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The seismic data written to tape in the dog house, whether on land or at sea, are not ideal for interpretation. To create an accurate picture of the subsurface, we must remove or at least minimize artifacts in these records related to the surface upon which the survey was performed, artifacts related to the instrumentation and procedure used, and noise in the data obscuring the subsurface image. Treatment of the data to achieve these ends is commonly referred to as ''seismic data processing''. Through processing, the huge volumes of data taken in the field are reduced to simple images for display on paper or the work station screen. This simple image, while it contains less data about the subsurface, is readily accessible to the interpreter and has many of the artifacts and errors just listed removed. [[:file:basic-seismic-processing_fig1.png|Figure 1]] shows a single, unprocessed (raw) field record taken from a line. [[:file:basic-seismic-processing_fig2.png|Figure 2]] is the same line of data after processing to illustrate how the field records are turned into an interpretable image.

The [[seismic data]] written to tape in the dog house, whether on land or at sea, are not ideal for interpretation. To create an accurate picture of the subsurface, we must remove or at least minimize artifacts in these records related to the surface upon which the survey was performed, artifacts related to the instrumentation and procedure used, and noise in the data obscuring the subsurface image. Treatment of the data to achieve these ends is commonly referred to as ''seismic data processing''. Through processing, the huge volumes of data taken in the field are reduced to simple images for display on paper or the work station screen. This simple image, while it contains less data about the subsurface, is readily accessible to the interpreter and has many of the artifacts and errors just listed removed. [[:file:basic-seismic-processing_fig1.png|Figure 1]] shows a single, unprocessed (raw) field record taken from a line. [[:file:basic-seismic-processing_fig2.png|Figure 2]] is the same line of data after processing to illustrate how the field records are turned into an interpretable image.

[[file:basic-seismic-processing_fig1.png|thumb|300px|{{figure number|1}}A single shot record as it is recorded in the field. The shot is at station 60. There were 120 geophones laid out in this “split” spread. Two seconds of data were recorded. © Landmark/ITA.]]

[[file:basic-seismic-processing_fig1.png|thumb|300px|{{figure number|1}}A single shot record as it is recorded in the field. The shot is at station 60. There were 120 geophones laid out in this “split” spread. Two seconds of data were recorded. © Landmark/ITA.]]

==Typical processing steps==

==Typical processing steps==

 

[[file:basic-seismic-processing_fig3.png|left|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_fig3.png|{{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_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.]]

file:basic-seismic-processing_fig4.png|{{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.

[[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-part1.jpg|{{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.]]

file:basic-seismic-processing_fig5-part2.jpg|{{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.

[[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.]]

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Given the broad categories of processing functions just described, this section briefly defines the common programs by their generic names in the order they would normally be applied. Some steps may be applied more than once at different times in the sequence, while others may be skipped for a particular dataset.

Given the broad categories of processing functions just described, this section briefly defines the common programs by their generic names in the order they would normally be applied. Some steps may be applied more than once at different times in the sequence, while others may be skipped for a particular dataset.

===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. [[:file:basic-seismic-processing_fig6.jpg|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.

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

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

NMO corrections are made under the assumption of horizontal planar reflectors. If the reflector has appreciable dip, then the actual movement will be slightly different. The DMO correction is a method for estimating the effect of dip on moveout and removing it from the records as well.

NMO corrections are made under the assumption of horizontal planar reflectors. If the reflector has appreciable [[dip]], then the actual movement will be slightly different. The DMO correction is a method for estimating the effect of dip on moveout and removing it from the records as well.

===Common midpoint (CMP) stack===

===Common midpoint (CMP) stack===

This is the single most effective step for noise reduction in the processing flow. The shooting procedure results in many traces being acquired with the point midway between [[source]] and [[receiver]] (called the midpoint) being coincident on the earths surface. The only difference between the traces is the distance between source and receiver (offset). Once these traces have been NMO (and DMO) corrected, they are really redundant samples of the same reflection. Adding them together increases the signal to random noise ratio by the square root of the number of redundant samples. The process reduces the field data to a stacked section consisting of one trace for each midpoint location, assumed to have been recorded with a shot and receiver coincident at the midpoint location (see [[:file:basic-seismic-processing_fig2.png|Figure 2]]).

This is the single most effective step for noise reduction in the processing flow. The shooting procedure results in many traces being acquired with the point midway between [[source]] and [[receiver]] (called the midpoint) being coincident on the earths surface. The only difference between the traces is the distance between source and receiver ([[offset]]). Once these traces have been NMO (and DMO) corrected, they are really redundant samples of the same reflection. Adding them together increases the signal to random noise ratio by the square root of the number of redundant samples. The process reduces the field data to a stacked section consisting of one trace for each midpoint location, assumed to have been recorded with a shot and receiver coincident at the midpoint location (see [[:file:basic-seismic-processing_fig2.png|Figure 2]]).

===Poststack filter===

===Poststack filter===

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]