that reality. Additionally, towed streamer image quality suffers due to natural water column variability (added 4D noise), random surface obstructions, shipping lane restrictions,
etc., that make shooting subsequent surveys
exactly as the previous survey – known as repeatability – virtually impossible. They cannot
recreate exactly all the environmental conditions present during the prior survey in order
to exactly compare the image generated from
the current survey to the image generated
from the prior survey. The 4D image quality
and comparability is inferior.
OBN and brownfield
OBN seismic technology is superior in the
brownfield. For seismic image quality, OBN
data for seismic image quality is unsurpassed.
The image shown here is a vertical section
derived from data captured via a deepwater
node survey in the Gulf of Mexico. The left
hand panel shows an image from the initial
baseline survey. The center panel shows an
image after redeployment and reacquisition
of 16 of the nodes in the same location as the
The “4D difference plot” image comparison, the right hand graphic, visibly captures
the initial deployment image characteristics
minus the time-lapsed redeployment. In
this case the 4D data analysis conclusively
shows that nodes provide highly repeatable
seismic acquisition suitable for 4D reservoir
monitoring in deepwater.
Nodes offer a distinct advantage over
streamer surveys in 4D analysis, as the latter
can be difficult to repeat in the unpredictable
currents of the Gulf of Mexico. The geologic
and imaging objectives of the survey are met
by all azimuth, all offset, high fold and dense
subsurface sampling that are now the stan-
dard requirements of new imaging and noise
reduction technologies. These types of acqui-
sition geometries are easily and efficiently
achieved using OBN systems.
Additionally, the careful deployment of autonomous nodes among crowded field developments balances the competing objectives
of subsurface imaging, affordability, acquisition efficiencies, and environmental impact.
Secondly, the use of node surveys is very
comparable in cost to surface-based, WAZ
streamer seismic acquisition, and the technology significantly lower costs when compared to other OBS systems, specifically
ocean bottom cable systems. OBC systems
use terminations, connectors, power distribution, and data telemetry equipment; and all
of these are subject to technical downtime,
which drives up seismic data acquisition
costs and risks, and HSE and financial risks.
Lastly, an ongoing reservoir characteriza-
tion solution must have the highest system
reliability. Reliability allows for repeatability,
and repeatability delivers 4D seismic data
that provides the highest image quality. Reli-
ability is therefore critical to quality. The re-
liability of the nodal systems and the lack of
technical downtime allows for more empha-
sis to be placed on improving the operation-
al performance and lowering expenditures
associated with production exploitation.
Moreover, new nodal technologies should
allow for OBN to become the effective, reliable, affordable permanent reservoir monitoring solution for the brownfield. These
systems can economically deploy a frequent
seismic monitoring solution, and deliver repeatable surveys for reservoir characterization for production optimization.
Nodes and PRM
The value of 4D seismic in reservoir management, increased recovery, and optimizing
and extending the life of the field, is just now
becoming well established. OBN is today
routinely used to acquire full azimuth 3D seismic data for some greenfield development as
well as providing effective brownfield initial
4D, or “baseline,” surveys.
Nodes can be economically deployed and
retrieved amongst infrastructure and during
simultaneous operations. OBN surveys have
proven to be highly repeatable, producing
time-lapse 4D data for reservoir monitoring that is the same image quality as data
acquired via cable-based PRM systems, but
with significantly favorable economics.
New nodal PRM has a deployment life of
five years with 300 days of active recording,
enough for twelve 25-day surveys. Installation involves little technical risk since it is deployed in the same manner as conventional
node deployment, which has had over 30,000
successful deployments in the last 10 years.
Image is everything
With the economic and operational stakes
higher than they have ever been for offshore
operators to deliver improved profitability
and lower risk thresholds for their maturing
offshore brownfields, seismic has emerged
as a critical part of the production exploitation game. The advantages of production-focused OBN extend beyond the seismological
benefits, too. The avoidance of the significant
potential negative environmental and HSE
impacts should be considered: less “traffic”
around the wellhead, fewer vessel-related
man days for injury to occur, minimal, if any,
seabed disturbance are just a few areas to
mention. P-seismic is fast becoming a necessity for the brownfield, and the data (
economic, environmental, and safety) indicates that
OBN technology may be the answer.
OBN provides both the image quality
needed and the permanent monitoring solution required to give operators the best
results to manage the most important asset
the operator has – the reservoir. It truly is
“all about the image.” •
Author’s note: FairfieldNodal is a member of the International Association of Geophysical Contractors (IAGC).
Image from a deepwater node survey (left panel), repeat survey (center panel), and difference
plot (right panel). Deepwater node surveys provide highly repeatable acquisition suitable for 4D
reservoir monitoring, evidenced by the very low NRMS values in the right panel. (Image courtesy