Velocity and the Seismic Tie
Learning objectives
- Compute velocity from the sonic and acoustic impedance from velocity and density
- Find the reflection coefficient as an impedance contrast
- Explain why a gas sand gives a soft, negative reflection
- Connect the well logs to the seismic image
From Slowness to Impedance
The sonic and density together carry the rock all the way to the seismic. The sonic gives velocity, ; multiply by the bulk density and you have the acoustic impedance
Impedance is the single property a seismic wave responds to: a hard rock (high Z) reflects differently from a soft one. Both ingredients, velocity and density, are exactly the logs of this chapter and the last.
The Reflection Coefficient
At a boundary between two layers, the fraction of the wave that bounces back is the reflection coefficient, the impedance contrast
When the lower layer is harder, RC is positive, a peak; when it is softer, RC is negative, a trough. The bigger the contrast, the brighter the reflection.
The Tie to Seismic
Now the bright spot makes sense. Put a gas sand under a shale: gas drops both the density and the velocity, so the sand's impedance falls well below the shale, the reflection coefficient turns strongly negative, and the seismic shows a strong soft trough. The amplitude and the polarity the seismic sees are set by the porosity and fluid the logs measured. This is the synthetic seismogram in miniature, and the handshake between this course and the seismic courses: petrophysics is what makes a seismic amplitude mean something.
References
- Sheriff, R. E. and Geldart, L. P. (1995). Exploration Seismology, 2nd ed. Cambridge University Press.
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.