Hence, the degraded seismic data transfer is assumed, as short-term packet loss may occur in moored OBS system. This requires a bandwidth of 0.5 to 30 Hz with 18 bits of resolution, with a signal to noise ratio above 60 dB, and a communication data loss rate less than 10%. Therefore, the moored OBS system should detect seismic events of low seismicity (1.5 < M < 5), located at depths up to 100 km, and the nominal natural period of 1 second. The real-time observations are useful to determine the magnitude and the hypocentre of the regional seismic events. In this paper we describe a moored OBS system, with continuous telemetry through the mooring line and buoy to the shore station, capable of providing good azimuth observations of regional seismic events. Finally, the conclusions drawn are presented in Section 4.Ģ.
#ORCAFLEX LISCENSE REGISTRATION#
The results include the registration of a seismic event located in the Hautes-Pyrénées, of magnitude 3.7, that occurred on 28 October at 19.06 UTC. The actual deployment of the standalone ocean bottom broadband station near the OBSEA observatory and the results are discussed in Section 3.
Section 2 focuses on the design philosophy of the standalone ocean bottom broadband system including the seafloor unit, the surface buoy, and the mooring and the inductive communication system it includes the dynamic simulations of the prototype for deployment near the OBSEA observatory at a depth of 20 m and in the Alboran Sea (Western Mediterranean) at a depth of 250–300 m.
First, an overview of the challenges for underwater monitoring of the regional seismic events in real time is presented. Within this context, we developed and implemented a new technology for standalone ocean bottom broadband systems. However, the standalone ocean bottom broadband stations, implementing an acoustic link between the OBS and the surface, are limited by the acoustic communication latency, power consumption, and bandwidth. More affordable solutions are the OBSs acoustically linked to surface buoys or surface vehicles. Although a permanent array of wired OBSs can be considered as the best technology for seismic surveys, having practically unlimited power and data transmission bandwidth, it is very expensive to implement and its deployment is restricted to the locations where such cable observatories can be deployed. In the last decade, various technologies have been proposed and tested to address this challenge, such as cabled Ocean Bottom Seismometers (OBSs), OBSs acoustically linked to surface buoys or surface vehicles and free-floating buoys. Monitoring the regional seismic events in real time makes it possible to achieve fast estimation of actual earthquake scales, since the measured seismicity directly gives us the true size of the earthquake. Recent seismic activity in 2013 (possible induced by a gas tank) on the coast of Vinaròs, or the intense underwater seismic activity associated with the eruption of El Hierro in the archipelago of the Canary Islands (2011–2012), shows the importance of controlling seismic events located in the sea that are not covered by the terrestrial monitoring networks. Variations in real-time seismicity provide knowledge of the state of local and regional stresses in the short and medium term, essential information to study the potential seismic risk that may affect infrastructures and population located in the area. A GPS receiver on the surface buoy has been configured to perform accurate timestamps on the seismic data, which makes it possible to integrate the seismic data from these marine seismometers into the existing seismic network. The seismometer transmits continuous data at a rate of 1000 bps to a controller equipped with a radio link in the surface buoy. In this paper we also present the first results and an earthquake detection of a prototype system that demonstrates the feasibility of this concept. Additional batteries are needed for the underwater unit. The power to operate the surface buoy is provided by solar panels. The seismometer used is a high sensitivity triaxial broadband geophone able to measure low vibrational signals produced by the underwater seismic events. Prior to the deployment the dynamics of the system have been simulated numerically in order to find optimal materials, cables, buoys, and connections under critical marine conditions. Inductive communication through the mooring line provides an inexpensive, reliable, and flexible solution. The system consists of an underwater seismometer, a surface buoy, and a mooring line that connects them.
An anchored marine seismometer, acquiring real-time seismic data, has been built and tested.
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