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Fiber optic 4C seabed cable field trials
S. Maas*, J. Bunn, B. Bunn, R. Metzbower, J. Bowlus
and J. Bielinski, PGS Marine Geophysical
Summary
A fiber optic 4C seabed cable has been successfully
demonstrated in the North Sea. The cable design and
performance is reviewed. A 2400 m cable with 4C sensor
stations located every 25 m was tested in a water depth of
300 m. While the cable tested was limited in seismic
terms, we have demonstrated the optical systems
capabilities beyond 3000 m depth and with channel counts
in excess of 2000 over 12 km in the lab. In the field trial,
the cable was tested along side an electrical 4C seabed
cable with comparable results. Data collected from the field
tests have proven the prototype optical system meets the
performance required of the deepwater seabed systems.
The optical system is an excellent fit for conventional 4C
seismic operations and would also be the preferred solution
for permanently installed reservoir monitoring systems.
Introduction
Traditional seismic acquisition hardware, whether it be
streamers or 4C seabed systems, rely on sensors that
produce a output voltage that is amplified, multiplexed and
transmitted up a cable to the recording system. The passive
nature of the optical telemetry system eliminates the need
for costly in-sea electronics and the problems associated
with them, providing a more reliable, less expensive, safer
system to deploy and operate. Optical sensor based
systems are beginning to replace the traditional technology
in the oil field especially in low channel count high stress
environments. The telemetry architecture utilized provides
a system that is expandable beyond the capabilities of
current seismic systems.
Optical sensors used in acquiring seismic data are typically
constructed from optical interferometers. Many
establishments have demonstrated the performance of
optical sensors, with the US Naval Research Laboratory
leading the technology in the late 70s and early 80s;
Giallorenzi (1987) and Dandridge et al (1991). Since then
the wide spread availability of fiber optic components and
subsystems have helped the optical sensor evolve and have
made an optical sensor system a reality; e.g. Bostick
(2000). In this paper we present the seabed system that
was tested in the North Sea, we describe the cable
construction, the optoelectronic system and the recording
interface. We also compare the data quality between the
electrical and the fiber optic cables.
The fiber optic seabed system
The cable utilizes a Dense Wavelength Division
Multiplexing (DWDM) telemetry scheme to optically
power 384 sensors used in the in-sea test. An
optoelectronic cabinet was assembled using 10 wavelengths
with capability to run 960 channels (only 4 wavelengths
were used for the demonstration cable). The basics of the
system include a phase modulate laser source passing
through an interferometer, where stress from the outside
world causes a phase shift in the light as it passes through
the interferometer. The light is then detected and the phase
information extracted to output a signal equivalent to the
input stress.
The sensors
The hydrophone and geophones in each sensor pad were
optical transducers fabricated using Michelson
interferometers. The hydrophone was an air-backed
mandrel wound with 55 m of optical fiber. The unit was
tested to have a scale factor of -140 dB re rad/µPa ±1dB
over environmental pressure and temperature, including
pressures equivalent to 3000 m depth. This translates into a
noise floor below 1µB. For the geophone we used an SM-
24 geophone to drive a PZT driven interferometer locally at
the geophone. The geophone assembly was a fully
gimbaled 3C sensor and mounted inside a pressure vessel
also capable of 3000 m operation.
The array construction
The array was designed to be used by a 4C exploration
crew, being continuously deployed and retrieved in
deepwater applications. A steel armored optical cable with
the optical fibers inside gel filled stainless steel tubes was
used in the construction. A sensor pad was attached to the
optical cable every 25m, the optical fibers in the cable were
extracted and fusion spliced to the sensors in the pad. A
protective cover and bending strain relief was attached to
the entire assembly. Mechanical stress test proved the
cable assemble can be deployed and retrieved thousands of
times without damage to the optical fiber over loads that
exceed deepwater deployments of 3000 m. Figure 1 shows
the sensor pad without the bending strain relief.

Field Trial of a 4C Fiber Optic Seabed System
The optoelectronic cabinet is assembled from cardfiles,
were each cardfile contains four laser boards (2 lasers
each), 6 demodulators, one clock reference generator and
an interface card. This is one wavelengths worth of
processing or 96 optical channels. Figure 3 shows the
cardfile up close and the rack used in the test. Adding an
additional cardfile means you add another 96 channels
worth of capability. The optoelectronic system fabricated
actually includes 960 channel capability or ten
wavelengths. Only four cardfiles were used to run the
2400m array
Figure 3: Optoelectronic system cardfile and rack
Figure 1: Sensor pad Assembly, side view of geophone housing on
sensor pad base, end on photo with protective housing shows
hydrophone mounted in pad.
The optoelectronic system
The optoelectronic system generates the optical power for
the array and processes the returned optical signals to
extract the seismic information. Light returning from the
array is routed to a select group of demodulation boards
that process the optical data and outputs a 32-bit digital
word equal to the seismic data. The data is then sent to a
network interface card where it is put into data packets and
sent to the recording system over an ATM network. Figure
2 shows the basic optical system architecture.
OPI
Opto-Electronics
Leadin Cable
Field Testing
The field test of the Seabed array was performed onboard
the Bergen Surveyor. The optical array was deployed
parallel to the PGS FOURcE seabed cable in 300 m of
water, 20 miles NW of Marstein, in the Norwegian Trench.
The arrays separation was 50 m and the location of the
arrays was monitored using external acoustic transponders.
The following two figures show the optical array during the
checkout and deployment stages. The data was acquired
while the gun boat traveled along the arrays firing every 25
meters and again while traversing perpendicular across the
center of the arrays.
32 Bit
Data
32 Bit Data
over ATM
PC with PGS
developed
gAS Software
PC with Viper
Time series
FFT
Optical Array
IBM 3590
Tape Drive
Figure 2: Optoelectronic system recording interface
Figure 4: Optical seabed array being prepared on floor in Bergen
Warehouse.

Field Trial of a 4C Fiber Optic Seabed System
Figure 5: Optical seabed array being prepared for the tests, array
on deployment reel and cable being deployed off the back deck of
vessel.
RESULTS
The data shows excellent correlation with that of the
electrical system. Figure 6 and 7 is a comparison of the
hydrophones in the arrays. The data presented in Figure 6
shows the corresponding 96 hydrophone channels for the
electrical and optical arrays for the same shot in common
shot-gathers. Figure 7 is the averaged signals from a
hydrophone channel, the red trace is the electrical channel
and green optical. The low frequency spike seen in the
optical channel isn’t present in the electrical data because
of the roll off filter used in the electrical acquisition system.
Finally, the noise response from the two systems is shown.
Figure 8 provides the same data as seen in 6 & 7 for the
vertical geophones.
Figure 7: Electrical hydrophones compared to optical hydrophones,
a) average signal output for a sensor (all shots on a line) electrical
(red) and Optical (green), b) average noise response electrical and
optical.
Figure 6: Electrical hydrophones compared to optical hydrophones,
a) shot record for 96 electrical channels, b) shot record for 96
optical channels
Figure 8: Electrical vertical geophones compared to optical vertical
geophones, a) shot record for 96 electrical channels, b) shot record
for 96 optical channels, c) average signal output for a sensor (all
shots on a line) electrical (red) and optical (green), d) average
noise electrical (red) and optical.

Field Trial of a 4C Fiber Optic Seabed System
Conclusion
We have successfully tested a 2400 m optical seabed cable
in the North Sea. A DWDM system allows for the
expandability to lengths greater than 12 km with channel
counts in excess of 2000. The optical cable was tested
along side an electrical 4C cable with comparable results.
Data collected from the field tests have proven the
prototype optical system meets the performance required of
the deepwater seabed systems. The optical system is an
excellent fit for conventional 4C seismic operations and
would also be the preferred solution for permanently
installed reservoir monitoring systems.
References
1) Giallorenzi, “All-optical towed and conformal arrays”,
United States Patent 4,648,083, March 3, 1987
2) Dandridge, A. and Cogdell, G., “Fiber Optic Sensors
for Navy Applications”, IEEE LCS, February, 1991.
3) Bostick III, F.X. (Tad), “Field Experimental Results of
Three-Component Fiber-Optic Seismic Sensors”, SEG
Expanded Abstracts, August, 2000
Acknowledgements
We would like to thank PGS Marine Acquisition for the
support and coordination during the tests and the crew
onboard the Bergen Surveyor for their excellent efforts.
We also thank PGS Marine Geophysical for permission to
publish the paper