Geology Reference
In-Depth Information
a)
b)
300m
2500m
Amplitude varying with offset
20000
15000
1000
5000
0
-5000
-10000
-15000
-20000
0
500
1000
1500
2000
2500
3000
Offset (m)
c)
Model from well data
Amplitude varying with angle
?
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
0
0.05
0.1
0.15
0.2
0.25
Figure 2.22
The issue of relating offset to sin
2
θ; (a) corrected gather showing a reflection from the top of a thick gas sand, (b) offset
versus amplitude crossplot from the reflection shown on the gather, (c) modelled change of reflectivity as a function of sin
2
θ based on
well data.
in longer computer run times and often requires
more time to be spent preparing the log data (e.g. in
devising a log blocking strategy). Given that in most
cases the seismic processor will have tried, with
varying success, to remove these events from the
data it may be that this type of modelling has
limited predictive value.
amplitude of various reflectors at the contact as well
as the subtle nature of the interference effects as the
fluid contact passes through the geological layering.
These types of model can be generated for different
angles of incidence to explore which angle gives the
clearest fluid response (
Chapter 5
).
2.3.5 Relating seismic data to models
In order to relate reflectivity models calculated at
the wells in terms of angle to seismic data which
have been collected as a function of source
2.3.4.4 2D models
Two-dimensional (2D) models (
Fig. 2.20
) can be
extremely useful for understanding the effects of lat-
eral changes in rock properties and/or layer thick-
nesses on the seismic signature. These types of
models are typically created using wireline well log
data interpolated along model horizons.
Figure 2.20d
shows an anticline with gas-bearing sands above a flat
gas
receiver
offset, a conversion scheme from offset to angle is
required (
Fig. 2.22
). This requires use of a velocity
model, usually constructed from velocity information
acquired in the course of seismic processing, i.e. in
correcting for moveout. If the geology can be approxi-
mated as a set of horizontal
-
water contact. Numerous effects are evident on
the section,
-
19
including changes in the polarity and
layers then reflection
