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5
Seismic Data Processing
data acquired in the field, or errors in the field parameters or malfunctions of the field equipment, computer software cannot retrieve such information because it is not present in the field seismic data. Data processing is a very powerful tool in manipulating the field information. It converts the raw data with all information, including noise and distorted information, to meaningful seismic cross-sections that represent vertical slices through the subsurface. These slices represent geological information and potential hydrocarbon traps. A generalized seismic data-processing flow has been presented. The intention is to explain the physical meaning of some “buzz” words that are heard, such as Cross-correlation that is used to extract information from seismic data acquired by the Vibroseis method; it is used to solve statics problems and for many more seismic data-processing applications. Gain is applied to compensate for spherical divergence and to preserve the relative true amplitude, which can be used as hydrocarbon indicator. Convolution is used to filter out some undesirable frequencies while deconvolution is a process to attenuate short-period multiple reflections and to enhance the vertical resolution. Normal moveout corrections are applied to correct for the field geometry and produce a zero-offset stack section as if the source and receiver were in the same location. Elevation statics are applied to minimize the effect of the variation of the surface elevations. Refraction statics are applied to solve the irregularities in the nearsurface formation called. “weathering layer” or “near-surface layers.” It may vary in thickness and velocities, both laterally and vertically. These near-surface layers distort the deep structural and stratigraphic features that are obtained from seismic sections. Data processing is both an art and science. The processing steps should be handled in a logical order. Every step should be checked carefully before proceeding to the next step. Remember that the goal of processing the seismic data is to obtain geologically sound sections and not just a pretty one with man-made information. The map is only as reliable as the seismic data. Many dry holes have been drilled on what appeared to be structural highs due to processing problems. As was discussed in Chap. 4, it is difficult to calculate the cost of acquiring a mile of seismic data. It also it is not easy to give a price range for processing a mile of seismic data. The cost per mile depends upon the type of data, whether it is marine or land, the signal-to-noise ratio of each type, the field problems involved, the field configuration such as the number of channels, fold, and sampling interval; and the processing sequence needed including special software programs to perform certain tasks. If a contractor is used to process newly acquired seismic data or to evaluate a prospect and a reprocessing job is needed, it is advisable to consult with several data processing contractors. Software applications for basic processing are very similar from one contractor to another. The knowledge and the experience of the processing geophysicist is the major factor in the quality and the validity of the processed data. The interaction of all staff members in the data processing sequence is very important. Seismic workstations may be used for post-stack enhancement programs