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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|left|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_fig2.png|thumb|300px|{{figure number|2}}A seismic section produced by processing six shots such as those in Figure 1. © Landmark/ITA.]]

The processing sequence designed to achieve the interpretable image will likely consist of several individual steps. The number of steps, the order in which they are applied, and the parameters used for each program vary from area to area, from dataset to dataset, and from processor to processor. However, the steps can be grouped by function so that the basic processing flow can be illustrated as follows:

The processing sequence designed to achieve the interpretable image will likely consist of several individual steps. The number of steps, the order in which they are applied, and the parameters used for each program vary from area to area, from dataset to dataset, and from processor to processor. However, the steps can be grouped by function so that the basic processing flow can be illustrated as follows:

==Typical processing steps==

==Typical processing steps==

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.

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 [[:file:basic-seismic-processing_fig3.png|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]].

===Deconvolution===

===Deconvolution===

Acoustic signals that are not reflections from subsurface layers appear in shot records ([[:file:basic-seismic-processing_fig1.png|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 ([[:file:basic-seismic-processing_fig1.png|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.

[[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.]]

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

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

[[file:basic-seismic-processing_fig6.jpg|thumb|300px|{{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.]]

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]]