Geology Reference
In-Depth Information
Lithology
Im
pedan
ce
Rc
Individual reflections
Synthetic
seismogram
Figure 2.9
Synthetic seismogram using
a symmetrical wavelet with positive
standard polarity (re-drawn and modified
importance of zero phase to the
interpreter; the main layer boundaries are
more easily identified with the processed
symmetrical wavelet compared to the
recorded asymmetrical wavelet shown
in
Fig 2.7
.
1234 56
- +
1
3
4
5
6
as a peak (i.e. positive number) then this is referred to as
'
For the interpreter who inherits a seismic project
it is evident that polarity and colour coding issues (as
well as uncertainties concerning the processing of the
data) introduce significant potential for misunder-
standing and error. It is critical that the interpreter
develops a good idea of the shape of the seismic
wavelet prior to detailed horizon picking (see
Chap-
ters 3
and
4
).
, whereas if it is represented
as a trough (i.e. negative number) it is referred to as
'
positive standard polarity
'
(
Fig. 2.8
). Historically the
usage of these two conventions has been broadly geo-
graphical and they have been referred to as
negative standard polarity
'
'
American
'
and
conventions are sometimes informally referred to
respectively as
'
European
'
'
'
'
'
SEG normal
and
SEG reverse
polarity.
'
'
It is evident, however, that the use of
normal
and
2.3.2 Isotropic and elastic rock properties
2.3.2.1 P and S velocities and bulk density
Seismic models for exploration purposes are con-
structed using velocities and densities, principally
from wireline log data (
Chapter 8
). As will be shown
in the following section the calculation of offset
reflectivity requires two types of velocity (compres-
sional (P) and shear (S)) as well as the bulk density (
'
'
terms can easily lead to confusion and it is
recommended that their use should be avoided.
From the point of view of seismic amplitude and
AVO studies it is recommended that positive stand-
ard polarity be used as it lessens the potential confu-
sion in the representation of amplitude data.
Following this convention will mean that AVO plots
will be constructed with positive numbers represent-
ing positive reflections and integration type processes
(such as coloured inversion (
Chapter 5
)) will produce
the correct sense of change in bandlimited impedance
traces (i.e. negative to positive for a boundary with a
positive reflection) (see
Section 5.5.3
)
.
With respect to colour, it is common for seismic
troughs to be coloured red and peaks to be coloured
blue or black. There are some notable exceptions,
however. For example, in South Africa the tendency
has been to adhere to positive standard polarity but
colouring the troughs blue and the peaks red. With
modern software the interpreter is not restricted to
blue and red and can choose from a whole range of
colour options. For more discussion of the role of
colour in seismic interpretation the reader is referred
reverse
ρ
).
Figure 2.10
illustrates the P and S waves of interest in
3D exploration. The P wave is characterised by par-
ticle motion in the direction of wave propagation. The
S wave travels in the same direction as the P wave (and
at approximately half the speed of the compressional
wave) but the particle motion is perpendicular to the
direction of wave propagation. Strictly speaking this
shear wave is the vertically polarised shear wave.
Whilst it is only compressional waves that are
recorded in marine seismic the reason that shear
information is important to the interpreter is because
changes in amplitude with angle are related to the
contrast in the velocities of P waves and the vertically
polarised S wave. By contrast, in land exploration a
comparison of vertically and horizontally polarised
shear waves (recorded with three component sensors)
10
