89 2. Surface geophysical methods a fundamental step to ensure the 3D data quality will meet structural, stratigraphy and lithology requirements. Pre-planning includes the evaluation of both geophysical and non-geophysical parameters such as environment considerations, health and safety requirements, etc. Specific pre-planning tools (Cordsen et al., 2000) were developed to estimate all characteristics of the future acquisition such as offset, fold and azimuth distributions, effects of surface obstacles, make up shots, etc. The pre-planning aims at defining the geological targets of the 3D with the associated geophysical parameters, design and costs. Figure 2.25 2D and 3D seismic imaging (after J. Meunier, 1998, 1999, IFP School course). In 2D the image obtained after processing is a vertical seismic section. The horizontal axis of the section represents the geographical abscissas of subsurface points along the acquisition profile and the vertical axis represents the record time. The seismic events that appear on the records correspond to the arrivals of waves reflected at normal incidence on the seismic horizons. The seismic horizons correspond to discontinuities of acoustic impedance; their picks provide a structural image of the subsurface. 3D seismic acquisition provides a full volume consisting of a collection of sections parallel to each other. Surface seismic has vertical and horizontal resolutions measured in tens of meters with lateral investigation distances only limited by the size of the area investigated by the seismic surveys. 2D seismic acquisition is achieved with spreads which are either end-on also called off-end or split dip spread (Figure 2.26a). The individual shot element is defined by the source to the first receiver distance, the number of receivers and the distance between two adjacent receivers. A receiver can be a single sensor (geophone for
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