American Oil and Gas Reporter - November 2016 - 51

SpecialReport: Oil & Gas Computing

FAZ Optimizes Multiple Attenuation
By Duane Dopkin
HOUSTON-As seismic waves travel
through the earth, they are dissipated
through both absorption (anelastic attenuation) and wave path scattering (elastic
attenuation).
Energy propagating through the earth
also can reverberate and create energy referred to as "multiple energy." This energy
can significantly impair a geoscientist's
ability to interpret and prospect with seismic data. Consequently, despite significant
advances in the science of multiple energy
attenuation, it remains one of the most
challenging areas for the seismic method.
Predictive, or data-driven, as well as
model-driven methods continue to be
modified to handle a complex range of
multiple generating conditions in the presence of irregular water bottoms, shallow
water, salts, volcanics, and high-reflectivity
carbonate and anhydrite sequences.
While all these conditions can generate
challenges for application geophysicists,
it is the interbed multiples, which are
generated from sequences of high-reflectivity formations, that can significantly
compromise the use of seismic data for
reservoir delineation and advanced characterization methods. Under these conditions, methods that attempt to exploit
move-out differences between primaries
and multiples fail.
One of the newer ways to address this
interbed multiple problem, together with
the broader range of multiple generating
conditions, is using full-azimuth depth
imaging and decomposition technology
to attenuate or remove this undesirable
energy from the seismic wave field. In
this model-driven process, primaries can
be distinguished from multiples, based
on their respective energy distributions
in "directivity" (dip-azimuth) space.
The full-azimuth imaging system is
able to map and decompose surfacerecorded seismic data into subsurface,
multidimensional prestack image gathers.
This mapping and decomposition technique is carried out with a special "bottom-up" ray tracing operator using an in
situ reference system referred to as the
local angle domain (LAD).
Five-Dimensional Gathers
Figure 1 illustrates the LAD imaging
method, with the source point shown in

red and the receiver point in green. The
outcome of the ray tracing and mapping
is a five-dimensional image gather described by full-azimuth subsurface directivity (the dip and azimuth of the local
reflecting surface) and by full-azimuth
subsurface reflectivity (the opening angle
and its associated azimuth defined by the
incidence and reflected ray pair).
Figure 2 shows examples of mapping
surface-recorded seismic data to five-dimensional gathers of full-azimuth reflectivity and directivity, and displaying the
results in 3-D or 2-D forms. This multidimensional decomposition provides the
highest levels of flexibility in designing
special imaging and inversion operators
to emphasize or attenuate different subsurface features. Specifically, the technology can be applied to:
* Enhance continuous reflections in
the presence of low signal-to-noise with
operators that emphasize "specular ener-

gy;"
* Recover the low energy associated
with small faults, discontinuities or feature
edges with operators that emphasize "diffraction energy;"
* Map in situ stresses and natural
fractures using full-azimuth reflectivity
data, and full-azimuth amplitude-variation-with-azimuth and velocity-variationwith-azimuth inversions; and
* Differentiate between primary and
multiple data.
Multiple Suppression
Two approaches can be used to attenuate the broad spectrum of multiples that
may be present in the fully recorded seismic wave field. Both methods exploit
the "directionality" of the imaging operator
and associated multidimensional gather
output.
The first approach is based on the internal (imaging) implementation of local

FIGURE 1
LAD Imaging System for Five-Dimensional Mapping of
Surface-Recorded Seismic to Subsurface Image Points

Displacement
azimuth
tance
ment dis
Displace

Offset
azimuth
Of
f
dis set
tan
ce

Incident
ray

Opening azimuth
Reﬂected
ray
Opening angle
Azimuth
Dip

NOVEMBER 2016 51
American Oil and Gas Reporter - November 2016

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