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  | isbn    = 0891816607
 
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The more control the geoscientist has in mapping the subsurface, the greater the accuracy of the maps. Control can be increased by the correlation of seismic data with borehole data. The synthetic seismogram (often called simply the “synthetic”) is the primary means of obtaining this correlation.
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The more control the geoscientist has in mapping the subsurface, the greater the accuracy of the maps. Control can be increased by the correlation of [[seismic data]] with borehole data. The synthetic seismogram (often called simply the “synthetic”) is the primary means of obtaining this correlation.
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Velocity data from the sonic log (and the density log, if available) are used to create a synthetic seismic trace. This trace closely approximates a trace from a seismic line that passes close to the well in which the logs were acquired. The synthetic then correlates with both the seismic data and the well log from which it was generated.
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Velocity data from the sonic log (and the [[density log]], if available) are used to create a synthetic seismic trace. This trace closely approximates a trace from a seismic line that passes close to the well in which the logs were acquired. The synthetic then correlates with both the seismic data and the well log from which it was generated.
    
==Calculating a synthetic seismogram==
 
==Calculating a synthetic seismogram==
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[[file:synthetic-seismograms_fig1.png|left|thumb|{{figure number|1}}Synthetic seismogram and associated well log data.]]
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[[file:synthetic-seismograms_fig1.png|400px|thumb|{{figure number|1}}Synthetic seismogram and associated well log data.]]
    
The calculation of a synthetic seismogram generally follows these steps:
 
The calculation of a synthetic seismogram generally follows these steps:
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The synthetic trace can now be compared to a trace from the seismic line. This is commonly done by laying the synthetic directly on top of the appropriate seismic trace and adjusting the synthetic vertically until the two coincide. Through a trial-and-error process, the interpreter determines at what point the synthetic trace “best fits” the seismic data.
 
The synthetic trace can now be compared to a trace from the seismic line. This is commonly done by laying the synthetic directly on top of the appropriate seismic trace and adjusting the synthetic vertically until the two coincide. Through a trial-and-error process, the interpreter determines at what point the synthetic trace “best fits” the seismic data.
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In an ideal world, there is an obvious agreement between the seismic line trace and the synthetic seismogram. A formation top or other correlation marker on the well log can then be tied to the corresponding seismic horizon with relative ease. In the real world, however, the interpreter may be, and often is, confronted with a synthetic trace and seismic data trace that bear little resemblance to each other. Variations in the quality of the well log data can have a major impact on the final synthetic display. A sonic log that was generated from a borehole containing numerous washed out zones will contain areas of unrealistic velocities. Careful editing of the well log data can help to smooth spurious data readings and generate a more realistic synthetic. Care should be taken, however, whenever well log data are edited. For more details about synthetic seismograms, the interested reader can refer to ''Seismic Exploration Fundamentals'' (Coffen, 1978).
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In an ideal world, there is an obvious agreement between the seismic line trace and the synthetic seismogram. A formation top or other correlation marker on the well log can then be tied to the corresponding seismic horizon with relative ease. In the real world, however, the interpreter may be, and often is, confronted with a synthetic trace and seismic data trace that bear little resemblance to each other. Variations in the quality of the well log data can have a major impact on the final synthetic display. A sonic log that was generated from a borehole containing numerous washed out zones will contain areas of unrealistic velocities. Careful editing of the well log data can help to smooth spurious data readings and generate a more realistic synthetic. Care should be taken, however, whenever well log data are edited. For more details about synthetic seismograms, the interested reader can refer to ''Seismic Exploration Fundamentals'' ([[Coffen, 1978]]).{{Citation needed}}
    
==Creating a synthetic seismogram==
 
==Creating a synthetic seismogram==
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  | isbn    = 0-89181-602-X
 
  | isbn    = 0-89181-602-X
 
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Once a stratigraphic model has been built using velocities and densities, a synthetic seismogram (or synthetic) can be constructed to identify seismic reflections. A synthetic seismogram is the fundamental link between well data and seismic data, and it is the main tool (along with a VSP, if available) that allows geological picks to be associated with reflections in the seismic data. As discussed, if a VSP is available for a particular well, a synthetic is not needed. The VSP directly measures both time and depth to a formation of interest. Usually synthetic seismograms are created using specialized software. The user may be unaware of the process that creates them. The table below lists the steps necessary to create a synthetic seismogram manually.
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Once a stratigraphic model has been built using velocities and densities, a synthetic seismogram (or synthetic) can be constructed to identify seismic reflections. A synthetic seismogram is the fundamental link between well data and [[seismic data]], and it is the main tool (along with a [[Checkshots_and_vertical_seismic_profiles#Vertical_seismic_profiles|vertical seismic profile [VSP]]], if available) that allows geological picks to be associated with reflections in the seismic data. As discussed, if a VSP is available for a particular well, a synthetic is not needed. The VSP directly measures both time and depth to a formation of interest. Usually synthetic seismograms are created using specialized software. The user may be unaware of the process that creates them. The steps necessary to create a synthetic seismogram manually are described below:
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{| class = "wikitable"
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# Edit the sonic and density logs for bad intervals.
|-
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# Calculate vertical reflection times.
! Step
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# Calculate reflection coefficients, R<sub>o</sub> .
! Action
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# Combine the last two items to create a reflection coefficient time series.
|-
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# Convolve the reflection coefficient series with the wavelet.
| 1
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| Edit the sonic and density logs for bad intervals.
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|-
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| 2
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| Calculate vertical reflection times.
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|-
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| 3
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| Calculate reflection coefficients, R<sub>o</sub> .
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|-
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| 4
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| Combine the last two items to create a reflection coefficient time series.
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|-
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| 5
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| Convolve the reflection coefficient series with the wavelet.
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|}
      
==Reflection coefficient==
 
==Reflection coefficient==
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The final simulated seismic trace can be summarized by the convolutional model:
 
The final simulated seismic trace can be summarized by the convolutional model:
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:<math>T(t) = R_{0}(t) * w(t) + n(t)</math>
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:<math>T(t) = R_{0}(t) \times w(t) + n(t)</math>
    
where:
 
where:
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==Example synthetic seismogram==
 
==Example synthetic seismogram==
[[file:interpreting-seismic-data_fig12-7.png|thumb|{{figure number|12-7}}From software by S. Hill, Conoco.]]
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[[file:interpreting-seismic-data_fig12-7.png|300px|thumb|{{figure number|2}}Simple synthetic seismogram. From software by S. Hill, Conoco.]]
The figure below shows a simple synthetic seismogram. We can see most of the components that go into the creation of a synthetic seismogram—the velocity model, reflection coefficient series, individual wavelets, synthetic trace, and simulated stack section (lower plot). The velocity model is from north-central Oklahoma. The density model is not shown.
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[[:file:interpreting-seismic-data_fig12-7.png|Figure 2]] shows a simple synthetic seismogram. We can see most of the components that go into the creation of a synthetic seismogram—the velocity model, reflection coefficient series, individual wavelets, synthetic trace, and simulated stack section (lower plot). The velocity model is from north-central Oklahoma. The density model is not shown.
    
==See also==
 
==See also==
* [[Building a stratigraphic model]]
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* [[Seismic data: building a stratigraphic model]]
* [[Matching synthetics to data]]
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* [[Synthetic seismograms: correlation to other data]]
* [[Identifying reflectors]]
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* [[Seismic data: identifying reflectors]]
 
* [[Seismic interpretation]]
 
* [[Seismic interpretation]]
 
* [[Vertical and lateral seismic resolution and attenuation]]
 
* [[Vertical and lateral seismic resolution and attenuation]]
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[[Category:Geophysical methods]][[Category:Interpreting seismic data]]
 
[[Category:Geophysical methods]][[Category:Interpreting seismic data]]
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[[Category:Methods in Exploration 10]]

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