Geology Reference
In-Depth Information
have been created by stacking the near-offset data and
the far-offset data separately, giving an immediate
visual impression of AVO effects but also providing
information that can be analysed quantitatively (see
Chapters 5
and
7
). In this particular case, the differ-
ence between near and far trace data is unusually large
but it illustrates the point that where there is only a
'
section available (i.e. created by adding
together all the traces of the gather) the interpreter
may be missing valuable information.
full stack
'
a)
Near stack (210-1800m)
100ms
2.3 Modelling for seismic
interpretation
Propagation of seismic energy in the Earth is a com-
plex phenomenon.
Figure 2.5
shows some of the
numerous factors related to geology and acquisition.
The goal of course is to relate seismic amplitude to
rock property contrasts across reflecting boundaries
but there are several other factors besides geology that
also have an influence on amplitude. Some of these
are associated with the equipment used for the survey;
these include variability of source strength and
coupling from shot to shot, variability of sensitivity
and coupling from one receiver to another, the direc-
tivity of the receiver array (i.e. more sensitive at some
b)
Far stack (1960-3710m)
Figure 2.4
Offset stacking example; (a) near stack, 210-1800 m
offset, (b) far stack, 1960-3710 m offset. Note that the choice of
offsets is dependent on the offset range available and the depth of
the target. In this case the offsets have been selected to avoid
stacking across a zone of phase reversal.
Instrument
response
Superimposed
noise
Geophone
sensitivity
Source strength
and coupling
Array
directivity
Scattering
Wavefront
divergence
Multiples
Absorption
Reflector interference
AVO
Reflection
coefficient
Refl
ector curvature and rugosity
Vertica
l p
olar
aniso
tropy
Small scale
horizontal
layering
Fractures
Azi
m
uthal
anisotropy
Elasticity &
isotropy
6
Figure 2.5
