# Real-Time Seismic Data from the Bottom Sea
## 4 Conclusions  
In this paper we described a new technology of a moored OBS, which can help to meet the long-term goal of telemetered ocean bottom seismic stations. A seismometer attached to a surface buoy has been proposed as an alternative to classical OBSs. In contrast to autonomous OBSs, the physical connection between the seismometer and the buoy allows real-time data communications with enough bandwidth to analyse seismic activity. It should be noted that inductive systems, such as the one included in this mooring OBS, are relatively easy to implement with a single anchor point. Moreover, this telemetry system has a very low latency and power consumption compared to acoustic telemetry, which is normally used as communication link between the seafloor and surface devices. Having a moored OBS, such as the one described in this paper, installed offshore in an active seismic zone will allow us to obtain almost in real time the first record of an earthquake, and to determine the arrival time of the P wave and its amplitude. Hence, this type of system is able to provide crucial information over the ocean that global seismic tomography so far desperately lacks. Two different mooring models, using a steel cable or a combination of steel cable plus elastomer, have been investigated. Numerical simulations, using Orcaflex, have been performed including the effect of the waves, winds, and currents, taken from the Spanish Harbor Authority records. Model 1 uses the inverse catenary to absorb movements and tensions whereas Model 2 uses an elastomer to do the same. The knowledge of these tensions is a key point in the design of the anchor system. Both models have pros and cons. Under normal conditions there is not a significant difference between models, although in extreme conditions Model 2 gives better results than Model 1, the seabed tension being about 4 times smaller. Under the studied conditions, the use of the elastomer reduced the  
Figure 14: Caption from the Garraf Seismic Station of a seismic event located at Hautes-Pyrenees, of magnitude 3.7 [17], that occurred on 28 October at 19.06 UTC. Figure 13: Caption from the Cabled Seismometer Station of OBSE of a seismic event located at Hautes-Pyrenees, of magnitude 3.7, that occurred on October 28 at 19.06 UTC [16]. A band filter between 4 and 10 Hz has been used to filter the raw seismic data. tensions at the anchorage, minimizing the risk of displacement, and it seems to be a good option in extreme conditions. Final design consists of a seafloor unit (Guralp 6 YC-OBS Gimbal seismometer) and a surface unit (EMS 2.0 buoy) connected by a steel cable. The steel cable allows inductive DPSK data transmission between the two units and consequently real-time data transmission through RF, GSM, or satellite link to the land station. First deployment and validation tests were done in June of 2017 at 2 nautical miles off the coast of Vilanova i la Geltru (Barcelona), near the OBSEA underwater observatory at a depth of 20 m. The reliability of the system has been proved by detecting a seismic event of magnitude 3.7 centered at Hautes-Pyrenees, that occurred on 28 October at 19.06 UTC. Finally, we plan to deploy the moored OBS system in an active seismic zone in the Carboneras Fault in the Alboran Sea (Western Mediterranean) at a depth of 250-300 m.  
Usually seismic Long-Period events (LP) range from 0.05 to 30 Hz. Our design, in the studied meteorological conditions, transmits seabed vibrations around 0.1, 0.2, and 0.6 Hz that partially overlap the seismic range. Special attention must be taken in order to identify low frequency seismic events. The vibrations of the nearby anchor due to the wave can be induced in the signal of the geophone, however, this behaviour can be characterized using the new elastic approach with a simple mass-spring-dashpot model of OBS/seabed interaction, that allows the analysis of amplitude coupling [18]. It is also possible to make corrections for this problem by the infidelity vector of the coupling [19].