# Event Coverage Detection and Event Source Determination in Underwater Wireless Sensor Networks
## 3 Event Detection and Sensory Data Aggregation
Without loss of generality, the network space is defined as a rectangular region, and \(SN\) is located at the center of the ocean surface with the geographical coordinate of \((0.5,1,0)\), where the \(z\)-coordinate corresponds to the depth of \(SN\) (or sensor nodes). The results of our experimental evaluation are presented and discussed in the following. Figure 2 shows the number of relay nodes when the transmission radius \(r\) is set to 0.1, 0.15, 0.2 or 0.25, respectively. The experiments are conducted over 10, 20 or 30 contiguous time slots, and the number of sensor nodes, whose sensory data may deviate from a normal sensing range, is set as 30 at each time slot. Note that \"number of relay nodes\" in this figure, as well as that in Figure 3 is the number in total for all relay nodes in these 10, 20 or 30 time slots. Due to the water dynamics, it is assumed that no more than five sensor nodes may drift away during each time slot, and their \(x\)- and \(y\)-coordinates may change no more than 3-5 m, while their \(z\)-coordinate may change no more than 0-3 m. Figure 2 shows that the number of relay nodes declines to a certain extent when \(r\) is set to a relatively large value, since the transmission region of a sensor node is relatively larger, and hence, more sensor nodes may select the same relay node according to Algorithm 2 for sensory data gathering and routing to \(SN\). This figure also shows that the number of relay nodes is non-linear with the value of the transmission radius \(r\), since the transmission region (_i.e._, \(\frac{4}{3}\times\pi\times(r_{qry})^{3}\)) increases much quicker than \(r\). Consequently, when \(r\) increases, the number of sensor nodes within the transmission region of a certain relay node may increase to a large extent, which results in a decrease in the number of relay nodes. Besides, the number of relay nodes shown in this figure is almost the same when \(r\) is set to 0.2 or 0.25. After examining the deployment of sensor nodes, it is found that, given two candidate relay nodes, the increase for the number of sensor nodes that one candidate node can relay is almost the same as that for another. This means that the determination of relay nodes \begin{table}
\begin{tabular}{l c} \hline
**Parameter Name** & **Value** \\ \hline Simulation network region (km\({}^{3}\)) & \(1\times 2\times 0.5\) \\ Number of sensor nodes (including one sink node) & 61 \\ Transmission radius \(r\) (km) & 0.15 \\ Time slots for experiments & 10, 20, 30 \\ EPING or \(HeartBl_{rot}\) control packet size (\(B\)) & 11 \\ EPONG or HELLO control packet size (\(B\)) & 7 \\ ACK and \(getData\) control packet size (\(B\)) & 6 \\ Data packet payload size (\(B\)) & 100 \\ Robustness factor for the parent-child relation determination & 0.7 \\ Smoothing factor for the link quality computation & 0.7 \\ Power for transmitting a data packet (\(W\)) & 2.8 \\ Power for transmitting a control packet (\(W\)) & 1.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Experiential parameters settings. Figure 2: Comparison of the number of relay nodes when the transmission radius \(r\) is set to various values, where 10, 20 and 30 specify the number of time slots for our experiments. This figure shows that the number of relay nodes declines when the transmission radius is set to a relatively large value. may not change for sensor nodes, and hence, the number of relay nodes may not change, as well. This fact indicates that the density of deviated sensor nodes (a more detailed discussion is presented in Figure 4), which is determined by the number of deviated sensor nodes and the communication radius, is the key factor for determining the number of relay nodes. Figure 3 shows the number of relay nodes when the number of sensor nodes (denoted _cnt_), whose sensory data deviate from a normal sensing range, is set to 20, 30, 40 or 50, respectively. The communication radius \(r\) is set to 0.1, 0.15 or 0.2, respectively.