IMAGING THE AFEN SLIDE FROM COMMERCIAL 3D SEISMIC –
METHODOLOGY AND COMPARISONS WITH HIGH-RESOLUTION DATA.
J. BULAT
British Geological Survey, Murchison House, West Mains Road,
Edinburgh EH9 3LA, UK
Abstract
The Afen Slide lies in deep waters within the Faroe Shetland Channel (FSC) and was
first recognised on TOBI data in 1996. Subsequently, it was observed on a 3D seismic
survey that had been acquired in 1995. This paper presents the latest image of the slide
generated from 3D seismic and the methodology used in attenuation of geophysical
artefacts. It is demonstrated that 3D seismic has the potential to produce highly detailed
images of seafloor features in deep-water areas comparable with swath or TOBI.
Keywords: Submarine slide, 3D seismic, imaging, Faroe Shetland Channel
Figure 1. Location of the Afen Slide superimposed
on bathymetry map (in metres) of the FaroeShetland Channel.
1. Introduction
The Afen Slide is located 95 km northwest of the Shetland Islands in water depths of
830-1120m (Wilson et al. 2002). It was first recognised on Southampton Oceanographic
Centre’s dual 32kHz sidescan sonar Towed Ocean Bottom Instrument (TOBI) data
acquired for the Atlantic Frontiers Environmental Network (AFEN) in 1996 (Masson,
2001). Figure 1 is a sketch map indicating the location of the slide and Figure 2 is part
of the TOBI data over the slide. The TOBI survey was conducted with a notional swath
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width of 3km each side of the towed vehicle
and a total swath width with overlay of 5 to
5.5km in water depths from approximately
800-1200m. The slide scour attains a
maximum width of 3km. The length of the
scour and debris lobe combined is 12 km. The
TOBI image shows the debris fan and part of
the scour well, but does not identify the head
wall in its entirety. This is due to insufficient
data coverage as a result of the wide track
spacing. As the only major slide in the FSC it
has been the focus of much study (Bulat and
Long 1997, Holmes et al. 1997, Holmes et al.
1999, Masson 2001, Wilson et al. 2002).
Figure 2. Part of a TOBI image over the Afen
Slide. Data supplied by Dr. D.G. Masson, SOC.
This paper presents the latest version of the
Afen Slide image and the methodology used
for the isolation and attenuation of survey
footprint effects on the seabed event.
Comparisons with high-resolution data are
also presented that illustrate the limits in
vertical and spatial resolution of such images.
NW
SE
Headwall of slide
Toe of debris slope
Figure 3. Arbitrary line along the axis of the Afen Slide from the 3D seismic volume used to generate
the images presented in this paper. The location of the line is shown in figure 4. Note the subtle nature
of the slide’s main features on the vertical profile.
Imaging the Afen slide
207
2. Imaging using 3D seismic surveys
In 1995, Shell U.K. commissioned a 3D survey for hydrocarbon exploration that was
subsequently found to cover the area of the Afen Slide. As the intended exploration
target was deep, temporal resolution and sampling is low (4ms sample rate and a
dominant frequency at the seabed of 30Hz). However, as is typical with 3D seismic, the
data has a high areal sampling (25m grid). As part of a regional imaging study of the
seabed using 3D seismic datasets over the FSC (Bulat and Long 1997), undertaken on
behalf of the Western Frontiers Association (WFA), a detailed study over the slide was
performed based on 3D horizon picks provided by consortium members. The results
from this study were very encouraging in that many seabed features were clearly
imaged, albeit with data artefacts. To investigate the slide further, Shell U.K. provided a
SEGY file of the 3D volume which was loaded at 16-bit resolution. Figure 3 shows an
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Figure 4. Depth converted raw horizon pick over the Afen Slide from 16-bit seismic data visualised
with ER Mapper. Water velocity of 1.5 km/s used for depth conversion. The image was generated using
a standard shading algorithm with illumination from the northeast. Although the image shows much
detail, the presence of northeast-southwest corrugations, presumed to be survey footprint, reduces the
overall image quality. White lines indicate the position of the seismic profiles presented in this paper.
White numerals indicate the associated figures numbers.
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example of the seismic data along the axis of the slide. A 3D horizon of the seabed was
generated using a Landmark Graphics workstation and then imaged using ER Mapper.
This raw 3D horizon is presented in Figure 4. Of particular note is that the subtle
features observed on the vertical profile form a consistent surface containing significant
detail. The form of the slide is clearly visible with headwall and debris lobe clearly
defined. However, finer detail is partially obscured by the presence of northeastsouthwest trending lineations or corrugations in the surface. The northeasterly
illumination direction was chosen to minimise the impact of these features on the image.
The linear noise correlates with the acquisition direction of the 3D survey and is
commonly referred to as survey footprint.
2.1 SURVEY FOOTPRINT
Marfurt et al. (1998) define survey footprint as any pattern of noise that correlates with
the survey acquisition geometry and describe some of the causes of this phenomenon.
These include poor survey design
aliasing
backscattered
noise,
imprecision in survey parameters
(such as feathering angle in marine
3D surveys) and inaccuracies in
certain seismic processing steps
such as migration. Survey footprint
distorts both amplitude and phase of
a reflector and exhibits itself in 3D
horizons as minor time shifts
between adjacent lines giving rise to
a corrugated effect. This type of
coherent noise is harmful to derived
products such as coherency volumes
and dip magnitude and azimuth
maps derived from 3D horizons.
The complex nature of survey
footprint generally makes it difficult
to remove from a whole seismic
data volume. However, on a strong
reflector such as the seabed, where
other noise is minor in comparison
to survey footprint, an empirical
method for estimating and removing
this
coherent energy over a limited
Figure 5. Published Afen Slide image generated using
area where the seabed is regionally
BLS correction on 8-bit horizon data. Illuminated from
SW.
simple does present itself.
2.2 SURVEY FOOTPRINT ATTENUATION
If it is assumed that the observed linear features are wholly artefacts and not genuine
features then the problem reduces to isolating the features and then subtracting them
Imaging the Afen slide
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from the image. Because the seabed is
regionally smooth, the short wavelength
of the linear anomalies can be used to
isolate them.
Figure 6. ProMAX display of 3D horizon
transformed into a pseudo-seismic profile after the
application of a 20 Hz high-pass filter, equivalent
to a spatial filter of 1250m in the cross-line
direction on the original horizon.
Our initial attempt at removing the
artefacts calculated a mean shift for each
line relative to a high order polynomial
surface fitted to the input 3D horizon
provided by Shell U.K. based on an 8-bit
data volume. The method was described
as Bulk Line Shift (BLS) and the
resulting image has been published
(Bulat and Long 1998, Holmes et al.
1997 and 1999).
The image is
reproduced here as Figure 5. The main
drawback to the BLS technique is that it
assumes that there is a single shift value
along the whole line. Closer examination
of the raw 3D horizon suggests that this
is not always the case.
Seismic processing packages, such as
ProMAX, have many types of data
analysis and enhancement tools. In
particular, spatial filtering can be
simulated by treating spatial data as time
series and using standard time series
filters. Thus, the raw 3D horizon was
converted into a pseudo-2D seismic
profile where the cross-line became the
Figure 7. ProMAX display of the result of applying
CDP and the in-line direction treated as
a weighted median trace mix over 101 traces to the
high-pass data in figure 6.(equivalent to an
two-way times, while the original twoaveraging over 2500m in the in-line direction).
way time became the amplitude in the
pseudo-2D profile. As the main variation
in slope is in the cross-line direction (i.e. two-way time) a simple high-pass frequency
filter suffices to remove the regional slope from the horizon. Figure 6 shows the result
of removing spatial frequencies longer than 1250m. The slide boundaries show up
clearly as do the linear footprint anomalies.
Fortunately, the slide boundaries are normal to the footprint anomalies. Thus, we can
discriminate against these by applying a weighted median trace mix. Figure 7 shows the
result of applying a trace mix over 101 traces as an additional processing step. The Afen
Slide outline has been removed and what remains is assumed to be footprint anomalies.
It is clear from Figure 7 that the assumption made in the standard BLS technique of a
single time shift for each line is an oversimplification and will generate errors in parts of
the image. The new refinement to the BLS approach, outlined above, is here termed the
weighted BLS correction. The estimated survey footprint is transformed into a new 3D
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Figure8. Weighted BLS corrected horizon illuminated from the northeast. The image was
generated using ER Mapper’s ‘shiny’ algorithm.
horizon which is subtracted from the original horizon. Figure 8 is the weighted BLScorrected horizon imaged using ER Mappers’s 'shiny' algorithm which treats the surface
as having reflection highlights as well as areas of shade. It is immediately apparent that
the image is far sharper than that presented in Figure 5 and that features previously
Imaging the Afen slide
211
unobserved in the adjacent seabed are now imaged. To date this is the best image of the
Afen Slide available. It is beyond the scope of this paper to present a geological
interpretation of the image; this is presented elsewhere in this volume (Wilson et al.).
However, it is clear that the image contains a high level of real information.
3. Comparison with deep-tow boomer records
An important consideration regarding the use of 3D seismic is that of its vertical
resolution. Deep-seismic 3D surveys typically employ low frequency sources.
Examination of the 3D
SW
NE
seismic volume shows
that
the
dominant
frequency of the seabed
event is 30Hz implying a
dominant wavelength of
50m (33ms two-way
25ms
time)
assuming
a
velocity of 1500m/s.
Thus,
the
observed
reflection is a composite
response
from
all
Figure 9. BGS deep –tow boomer line 0002-07. The line traverses the
slide. See Figure 4 for location. The green horizon is the weighted BLS
corrected seabed event projected onto the 2D profile. The occasional
data spikes are a product of the projection process. Otherwise there is
good agreement with the boomer profile.
NW
SE
25ms
Figure 10. BGS deep-tow boomer profile 0002-02. The line runs
parallel to the slide. See Figure 4 for location. Note that the green
horizon agrees not with the seabed but with a package of events just
below it.
reflectors within the first
50m of the seabed.
In 2000, BGS acquired a
grid of 20khz deep-tow
boomer lines over the
Afen Slide. Although
there are problems with
the determination of the
exact position of the
boomer, inherent in a
deep-tow device, the
survey does provide an
opportunity to compare
the 3D seabed horizon
with the boomer profiles.
Overall there is good
agreement between the
boomer lines and the
seabed image. Figure 9
shows
an
example
traversing the Afen Slide
with the seabed horizon
projected
onto
the
profile. There is some
smoothing but otherwise
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good agreement. An example of the seabed image not in full agreement with the deeptow boomer is presented in Figure 10. Here the deep seismic seabed horizon clearly
mimics the topography of a buried package of reflections just below the seabed. The
difference is explicable by consideration of the seismic frequencies used in the survey.
The 3D seabed horizon is an average response over a 33ms window. Where the
geometry of shallow buried features dominate the time window, as seen in figure 10, the
seabed image reflects these geometries. This has implications for the interpretation of
the Afen Slide and indeed any study involving images derived from 3D surveys. Faults
with minor vertical displacements, but persistent over the time window will be imaged,
whereas very thin sedimentary units or point sources such as shipwrecks will not be
observed.
4. Conclusions
Commercial 3D seismic data are increasingly being used as a primary exploration tool
in many parts of the world. This study shows that despite the low frequencies inherent
in such data, images of deep-water seafloor features such as slides can be generated that
stand comparison with those produced using other high-resolution systems, such as
TOBI or swath bathymetry.
3D seismic data can contain artefacts, in particular survey footprint. However, these can
be overcome with the use of processing techniques such as weighted BLS where
circumstances permit the ready isolation of such artefacts.
Comparison with high-resolution seismic reflection data illustrates the limits in vertical
resolution of such images. It is important to understand these limits before interpretation
as the images are highly detailed and can seduce the interpreter into over interpretation.
In particular, it must always be understood that the image is that of the composite
response of the first 50m (i.e. the dominant wavelength) of the seabed and near-seabed
sediments.
5. Acknowledgments
Thanks are due to the members of the Western Frontiers Association for funding the
work and to Shell U.K. for providing the 3D seismic data volume. This paper is
published with the permission of the Executive Director of the British Geological
Survey (NERC).
6. References
Bulat, J. and Long, D., 1998. Creation of seabed feature maps from 3D seismic horizon data sets. British
Geological Survey Technical Report WB/98/38C.
Bulat, J., and Long, D., 2001. Images of the seabed in the Faroe-Shetland Channel from commercial 3D
seismic data. Marine Geophysical Researches 22: 345–367, 2001.
Holmes, R., Bulat, J., Gillespie, E., Hine, N., Hobbs, P., Jones, S., Riding, J., Sankey, M., Tulloch, G., and
Wilkinson, I.P., 1997. Geometry, processes of formation and timing of the AFEN submarine landslide
west of Shetland. British Geological Survey Technical Report WB/97/33C.
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Holmes, R., Masson, D.G. and Sankey, M. 1999. Geometry and timing of the AFEN submarine landslide west
of Shetland. In: abstracts North-east Atlantic Slope processes: multi-disciplinary approaches.
Southampton Oceanography Centre, Southampton, p42. 1999.
Marfurt , K.J., Scheet, R.M., Sharp, J.A and Harper, M.G., 1998. Suppression of the acquisition footprint for
seismic sequence attribute mapping. Geophysics, 62: 1774-1778.
Masson, D.G., 2001. Sedimentary processes shaping the eastern slope of the Faeroe-Shetland Channel.
Continental Shelf Research, 21: 825-857.
Wilson, C.K., Bulat, J. and Long, D., 2002. The Afen Slide. British Geological Survey Technical Report
CR/02/291.
Wilson, C.K., Long, D. and Bulat, J., 2003. The Afen Slide - a multistage slope failure in the Faroe - Shetland
Channel. In: Submarine mass movements and their consequences. J.Locat and J Mienert Ed.,
Advances in Natural and Technological Hazards Research Series, KLUWER, (this volume).