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|Authors: ||D'Anna, G.*|
|Title: ||The OBS experience at INGV|
|Editors: ||D'Anna, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia|
|Issue Date: ||11-Feb-2010|
Ocean Bottom Seismometer
|Abstract: ||It was the 2005 when management of the Centro Nazionale Terremoti, department of the Istituto
Nazionale di Geofisica e Vulcanologia, thanks to funds of the Italian Civil Protection national
service, decided to build its own first group of OBS’s. On July 2006, technicians of the Gibilmanna
Observatory, seat of the “OBS Lab”, deployed the first prototype of OBS/H on the flat top of the
Marsili submarine volcano. The nine days test, allow to record more than 1000 volcano-tectonic and
regional events, among them, only nine were also recorded by the on-land seismic network, the
others were related to the volcano activity.
The successful test led to the construction of seven OBS’s equipped with: i) Nanometrics
Trillium 120p seismic sensors (120s-175Hz), installed on a passive levelling system inside a 17”
pressure glass sphere; ii) Cox-Webb Differential Pressure Gauge (500s – 2Hz) or HTI-04-PCA/ULF
hydrophone (100s – 8kHz); iii) four channels, 21 bits, Send Geolon MLS digitizer.
The ballast “burn-wire” release system is based on two different acoustic releasers, one backup
of the other: i) Ixsea AR816S-MR, ii) Ore Offshore C980102. This solution, with two different
releasers, installed in two different canisters, showed itself to be very reliable, allowing the recovery
of the instrument also when one of the two releasers didn’t reply to the commands sent from the
At the end of the experiment, owing to the release of the ballast, the OBS’s come up to the
surface thanks to the buoyancy of McLane and Vitrovex glass spheres and they are recovered with
the help of Novatech radio beacons and xenon flashers.
In case of unwished ballast release, a GPS based tracking system will supply, through a web
platform, data for recovering of the instrument. This system allowed to recover one of the OBS’s
deployed in the Aeolian islands area on July 2008: because of an electronic issue, the Ore Offshore
releaser board gave voltage to the burn wire system and the instrument came up to the surface few
hours after the deployment; the OBS was recovered three days after nearby the Straits of Messina.
Trillium 120p seismometers, installed until the end of the 2008 on our OBS’s, have a very
narrow operational tilt range: ±0.2°. Out of the range of ±0.1°, power consumption increases to
2.5W from the nominal 600mW. This high power consumption in the presence of tiny mechanical
problems on the leveling system, leaded us to replace the Nanometrics sensors with Guralp
CMG40T-OBS (60s – 100Hz), a low power sensor (about 150 mW) gimbaled and housed in a glass
sphere with a diameter of 16 cm.
First months of 2010 will see the birth of a new prototype of OBS. This new project provides
the capability to communicate from the sea bottom to the surface: the instrument will be equipped
with an embedded system, based on ARM processor, which will store and process seismic data
coming from the seismometers and the DPG/hydrophone; a full depth acoustic modem will allow to
transfer to the surface portion of seismic data (e. g. triggered earthquakes traces) or tsunami alarm.
A new frame with syntactic foam instead of glass spheres was drawn to obtain the necessary
buoyancy for the modem and its batteries pack.|
|Appears in Collections:||Conference materials|
04.06.10. Instruments and techniques
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