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Seismic in arctic environs: Meeting the challenge
RICK TRUPP, JEFF HASTINGS, SCOTT CHEADLE, and RADIM VESELY, CGGVeritas
A
laska and the Canadian arctic hold significant estimated
oil and gas reserves but the area also has a unique
and fragile natural ecosystem. Specialized equipment
and methods deployed by exceptional crews working
closely with the local agencies are required to meet the
environmental challenges.
This region also has one of the harshest climates in
the world. Seismic crews need extreme dedication and
exceptional QHSE awareness to overcome the complex
logistical and operational constraints faced daily. Our
crews, who have been operating in Alaska and the arctic
since 2001, have learned that flexibility and ingenuity are
critical in the extreme conditions.
Permafrost, ice, and a highly variable near-surface
present unique geophysical challenges. These require
specific processing techniques to obtain reliable images
Fig ure 1. Working in the
of the subsurface.
wide-open spa
ces of Alaska.
Seasonal and terrain constraints
Any seismic survey in the far north faces seasonal, environmental, and safety constraints prior to its start, throughout
its duration, and after completion. In Alaska, for example,
crews cannot set up camp until the state Department of Natural Resources (DNR) declares “the tundra open to off-road
travel.” This means that along the coastal region, the snow
cover must meet the 15-cm rule (6 inches) and the ground
must be frozen so permitted vehicles can operate without significantly damaging the tundra. In recent years, the tundra
has opened as early as 12 December and as late as 21 January. When the DNR closes the tundra in the spring, crews
have only 72 hours to stop operations and evacuate. The crew
keeps in frequent touch with the department throughout late
April and early May, when the snow becomes soft. The crews
and DNR work together so that everyone returns safely and
without harming the environment.
During the winter, the Alaskan terrain can be characterized as a treeless plain within a frozen wetland. Temperatures
typically range from -13°C to -53°C (plus windchill). The
flat land, wind, and temperature make the climate very dry
and acquisition crews must consume more than 4–6 liters of
water a day (Figure 1).
Zero impact on a fragile environment
For operations in Alaska and the arctic, minimal impact
on the fragile environment is fundamental. Access to sites
is highly regulated and crews liaise closely with government
agencies.
In the Canadian arctic, a minimum of 10 cm of snow/
ice is required before a path can be built to the staging area.
Crews travel on a single-lane road, with dedicated spots for
turnarounds, and with low ground pressure (LGP) vehicles,
equipped with wide tires or tracks. In the mid 1990s, sleighmounted camps would be moved every 2–3 days using a
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Figure 2. Vibrators operating across the Alaskan tundra.
dozen steel-tracked bulldozers. However, today only a small
number of steel-tracked bulldozers are used; they have been
replaced with modified rubber-tracked equipment.
To minimize long-term effects on the ground, staging
fueling and camp areas may be completely iced-over manually or, preferably, set up on frozen pools after profiling has
indicated the ice can support the weight. In Alaska, camps
or fueling areas are not iced over unless the crew expects to
be there for several weeks and the location is central to the
program. Camp strings are on skis, and a move is made every
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Geophysics of the northern frontiers
a visit made the following summer to check possible
impac Water withdrawal amounts and locations have
impact.
to be documented. All sewage has to be treated, incinerate or trucked to an approved facility.
erated,
E
Each
survey season begins with a three-day health,
safet and environment (HSE) seminar for the crew.
safety,
This covers rules specific to arctic operations (such
ic management, arctic survival, and safe conduct
as ice
with wildlife) and how to handle a simple scuff in
the snow and to report any drop of oil spilled on the
gro
ground.
The rule is simple: Zero spills are allowed.
Eve drop of oil has to be picked up and properly
Every
dis
disposed.
ne.
g Beaufort shoreli
-zone survey alon
on
iti
ns
tra
,
er
at
w-w
Figure 3. Shallo
3–5 days as the acquisition spread moves.
The mantra of working in the snow is “take only data,
leave only footprints.” No garbage, no petroleum, oils or lubricants, and no contaminants of any kind are to remain on
the site, so that, when the snow melts, there is no evidence of
the winter’s work. Drivers inspect vehicles daily for leaks and
maintenance concerns. In Alaska, drip pans must be on all
vehicles stationary for more than 15 minutes. Another common precaution is self-contained double-walled fuel storage.
If a leak occurs, a photo is taken, the incident recorded, and
Cr operations
Crew
GPS units provide vehicles with maps identifying
G
h
hazard
areas including polar bear or grizzly bear
d
dens,
whose locations are communicated by gove
ernment
agencies. If a vehicle gets closer than one
mile from a polar bear den, an alarm goes off
i the recorder truck, and in the offices of the
in
c
client
and crew manager.
Surveyors ensure zero impact on the envir
ronment
by using GPS references instead of
s
stakes
to mark source points and by scouting
t area in Haaglund BV 206 vehicles. These
the
a
all-terrain
vehicles, used by the Swedish
A
Army
throughout the 1970s and 1980s, are
l to the ground (which allows the surveyor
low
t stake the lathe out the door of the vehicle)
to
a are also used to confirm ice depths on
and
l
lakes
and rivers with ground-penetrating rad coupled with drilling.
dar
Vibroseis is the preferred source (Figure
2 but when dynamite is required, drilling
2),
i performed in water zones only. Vibrators
is
a must remain at least 100 m from pingoes
also
(
(large
conical ice mounds covered with soil).
G
Geophysical
challenges
The arctic presents many geophysical challlenges. For example, during the winter of
22007 a client commissioned an on-ice seism
mic survey to determine the merits of acquiriing offshore seismic from floating sea ice. The
aaim was to provide a means of eliminating
aany possible conflicts between seismic operations and subsistence hunting by local communities.
Several different source/receiver pairings were tested to
reduce the effects of flex wave including standard and lightweight vibrators, an accelerated weight drop on the ice, and
small-volume air-gun arrays deployed through holes drilled
in the ice. Receivers were placed directly on the sea ice, below
the ice suspended in the water, and on the seabed floor.
During the summer of 2008, another client needed 3D
imagery to plan an extended-reach offshore drilling program.
The survey had a narrow data-acquisition window in a rugAugust 2009
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937
Geophysics of the northern frontiers
ged and environmentally sensitive
area of the Beaufort Sea shoreline.
CGGVeritas responded by putting
together a complement of vessels
and a shallow-water transitionzone team. Recent lease sales in
the Chukchi Sea will let us apply
this expertise on hazard surveys,
sparker surveys, bathymetry modeling, sidescan sonar, and sub-bottom profiling (Figure 3).
In this 2009 season, CGGVeritas successfully completed a
program within the infrastructure
of an oil field on the North Slope.
Special precautions let the crew
work safely in and around “hot”
pipelines, cut road for cable crossings, and deal with the simultaneous operations of an active field.
The crew also had to deal with
multiple crossings and overflows
of the Sagavanirktok River while
safely transferring over 15,000 liters of fuel daily.
Processing data from arctic and
permafrost environs
Processing arctic data presents
unique challenges, such as handling data acquired over permafrost. Permafrost, a layer of soil or
rock where the temperature has
been below 0°C for some years,
exists where summer heating fails
to reach the base of the frozen
ground layer. Velocity in permafrost-affected areas can vary from
1.9 to 4.0 km/s, depending on water content and temperature. Permafrost can occur in layers or in Figure 4. (a) An aerial view of a permafrost-affected project area; (b) a slice through the
scattered patches, and can vary in tomographic inversion at a depth of 200 ft below the surface, overlain on the aerial view. The model
thickness from 40 to 400 m in a velocities correspond well with observed surface features. Velocities range from 5200 ft/s (purple) to
12,200 ft/s (red).
short distance.
To the geophysicist, permafrost
presents unique difficulties such as determining the velocity consuming and often interpretive process under these condistructure of the near surface for weathering static corrections. tions. The second problem is long-wavelength time structure
Usually, the near surface is unconsolidated low-velocity mate- related to the variable thickness of the overall permafrost
rial such as sand dunes, glacial till, deltaic deposits, or muskeg zone. This is most effectively dealt with by careful velocitymodel building and depth-domain imaging.
which delay, distort, and attenuate the seismic signal.
Turning-ray tomography has successfully inverted firstPermafrost can cause at least two distinct problems rearrivals
to derive a high-resolution near-surface model. In
lated to imaging. The first is a short-wavelength static probvarious
locales
in the Mackenzie Delta in Canada’s arctic and
lem caused by low-velocity anomalies associated with melting
northern
coastal
regions of Alaska, careful picking of first
beneath perennial water bodies or ice lakes. The rapid spatial
breaks
and
appropriate
parameter selection has produced nearvariation of velocity not only causes large statics but also ersurface
models
that
clearly
capture the low-velocity anomalies
ratic wavefield behavior which complicates imaging. Even
and
allow
tomographic
statics
to be calculated and applied to
picking first breaks for near-surface analysis becomes a time938
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Geophysics of the northern frontiers
model and the disposition of the
water bodies that cause melting of
the permafrost.
A cross section through the
model also showed the expected
thinning of the permafrost package offshore. Offshore to the
north, the permafrost zone is
warmed from the top down by the
ocean and from the bottom up by
the local geothermal gradient and
is melted into a wedge shape with
a downward-sloping top and upward-sloping belly. The boundaries
of the wedge are typically blurred
as the melting causes sporadic
lenses of “slush,” the shape and lateral extent of which are driven by
the local mineralogy of the matrix,
porosity, and brine content. This
presents a processing challenge
because the long-wavelength statics change significantly as the data
are recorded within and beyond
the perimeter of the wedge. If this
is not accounted for, a significant
long-wavelength time structure
remains in the data which masks
the true geological position of the
rock strata. Turning-ray tomography accurately modeled this wedge
and successfully removed the longwavelength statics in a surfaceconsistent manner. This has been
verified by well data on and off
the wedge zone. The weathering
statics applied on the basis of this
model substantially resolved both
the short- and long-wavelength reFigure 5. (a) Satellite image of surface conditions, seismic grid overlain. (b) Depth slice (20 m
sidual time structures where conbelow the surface) through the tomographic velocity model, with slow velocities in blue and faster
ventional methods failed.
velocities in red.
Another example shows a satelremediate the short-wavelength discontinuities. While hand lite image of surface conditions, with a seismic grid plotted
statics may be required in the worst cases, our turning-ray (Figure 5a) and a depth slice at 20 m through the tomography
tomography method has consistently reduced the effort and depth-velocity model (Figure 5b).
The long-wavelength problem is best dealt with in the
uncertainty associated with this particular challenge.
Statics in permafrost-affected areas are notoriously resis- depth domain. Only prestack depth migration (PSDM),
tant to conventional refraction solutions primarily because which properly handles the lateral variations in velocity, can
there is no simple layering of the shallow-velocity structure. remove the apparent structure related to the permafrost layer.
Rather, the entire area has a generally fast (frozen) background Again, tomographic inversion can play an important part in
punctuated by low-velocity pockets associated with melted deriving the shallow part of the depth velocity model used
zones. The melting typically occurs beneath long-standing to drive the depth migration. However, often the permafrost
bodies of water. Figure 4a, an image of the project area, layer is thicker than the diving depths of the turning rays asshows the distribution of river channels and ice lakes. Figure sociated with the first arrivals. In these cases, conventional
4b shows a slice through the shallow portion of the depth residual moveout analysis of PSDM image gathers may detervelocity model derived by turning-ray tomography. There is mine velocities for reflection events at or near the base of peran excellent correspondence of low-velocity pockets in the mafrost as well as the deeper portions of the depth-velocity
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Geophysics of the northern frontiers
Figure 6. (a) Fast-track flow with poststack migration. (b) AVO-compliant flow with PSTM.
model. Ideally, shallow log data are available to help determine the thickness and sonic velocity of the permafrost below
the region supported by first-break analysis.
Other processing issues in handling data from arctic regions typically include:
• Data acquired in transition zones, from land to sea ice,
with variable source types
• Noise patterns and strength that vary significantly between
onshore and offshore records
• Designing a processing flow to preserve primary amplitudes keep AVO options open
• Both shallow and deep objectives, with variable dips
• Imaging shallow stratigraphic detail (critical since the shallow section is often structurally controlled by deeper features)
• Sparse or irregular source and/or receiver spacing due to
acquisition constraints
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Geophysics of the northern frontiers
Amplitude preservation
Deterministic amplitude corrections are an important element of amplitude-preserving processing. Surface-consistent
scaling accounts for source-to-source and receiver-to-receiver
amplitude variations that may arise from physical and coupling differences. Surface-consistent scalars are usually calculated and applied at several stages in the processing sequence
to preserve amplitude integrity throughout the flow, particularly in conjunction with pre- and post-deconvolution noise
attenuation. All noise attenuation schemes must be AVOcompliant, with careful QC to ensure that primary signal is
preserved.
Another important correction is the free-surface loss
related to the angle of emergence. Because of refraction by
near-surface material, the angle of emergence of seismic energy recorded at a receiver is likely oblique to the receiver axis
and the geophones will “capture” only the vertical part of the
emerging energy. The amplitude loss due to angle of emergence is a function of offset, time, the velocity field, and of
dip. A time-variant and offset-variant amplitude correction is
determined by ray tracing through the velocity field. This approach will properly account for any velocity variations such
as high-velocity zones associated with permafrost.
Several algorithms are applicable for directly enhancing
bandwidth, but it is critical that they are supported by a foundation of carefully applied traditional processing techniques.
Bandwidth can unintentionally be decreased by ineffective
deconvolution, NMO stretch, residual moveout, interference
from noise, and anything that causes events to be less than
perfectly flat in the gather before stacking. Ultimately, good
statics and velocities combined with careful noise attenuation and signal whitening (such as AVO-compliant spectral
balancing) are the cumulative elements required for optimal
resolution in the seismic image. AVO-compliant spectral
balancing uses a reference signal band and is common-offset
ensemble-based. The advantages of this method include no
requirement for an estimate of the wavelet or prior knowledge
of the AVO. The operator is nonstationary in offset and time,
and helps mitigate NMO stretch (Nagarajappa and Downton, 2009).
5D interpolation
Sparse or irregularly sampled seismic acquisition is typical in areas with many access constraints such as Alaska
or the Canadian arctic. These spatial sampling limitations
will negatively impact prestack migration. To overcome this
problem, CGGVeritas innovated the REVIVE 5D Interpolation process, a global multidimensional interpolator to perform simultaneous prestack interpolation in five dimensions
(offset, azimuth, inline, crossline, and frequency) to predict
new shots and receivers at locations determined necessary
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to improve wavefield reconstruction. 5D Interpolation can
help infill sampling gaps and increase spatial sampling while
preserving all the original recorded traces. Tests have shown
that this innovative algorithm preserves both offset- and azimuth-dependent amplitude variations to help support subsequent AVO and AVAZ analysis.
Project planning and efficiency
The importance of project management cannot be overestimated. For example, for a recent 1680 km2 onshore 3D from
the North Slope, testing and initial processing were carried
out as data were received at the processing center throughout the two months of acquisition. Fast-track processing was
completed in four weeks to give a “quick look” at the data.
To expedite the fast-track volume, AVO-compliancy was
not adhered to for the flow; rather, mean scaling and traceby-trace deconvolution was performed as the data arrived.
Quick static solutions were used for the fast-track processing, and more thorough surface-consistent statics and even
hand-picked statics across two lakes as needed for the main
processing sequence. The full AVO-compliant processing
flow was begun once the final data shipment was received,
even while the fast-track processing was being completed. As
per client request, the full AVO-compliant processing was
completed within 14 weeks.
Figures 6a and 6b compare fast-track poststack migration
to the full AVO-compliant processing PSTM stack.
Conclusion
Seismic exploration in Alaska and the Canadian arctic involves considerable operational and geophysical challenges. It
is important to deploy the highest possible standards of specialized equipment, crew training, and zero-impact methods
to preserve the environment and delicate ecosystem. Once
the data are recorded, the challenge shifts to the processing
center where expertise in handling the unique problems of
data recorded over permafrost, ice, and a highly variable near
surface is needed.
Suggested reading. “AVO compliant spectral balancing” by
Nagarajappa and Downton (presented at 2009 CSPG CSEG
CWLS Joint Convention).
Acknowledgments: The authors thank Tess Ingalls, Sara PinkZerling, and Sylvie Austrui for their contributions to this article.
Jan Dewar and Jonathan Miller are gratefully acknowledged for
reviewing and improving the manuscript. Photos courtesy of
CGGVeritas (Dominique Lecuivre productions).
Corresponding author: [email protected]
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