The new broad band OBS/H of the Istituto Nazionale di Geofisica e Vulcanologia

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) has successfully completed the tests of a new deep sea broad band sismometer with hydrophone (OBS/H Ocean Bottom Seismometer with Hydrophone).


The prototype has been entirely planned and assembled at the Gibilmanna Observatory (PA), a unit of the INGV National Earthquake Centre (CNT). It is the first braod band OBS/H realized in Italy.  The project has been carried out in the framework of the 2004-2006 convention between INGV and the National Department of Civil Protection (DPC), and will be used for studies of active faults  and submarine volcanoes, presently studied only with far, on-land instruments, therefore with low precision.
After the tests in laboratory, in hyperbaric chamber up to 600 atm, and at the harbour of Cefalù (Sicily), the first deployment on the sea bottom was carried out in July 2006 in the Tyrrhenian sea at 3.412 meters depth, for a test of operability, release, and recovery. Subsequently, once refited, the OBS/H has been deployed on the flat top of the submarine volcano Marsili, at 790 meters depth (coordinates 39° 16,383' N and 14° 23,588' E)
.



Figure 1. Some shots of the deployment and recovery of INGV- OBS in the Tyrrhenian Sea, in July 2006.


The instrument has acquired seismic data and from hydrophone for 9 days, recording interesting data, including the M7.2 earthquake near Java on July 17, the M=3.4 earthquake of the Gulf of Patti (Sicily) on July 18, and the M=2.9 earthquake in Ionian Sea on July 20. Moreover, the new instrument has recorded tremor and microseismicity related to the Marsili volcano, not recorded from the seismic stations installed on land.



Figure 2. Location of the INGV OBS-H during the campaign of July 2006.


The project chief, Dr. Giuseppe D' Anna, has declared that “all the goals  of the project have been achieved. The acquired data, with a Nanometrics Trillium 40 second seismometer and with a Hydrophone OAS E-2PD, are of high quality. In the next three months we will have the first pool of 6 instruments, that will be ready to operate within the end of this year. These instruments” says D’Anna, “will help the Italian and Euro-Mediterranean seismological community to the monitoring of the marine areas. Our OBS’s have an autonomy up to 24 months, and therefore can be used for long passive seismological campaigns, for active seismic experiments, and for temporary deployments in the event of seismic sequences.


“This result” has declared Alessandro Amato, director of the National Earthquake Center of the INGV, “opens a new phase in the seismic monitoring of the Mediterranean. The next challenge will be the development of real time instruments that from the sea bottom transmit the data directly to a surface buoy and then to a shore station, in order to monitor the submarine seismic areas and to prevent the devastating effects of possible tsunamis.”






Figure 3. Some recordings of the INGV OBS/H in the period of operation (12-21 July 2006). The three upper traces represent 5 hours of recording of the seismic waves generated from the M 7.2 earthquake in Java on July 17, that generated also a
tsunami wave.
Below, the two traces are relative to a M 2.9 earthquake in the Ionian Sea and to local microseismic activity, probably related to the Marsili volcano.


Technical specifications: Long term deployment INGV OBS/H

Seismometer gimbal mounted Nanometrics model Trillium 40s or Trillium 120s or Guralp CMG40T-OBS
Pressure

OAS E-2PD Hydrophone, Omnidirectional Frequency Response: Flat (+/- 1dB) 0 – 5 KHz, to 25 KHz. Sensitivity (re 1V/mbar): -92 dB
Or
Differential Pressure Gauge Cox-Webb 500s-2 Hz gain 1 mV/Pa at 1 Hz

Digitizer SEND Geolon MLS 21 bits digitizer 4 channels (3 for seismic signal, 1 for hydrophone or DPG)Up to 200 sps

Clock

Seascan Precision Time base. Drift rate is 3-5 x 10-8 (<0.5 ms drift a day before correction).

Disk Capacity

12 PCMCIA slots, up to 24 GB with 2GB flash cards or microdrive

Battery Pack

2 Primary Lithium-Thionyl Chloride battery packs of 14,4 V and 350 Ah

Recording Duration

12-24 months autonomy depending from the installed instrumentations

Weight

450/30 Kg in air/in water

Pressure Case

Canister Aluminium alloy Ergal 7075 T6 with hard anodization

Release

Single burn wire operated acoustically IXSEA AR861S-MR, modified from INGV technicians for Burn-Wire release Inox steel canister full depth (6000 m) applications Autonomy: up to 20 months

Seismometer release

Single burn wire operated from digitizer facility

Emergency localization system

GPS receiver integrated with a sat-phone which transmits the position of the OBS every 3-6 hours in emergency when the OBS are floating

Dimensions

1200x800x1500 h

Power

0,56-1,15 W four channels at 100 samples/s


Acknowledgments: the INGV team is grateful to Dr. Spahr Webb and Eng. Patrick Jonke of the Lamont-Doherty Earth Observatory for the encouragement and the precious advices.