1. A method for stacking receiver seismic data, the method comprising: receiving a gather of receiver seismic data generated by receivers of a seismic data acquisition system; calculating a smoothed-amplitude gather from the gather of receiver seismic data, the smoothed-amplitude gather having smoothed-amplitude traces that each correspond to a trace of the gather of receiver seismic data; and stacking the traces of the gather of receiver seismic data to generate a noise-reduced trace, amplitudes of the noise-reduced trace calculated as a sum of amplitudes of the traces weighted by smoothed amplitudes of corresponding smoothed-amplitude traces.