GAISER, JAMES, NEIL JONES, and PHILIP FONTANA,Western Geophysical Co.
Abstract: Sub-Salt and Sub-Basalt Imaging Using Long Offset and Multicomponent Seismic Data
Introduction
Exploration in potential
hydrocarbon producing areas beneath high-velocity layers (HVL) is typically
hindered by a variety of seismic propagation effects. Dramatic elastic-impedance
contrasts between the HVL and regional sedimentary framework cause P-wave
energy to be converted to shear-
wave
(S-
wave
) energy. The associated loss
of frequency content, unusual amplitude versus offset effects, and numerous
multiple reverberations act to limit the signal-to-noise (S/N) ratio of
reflections and the ability to image structures below the HVL.
The objective of this paper
is to present an overview of a number of different approaches that have
proved useful in overcoming some of the above problems to obtain accurate
images. The approaches involve techniques in both acquisition and processing
of seismic data. For example, whether we should collect data at the sea
surface (conventional towed streamer or long-offset towed streamer) or
on the sea floor, such as ocean-bottom cable (OBC) data, which is itself
dependent on the type of seismic body wave
we intend to investigate. Conventional
P-waves are always important but S-waves can be used to image below HVLs
in conventional and OBC data. Processing of these data always requires
more accurate information regarding multiples and velocity/depth structure
in order to transform and enhance our reflection data to properly image
below the HVL.
Conventional Steamer Data
Prestack depth migration
has been successful in imaging conventional P-wave
towed-streamer data.The
biggest problem is that the HVL is not only a good P-
wave
multiple generator
but it also generates significant S-
wave
energy. The implication is that
velocity filters designed for removing P-
wave
multiple
wave
trains may
have little if any effect on the S-
wave
mode-converted noise.
The Tertiary basalts of the
Faeroes-Shetland region vary in thickness from tens to thousands of meters
and overlay potential hydrocarbon-bearing sediments. The velocities vary
both vertically and laterally within the region (Fontana and Jones, 1998)
and can result in strong impedance contrasts with the adjacent sediments.
Ray-based modeling is used to identify P-wave
multiples and S-
wave
converted
modes that interfere with the underlying sediment reflections. The models
aid in designing Radon filters for multiple attenuation. Figure 1a shows
parabolic Radon transforms of the field data from the Faeroes area and
the corresponding synthetic data. Primary reflections are the seafloor
(P1), top Oligocene (P2), and top basalt (P3), that can easily be identified
and separated from multiples. Diagonal lines indicate the approximate mute
boundary. Along with the improved S/N ratio of primaries, the ray-based
models help in building improved P-
wave
velocity/depth models for prestack
imaging of the base of basalt and below. Figure 1b is a common image-point
gather, offset increasing to the right, illustrating the base basalt-reflector
at 2700 m and base Cretaceous at 4000 m. Note the large amount of under-corrected
mode-converted S-
wave
energy that curves downward below top basalt and
interferes with primary reflections.
Alternatively, base of basalt
(as well as deeper horizons) can be effectively imaged using.the mode-converted
S-waves by replacing the P-wave
velocity in the HVL with the appropriate
S-
wave
velocity (Purnell, 1992). Figure 1c illustrates such an approach
where we have derived an S-
wave
velocity structure within the high-velocity
basalt layer and used that velocity/depth model to drive a PSSP depth migration.
PSSP waves are P-waves that convert to S-waves at the top of basalt, reflect
off horizons below, and are subsequently converted back to P-waves at the
top of basalt. The result is that slow events in the P-
wave
common image-point
gather tend to map to comparable depth structures, e.g., base basalt and
base Cretaceous.
Figure 2a is a portion of the velocity/depth model used to drive multiple suppression and depth migration, yielding the associated seismic depth-stack (Figure 2b). Over this limited portion of the model, the basalt thickness varies between 200 m and 280 m. The estimates of thickness are in agreement with values derived from PSSP imaging. Basalt velocities vary laterally between 4100 m/s and 4300 m/s. Notable structures (arrows) are base basalt at 3000 m, the strong unconformity (base Cretaceous) at about 4200 m depth, and the dipping event below 5000 m depth, that is thought to be a pre-rift feature.
Long-Offset Streamer Data
Another method of deriving
velocity/depth information in HVL areas makes use of long-offset arrivals
rather than dealing with conventional spreads. In this method we make use
of diving-wave
arrivals associated with energy that has continuously turned
along ray paths within the subsurface. By deriving traveltime attributes
associated with these first breaks we tomographically invert for P-
wave
velocity/depth structure in areas that suffer poor S/N due to multiples
and coherent mode-converted noise.Acquiring such towed-streamer data necessitates
multi-boat and multi-pass 2-D profiling.We present an example from an experiment
conducted in the Faeroes-Shetland region where dual-vessel acquisition
gives rise to the formation of super shot gathers ranging in offsets from
0-36 kilometers.Tomographic inversion of the traveltime data gave rise
to sub-basalt velocity/depth models, allowing an independent means of corroborating
models derived from prestack depth migration.
Ocean-Bottom Cable Data
The recent advent of OBC
technology has brought with it the ability of recording high-fold, regularly
sampled S-wave
data using multicomponent instruments. This involves detecting
sea-bottom particle motion with three-component geophone receivers (two
horizontals and a vertical) and a hydrophone. Combining the vertical geophone
and hydrophone data appropriately attenuates water-born multiples to provide
high S/N P-
wave
reflections for prestack imaging the base of an HVL as
well as structures below (Kendall et al., 1998). The above allows us to
record the same
wave
types
as observed in our conventional towed-streamer
data (e.g., P-waves and PSSP-waves). On the other hand, combination of
the two horizontal geophones (based on survey geometry) allows us to analyze
S-waves that have converted from P-waves at the target reflectors (PS-waves).The
PS-
wave
reflections travel back to the sea floor where they are detected
by horizontal receivers. This is in contrast to S-waves used for imaging
in towed-streamer data that must convert back to P-waves on their journey
to the hydrophones.
Imaging with PS-wave
data
from OBC surveys requires additional processing effort compared to that
used in P-
wave
depth migration. Using the P-
wave
velocities and Vp/Vs information
from the converted-
wave
processing, prestack depth migration of the PS-
wave
data can also be accomplished to give independent subsurface images of
base salt and the structure beneath to complement the P-
wave
images. Figure
3 illustrates such an example of P-
wave
and PS-
wave
depth migrations of
an OBC experiment from the Gulf of Mexico. Although no parabolic Radon
multiple attenuation was performed, possible sub-salt reflections are present
(arrows) in both sections.
Conclusions
Ray-theoretical modeling
has been effective in aiding multiple attenuation of data from the Faeroes
region where thick basalt layers overlay sediments. The result is improved
P-wave
S/N and velocity/depth models for prestack imaging of the base of
basalt.Also, base of basalt (as well as base of salt intrusions in the
Gulf of Mexico) can be effectively imaged using mode-converted shear waves
by replacing the P-
wave
velocity in the HVL with the appropriate S-
wave
velocity. In addition, depth/velocity models of sediments below HVLs can
be obtained from long-offset streamer data by tomographically inverting
refraction first-breaks. Finally, OBC technology has brought the ability
to image the base of an HVL with PS-waves to complement P-
wave
images.
Acknowledgments
The authors thank Gillian Brown at Western Geophysical and Doug Toomey of the University of Oregon for their help in creating the velocity models for the Faeroes data. A special thanks goes to Greg Wimpey of Western Geophysical for his assistance in depth imaging the Gulf of Mexico data. Robert Bloor and Rich Van Dok, also of Western Geophysical, provided much support in processing the Gulf of Mexico data.
References
Fontana, P. and Jones, N., 1998, Long offset and multicomponent recordings: application to sub-basalt imaging, Presented at the Advances in Seismic Technology Conference, Stavanger Norway.
Kendall, R.R., Gray, S.H., and Murphy, G.E., 1998, Subsalt imaging using prestack depth migration of converted waves: Mahogany Field, Gulf of Mexico, 68th Ann. Internat. Mtg., Soc. Expl. Geophys., New Orleans, Expanded Abstracts, 2052-2055.
Purnell, Guy W., 1992, Imaging beneath a high-velocity layer using converted waves, Geophysics, 57, p. 1444-1452.
Figure 1: Parabolic Radon
transforms (a) of field data from the Faeroes (upper) and synthetic data
(lower). P1, P2, and, P3 are primary reflections and all other events are
multiples. Diagonal lines indicate the approximate mute boundary. P-wave
common image gather (b), and PSSP common image gather (c) of Faeroes data.The
PSSP mode conversions are P-waves that convert to S-waves within the basalt
and after reflection convert back to P-waves at the top of basalt.
Figure 2: Portion of the
P-wave
velocity/depth model (a) derived from prestack depth migration (b)
in the Faeroes area. Good similarity exists between the model and the depth
section in the character and structure of the top and base-basalt reflector,
the base Cretaceous unconformity, and the steeply dipping event.
Figure 3: P-wave
(a) and
PS-
wave
(b) depth sections from an OBC experiment in the Gulf of Mexico.
Using the P-
wave
velocities and Vp/Vs information from the PS-
wave
processing,
prestack depth migration of the PS-
wave
data can also be accomplished to
give independent subsurface images of base salt and structure beneath to
complement the P-
wave
image. Although no parabolic Radon multiple attenuation
was performed, possible sub-salt reflections are present in both sections
(arrows).
AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England