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GeoSeis Pty Ltd provides seismic, acquisition and interpretation
services for both petroleum exploration and production,
concentrating on wide aperture seismic, converted wave seismology, and accurate depth conversion.
See Example below.
Accurate knowledge of the subsurface velocity is essential, both for seismic migration,
and for depth conversion of reflection data. In conjunction with conventional reflection seismic processing,
wide aperture seismic can provide this control.
Three component (3-C) seismology uses both the compressional (P) and
the shear (S) waves to probe the sub-surface. Ocean Bottom Seismic (OBS) technology
records both the P and S seismic wavefield, acquiring exceptional quality
data in the low noise environment of the seafloor below wave base.
Converted wave seismology provides additional information on the
physical properties of the sub-surface rocks, and the pore fluids.
For marine application, the PS converted phase is generally employed, using
a standard airgun array to
generate the down-going P waves, and an array of seafloor OBS units to record
the shear-wave reflections.
SEABED
SEISMIC
Seabed seismic acquisition is providing solutions to new challenges. Key
developments are now making this ocean bottom approach more
efficient, increasing its ability to target specific
questions that are not adequately addressed by streamer
technology. However, both the acquisition and processing of
ocean bottom seismic require a paradigm shift from conventional
marine seismic.
Advantages
OBS methods have the
potential to solve a variety of seismic imaging problems that can
reduce the risks involved in oil & gas exploration operations.
Conventional steamer technology provides very high
multiplicity and very fine sampling along the streamer(s), but it
usually suffers from relatively poor sampling in the cross line
direction. In seabed seismic acquisition, the philosophy is
different. Autonomous recorders (or nodes) record the signals from an
array of shots at the full range of offsets and azimuths. Each node
records from both hydrophones and geophones for acquiring pressure
(P) field and the three orthogonal components of motion, providing
the PP and PS waves, usually called P and C waves. This
multicomponent seafloor acquisition improves resolution and is one of
the most important factors in increasing the value of seismic data.
Seafloor seismic can offer:
- Full vector waveform recording, including shear waves
- Extended spectral range of the seismic, especially high frequencies
- Reduction of ghost and multiple phases through the use of P-Z summation technology
- Lower noise levels
- Better control on the position of receivers
- Superior coupling
- Better resolution over gas chimneys
The advantages of seafloor seismic over towed
streamer seismic, include acquisition in obstructed areas, increased
resolution and signal noise ratio, full wave imaging (shear waves),
full wave imaging (wide imaging) and improved repeatability (4D).
Both 2C and 4C data provide greater seismic quality in comparison
with streamer acquisition, so that using ocean bottom in difficult
areas is the main target. In general, OBS surveying is
target-orientated and typically encompasses smaller areas compared to
conventional towed streamer seismic programs.
With this approach, the receiver nodes have a sparse
geometry and the fine sampling is given by the shot density. Each
node records every shot at all azimuths and offsets. Fold is a
function of the distance between the nodes and the distance between
the shots, and these are adjusted according to the objective. There
is a tremendous advantage in understanding this concept of the node
geometry and taking full advantage of it.
Multicomponent data
Acquisition of very high quality seabed 4C-3D data
is now possible using node technology. Largely due to economic
restraints, seabed acquisition has often not achieved adequate data
density, but this need no longer be the case, as demonstrated by the
striking images of the Cantarell field of Pemex in Mexico. Processing
and interpretation of these 3-D node data requires node oriented
algorithms, and recently more effective tools for the interpretation
of the converted waves have become available. For compressional
waves, OBC data is of better quality than streamer data because of
better multiple attenuation, multi azimuth and high fold coverage.
While the quality of multi-component data can be
superior to that of conventional streamer data acquired near the sea
surface, operationally, seafloor seismic is more complex than towed
steamer acquisition, specifically in regard to its deployment,
retrieval and positioning. Node technology aims to record the best
possible quality data from the sea bottom, and this requires the best
possible coupling of the sensor to the sea bottom. By deploying the
OBS using an ROV, the seismic sensor can be planted in the seabed,
and linked by a short cable to the recording unit, maintaining full
isotopic conditions of the sensor. Optimization of OBS deployment
recovery by ROVs is making this technology more cost effective. Once
the nodes are positioned, ocean bottom seismic acquisition can
benefit by using a high source density, which can be provided without
mobilising a conventional seismic vessel.
Solutions to challenges
An application that has achieved increasing
attention over the last few years is the use of nodes for complex
imaging beneath salt pillows combined with ultra deep waters, such as
in the Gulf of Mexico. In the case of exploration in complex areas
and very deep water, full azimuth ocean bottom acquisition, with
nodes may be the only methodology which can solve the problem.
There is also an increasing focus on highly
repeatable 4D seismic services in conjunction with enhanced oil
recovery. Node-based 4C solutions are very well suited for this
application, due to the repeatable acoustic coupling and positioning
accuracy. Further developments are on the horizon in the use of
shear wave data. At present, these shear waves are often not used,
largely due to the lack of suitable processing and interpretation
techniques.
There are many reasons why nodes are particularly
suited for 4D use:
- Significantly lower
installation costs where no trenching/burying is needed and lower
costs than for permanent buried cable installations.
- Complete flexibility
in obstructed areas and easy relocation of nodes at any time in the
oilfield’s life-cycle.
- Easily maintainable
and fault-tolerant with no system degradation over time.
- Modular node architecture makes new system
adaptations relatively cost effective where the system can evolve
with time without major re-investments.
In another application, OBS technology can assist in
the static and dynamic characterization of petroleum reservoirs,
providing critical information for optimal production and recovery.
OBS technology provides more effective methods necessary to address
the challenge of identifying un-drained hydrocarbons compartments in
mature fields.
Improvement in understanding how changes in the
reservoir influence the seismic response is currently an important
research topic. Improved modelling techniques need to be developed
for fluid flow and wave propagation in anisotropic multi-component
and multi-fluid phase solids with fractures. New seabed seismic
techniques, and inversion techniques for the physical parameters of
the reservoir will improve reservoir management. 3D visualization
tools provide an important technology for synergies between reservoir
geology, reservoir simulation and geophysical interpretation.
Expansions of OBS technology will depend on the
specific problems targeted for exploration by the oil company.
Evaluation and extension of discovered fields is on the rise due to
world’s demand for hydrocarbons. Although seabed 4C technology
definitely has a great future, its growth faces some challenges.
These include reducing acquisition costs, improving the operational
field method and improving processing quality.
OBS Technology
To answer these challenges, seabed exploration
techniques have evolved to use multi-component or 4C (measures P
pressure + vector response of compressional (P) and the shear (S)
from the converted (C) waves) evolved in the last decade, using
sensors on the seafloor, rather than towed behind a vessel.
Advances in the capacity of data storage and battery
packs have increased the volume of seismic data recorded on each
seabed node, which in turn translates into more economical solutions
when designing exploration programs. Modern OBS units incorporate a
small, low-mass sensor connected to a control and data acquisition
unit, which contains a processor, power supply, high accuracy clock,
data storage medium and telemetry system.
Conclusion
Ocean bottom seismic (OBS) technology is a tool which can complement
conventional 3D and 4D seismic techniques, and be used to solve
specific problems. Currently, there is a conservative attitude among
operators and contractors, and a reluctance to adopt new ways - but
this will change as this new ocean bottom seismic approach proves its
worth.
James Leven,
Director, GeoSeis
Short Tutorials
Wide Aperture Seismic & OBS acquisition
The following short "tutorials" have been generated to illustratei the utility of
acquisition, processing and interpretation of wide aperture
seismic data.
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Reflected and Refracted phases
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This simple example looks at the pre-critical,
post-critical and refracted phases, and the respective
amplitudes of these phases.
Reflect/Refracted phases
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WARRP Principles
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This tutorial looks at the basics of Wide Aperture Seismic
and how this can be applied to determine sub-surface seismic wavespeeds.
Wide Aperture Seismic
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The effect of low-speed zones
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This provides a illustration of the effects of
low speed zones, showing the travel time and amplitude
anomalies generated by these low speed zones.
Low speed zones
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GeoPro's OBS technology
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A look at the design and construction of an modern
OBS recorder.
OBS design
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Ray tracing in a simple structure
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This provides an illustration of the propagation of
primary compressional wave rays through a simple anticlinal
layered model.
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Ray tracing through the Marmousi structure
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This provides an illustration of ray propagation of
primary compressional wave rays through the Marmousi structure.
Marmousi-2 ray tracing
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Near-offset Interpretation - Marmousi-2
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This provides an illustration of the process of
near-offset modelling of individual shot gathers - using the synthetic data from
the Marmousi-2 model (Martin, 2004).
Marmousi-2 modelling
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WAS used to confirm and improve vel-depth models
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This tutorial illustrates the utility of WAS interpretation
to check a velocity - depth model. It demonstrates this using both
synthetic and marine streamer data.
WAS vel-depth confirmation
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Reflection Event Flattening
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This tutorial illustrates Reflection Event Flattening
for oil and gas reservoirs using the Marmousi Model for both PP and
converted PS waves.
Reflection Event Flattening
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GeoSeis