98 Geophysics in Geothermal Exploration For seismic onshore data, and particularly in the centre part of Paris basin (mainly on Tertiary cover in the Ile de France), the quality of the seismic processing is particularly dependent on static corrections. The Tertiary units have a thickness never exceeding 250 m. But the wide variability of seismic units (limestones, marls, evaporites, sands, clays) make the velocity variations very strong and dependant on the different aquifers. Many velocity inversions make totally forbidden the use of refraction statics to compute primary statics. Indeed, such use of refraction statics is totally unsuitable in this area and provides wrong structural shapes, cycle skips and loose of signal noise ratio. Such wrong artefact caused by refraction statics have induced in the past very strong artefacts and misunderstanding in reservoir characterization. They also lead in the past to drill a lot of wrong structures in time that have no reality in depth and cause the main failure of the oil and gas exploration in Paris Basin. (a) (b) Figure 2.32 Example of geological modelling for static computation based on geological data (up holes). (a) Geological model, (b) geological velocity model (CDP Consulting document). The only way to optimize the stack quality and get confidence in final seismic data needs to follow an accurate processing sequence. This needs to integrate a modelling of primary statics supported by a well constrained geological velocity model of the Tertiary units. CDP Consulting has developed a dedicated methodology for primary static computation based on Franck Hanot experience and widely discussed in many publications (Hanot, 1992; Hanot et al. 2012; Miquelis et al., 2016, 2019; Nosjean et al, 2017). Consequently, the pitfalls induced by static problem can be solved and need to be integrated in derisking of geothermal exploration (depth/ temperature well design and reservoir characterization).
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