182 Seismic Imaging 7.4 Amplitude quality control A Stochastic Quality Assessment (SQA) workflow has been designed to assess the reliability of the stacked amplitudes and to optimize further modelling of mechanical and hydrogeological properties. Figure 7.15 Profile XL217. Instantaneous amplitude (left) and SQI factor (right). SQA processing of seismic data works under a mathematical framework known as geostatistics, where the observed (measured) seismic amplitude is considered as a realization of a random function (RF) defined in space and time (Shtuka et al., 2011 and 2009). The added value of using this type of theoretical framework is that it takes into account the spatial correlation between measured seismic amplitudes at different locations using variogram or spatial covariance. Interpretation and modelling of the experimental variogram computed in the offset direction (for fixed time) on pre-stack gather data enables the assessment of the signal and noise content of the seismic measurements in terms of the contributive (signal) and noncontributive (noise) part of the stacking process. The signal/noise interpretation of the variogram is quantified by modelling the experimental variogram accordingly in both horizontal (offset or angle) and vertical (time) directions. SQA handles the global non-stationary behaviour of the gather, by relying only on a local stationarity assumption: local computations and modelling of variogram parameters (sill and ranges) are performed inside a local neighbourhood defined around each sample
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