99 2. Surface geophysical methods Figure 2.32 is an example of geological modelling for static computation based on geological data (up holes). The methodology is particularly efficient when the merge of seismic data with different parameters is strong, and when the quality of refraction is very poor as in Fontainebleau sands area or noisy locations in densely populated areas. A particular attention must be also to tackle the noise effect depending on the location and phenomena of signal absorption due to thick deposit of dry sands (west part of Paris basin). Figure 2.33 shows the effects of static corrections on a vintage line of a seismic campaign “Paris Ile de France” (1986). Figure 2.33a shows the line processed with conventional refraction static corrections (paragraph 2.4.1) and highlights the strong difficulty to stack below the Fontainebleau sands (central part of the figure). Figure 2.33b shows the same line processed with the CDP Consulting methodology for the primary statics. One can notice a good continuity of the seismic horizons below the Fontainebleau sands, sustaining a good reservoir quality approach. (a) (b) Figure 2.33 Effect of static corrections on the seismic stack (CDP Consulting document). (a) Conventional static method, (b) CDP Consulting method. In Paris Basin, static problems due to the chalk diagenesis, already described in 1961 by Millouet, is superposing to the static problems induced by tertiary deposits. In the same way than the tertiary deposits, the chalk effects could lead to strong artefacts of dogger reservoir imagery. The problems are particularly difficult to master because they are very often wider than the seismic line. The chalk problem needs to be accurately considered for geothermal exploration in sensitive place of Paris Basin, particularly under tertiary deposits (Hanot et al., 2012; Miquelis et al., 2016). After having solved individual static problem and stack quality of individual line, producing regional line in true amplitude processing needs to follow a very detailed specific sequence including homogenous static modelling over the whole area, and a very specific approach in terms of geometry, noise removal, velocity picking and migration parameter. Once this very specific workflow is successful, extended regional lines could be used for the seismic reservoir quality quantification, notably for geothermal energy. Figure 2.34 is an example of a regional line of more than 100 km in true amplitude PSTM processing composed of 12 vintage lines belonging to 8 seismic campaigns with different parameters (sweep, number of vibrators, distance between shots, distance between seismic traces, geophone filtering, etc.).
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