A Frequency Criterion for Smoothing with Optimal Cubic Splines

Marine surveys acquired in deep-water areas often exhibit static variations between sail-lines that are the result of changes in sea level elevation (due to tides) or/and in water velocity. They manifest themselves as lateral discontinuities (or jitters) on cross-line sections or on 3D CDP gathers and can lead to stack deterioration. In this paper, we review the methodologies available to correct for these two types of variations. We will first show how GPS data can be used to compensate accurately for tidal statics. We then propose to correct for water velocity variations using a direct measurement on seismic gathers. The objective is to replace the measured water velocity (varying from sail-line to sail-line) by a constant water velocity that is consistent across the whole survey. This is the key point of the methodology as all the sail-lines (and all the vintages in 4D) will be re-aligned to this spatially constant water velocity. The main advantage of this method is that it does not rely only on a water bottom time measurement, which could be biased by residual tidal statics.