Seafloor Observatory Science: a Review

The ocean exerts a pervasive influence on Earth’s environment. It is therefore important that we learn how this system operates (NRC, 1998b; 1999). For example, the ocean is an important regulator of climate change (e.g., IPCC, 1995). Understanding the link between natural and anthropogenic climate change and ocean circulation is essential for predicting the magnitude and impact of future changes in Earth’s climate. Understanding the ocean, and the complex physical, biological, chemical, and geological systems operating within it, should be an important goal for the opening decades of the 21st century. Another fundamental reason for increasing our understanding of ocean systems is that the global economy is highly dependent on the ocean (e.g., for tourism, fisheries, hydrocarbons, and mineral resources) (Summerhayes, 1996). The establishment of a global network of seafloor observatories will help to provide the means to accomplish this goal. These observatories will have power and communication capabilities and will provide support for spatially distributed sensing systems and mobile platforms. Sensors and instruments will potentially collect data from above the air-sea interface to below the seafloor. Seafloor observatories will also be a powerful complement to satellite measurement systems by providing the ability to collect vertically distributed measurements within the water column for use with the spatial measurements acquired by satellites while also providing the capability to calibrate remotely sensed satellite measurements (NRC, 2000). Ocean observatory science has already had major successes. For example the TAO array has enabled the detection, understanding and prediction of El Niño events (e.g., Fujimoto et al., 2003). This paper is a world-wide review of the new emerging “Seafloor Observatory Science”, and describes both the scientific motivations for seafloor observatories and the technical solutions applied to their architecture. A description of world-wide past and ongoing experiments, as well as concepts presently under study, is also given, with particular attention to European projects and to the Italian contribution. Finally, there is a discussion on “Seafloor Observatory Science” perspectives

Rock properties of the upper-crust in Central Apennines (Italy) derived from high-resolution 3-D tomography

High-resolution 3-D P and S-wave velocity models of a central sector of the Apennines (Central Italy) are computed by inverting first arrival times from an aftershock sequence (September–December, 1997) following the Mw 5.7 and Mw 6.0 Umbria-Marche earthquakes that occurred on September 26, 1997. The high quality of the data set, especially for the S-wave, allows us to compute 3-D variations in Vp, Vp/Vs and Vp · Vs. The anomalies can be interpreted as lateral changes in rock type and fracturing, which control fluid diffusion and variation in pore pressure. This is in agreement with a poro-elastic view that can be inferred from the spatio-temporal evolution of the seismic sequence

Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea

We present the first investigation performed on the seismicity of Southern Tyrrhenian Sea, off-shore Sicily with the contribution of data from broad-band ocean bottom seismometers and hydrophones (OBS/H). Offshore data were recorded during the TYrrhenian Deep sea Experiment (TYDE) from December 2000 to May 2001 in the Southern Tyrrhenian Sea. Hypocenter locations of a cluster of 53 seismic events occurred in March 2001 in north-eastern Sicily were estimated by the integration of land (permanent network) and offshore (temporary network) data and compared with locations estimated from land data only. The scatter of the cluster was evaluated by dispersion parameters. The off-shore data significantly reduced the scatter of the swarm hypocenters also restricting the depth range of the cluster. Moreover, space trends of the event distribution originally shown by the land data were only partially confirmed by the land-sea joint data. In order to assess the efficiency in terms of hypocenter mislocations in the subject area, of a land-sea integrated network with respect to a land-based network, we performed simulations by assuming a grid distribution of earthquakes and a recent local 3D velocity model, computing synthetic arrival times of body waves to the stations of both network configurations (integrated and land-based) perturbing the computed times and relocating earthquakes by inversion. The results of the synthetic tests demonstrated that the presence of sea bottom stations in the Tyrrhenian basin can reduce the mislocations of large magnitude and/or superficial earthquakes in the southernmost Calabria and Messina Strait and of low magnitude and/or deep earthquakes in north-eastern Sicily. The major accuracy of synthetic earthquake locations obtained including OBS/H data provides an additional support to the interpretation of the cluster occurred in March 2001 and to the opportunity of long-term installation of an off-shore network like TYDE in the study region

CUMAS: a seafloor multi-sensor module for volcanic hazard monitoring - First long-term experiment and performance assessment

A seafloor multi-sensor module with real-time data transmission, named CUMAS (Cabled Underwater Module for Acquisition of Seismological data), has been deployed in January 2008 in the Gulf of Pozzuoli, in the Campi Flegrei caldera (southern Italy), which is one of the most active volcanic areas in the world. The sensors installed in CUMAS were selected to monitor a set of signals related to the local seismicity as well as the ground uplift and subsidence of the seafloor that are related to the bradyseismic phenomenon. In particular, together with a broad-band three-component seismometer and a low-frequency hydrophone, a seafloor water-pressure sensor is used to assess the feasibility of measurements of the slow vertical movement of the seafloor (bradyseism). Further sensors are acquired by two embedded Linux computers, namely tilt and heading sensors for the measure of the actual module orientation on the seafloor, and status sensors that monitor the state of health of the vessel (e.g., internal temperature, power absorption, water intrusion). The underwater acquisition systems are linked to a support infrastructure, a floating buoy (elastic beacon), through an electro-mechanical cable with an Ethernet line. The buoy provides the needed power supply thanks to batteries charged by solar panels and a wind- generator. A Wi-Fi antenna on the buoy is used to transmit the seafloor data from the sea surface to the land acquisition centre in the city of Naples. A meteorological station is also mounted on the buoy, to allow the correlation of the air and seafloor data. CUMAS, although based on commercial sensors, relies on an original system for the centralized management of a wide set of geophysical and physical oceanographic sensors, that handles the continuous data acquisition and real-time data transmission. After the installation in the Gulf of Pozzuoli at about 100 m w.d., and after a test period, CUMAS uninterruptedly operated from May 2008 to June 2009, thus providing continuous geophysical data to the Monitoring Center of the Campi Flegrei volcanic areas, managed by the Istituto Nazionale di Geofisica e Vulcanologia. The long-term operational performance of CUMAS is presented here, together with the first results from the analysis of the geophysical long time-series acquired. Examples of the acquired signals, especially geophysical data, will be presented to point out the high quality in term of signal-to-noise ratio. In particular, earthquake recordings obtained from the hydrophone resulted of comparable quality to the seismic data acquired on land by the permanent network, thus demonstrating the suitability of hydrophones to monitor the seismic activity of the caldera

Tsunami Early Warning System: Deep Sea Measurements in the Source Area

In the framework of the EU project NEAREST, a new Tsunami Early Warning System (TEWS), able to operate in tsunami generation areas, was developed and installed in the Gulf of Cadiz. The TEWS is based on the abyssal station GEOSTAR, placed above a major tsunamigenic structure, and on three seismic centres of Portugal, Spain and Morocco. The core of the system is a tsunami detector installed onboard of GEOSTAR. The tsunami detector communicates with a surface buoy through a dual acoustic link. The buoy is connected to land stations via satellite link. The system was designed for near-field conditions and successfully operated from August 2007 to August 2008, 100 km SW of Cabo de Sao Vincente (Portugal). A new mission started on November 11th, 2009 in the same location. The tsunami detection is based either on pressure events either on seismic events. The bottom pressure data are analysed in real-time at the seafloor by a new tsunami detection algorithm, which can recognize tsunami waves as small as one centimetre. At the same time it was developed a new theoretical approach to account for tsunami generation in compressible water and in presence of a porous sediment. This model showed that hydro-acoustic waves, travelling much faster than the tsunami, are caused by the seafloor motion. These waves can propagate outside the generation area and are characterised by a modulation carrying valuable information on the seafloor motion, which can be recovered from their first arrival

An innovative tsunami detector operating in tsunami generation environment

On August 25th 2007 a tsunami detector installed onboard the multi-parameter observatory GEOSTAR was successfully deployed at 3200 b. s. l. in the Gulf of Cadiz, Portugal. This activity is within the NEAREST EC Project (http://nearest.bo.ismar.cnr.it/ ). Among other deliverables, the NEAREST project will produce and test the basic parts of an operational prototype of a near field tsunami warning system. This system includes an onshore warning centre, based on the geophysical monitoring networks which are already operating, and a tsunami detector deployed on board GEOSTAR at the sea bottom. On land the warning centre is in charge of collecting, integrating, and evaluating data recorded at sea. At the sea bottom data is recorded and processed by an advanced type of tsunami detector which includes: a pressure sensor, a seismometer and two accelerometers. The detector communicates acoustically with a surface buoy in two-way mode. The buoy is equipped with meteo station, GPS and tiltmeter and is connected to a shore station via satellite link. The prototype is designed to operate in tsunami generation areas for detection-warning purpose as well as for scientific measurements. The tsunami detector sends a near real time automatic alert message when a seismic or pressure threshold are exceeded. Pressure signals are processed by the tsunami detection algorithm and the water pressure perturbation caused by the seafloor motion is taken into account. The algorithm is designed to detect small tsunami waves, less than one centimetre, in a very noisy environment. Our objective is to combine a novel approach to the tsunami warning problem, with a study of the coupling between the water column perturbations and sea floor motion, together with the long term monitoring of geophysical, geochemical and oceanographic parameters

NEMO-SN1 (Western Ionian Sea, off Eastern Sicily): A Cabled Abyssal Observatory with Tsunami Early Warning Capability

The NEMO-SN1 (NEutrino Mediterranean Observatory - Submarine Network 1) seafloor observatory is located in the central Mediterranean, Western Ionian Sea, off Eastern Sicily Island (Southern Italy) at 2100 m water depth, 25 km from the harbour of the city of Catania. It is a prototype of cabled deep-sea multiparameter observatory, and the first operating with real-time data transmission in Europe since 2005. NEMO-SN1 is also the first-established node of EMSO (European Multidisciplinary Seafloor Observatory, http://emso-eu.org), one of the European large-scale research infrastructures. EMSO will address long-term monitoring of environmental processes related to marine ecosystems, climate change and geo-hazards. NEMO-SN1 will perform geophysical and environmental long-term monitoring by acquiring seismological, geomagnetic, gravimetric, accelerometric, physico-oceanographic, hydro-acoustic, bio-acoustic measurements to study earthquake and tsunami generation, and to characterize ambient noise which includes marine mammal sounds, and environmental and anthropogenic sources. NEMO-SN1 is also equipped with a prototype tsunami detector, based on the simultaneous measurement of the seismic and bottom pressure signals and a new high performance tsunami detection algorithm. NEMO-SN1 will be a permanent tsunami early warning node in Western Ionian Sea, an area where very destructive earthquakes have occurred in the past, some of them tsunamigenic (e.g., 1693, M=7.5; 1908, M=7.4). Another important feature of NEMO-SN1 is the installation of a low frequency-high sensibility hydrophone and two (scalar and vector, respectively) magnetometers. The objective is to improve the tsunami detection capability of SN1 through the recognition of tsunami-induced hydro-acoustic and electro-magnetic precursors.SubmittedRhodes, Greece3A. Ambiente Marinorestricte

Tsunami Warning prototype in the frame of the EC NEAREST project.

Nell' ambito del progetto NEAREST finanziato dalla EC sono stati sviluppati alcuni elementi di un sistema di allerta per tsunami, fra i quali un prototipo di detector di onde anomale istallato a bordo dell' osservatorio abissale GEOSTAR: l' osservatorio con il detector di onde anomale ha operato per un anno nel Golfo di Cadice, a 3200m di profonditàPublishedSassari1.8. Osservazioni di geofisica ambientaleope

Observing Volcanoes from the Seafloor in the Central Mediterranean Area

The three volcanoes that are the object of this paper show different types of activity that are representative of the large variety of volcanism present in the Central Mediterranean area. Etna and Stromboli are sub-aerial volcanoes, with significant part of their structure under the sea, while the Marsili Seamount is submerged, and its activity is still open to debate. The study of these volcanoes can benefit from multi-parametric observations from the seafloor. Each volcano was studied with a different kind of observation system. Stromboli seismic recordings are acquired by means of a single Ocean Bottom Seismometer (OBS). From these data, it was possible to identify two different magma chambers at different depths. At Marsili Seamount, gravimetric and seismic signals are recorded by a battery-powered multi-disciplinary observatory (GEOSTAR). Gravimetric variations and seismic Short Duration Events (SDE) confirm the presence of hydrothermal activity. At the Etna observation site, seismic signals, water pressure, magnetic field and acoustic echo intensity are acquired in real-time thanks to a cabled multi-disciplinary observatory (NEMO-SN1 ). This observatory is one of the operative nodes of the European Multidisciplinary Seafloor and water-column Observatory (EMSO; www.emso-eu.org) research infrastructure. Through a multidisciplinary approach, we speculate about deep Etna sources and follow some significant events, such as volcanic ash diffusion in the seawater