Small waveform and directivity variations of marine airgun signatures due to waves interacting with the source float are a source of 4D noise. We are assessing the magnitude of this noise by first measuring the amount of variability from near-source auxiliary data and then modeling synthetic time-lapse ocean bottom seismic data with realistic source variations based on the measured statistics and standard ocean wave models. We quantify the contribution of source variations to 4D noise as a function of sea state by calculating the NRMSD attribute in the image domain. We find that up to 4% NRMSD can be attributed to source variations under realistic scenarios, with two main contributing effects: variations of individual gun signatures due to pressure changes, and array directivity variations due to the wave-induced pitch and roll of the source floats. The latter effect has a larger impact on the 4D noise in our simulations and depends more on the wave steepness rather than the wave height. While waveform variations can be addressed by a nearfield-based shot-by-shot designature, directivity variations are difficult to correct without knowledge of the sea surface shape.


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