GEODIS (GEOphysical DIving Saucer):
A portable ocean bottom broadband seismic station

L. Beguery∗, JP. Montagner ♦, JF. Karczewski ∗ , S. Cacho *,JC. Koenig ∗ , J. Savary ∗ , E. Stutzmann ♦, P. Lognonne*, G. Roult ♦Institut de Physique du Globe de Paris
Departements Sismologie: ∗ OFM / ♦GEOSCOPE
Département des Etudes Spatiales: *4, avenue de Neptune
94107 Saint Maur des fossés Cedex
Francebeguery@ipgp.jussieu.fr

1/ Scientific goals
The last ten years have seen the simultaneous developmentf a global seismic network coordinated through theFDSN (Romanowicz and Dziewonski, 1986) and of portable broadband seismic arrays. The same approach canbe followed for improving our scientific understanding of the Earth processes below oceanic areas. These twocomponents of ocean bottom geophysical networks, might be coordinated by ION (International Ocean Network;
Suyehiro et al., 1995). They are complementary they enable to investigate the Earth structure and active
processes at different spatial and temporal scales. Permanent Ocean Bottom Observatories and temporary
portable seismic stations are sharing common technological problems. However, issues of power-supply and ofreal-time transmission are more crucial for an observatory than for a temporary station. Following therecommendation of ION (Montagner and Lancelot, 1995) concerning the operation on the bottom of the sea ofGeophysical instruments and particularly VBB Seismometers, the Geoscope/OFM/VBB group of the IPGP has
developed a prototype of an autonomous VBB seismological station named GEODIS. This station might be one
basic and central element of a permanent observatory. It relies on the use of adapted VBB sensors issued fromspace experiments and technology and on improved electronics compared with previous ocean bottom
experiments (Montagner et al., 1994a,b; Stakes et al., 1998; Romanowicz et al., 1998; Stutzmann et al., 2001)

2/ Major characteristics of the station
2.1/ GEODIS: GEOphysical DIving Saucer•Qualified for 6000 meters depth•Size 930x930x430 mm (37x37x17 inch)•Weight in air is 135 kg (298 lb) and 90kg (200 lb) inwater•3 axes Very Broad Band sensors ( 120 sec- 0.2 sec)
•1 infrasonic pressure wave sensor (sensitivity 1E-2 Pa)•1 scientific temperature (sensitivity 1.E-3 °K )•Energy Li batteries 75 KWh•Data acquisition on RAM disk 2Go

2.2/ GEOMAS : GEODIS Module of Assistance and Services
To install the GEODIS without the assistance of ROV (RemoteOperated Vehicle) or Submarine and to demonstrate the ability of thesystem to operate at the bottom of the sea, we have designed andassembled a module called GEOMAS.
The system is basically a sphere (heritage of previous experiments
such as SISMOBS 1992) associated with a structure made of Al tube
supporting 2 acoustic releases and transponders to take care of releasing
the dead weight and of the extra buoyancy . The third one is redundant of
the most important one. It will be used as a telemetry system to send few
words and to give status of the installation process and successful run of
the acquisition system (flash card and mass memory).

3/ Instruments on board GEODIS :3.1/ Very broadband seismometer .
•Inverted pendulum with a moving mass of 125 grams.
•Resonant frequency : 1.28 Hz.
•Quality factor of the sensor : around 150 inside vacuum.
•Displacement sensor :differential capacitive transducer with high gain

1V/µ and very low noise (better than 1 nm at 0.01 Hz).
•Feedback force ( around 30N/A) produced by a set of 4 coils each ofthem with 250 turns of Copper wire (17 ohms) inside housing in soft ironused as a shield and concentrator of flux for 2 sets of 2 SmCo5 magnets
mounted in opposition with an optimized gap in between.
•Centering by moving (Portescap motor) a small mass of 14 grams witha resolution around 1 µm.
•Mechanical locking of the pendulum by an extended coarse of theequilibrium device.
•Pivot 16 blades of CuBe2 alloy (similar to the one designed for theVBB Martian sensor provided by company SODERN-EADS).
•Moving and fixed parts of the pendulum made of Ti alloy.

3.1.1/ Leveling platform
The triaxial VBB sensors are mounted on a platform mechanically linked to a set of a 2 axes motors . They areactuated by a device which generates pulses of 2msec width and of variable number and polarity. For that
purpose, the microprocessor, devoted to manage GEODIS, receives the output of a digital electronic
inclinometer (provided by APM instrumentation) with each axis well aligned (better than 0.1 degrees) on thesame axis as the motor. A soft locking device ensures a very good stability of the leveling system whichconsequently makes the leveling automatic.

3.1.2/ Sphere under vacuum.
The Archimedes effect of the air affects the seismometer performances. A vacuum must be done in the sphere inorder to ensure a good performance of the whole system.

3.2/ Infrasonic wave sensor
An infrasonic sensor was built at the laboratory by using a differential pressure gauge provided by EFFADRUCK with a sensitivity of 1.E-2 Pa and a dynamic range of 2000 Pa ; the detector is immersed inside oilshell Morlina 10.

3.3/ Temperature sensors
A small temperature sensor has been developed at the laboratory: Platinum sensor PT 100 (4 ohms / degrees K;
1000 ohms at zero °C) Sensitivity in the range better than 1milli Kelvin.
The sensor is inside a small tube made of steel in order to be qualified for 6000 meters depth.

4/ Energy
The energy is given by Lithium batteries stored inside either the GEOMAS sphere : around 75 KWH
devoted to power the main electronics units or inside 3 of the cylinders inside GEODIS to power permanently
the real time clock (SEASCAN) and the sensors.
The following table gives the power consumption for each module:
Alim.
Module Désignation
Origin of Power
Temperature :10.5v

Pressure Sensor :10.5v

Seismos module:+/-7v

Cont Bat/Temp
container Temp/Bat

mA- W
5mA- 50mW

Available power
2x3=6 batteries Li
i.e. 36Ah=185days

container Temp/Bat

2x50mW(2x3)x2=12batt. Li
i.e. 2x36Ah=185 days

Geomas Sphere
Ageco module:+/-5v_+5v

Geomas

100mA100mA- 0.5W

(USO)

+7.0v

autonomous

0.6mA- 5mW

Acquisition module:
SEASSMO

+5v

Geomas

200mA200mA- 1W

Data logger:CADO+5V/12V

Geomas

600mA-3W

Total Poweraverage=3W

Consumption:

margin=0.5W

12Ah/7v Li =833jrs

peak=7.5W
AUTONOMY::
100 days24v

Geomas(52Ah/0.13)x6

0.13A -3.2W

6 pack Li -24V
(52Ah/24v)x6

5/ Electronics
The central sphere in Titanium alloy contains VBB sensors and proximity electronics ; the digitizer (4 channels
with 24 bits at 20 SPS for the 3 VBB sensors and the infrasonic signal, and 16 channels with 16 bits at 1 SPS forhousekeeping channels such as 3 POS, 3 temperatures, 2 inclinometers, and others HK) in association with thereal time clock from Seascan. They are all located in cylinders.
The temperature sensor and Li batteries to power all the GEODIS sensors are located in another cylinder .
The micro-controller and the data logger are located inside the third cylinder. The two other cylinders are filledwith Li batteries.
a block diagram of the full electronics is given on the figure on the next page.

6/ Schedule
A campaign of validation of the installer and recovery system is programmed for June 01in theMediterranean sea and a first campaign of operational measurements is scheduled in September 01 certainly in
the Thyrenean sea at the place where GEOSTAR2 was installed during the campaign (Sept00 to April 01).

7/References.
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