Changes

The association by unique identifier of each recorded trace with shot and receiver locations.

The association by unique identifier of each recorded trace with shot and receiver locations.

===Antialias filter===

===Antialias filter===

A low pass filter applied before resampling the data to a coarser time scale to prevent aliasing. Aliasing is a phenomenon in which high frequency data masquerades as low frequency energy as a result of undersampling. To sample a signal properly, there must be at least two samples within the shortest period of interest. Antialias filters remove frequencies above the sampling limit (Nyquist frequency) of the new sampling time. The operation is performed before the sampling is reduced.

A low pass filter applied before resampling the data to a coarser time scale to prevent aliasing. Aliasing is a phenomenon in which high frequency data masquerades as low frequency energy as a result of undersampling. To sample a signal properly, there must be at least two samples within the shortest period of interest. Antialias filters remove frequencies above the sampling limit (Nyquist frequency) of the new sampling time. The operation is performed before the sampling is reduced.

===Gain recovery===

===Gain recovery===

The correction for the loss in amplitude of a signal as it travels through the earth and spreads its energy over a larger surface area. This involves multiplication of the signal by a number that increases with time. The exact time variant multiplier can be based on the theoretical concept of spherical spreading (related to the square of the distance traveled), can be based on measurements of amplitude decay with time made on the data itself, or can be entirely arbitrary. An example of the effect of gain recovery is given in Figure 3.

The correction for the loss in amplitude of a signal as it travels through the earth and spreads its energy over a larger surface area. This involves multiplication of the signal by a number that increases with time. The exact time variant multiplier can be based on the theoretical concept of spherical spreading (related to the square of the distance traveled), can be based on measurements of amplitude decay with time made on the data itself, or can be entirely arbitrary. An example of the effect of gain recovery is given in [[:file:basic-seismic-processing_fig3.png|Figure 3]].

[[file:basic-seismic-processing_fig3.png|thumb|{{figure number|3}}The shot record of Figure 1 after the application of a gain recovery algorithm to replace the energy lost as the signal traverses the earth. © Landmark/ITA.]]

[[file:basic-seismic-processing_fig5-part1.jpg|left|thumb|{{figure number|5a}}The application of statics corrects for differences in arrival time caused by elevation or weathering. (a) The valley in the data to the left of station 1500 represents an anomaly that persists throughout the time length of the record. Copyright Landmark/ITA.]]

[[file:basic-seismic-processing_fig5-part2.jpg|left|thumb||{{figure number|5b}}The application of statics corrects for differences in arrival time caused by elevation or weathering. (b) This “static” effect has been corrected. Copyright Landmark/ITA.]]

===Deconvolution===

===Deconvolution===

The removal of the frequency-dependent response of the source and the instrument. The instrument response is normally known and can be removed exactly. The source shape is not usually known but can be measured directly (marine air gun signatures) or estimated from the signal itself under certain assumptions. Signature deconvolution, wavelet deconvolution, spiking deconvolution, gapped deconvolution, predictive deconvolution, maximum entropy deconvolution, and surface consistent deconvolution are various manifestations of the attempt to remove the source width from the observed reflections.<ref name=pt07r64>Yilmaz, O., 1987 Seismic Data Processing: Society of Exploration Geophysicists, Tulsa, OK, 525 p.</ref> The resulting reflection sequence always has some smoothing function left, usually called the ''residual wavelet''. Attempting to be too exact about deconvolution usually results in a very noisy section. The effect of deconvolution is seen in Figure 4.

The removal of the frequency-dependent response of the source and the instrument. The instrument response is normally known and can be removed exactly. The source shape is not usually known but can be measured directly (marine air gun signatures) or estimated from the signal itself under certain assumptions. Signature deconvolution, wavelet deconvolution, spiking deconvolution, gapped deconvolution, predictive deconvolution, maximum entropy deconvolution, and surface consistent deconvolution are various manifestations of the attempt to remove the source width from the observed reflections.<ref name=pt07r64>Yilmaz, O., 1987 Seismic Data Processing: Society of Exploration Geophysicists, Tulsa, OK, 525 p.</ref> The resulting reflection sequence always has some smoothing function left, usually called the ''residual wavelet''. Attempting to be too exact about deconvolution usually results in a very noisy section. The effect of deconvolution is seen in [[:file:basic-seismic-processing_fig4.png|Figure 4]].

 

[[file:basic-seismic-processing_fig4.png|thumb|{{figure number|4}}The shot record after a statistical deconvolution process has been applied to “shorten” the wavelet and increase time resolution. Copyright Landmark/ITA.]]

===Statics===

===Statics===

The removal of traveltime artifacts relating to the placement of the source and receiver at or near the earth's surface. Differences in traveltime to the same reflector which result from elevation differences and near-surface velocity changes at different source and receiver stations must be removed. The relative elevation of each shot and receiver location and the near surface velocity must be known to make these corrections. An elevation datum is chosen, and the distance above or below that datum is measured for each source and receiver. The difficulty is in knowing what velocity to use to convert this elevation difference to a time correction to be added to or subtracted from the entire trace (hence the term ''statics''). Refraction statics, surface consistent statics, and residual statics are all techniques used to estimate and apply the appropriate velocity and time corrections (Figure 5).

The removal of traveltime artifacts relating to the placement of the source and receiver at or near the earth's surface. Differences in traveltime to the same reflector which result from elevation differences and near-surface velocity changes at different source and receiver stations must be removed. The relative elevation of each shot and receiver location and the near surface velocity must be known to make these corrections. An elevation datum is chosen, and the distance above or below that datum is measured for each source and receiver. The difficulty is in knowing what velocity to use to convert this elevation difference to a time correction to be added to or subtracted from the entire trace (hence the term ''statics''). Refraction statics, surface consistent statics, and residual statics are all techniques used to estimate and apply the appropriate velocity and time corrections ([[:file:basic-seismic-processing_fig5-part1.jpg|Figure 5a]] and [[:file:basic-seismic-processing_fig5-part2.jpg|b]]).

 

[[file:basic-seismic-processing_fig5-part1.jpg|thumb|{{figure number|5}}(previous page) The application of statics corrects for differences in arrival time caused by elevation or weathering. (a) The valley in the data to the left of station 1500 represents an anomaly that persists throughout the time length of the record. (b) This “static” effect has been corrected. Copyright Landmark/ITA.]]

[[file:basic-seismic-processing_fig5-part2.jpg|thumb]]

===Demultiple===

===Demultiple===

Acoustic signals that are not reflections from subsurface layers appear in shot records (Figure 1) as straight lines rather than hyperbolic curves. These events have a constant “apparent velocity” as they travel along the receiver cable. This simple organization allows them to be isolated from the reflection signal and to be removed from the record. A common way to do this is with the FK (sometimes called pie slice) filter. Judicious selection of the range of apparent velocities to be removed can eliminate linear noise. Too wide a filter can remove too much information from the section and causes serious interpretation problems.

Acoustic signals that are not reflections from subsurface layers appear in shot records (Figure 1) as straight lines rather than hyperbolic curves. These events have a constant “apparent velocity” as they travel along the receiver cable. This simple organization allows them to be isolated from the reflection signal and to be removed from the record. A common way to do this is with the FK (sometimes called pie slice) filter. Judicious selection of the range of apparent velocities to be removed can eliminate linear noise. Too wide a filter can remove too much information from the section and causes serious interpretation problems.

===Normal moveout (NMO) correction===

===Normal moveout (NMO) correction===

The reflection from a given horizon does not arrive at the same time at different receivers along the length of the seismic cable or spread (see “Seismic Migration”). However, if the velocity at which the sound traveled is known, the arrival time difference (moveout) at each station can be predicted. Conversely, knowing the arrival time difference, the velocity the sound traveled can be determined under certain model assumptions. Usually the velocity of the earth as a function of time is determined at a few locations over the survey. This model can then be used to calculate moveout as a function of time everywhere in the survey. The moveout is subtracted from each seismic record such that the reflections from a given horizon will appear flat. This facilitates identification of reflectors and stacking. Figure 6 demonstrates the NMO process.

The reflection from a given horizon does not arrive at the same time at different receivers along the length of the seismic cable or spread (see “Seismic Migration”). However, if the velocity at which the sound traveled is known, the arrival time difference (moveout) at each station can be predicted. Conversely, knowing the arrival time difference, the velocity the sound traveled can be determined under certain model assumptions. Usually the velocity of the earth as a function of time is determined at a few locations over the survey. This model can then be used to calculate moveout as a function of time everywhere in the survey. The moveout is subtracted from each seismic record such that the reflections from a given horizon will appear flat. This facilitates identification of reflectors and stacking. [[:file:basic-seismic-processing_fig6.jpg|Figure 6]] demonstrates the NMO process.

 

[[file:basic-seismic-processing_fig6.jpg|thumb|{{figure number|6}}(a) A gather of processed traces with a common surface location. Shot-to-receiver offset is zero at the center of the gather and increases to about 2000 m deep on either end. The offset related curvature of the reflections is due to normal moveout. (b) Normal moveout correction (NMO) has been applied and the horizons are flat. The gather is now ready to be summed or stacked to produce one trace on Figure 2. Copyright Landmark/ITA.]]

===Dip moveout (DMO) correction===

===Dip moveout (DMO) correction===