# Event Coverage Detection and Event Source Determination in Underwater Wireless Sensor Networks
## 3 Event Detection and Sensory Data Aggregation
Note that the experiments for \(r\) as 0.25 are not discussed, since \(r\) as 0.25 is relatively too large with respect to the network region, and the results of the experiments may not be convincing. This figure shows that the number of relay nodes increases to an extent when _cnt_ is set to a relatively large value. Note that when _cnt_ is quite large (e.g., 40 or 50), the increasing of the number of relay nodes is relatively small. This is due to the fact that when _cnt_ is relatively large, sensor nodes, whose sensory data deviate from a normal sensory range, are densely distributed in the network. Consequently, a relay node may have to relay sensory data for a relatively larger number of deviated sensor nodes, but relay nodes may not need to be newly added. In this setting, the workload of relay nodes should be increased, which should cause the increase of energy consumption in total. However, the number of relay nodes may not increase to an extent. Figure 4: Comparison of the number of relay nodes when the transmission radius \(r\) is set to various values and the number of deviated sensor nodes is set to 20, 30, 40 or 50, respectively. This figure shows that the number of relay nodes is mostly impacted by the density of deviated sensor nodes, which is determined by \(r\) and the number of deviated sensor nodes in the network region. Figure 3: Comparison of 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 various values. This figure shows that the number of relay nodes increases significantly when _cnt_ is set to a relatively large value. Figure 4 shows the number of relay nodes when the transmission radius \(r\) is set to 0.1, 0.15, 0.2 or 0.25, respectively, while the number of deviated sensor nodes is set to 20, 30, 40 or 50, respectively. The other parameters are set to the same values as those in Figure 3. Figure 4 shows that the increase of the number of relay nodes is non-linear with that for the number of deviated sensor nodes. In fact, the density of deviated sensor nodes in the network region is the key factor for determining the number of relay nodes, where the density can be represented as the average number of deviated sensor nodes contained in a sphere whose radius is \(r\). Generally, when deviated sensor nodes are relatively sparely deployed in the network region (for instance, the number of deviated sensor nodes is 20 or 30), newly-added deviated sensor nodes \(UV_{dev}\) may require newly-added relay nodes for sensory data gathering and routing to \(SN\), since existing relay nodes may hardly cover \(UV_{dev}\). On the other hand, when the number of deviated sensor nodes is large enough (for instance, 40 or 50), relay nodes may have covered the whole network region already. Consequently, newly-added deviated sensor nodes can be relayed by existing relay nodes, although their workload is much heavier than before. As argued in Figure 2, the number of relay nodes is mostly decided by the density, rather than the number of deviated sensor nodes. Figure 5 shows the number of relay data packets when the transmission radius \(r\) is set to 0.1, 0.15, 0.2 or 0.25, respectively. Similar to \"number of relay nodes\" in Figure 2, \"number of relay data packets\" in this figure specifies the number in total for all relay nodes in these 10, 20 or 30 time slots. The other parameters are set to the same values as those in Figure 2. Figure 5 shows that the number of relay data packets decreases when \(r\) is set to a relatively large value. However, when \(r\) is large enough (e.g., 0.2 or 0.25), the number of relay data packets is almost the same. Similar to the explanation in Figure 2, the relation for sensor nodes and the corresponding relay nodes may not change somehow when \(r\) is large. This means that the number of relay data packets may not change, as well, although hop counts may be decreased when routing relay data packets to \(SN\), which may cause the decrease of energy consumption.