170 Seismic Imaging seismic horizons and is the source of the long wavelength static anomalies. The velocities in the first 20 m are sensitive to: • the nature of the formations affected by the alteration (Valanginian sands, Portlandian limestones or Kimmeridgian marls), • the geomorphology (difference in velocities observed between plateaus, hillsides and valleys). To compensate the long wavelength static anomalies, two procedures can be carried out: • obtain a new set of static corrections based on refraction surveying (weathering shots) and up-hole surveying (VT: vertical times); including all the geological information available (geological maps). The seismic dataset must be completely reprocessed, • estimate the geological time variation of the seismic horizons. The methodology is based on an a priori knowledge of the structural shape of the geological model. In 3D, a map of LWL static corrections has to be computed, the more appropriate method being a kriging method which allows the filtering of the map of the picked times of a reference horizon into a trend map and a residual map. Figure 7.5 Long wavelength wave static correction - trend and LWL static curve. To compensate the long wavelength anomalies, the picked times of major seismic horizons (0.3 – 0.6 s) have been considered as the stack of 2 functions: a trend which represents the geological time variation of the seismic horizon and a residual time function which represents the long wavelength anomaly correlated with
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