Seismic data: building a stratigraphic model

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Interpreting seismic data
Author	Christopher L. Liner
Link	Web page
Store	AAPG Store

The problem addressed in seismic modeling, or simulation, is calculating the seismic response (traveltime and amplitude) for a given stratigraphic model. The stratigraphic model consists of those physical properties that influence seismic wave propagation—typically compressional wave speed, shear wave speed, and mass density. This set of parameters can describe the simplest possible solid, called an isotropic elastic solid. For some purposes, it is sufficient to consider the earth as an acoustic (fluid) medium characterized by only two parameters: sound speed (υ) and mass density (ρ). Seismic reflections are generated where there is a contrast in impedance (which is the product of velocity and density).

Velocity data sources

Depth-dependent velocity and density models are needed to identify events or to create a synthetic seismogram. Velocity information can come from a variety of sources. Here is a list, in order of preference:

• Vertical seismic profile (VSP)
• Sonic log with checkshots
• Sonic log without checkshots
• Checkshots only

See also Checkshots and vertical seismic profiles.

VSPs

Figure 1 Example of a zero-offset VSP. Copyright: Landmark Graphics.

A vertical seismic profile (VSP) yields the best connection between geologic horizons and seismic events. It is recorded by using a source at the surface and many receiver locations down a wellbore, or vice versa. The receivers record full traces for interpretation. The receiver spacing is usually length::10 ft. This gives actual traveltimes from the surface to points in the earth, and it is the best and most direct method of associating seismic events with geological horizons. The kind of VSP shown in Figure 1 (produced by commercial software) is often called a zero-offset VSP, meaning that only a single source position is used as close to the wellhead as possible. It is relatively inexpensive. There are also multioffset and multiazimuth VSPs, which use many source locations. These are much more expensive and sometimes are useful for local, high-resolution imaging. However, a zero-offset VSP is sufficient for event identification and 3-D seismic data calibration.

Velocities from sonic logs

Velocities can be acquired from sonic logs with or without a checkshot survey. A checkshot survey is like a baby VSP. The receivers are sparsely located down the well, usually on key geologic boundaries. Also, the information recovered is limited to arrival time (a number), unlike the full trace a VSP gives. The checkshots help correct for any drift in a sonic log due to missing log intervals or hole problems. This makes the calculated traveltimes more reliable. One can obtain good velocity data from sonic logs without checkshots. However, if there are any hole integrity problems, significant errors can exist in the sonic data. If a sonic log is not available, very coarse velocity data can be obtained from a checkshot survey with only 5 or 6 traveltimes per well.

Density data

Density information can improve the accuracy of a stratigraphic model for seismic analysis.

Velocity can be estimated from a density log if a sonic log is not available, but data from a sonic log is much better.

See also

• Seismic data
• Synthetic seismogram
• Synthetic seismograms: correlation to other data
• Seismic data: identifying reflectors
• Checkshots and vertical seismic profiles

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