Geology Reference
In-Depth Information
Polar Anisotropy (with vertical symmetry) or Transverse
isotropy with vertical symmetry (VTI)
E.g. shale units with aligned
particles or horizontally organized
beds or layers (scale <
λ
).
Vertical axis of symmetry Amplitude
varies with angle but no azimuthal
variation in properties.
Azimuthal anisotropy
Rock properties vary
depending on angle of
incidence and direction.
E.g. dipping zones of thin
beds or more usually to
fractured rock.
The simplest form of azimuthal anisotropy is Polar
anisotropy with horizontal symmetry or HTI
(transverse isotropy with horizontal symmetry).
Figure 5.38
Two types of anisotropy important to the seismic interpreter.
reservoirs tends to have a greater effect than decreas-
ing reservoir pressure.
Lynn and Michelena (
2011
) for a more detailed
treatment.
By definition the characteristic of anisotropic
rocks is that velocities measured in different direc-
tions (e.g. vertical vs horizontal) are not the same. In
essence this is due to the effect of preferred align-
ments of geological components. For example, at the
pore scale, shale prone rocks are anisotropic owing to
the alignment of clay minerals. Anisotropic effects
can also occur at the seismic scale owing to sediment-
ary layering and vertical fracturing.
There are essentially two types of anisotropy that
are relevant for the seismic interpreter (
Fig. 5.38
).
These are as follows.
5.3.7 Anisotropy
Whilst the amplitude interpreter
s model tends to be
focussed on an isotropic view of the world, an under-
standing of anisotropy is important in seismic pro-
cessing and AVO modelling. The aim of the following
discussion is designed to introduce concepts and
present useful first order observations and is not
meant as a detailed treatment of the subject. Readers
are referred to the works of Thomsen (
2002
), Lynn
(
2004
), Tsvankin et al.(
2010
), Grechka (
2009
) and
'
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