If a seismic survey is performed twice in quick succession the two images will not be exactly the same. Part of the difference results from ambient noise and part is shot-generated. The shot-generated noise originates both from the previous shot and from perturbations to the acquisition, such as small differences in shot and receiver positions. Only if the ambient noise is higher than the shot-generated noise will increasing the source output improve the overall signal-to-noise ratio of the image. We used data from a repeated survey, together with ambient noise measurements, to determine the power spectra of the signal, the ambient noise and the shot-generated noise. We did these tests in both calm weather and poor weather. We show that, in this example, the noise in the seismic image is mostly shot-generated not ambient. This means that the signal-to-noise ratio of the image would be largely unaffected if the source power were reduced. In our test example, for most of the bandwidth, the seismic source is unnecessarily loud.


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