98 Seismic Imaging 4.2 Comments on the reflection seismic processing sequence The classical approach to seismic processing can be summarized in two main steps. 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 parasitic events created by direct and refracted arrivals, surface waves, converted waves, multiples and noise. It is intended to compensate for amplitude losses related to propagation. Deconvolution operators are applied to improve resolution and harmonize records by taking into account source efficiency variations and eventual disparities between receivers. Any deconvolution is sensitive to noise. A classically used method that is relatively robust to noise is deconvolution with the Wiener filter. The Wiener filter allows the processing of a measured signal to obtain a desired signal. It minimizes (least squares conditions) the difference between the desired signal and the signal estimated by the filter. The desired signal can be a Dirac impulsion. In this case, spiking deconvolution is necessary. Static corrections, which are specific to land seismic surveys, are intended to compensate for weathered zone and topographic effects. Seismic records are sorted in common midpoint gathers or common offset gathers. The second processing step is the conversion of common midpoint 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, in particular 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. In near-surface experimentation, the separation of interfering wavefields is a crucial step to enhance reflected waves. To achieve this, wave separation filters such as F-K filters or SVD (Singular Value Decomposition) filters should be used. 4.3 Near-surface imaging An experimental site at Vesdun (situated in the Cher region in central France) has been developed to train IFP School and university students, along with professionals. The geophysics training relates to the acquisition and processing of surface seismic data in 2D or 3D. A borehole has been drilled on site. It allows the acquisition of well seismic data such as vertical seismic profiles (VSP), and logging data such as full waveform acoustic data. The site is also used for experimental studies in near-surface geophysics.
RkJQdWJsaXNoZXIy NjA3NzQ=