Installation of the multi-disciplinary VENUS observatory
at the Ryukyu Trench using Guam-Okinawa geophysical submarine cable
Junzo

Kasahara*, Katsuyoshi Kawaguchi ** , Ryoichi Iwase ** and Yuichi Shirasaki ****Earthquake Research Institute, University of Tokyo
1,1,1, Yayoi, Bunkyo, Tokyo 113-0031, Japankasa2@eri.u-tokyo.ac.jp**JAMSTEC*** Kokusai Marine Engineering Corp
Cable-1), an ocean bottom seismometer was installed
Abstract. A
multi-disciplinary VENUS (Versatile Ecoon the forearc slope of the Izu-Bonin Trench by themonitoring Network by Undersea-cable System) observatory was
GeO-TOC project, showing effectiveness to realize
installed at the depth of 2,170 meters on the slope of the Ryukyu
real-time ocean bottom measurement by reasonable
Trench. It equips seven geophysical instrument groups. Prior tocost (Kasahara et al., 1998a,b). Although thisthe installation of the VENUS multi-disciplinary ocean bottom
deployment was based on well-developed method in the(MDOBO) observatory, an ocean-bottom telemetry system,
submarine cable works, it is difficult to add somewhich has functions to supply electrical power to the MDOBO,
branches from the main cable if multi-sensors are
and the submarine coaxial cable were installed at ocean bottom.
required within nearby-locations.
The installation of the multi-disciplinary ocean bottom
The TPC-2 submarine cable was the second Japanobservatory was done by use of deep-towing unit and ROV
US submarine cable constructed in 1976 and it
Kaiko-10K. During the period of August- September 1999,
terminated its long commercial service in 1994 due toseven instrument groups of MDOBO were deployed at the target
the replacement by a new generation cable. In 1995,
position, at 80-1000 meter distances from the telemetry system,
the VENUS (Versatile Eco-monitoring Network bywith several meters allowances using a deep-towing unit. To
Undersea-cable System) project was created to
install the instrument at the exact location, the mother ship ofdevelop a new multi-disciplinary geophysical station
deep-towing unit was precisely navigated. The extension cables
using the TPC-2 cable (Guam Okinawa Geophysicalwere also dropped from the deep-towing unit. The ROV KaikoCable; hereafter GOGC) (Kasahara et al., 2000a). The
10K extended multi-conductor extension cables from instrument
GOGC passes geophysically important regions and it
units towards the ocean-bottom telemetry system and connected
is routed from the Mariana Trough to the midthem to undersea mateable connectors on the junction box. The
Philippine Sea plate to the Ryukyu Trench. TheseMDOBO collected one and half month records. Some useful data
areas contain seismically very active subduction zone
were observed since the installation.
and a rifting back-arc basin. A multi-disciplinary
ocean-bottom observatory (hereafter MDOBO) was
installed at the Ryukyu Trench during August-October,
1. Introduction
It has been widely recognized that studies of1999.
temporal variations are extremely important sincegeophysical phenomena have episodic natures and
2. VENUS System Descriptionsome phenomena such as earthquake occurrence or
The TPC-2 (current GOGC) used 1.5"-diametersubmarine volcanism require real-time observation.
coaxial cables (SF system) and had 845 voice channels
One of the best technologies to achieve real time and(KDD, 1976). The +1,080 V DC from Okinawa and long-time observations is the use of submarine cables.
1,080 V DC from Guam were supplied to the cables at
The use of new fiber-optic submarine cables is costly.
136 mA constant current during the commercial use.
Another kind of submarine cable is the coaxial cable,
Voices were transmitted from one end to the other as
which can provide electrical power and real-timeanalog signals by use of 3 kHz carrier band superposed
telemetry. By reuse of coaxial submarine cable resource,
on the high voltage DC. To convert the TPC-2 to aa real-time geophysical observatory on the deep-sea
geophysical tool, the cable system was greatly modified
floor can be realized by high reliability (e.g., Nagumo
on the power supply, the method of communication,and Walker, 1989; Kasahara et al., 1995). By the use ofand the configuration of the main cable.
old coaxial submarine cable TPC-1 (Trans PacificVENUS Project using
subm arine cables
Honsyu,Japan

OBTS

SM CC

JA MSTEC,
YokosukaDTS
JBX
Okinaw a,
Japan

OBTS

V E NUS
Instrum ents

Okinaw a,
Shore
stationPow ersupplyJA MSTEC,
Yokosuka
Datacenter

MDOBO

M DOBOSM CC

Guam Okinaw a Geophysicalcable(TPC-2)

Guam
S ea
Earth

Fig. 1:The VENUS system from the MDOBO to thedata center in Yokosuka. MDOBO: Multi-Disciplinary
Ocean-Bottom Observatory; JBX: Junction BoX; DTS:
Data Telemetry System; OBTS: Ocean Bottom
Telemetry System and SMCC: Sub-Marine Coaxial
Cable.
The VENUS system comprises the MDOBO, theocean-bottom telemetry system (hereafter OBTS), submarine coaxial cables (hereafter SMCC) of GOGC, andland equipment (Fig. 1). The MDOBO comprises sevenbottom sensor groups with one land-based electricalpotential measurement system developed by six
institutions 1. Since the details of the VENUS system
are described in Kasahara et al (2000a), the outline of
the system is described in this paper. The OBTS
comprises a data-coupling unit (hereafter DCU), a datatelemetry unit (hereafter DTU) and a junction box(hereafter JBX) developed by the KDD researchLaboratory. The methods for deploymental works weredeveloped by the joint efforts of the KDD researchLaboratory and JAMSTEC (Japan Marine Science andTechnology Center). The cable length of the GOGC is2,527 km.
The DTU multiplexes multi-component data by
TDM (Time Division Multiplexing) and sends them toshore using a 240-kHz-carrier bandwidth. The
transmission rate for the multiplexed data is 96 kbps,
which is shared by seven instrument groups. If the
instrument does not work correctly, users can shut
down a malfunctioning unit remotely from a land base.

1
Six institutions (Earthquake Research Institute, JAMSTEC,
Hydrographic Department Japan, Geological Survey of Japan,
Japan Meteorological Agency and Electronic Technicallaboratory) jointly worked.

Each instrument of the MDOBO and the JBX wasconnected by an eight-conductor extension cable. The
data from an instrument transmitted to the JBX throughthe extension cable by the RS422 protocol. The JBX
has nine so-called "ROV (Remotely Operated Vehicle)
undersea mateable connectors". The ROV connectors
allow units to be plugged in and unplugged on theocean floor with the assistance of a manned
submersible or an ROV.

3. Deployment of whole bottom units
The target site for the MDOBO was selected at 25°44'N
and 128°02.5'E at a water depth of 2,200 meters, whichis halfway between two repeaters, R139 and R138. The
route of the GOGC cables was identified at more than
20 locations by the deep-tow camera of R/V Yokosukain 1998 (Kasahara et al., 2000c). After the careful
examination on bottom units, components, anddeployment method, the OBTS was successfully
installed at the location of 25 44.397 N, 128

03.748’E at the depth of 2,157 meters by the cable ship C/SKuroshio-Maru on August 29, 1999. The location of the
OBTS is almost the same as the target site (Fig. 2). At
the same time, the hydrophone array and the tsunami
gauge were also deployed to the ocean bottom. The
tsunami gauge was installed in the same frame with
OBTS and the hydrophone array was connected to the
OBTS in series. We confirmed that the data from theOBTS, the tsunami gauge and the hydrophone array
were correctly transmitted to JAMSTEC.
In middle of September 1999, five instrument sets of
the MDOBO other than the hydrophone array and thetsunami gauge were deployed by the deep-towing unit
on board of the R/V Kaiyo of JAMSTEC , which
equipped 6000m-long tethered cables, and two TV
cameras, one for watching the bottom and one forwatching the ahead. The MDOBO instruments
occupied the area with an approximately 1-km radiusaround the telemetry system as shown in Fig. 2. The
positions of instruments were decided by considering
the geophysical importance and easiness of deployment.
The deep-towing unit also had two releases systems,
which could be operated from the Kaiyo. It was thoughtthat a precise navigation of deep-towing unit was
extremely difficult task because of difficulty to know
the precise distance between vehicle and the target, andmaneuver the vehicle directly from the mother ship R/VKaiyo. It, however, was carried out with accuracy ofapproximately three meters by use of ROV homers,which could tell us the off-center and the distance to thetarget. The Kaiyo was also navigated by watching thepositions of the deep-towing unit and targets on thevideo-plotter screen and the deep-towing unit was
precisely controlled by Kaiyo’s speed and heading withseveral minutes time lags.
For the ocean bottom broad-band seismometer (OBBS),
the multi-conductor extension cable with 100m-long on
the wooden reel and eight lead weights with 20 kg eachwere attached to bottom of the deep-towing unit frame,
and they were released from the deep-towing unit by
one by one (25-44.436’N, 128-03.748’E, 2150m. The
OBBS unit was placed at 80 meter north of the bottom
telemetry system, within 5-meter distance allowance
using a deep-towing unit. The longest extension cables
were 1,000 meters long for the three precise-baseline
measurement systems (PBMS) and these long cables
were extended toward the JBX by the deep-towing unit
after the detachment of the measurement unit. The final
positions of the PBMS cable drums were less than 40
meters, which were just suitable for the length of thesecond stage extension cables.

Kaiko dived to the target site in the end of September(KR99-09; dive #128) During the Kaiko-operation, thelauncher was kept at 100 meters above the center of theseafloor operation and the shuttle part reached to theseafloor. All the instruments were found to sit correctlyat the seafloor. Although each instrument required
specific works, each ROV-dive-time was sufficient for
the necessary works for each instrument set. Most of
instruments excepting the tsunami gauge and thehydrophone array required cable extension works. The
Kaiko uncoiled the multi-conductor extension cablesfrom each reel and approached slowly to the OBTS.
The longest extension cable was 300 meters for thegeoelectric and geomagnetic system. The tight andslack situation during the cable extension works was
the most difficult one among the Kaiko undersea
operations. At the OBTS, the manipulators of the
Kaiko were used to penetrate the male ROV connectors
into the receptacles on the JBX (Fig. 3).. Someamount of pelagic sediment was found in the ROV
connector cover and it was removed by several
operations of plug in-plug out of the ROV connector.

Fig. 3: Photographs of JBX and manipulator
operation at the ocean-bottom

Fig. 2. Geographical arrangements for instruments of
MDOBO, DCU and SMCC at the VENUS site.
We also used the ROV Kaiko-10K, which was an
ROV with capability to dive down to 12,000m deep.
The Kaiko-10K system comprises a tethered launcher
and a shuttle vehicle with 200 meters-long extension
cable. The Kaiko had missions to find the instruments,
extend seven sets of extension cables and connect theundersea meteable connectors (so called ROV
connector) of the extension cables and the JBX. The

A shore station is located in Okinawa. In order toexamine the successfulness of each operation, the DC
power of 2,900V and 135 mA was supplied to the
SMCC, the OBTS and the MDOBO from Okinawa
while the Kaiko stayed in front of the OBTS. By thepower supply to the whole system, we confirmed thecorrect operation of each instrument. All necessary
undersea operations were finished before early October
1999.
The shore-receiving unit (SRU) demodulated
signals and sent them to Yokosuka, Japan, using two
64-kbps commercial telephone lines. Some controls of

instruments such as level adjustment and clampunclamp of the seismic unit were done from Yokosuka.
Timing by GPS clock was supplied to each instrumentfrom the shore station. Whole data from the bottom
were monitored at Yokosuka. Data from ocean bottom
instruments were stored in the data storage at the
JAMSTEC.
Some problems arose at two and half months after
the deployment of the OBTS. On November 21, 1999,
the bottom system was abruptly down. The problem
seemed on 24V DC converter in the data-telemetry unit.
By two diving cruises of the ROV Dolphine 3K, whichis the 3,000m-class ROV, and the Kaiko-10K, it was
found that there were a trace of corrosion on thepressure case of the data-telemetry unit and insufficientinsulations on instruments and the OBTS (Kasahara,
2000). The problems seem to be generated during the
deployment due to weakness on extension cables andsome other causes.
In January 2001, the OBTS, tsunami gauge and thehydrophone array were retrieved to analyze the cause of
malfunctioning of the VENUS system using C/S KoukiMaru with assistance of KDDI Co. Ltd. The visualinspection of the OBTS showed serious corrosions on
the titanium pressure case of the DTU in contrast to nocorrosions on the DCU and the JBX. In addition tosome defects on the OBTS, the hydrophone array hadsome damages on connectors between each sensor andthe main electrical cable. It seems that the electricalinsulation in such systems was damaged by unknown
reasons. The more detail causes will be analyzed by theexamination of the retrieved system.

4.

and geo-electric field show good results (Nakatsuka etal., 2000). During the observation period, a magnetic
storm on Oct. 20, 1999 was observed. Bothgeomagnetic and geo-electric showed simultaneous
variation.
The multi-sensor system showed that the bottom
current was quite high (about 20 cm/s) and the bottom
temperature change at depth of 2,200m was 0.1 °C(Iwase et al., 2000) Although the pressure change
measured by the tsunami gauge showed semi-diurnal
variation suggesting large effects by tidal forces(Katsumada et al., 2000), the bottom temperature
variation (Iwase et al., 2000) showed diurnal changes,
which are less effected by tidal forces. The PBLM
instrument showed a possibility of crustal deformation
measurement at the forearc basin with a few
centimeters accuracy over 1km distance whenmeasurements were corrected by bottom temperature
variations (Nagaya,2000). Those data are being
analyzed.

5. Conclusions

Preliminary results

Before November 20, 199, the MDOBO collected
one and half month records (Iitaka, et al., 2000; Iwase
et al., 2000; Kasahara and Sato, 2000; Kasahara et al.,2000b; Katsumata et al., 2000; Nagaya, 2000;
Nakatsuka et al, 2000; Watanabe, 2000). During theobservation, an earthquake of Ms= 6.1, which is one of
the major aftershocks of the Chichi Earthquake in
Taiwan on September 21, 1999 (Ms 7.7), was observedby OBBS (Fig. 4) (Kasahara and Sato., 2000b)
However, the noise level of OBBS at period of 100-200
seconds is surprisingly large due to strong influencesfrom the infra-gravity waves currents (Webb, 1998),
although it was estimated because the OBBS could not
be buried in hard-rock base below the ocean bottom. By
comparison with land geomagnetic observation at
Kakioka, JMA, 40 days records of geomagnetic field

Fig. 4: Photograph of OBBS and cable connecting to JBXThe VENUS project was started in 1995 to develop a
new concept for a submarine observatory. In thisproject, a geophysical MDOBO was developed using
the decommissioned submarine cable GOGC. The
system comprises MDOBO, OBTS, SMCC and landsystem. Digital data transmission using the coaxial
analog cable is used. The OBTS and the seven
instruments of the MDOBO were developed. In order
to increase flexibility of installation and modification,
the JBX with nine undersea mateable connectors wasused, which enabled us to connect any instruments to a
JBX in the deep sea. Several requirements arose due tothe use of analog submarine cables, which weresatisfied by mechanical and electronics designs. Usinga cable ship, a deep-towing unit and an ROV, theOBTS and MSOBO were successfully installed at a
depth of 2,157 m in the Ryukyu Trench during theperiod between August and September 1999. All data
were successfully transmitted to JAMSTEC data centerand stored in the mass-storage. The seven instrument of
the MDOBO collected one and half month records.

cable OBS using a retired submarine telecommunication
cable: GeO-TOC program, Phys. Earth Planet.Intr.,108, 113128, 1998a.
Kasahara, J., T. Sato, H. Momma, and Y. Sirasaki, A new
approach to geophysical real-time measurements on a deepsea floor using decommissioned submarine cables, Earth
Planets Space, 50, 913-925, 1998b.
Kasahara, J., Y. Shirasaki, and H. Momma, Multidisciplinary

Acknowledgements
This project was supported by STA (Science andTechnology Agency), Japan, and Earthquake ResearchInstitute, University of Tokyo. We give greatly thanks
JAMSTEC for their kind assistances during the field
operation by R/V Kaiyo, ROV Kaiko-10K, manned
submersible Shinkai 6500 and ROV Dolphine 3K. We
also give our great thanks people of the KDD Co. Ltd.,for their kind helps on deployment of the VENUS
systems.
We express our great thanks KDDI Co. Ltd and KCS
Co. Ltd for their great helps on the retrieval of the
OBTS and some instruments.

geophysicalmeasurements

on

the

ocean

floor

using

decommissioned submarine cables: VENUS project, IEEE J.
Oceanic Eng., 25, 111-120, 2000a.
Kasahara, J., et al., A new era of observations: Real-time multidisciplinary observation at the ocean floor using submarine
cables, J. Geography, 109, 827-840, 2000b.
Katsumada, A., et al., Study on the development of the tsunami
observation system by use of a precise bottom pressure sensor,
in “Final Report of the VENUS Project, Steering Committee of
the VENUS Project”, 165-170, 2000.
KDD, Special issue on the Trans Pacific Cable-2, KDD
Technical Jour., 88, 3-75, 1976.
Kojima, .J.,

Kato, and Y. Shirasaki, Development of thebottom telemetry system, in ”Final Report of the VENUS

6. References

Project, Steering Committee of the VENUS Project”, 67-73,

Iitaka, H., et al., Maneuverable and flexible deep ocean

2000.

observation system by acoustic transmission, in “Final Report

Nagaya, Y., Observation with seafloor acoustic ranging systemof the VENUS Project, Steering Committee of the VENUS

linked to submarine telecommunication cable, in “Final

Project”,152-160, 2000.

Report of the VENUS Project, Steering Committee of the

Iwase, R., K, Kawaguchi, and K. Mitsuzawa, Observation by
Multi-sensor in “Final Report of the VENUS Project, Steering
Committee of the VENUS Project”, 142-147, 2000.
Kasahara, J., ed., “Final Report of the VENUS Project”, Steering
Committee of the VENUS Project, pp.206, 2000.

VENUS Project”, 148-151, 2000.
Nagumo, S., nd D. A. Walker, Ocean bottom geoscienceobservation: Reuse of transoceanic telecommunication cables,
EOS Trans. AGU, 70, 673-677, 1989.
Nakatsuka T., et al., VENUS deep-sea geomagnetic andKasahara, J., and T. Sato, Observation by the VENUS OBBS

electrical filed observation at the Okinawa site, in “Final(Ocean Bottom Broadband Seismometer), in “Final Report ofReport of the VENUS Project, Steering Committee of thethe VENUS Project, Steering Committee of the VENUS
Kasahara, J., H. Utada, and H. Kinoshita, GeO-TOC projectReuse of submarine cables for seismic and geoelectricmeasurements, J. Phys. Earth, 43,

VENUS Project”, 134-141, 2000.
Watanabe, K., Observation by the hydrophone array, in “Final

Project”, 127-133, 2000.

619-628, 1995.

Kasahara, J., H. Utada, T. Sato, and H. Kinoshita Submarine

Report of the VENUS Project, Steering Committee of the
VENUS Project”, 123-126, 2000.
Webb, S. C., Broadband seismology and noise under the ocean,
Rev. Geophysics and Space Phys., 36, 105-142, 1998.