• Geometric spreading and attenuation loss combine to cause a rapid decay of the seismic signal with time (depth). If these losses are left uncorrected, only the uppermost reflectors would be visible on the final section.
• The two images shown above show a shot gather as originally recorded (left), and the same gather after gain has been applied (right).
Data collected on land need to have the effects of changes in ground surface elevation removed and, potentially, the effects of changes in thickness (and velocity) of the
“surface layer” (low-velocity weathered rock, till, aeolian dunes, etc.). An arbitrary seismic datum is defined at this time. These processes are collectively known as static corrections (“statics”). In addition to defining the near-surface velocity, the elevation of sources and receivers must be known.
Short-period statics occur within the length of the spread, whereas long-period statics are associated with lateral velocity variations that are longer than the length of the spread.
Static corrections may be undertaken in two steps. Field statics (“refraction statics) are traveltime corrections to account for irregular topography and near-surface weathering layer. They remove a significant part of distortions (especially long-wavelength
anomalies), but not all. Residual statics are applied later and account for rapidly varying near-surface effects.
Failure to account for statics problems can lead to poor stacking (next slide) and to false structures at depth (see later).
The upper image shows a gather with a prominent “valley” to the left of station 1500.
The “valley” persists throughout the length of the section. These data will stack poorly after NMO corrections are made and the final image will not be clear.
The lower image shows the same gather after static corrections have been applied. This version of the data will stack better, giving a more interpretable image.
Duncan, 1992
These two images show the effects of static corrections on data quality and the apparent subsurface structure. The upper image shows a stacked seismic section without static corrections. The lower image shows the same data but with static corrections applied.
Note the improvement in data interpretability in the lower image and the disappearance of some low-amplitude structures seen in the upper image; these were due to statics problems.
Yilmaz, 2001
As the seismic wavelet travels from the source to reflecting interface and back to a receiver, its shape is modified by a variety of factors, including loss of certain frequency ranges, interference effects (including short-period multiples), noise, etc. The receivers themselves have a characteristic response function and the wavelet generated is not an infinitely sharp pulse. Because of these problems, reflections are smeared out over a period of time and the true position of the reflecting interface may not be clear.
Deconvolution refers to seismic processing techniques that attempt to correct for these problems. The objectives are to: a) compress the basic wavelet, b) attenuate
reverberations (“ringiness” of the data), c) attenuate short-period multiples, and d) wavelet shaping (produce a wavelet with known phase). The result will be improved temporal (i.e., stratigraphic) resolution.
The images shown above compare a stacked section without deconvolution (top) with a stacked section after deconvolution (bottom). Stratigraphic features are clearer in the lower image.
Yilmaz, 2001
Different types of deconvolution are available. Statistical deconvolution assumes that the source wavelet was minimum phase and that the subsurface reflectivity is random. If these two assumptions are not valid, the result will be a mixed-phase wavelet. For signature deconvolution the source wavelet must be known, but the result will be a zero phase wavelet.
The images shown above, from Henry (2001), compare seismic data processed using statistical deconvolution (left) with signature deconvolution (right) that produced mixed-phase and zero-mixed-phase wavelets respectively. The zero-mixed-phase data have improved definition of faults and stratigraphic features.
Henry, 2001
Most areas of interest to petroleum geoscientists are not characterized by layer-cake geology. Subsurface structures are present, and these can lead to problems using the seismic method. The figure above shows one such problem. The upper panel shows a buried channel. The lines show raypaths from source (at left) to receivers (at right). Notice that reflections do not come from half way between source and receivers (as expected using the common midpoint method) and that a receivers record reflected energy from three places along the interface. The
expected seismic expression of the channel is shown in the lower image. Note the classic “bowtie” appearance.
Properly accomplished, seismic migration accomplishes three objectives: 1) repositioning reflected energy to its true subsurface location, 2) collapsing diffractions, and 3) shrinking the Fresnel zone.
These images show the impact that seismic migration can have on a seismic image. The upper image is a stacked section without migration, the lower image shows the same data but migrated. Note that the “bowties” evident at left in the upper image have disappeared and the anticlines in the right-center portion of the image are smaller (“tighter”) in the migrated image.
Migration is typically one of the last processing steps. Different types of migration are available with the commercial cost being proportional to the amount of computer time required for the work. To migrate seismic data, processors need to know, or be able to estimate, the subsurface velocity field. The accuracy of the results of implementing any migration algorithm will be dependent on the accuracy of the velocity model used.
Different types of migration algorithms are available. We will come back to these following the section on 3-D migration
Yilmaz, 2001