Page 123 - Principles of Applied Reservoir Simulation 2E

108 Principles of Applied Reservoir Simulation

illustrates a correlation between seismic wave velocity and the bulk density of
different types of rock. Further discussion of rock properties and their relation-
ship to seismic variables can be found in the literature [for example, Schon
1996],
A change in acoustic impedance will cause a reflection of the sound wave.
The ability to reflect a sound wave by a change in acoustic impedance is
quantified in terms of the reflection coefficient. The reflection coefficient R at
the interface between two contiguous layers is defined in terms of acoustic
impedances as
- Z
Z 2
n _ / £ _

where subscripts 1 and 2 refer to the contiguous layers.
Reflection coefficient magnitudes for typical subsurface interfaces are
illustrated in Table 12-1. Values of reflection coefficients at the sandstone/lime-
stone interface show that reflection coefficient values can be relatively small
In addition to reflection coefficient, a transmission coefficient can be defined.
The transmission coefficient is one minus the reflection coefficient.

Table 12-1
Typical Reflection Coefficients
Interface Reflection Coefficient

Sandstone on limestone 0.040
Limestone on sandstone - 0.040
Ocean bottom 0.11 (soft) to 0.44 (hard)

Nonzero reflection coefficients occur when a wave encounters a change
in acoustic impedance, either because of a change in compressional velocity of
the wave as it propagates from one medium to another, or because the bulk
densities of the media differ. If the change in acoustic impedance is large enough,
the reflection can be measured at the surface. That is why gas tends to show up
as bright spots on seismic data - there is a big change in the density of the fluid.
By contrast, the presence of an oil/water contact is harder to observe with seismic