169 7. Integrated seismic study to surface conditions. One of the main challenges of the processing was to effectively attenuate the noise and compensate source and receiver coupling as part of an amplitude preserving processing sequence. A methodical multi-domain approach, including shot, common offset and post stack domains, was used to analyse and apply suitable algorithms to progressively attenuate the observed noise. An iterative approach was seen to be the best method to optimize the noise attenuation and the compensation of source and receiver variability. The final pre-PSTM (Pre Stack Time Migration) sequence dedicated to noise attenuation and source-receiver variability compensation proved effective and achieved the aim of creating amplitudepreserved PSTM data. The main seismic processing sequence steps were: • data editing, • minimum phase conversion, • amplitude compensation (spherical divergence correction), • surface consistent amplitude compensation (source and receiver), • de-noise and wave separation on shots, • statics application (data shifted to floating datum), • surface consistent deconvolution, • velocity analysis, • surface consistent residual statics, • second pass velocity analysis, • surface consistent amplitude compensation (source, receiver and offset), • interpolation and regularization in offset planes and noise attenuation, • velocity model updating (residual move out), • QC: 60 fold CMP stack (0-1,400 m offsets) with static to the final datum (450 m MSL), • pre-stack time migration (time shifted to the final datum: 450 m MSL), • Q compensation (Q =100), • Noise attenuation and phase conversion (statistical to zero phase), • Band pass filter (15, 20-140, 160 Hz). However, using the processing sequence described, some undulations of the seismic horizons in time were observed on the obtained seismic lines. The undulations, which have no geological meaning, are characteristic of long wavelength static anomalies, which do not degrade the stack but can introduce structural anomalies. They are due to lateral variations in the properties of the weathering or shallow layers (elevation effects, lateral velocity variations, dip) which are negligible at the scale of the recording spread or CMP gather but not negligible at the scale of the seismic line. Short wavelength anomalies are detectable at the spread length and they degrade the CMP stack. They can be compensated for by the residual static correction methods. In this area of the Paris Basin, the layer of the first 20 meters of variable velocity is the most sensitive layer of the static model. It largely conditions the shapes of
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