiver lines; splitting the seismic data into groups, each group including subsets of seismic data for which a distance from a shot point where a seismic wave is generated to a receiver line is within a predetermined range for the group; extracting signal data by subtracting noise from the seismic data for each group of seismic data; and outputting the signal data useable to generate an image of the subsurface.