Changes

An example of the use of modeling to aid in stratigraphic interpretation is the use of amplitudes of seismic data to infer the thickness of a thin sand layer. If layer thickness is less than a wavelength, variations and thickness appear as tuning effects, with systematic changes in amplitude and wave shape correlating directly with sand thickness. Producing forward models with varying sand thickness using well data to calibrate, and comparing these synthetics with observed seismic sections, provides a means of accurately determining sand thickness where well data are sparse or absent.

An example of the use of modeling to aid in stratigraphic interpretation is the use of amplitudes of seismic data to infer the thickness of a thin sand layer. If layer thickness is less than a wavelength, variations and thickness appear as tuning effects, with systematic changes in amplitude and wave shape correlating directly with sand thickness. Producing forward models with varying sand thickness using well data to calibrate, and comparing these synthetics with observed seismic sections, provides a means of accurately determining sand thickness where well data are sparse or absent.

Another use of forward modeling is the determination of the seismic expression of expected features of the geology. Figures 1 and 2 illustrate the use of seismic modeling to determine if [[porosity]] in a carbonate reef interval is detectable on seismic data. Figure 1 is a strike cross section through a 3-D geological model representing a carbonate reef play. Reef structures are located in the interval between the depths of 2500 and [[depth::3500 ft]]. The structure located on the left side of the cross section contains porosity, while the structure on the right is tight. [[Porosity]] is represented in the model by a lower interval velocity and lower density than the surrounding rock (see [[Porosity]]).

Another use of forward modeling is the determination of the seismic expression of expected features of the geology. [[:file:forward-modeling-of-seismic-data_fig1.png|Figures 1]] and [[:file:forward-modeling-of-seismic-data_fig2.png|2]] illustrate the use of seismic modeling to determine if [[porosity]] in a carbonate reef interval is detectable on seismic data. Figure 1 is a strike cross section through a 3-D geological model representing a carbonate reef play. Reef structures are located in the interval between the depths of 2500 and [[depth::3500 ft]]. The structure located on the left side of the cross section contains porosity, while the structure on the right is tight. [[Porosity]] is represented in the model by a lower interval velocity and lower density than the surrounding rock (see [[Porosity]]).

[[file:forward-modeling-of-seismic-data_fig1.png|thumb|{{figure number|1}}A strike cross section of a carbonate reef play. The reef structure on the left contains porosity, while the reef structure on the right is tight.]]

This study included the generation of two synthetic seismic sections using 3-D ray tracing techniques. One section represents a synthetic stacked seismic section (created using normal incidence ray tracing), while the second represents a synthetic migrated section (created using image ray tracing). The objective of modeling is to determine if there is a noticeable difference between the seismic signature of the tight carbonate and that of the porous carbonate. The synthetic migrated section is illustrated in Figure 2. The positive event at traces 20 through 26 and between 325 and 375 msec represents the top of the tight anticlinal structure. There is a distinct character change of this seismic event corresponding to a change in porosity in the geological model. The porosity is identified by the dim spot on traces 6 through 11 and the velocity pulldown of the underlying event. The results from this modeling indicate that seismic data could be used to identify porosity in this geological setting. A good description of modeling methods and a number of case study examples can be found in Fagin.<ref name=pt07r12>Fagin, S. W., 1991, Seismic Modeling of Geologic Structure: Tulsa, OK, Society of Exploration Geophysicists, Geophysical Development Series, v. 2.</ref>

[[file:forward-modeling-of-seismic-data_fig2.png|thumb|{{figure number|2}}[[Synthetic seismograms]] for the model in Figure 1. By synthetic modeling of a migrated section, the expected seismic signatures of reefs containing porosity and tight reefs have been obtained.]]

==Methods of forward modeling==

This study included the generation of two synthetic seismic sections using 3-D ray tracing techniques. One section represents a synthetic stacked seismic section (created using normal incidence ray tracing), while the second represents a synthetic migrated section (created using image ray tracing). The objective of modeling is to determine if there is a noticeable difference between the seismic signature of the tight carbonate and that of the porous carbonate. The synthetic migrated section is illustrated in Figure 2. The positive event at traces 20 through 26 and between 325 and 375 msec represents the top of the tight anticlinal structure. There is a distinct character change of this seismic event corresponding to a change in porosity in the geological model. The porosity is identified by the dim spot on traces 6 through 11 and the velocity pulldown of the underlying event. The results from this modeling indicate that seismic data could be used to identify porosity in this geological setting. A good description of modeling methods and a number of case study examples can be found in Fagin.<ref name=pt07r12>Fagin, S. W., 1991, Seismic Modeling of Geologic Structure: Tulsa, OK, Society of Exploration Geophysicists, Geophysical Development Series, v. 2.</ref>

[[file:forward-modeling-of-seismic-data_fig1.png|left|thumb|{{figure number|1}}A strike cross section of a carbonate reef play. The reef structure on the left contains porosity, while the reef structure on the right is tight.]]

==Methods of forward modeling==

[[file:forward-modeling-of-seismic-data_fig2.png|thumb|{{figure number|2}}[[Synthetic seismograms]] for the model in Figure 1. By synthetic modeling of a migrated section, the expected seismic signatures of reefs containing porosity and tight reefs have been obtained.]]

A number of forward modeling methods are available, and the choice of method generally depends on a tradeoff between the accuracy necessary and the desired computing time. In general, the type of data to be modeled, the complexity of the model, and the aspects of the data that need to be accurately modeled dictate the method that should be used.

A number of forward modeling methods are available, and the choice of method generally depends on a tradeoff between the accuracy necessary and the desired computing time. In general, the type of data to be modeled, the complexity of the model, and the aspects of the data that need to be accurately modeled dictate the method that should be used.