91 2. Surface geophysical methods The first step includes pre-processing of the data and the application of static corrections. The purpose of pre-processing is to extract reflected waves from individual shots, by filtering out the waves which are not reflected waves: direct and refracted arrivals, surface waves, converted waves, and noise. The conventional wave separation methods are the F-K method and the SVD method (Singular Value Decomposition). Pre-processing is intended to compensate for amplitude losses related to propagation. Deconvolution operators are applied to improve resolution (as example spiking deconvolution), harmonize records by considering source efficiency variations and eventual disparities between receivers, and attenuate multiples (predictive deconvolution). Any deconvolution is sensitive to noise. Some specific processes, such as SVD decomposition, are used to enhance signal to noise ratio, by splitting the data in a noise space and a signal space. Static corrections, that are specific to land seismic, are intended to compensate for the effects of the weathered zone and topography. Records are then sorted in common mid-point gathers or common offset gathers. The second processing step is the conversion of common mid-point gathers or common offset gathers into time or depth migrated seismic sections. This second step includes the determination of the velocity model, with the use of stacking velocity analyses, or tomography methods. The role of migration is to place events in their proper location and increase lateral resolution, by collapsing diffraction hyperbolas at their apex. Proper migration requires the definition of a coherent velocity field, which must be a field of actual geologic velocities in migrated positions. Determination of the velocity field is the most critical aspect of migration. The migration process can be done post or pre stack in time or depth. After migration, vertical and horizontal resolutions can be estimated by a quarter of the dominant wavelength of the seismic signal. An inversion process can be applied to post stack migrated sections to recover acoustic impedance distribution Ip (Ip = ρ VP, with ρ density and VP P-wave velocity of the formation). An inversion process, which considers the amplitude variations versus offset of the reflected signal, can be applied to pre stack migrated sections to recover elastic impedance distributions Ip and Is (Ip = ρ VP, Is = ρ Vs with ρ density, VP and VS respectively P-wave and S-wave velocities of the formation). Figure 2.27 is an example of 3D seismic spread for near surface imaging. The seismic spread is composed of a receiver spread and a source spread (Figure 2.27a). The receiver spread, displayed in green, is composed of 2 receiver lines. Receiver line direction is called the in-line direction. Distance between receiver lines is 4 m. There are 24 geophones per line. Distance between geophones is 2 m. The source spread, displayed in yellow, is composed of 11 source lines oriented perpendicularly to the receiver lines. 11 shots are fired per line. Distance between shots is 2 m. Distance between source lines is 4 m. The source lines and the receiver lines are oriented perpendicularly. The distance between receiver spread and source spread is 4 m. There is no overlap between the source and the receiver spread. Due to the geometry of acquisition, the geometry fold is symmetric. Figure 2.27b shows the fold variation. It varies from 0 to 22.
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