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3-D seismic coherency techniques applied to the identification
anddelineation of slump features
Susan E. Nissen*, Norman L. Haskell, Amoco Exploration &
Production Technology; John A.Lopez, Thomas J. Donlon, and Mike S.
Bahorich, Amoco Corporation
SS3.
Summary
3-D seismic coherency techniques (Bahorich and Farmer,1994,
1995; Marfurt et al., 1995) and associated calculations ofdip and
azimuth of the coherent seismic reflections (Marfurt etal., 1995)
have been used successfully to aid in the identificationand
delineation of slump blocks with both high and low
seismicamplitudes within a submarine canyon in the South
MarshIsland area of the Gulf of Mexico. Block faces with low
ampli-tude, but relatively high coherence, which are not evident
onstandard seismic time slices, can be seen on coherency timeslices
and dip/azimuth plots. These slump blocks produce a dis-
tinctive mottled pattern on coherency time slices and
corre-spond to areas of high dip and varied azimuth on
dip/azimuthplots. Similar coherency and dip/azimuth patterns are
foundelsewhere in the world, associated with slumping in areas
offaulting, dewatering, and mass wasting.
Introduction
The seismic coherency method of Bahorich and Farmer(1994, 1995)
quantitatively measures the similarity or dissimi-larity of
adjacent traces. In addition, Marfurt et al. (1995) havedeveloped a
method for determining the dip and azimuth ofcoherent seismic
reflections. These methods have been success-fully used as an aid
in the identification of both structural andstratigraphic features
on seismic data.
The identification of slump features in South Marsh IslandAt the
western edge of the South Marsh Island area of the
Gulf of Mexico, 3-D seismic coherency time slices between 800and
1100 msec show an elongate region of low coherencywhich covers an
area of approximately 14 miles in the north-south direction by 5
miles in the east-west direction (Figure 1).The low coherency zone
has a three-lobed northern boundary,and extends southward to the
southern edge of the seismic sur-vey area. Although this zone is
also visible on a standard seis-mic time slice (Figure 2), it is
not readily identified on thisdisplay. As part of a study to
geologically calibrate coherencyresults, (see also Haskell et al.,
1995) the low coherency featurehas been examined in detail using
coherency time slices, dip/azimuth plots, standard seismic
profiles, and well logs. A stan-dard seismic profile, A-A, down the
axis of the feature shows agenerally chaotic seismic character,
with a number of steep-sided blocks (e.g., A, B) apparent (Figure
3). The feature hasalso been penetrated by five wells. Logs from
these wells showa distinctive character associated with the
seismically chaoticfill. The fill is approximately 1000 ft. thick,
on average, andextends from approximately 2300 ft. to 3300 ft.
below sea level.It is Late Pleistocene in age. The shape of the low
coherencyfeature in map view, its seismic character, and its
thickness haveled to the interpretation that this is the head of a
submarine can-yon produced by mass wasting, similar to the Late
PleistoceneMississippi Canyon (Coleman et al., 1983). The blocks
identi-fied on the seismic profiles are proposed to be slump
blocks.
Upon close investigation of the coherency time slice (Fig-ure
l), it can be seen that the coherency within the canyon is
notuniform, but instead exhibits a somewhat mottled pattern.
Also,at several discrete locations, small, bright (high
coherence)regions (e.g. B, C, D) appear within a darker (low
coherence)matrix. These high coherence regions correspond to high
ampli-tudes found at the crest of certain slump blocks (e.g., B on
Fig-ure 3). The high amplitudes may result from shallow gastrapped
within the slump blocks. Other blocks (e.g., A) do nothave such
high amplitudes and are not as strongly delineated onthe coherency
plot. On a dip/azimuth plot coincident with seis-
mic profile A-A, however, the dipping faces of block A andother
relatively low amplitude slump blocks within the canyonare readily
apparent. On a dip/azimuth time slice, the slumpblocks show up as a
zones of high dip, with azimuth varyingfrom block to block. The
lateral extent of individual blocks canbe easily traced on this
dip/azimuth plot. Block A, for instanceis measured to have lateral
dimensions of approximately 2500feet by 3000 ft. at the depth of
the 984 ms time slice.
Conclusions
Slump blocks with both high and low seismic amplitudeshave been
identified and delineated using coherency techniquesin the South
Marsh Island area. The distinctive mottled coher-ency pattern and
the high dips and varied azimuths of the dip/azimuth plots are also
found elsewhere in the world, associated
with slumping due to faulting, dewatering, and mass wasting.
References
Bahorich, M. S., and Farmer, S. L., 1994, 3-D seismic
disconti-nuity: The coherence cube for faults and stratigraphic
fea-tures: U. S. and Foreign Patents Pending.
Bahorich, M. S., and Farmer, S. L., 1995, 3-D seismic
disconti-nuity for faults and stratigraphic features: The
coherencecube: SEG/EAGO/EAEG Intemat. Geophys. Conferenceand
Exposition, Expanded Abstracts.
Coleman, J. M., Prior, D. B., and Lindsay, J. F., 1983,
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Haskell, N. L., Nissen, S. E., Lopez, J. A., and Bahorich, M.
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features: submitted for presentation at 65th Ann.Internat. Mtg.,
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Marfurt, K. J., Kirlin, R. L., Farmer, S. L., and Bahorich, M.
S.,1995, 3-D seismic attributes using a running window multitrace
coherency algorithm: submitted for presentation at65th Ann.
Internat. Mtg., Soc. Expl. Geophys.
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