Synthetic seismograms: correlation to other data
| 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 goal of using a synthetic seismogram is to match stratigraphy as seen in well logs or outcrops to seismic field data. The field data need to have been migrated, since only then does the time axis represent vertical traveltime, which is calculated from sonic log measurements.
The matching process
The matching process involves simulating a seismic trace from well logs and user parameters waveform, frequency, phase, then manually aligning the simulated trace with the field trace(s) in the vicinity of the well. This is normally done over some limited interval in the well, probably centered on the reservoir target. If the fit is not good enough, then the parameters are changed (updated) and another comparison is made. This continues until a match is achieved. It can be a tedious and time-consuming job, particularly for large projects involving many wells. But if not done properly, it is possible to incorrectly associate seismic events and geological horizons.
Problems with matching
In practice, synthetic seismograms are rarely a perfect match to field data. There are many reasons for this.
- Frequency—Sonic logging operates in the kilohertz frequency range (high frequency short wavelength), while seismic data are typically 10–90 Hz (low frequency, long wavelength). This means the sonic log is influenced by a tiny volume of rock compared to a seismic wave passing the borehole.
- Anisotropy—Sonic logs measure velocity in the vertical direction, while seismic waves travel at significant angles away from the vertical. If anisotropy is present (and it usually is), then velocity depends on the direction the wave is traveling. It is not uncommon to see a 10–15% difference between horizontal and vertical velocities.
- Hole—Sonic logging is sensitive to washouts and other hole problems, while long-wavelength seismic waves are not.
- Wavelet—The user is required to specify the wavelet, and it is very easy to get it wrong. Some advanced software products can scan the data and attempt to extract the wavelet. But these scanners involve many user parameters.
See also
- Seismic data
- Seismic data: building a stratigraphic model
- Synthetic seismogram
- Seismic data: identifying reflectors
External links
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