# Real-Time Seismic Data from the Bottom Sea
## 2 Standalone Broadband Ocean Bottom System
### The Mooring and Inductive Communication System
#### 2.3.3 Simulation Results  
Table 3 displays values of the horizontal displacement of the surface buoy (surge), the maximum tensions induced by Line 2 to the anchor, and the inclination angle (referred to the vertical at seabed). The most critical value is the tension supported by Line 2 at the seabed (last column in Table 2). This tension, if large enough, could produce movements and displacements in the anchor and also vibrations that could affect the measurements of the nearby seismometer. In normal conditions, we observe that the range of displacement of the surface buoy (measured with the \(x\) variable) do not depend on the line ratio, being around 12 m in Model 1 and 8 m in Model 2. The vertical angle of Line 2 remains small and similar in both models, being the maximum 3.5\({}^{\circ}\) (see left profile in Figure 7). The tension at the seabed is also small in all cases, with a maximum of 2.2 kN. We can conclude that, in normal conditions, there is no substantial difference between the two models. Taking into account that our purpose is to minimize the angle and the tension in the seabed, we can accept that a bigger ratio gives better results. In extreme conditions, due to the strong tension involved, the Line 1 inverse catenary disappears and acts like a quasi-straight line (see right profile in Figure 7). In Model 1 the surge range of the surface buoy (about 19 m) does not depend on the ratio of lines and only slightly depends on the ratio (36.6-41.4 m) in Model 2. A noticeable difference, regarding maximum surge, Model 1 always shows higher values than Model 2 (about 4 times bigger). The angle of Line 2 is similar in both models under the same meteorological conditions. The proposed design consists of a seabed unit (containing the seismometer), an intermediate underwater buoy (that minimizes displacements and tensions in the system), and a surface buoy (containing the offshore communication system). Under normal conditions there is not a significant difference between models, though in extreme conditions Model 2 gives better results than Model 1, as the seabed tension is about 4 times smaller. Under the studied conditions, the use of the elastomer is a good option in extreme  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Line Configuration**} & \multicolumn{3}{c}{**Surface Buoy**} & \multicolumn{3}{c}{**Line 2**} & **Seabed** \\ \cline{2-9}
**Meteo** & \begin{tabular}{c} **Ratio** \\ **Line 1** \\ \end{tabular}  & \begin{tabular}{c} **Line 1** \\ \end{tabular}  & \begin{tabular}{c} **Line 2** \\ \end{tabular}  & \begin{tabular}{c} **min \(x\)** \\ \end{tabular}  & \begin{tabular}{c} **max \(x\)** \\ \end{tabular}  & \begin{tabular}{c} **Ax** \\ **m** \\ \end{tabular}  & \begin{tabular}{c} **Angle** \\ **degrees** \\ \end{tabular}  & \ \begin{tabular}{c} **max T** \\ **kN** \\ \end{tabular}  \\ \hline \multirow{4}{*}{ \begin{tabular}{c} normal \\ 1.6 \\ \end{tabular}  } & 1.2 & 120 & 200 & 66.9 & 79.1 & 12.2 & 3.55 & 2.10 \\  & 1.4 & 140 & 200 & 85.9 & 97.1 & 11.2 & 2.52 & 1.42 \\  & 1.6 & 160 & 200 & 101 & 113 & 12.1 & 2.36 & 1.37 \\  & 1.2 & 120 & 200 & 113 & 132 & 19.2 & 22.6 & 25.9 \\  & 1.4 & 140 &conditions as it reduced the tensions at the anchorage, minimizing the risk of displacement. The buoy has also been over-dimensioned to support extreme conditions. A large anchor and a strong steel cable ensure their stability and the seismometer has been placed as far as possible from the anchor to avoid interferences. There is also a significance difference between maximum tensions at the seabed depending on the model, ranging from 25.9 kN in Model 1 to 7.5 kN in Model 2 (see Figure 8). Figures 9 and 10 show the Fourier spectral density of tensions induced to the seabed using Model 1 and Model 2 respectively (notice the different scales in figures and between them). Dominant frequencies are always smaller than 1 Hz. The spectral analysis is basic in the study of dynamic behaviour.