# Event Coverage Detection and Event Source Determination in Underwater Wireless Sensor Networks
## 3 Event Detection and Sensory Data Aggregation
The experiments are conducted for 20 contiguous time slots, and the other parameters are set to the same values as those in Figure 2:2 Twelve relay nodes are selected for studying the number of sensory data packets to be forwarded, and these 12 relay nodes are represented as \(r1\),..., \(r12\) in Figure 2:3 Note that these 12 relay nodes include those having the largest, and the smallest, number of sensory data packets. Figure 8 shows that the distribution of the number of sensory data packets is relatively even when the variance is relatively smaller (_i.e._, the variance is 60 km), although the number of sensory data packets for certain relay nodes may be smaller when the variance is relatively larger (\(r7\), for instance). This is due to the fact that sensor nodes are distributed in a more skewed fashion when the variance is relatively larger. Therefore, the workload of relay nodes is more uneven, which is reflected by the number of sensory data packets in Figure 2:4 Note that relay nodes with a larger number of sensory data packets should consume more energy and may die earlier than the others. This is harmful to the network lifetime. Therefore, a relatively even distribution of sensor nodes is beneficial for balancing the energy consumption of relay nodes. Figure 9 shows the number of sensory data packets, when sensory data are gathered by relay nodes, or are not gathered, and will be routed by individual sensor nodes through the routing tree independently. The transmission radius \(r\) is set to 0.1, 0.15, 0.2 or 0.25, respectively. The experiments are conducted for 20 contiguous time slots. There are 10, 30 or 50 sensor nodes in each time slot whose sensory data deviate from a normal sensing range, which are specified in Figure 9 and in Figure 10, as gathered, 10/30/50, and non-gathered, 10/30/50, respectively. The other parameters are set to the same values as those in Figure 2:5 Figure 9 shows that the number of sensory data packets is smaller for the gathered cases, when \(r\) is relatively smaller. This is due to the fact that sensory data of several sensor nodes can be gathered as a single data packet, which should be routed to \(SN\) with relatively large hops through the routing tree. Hence, the number of sensory data packets can be reduced to a large extent, in comparison with that of the non-gathered cases. This experiment shows the advantage

Figure 2:6 Comparison of the number of sensory data packets when sensor nodes are deployed in diverse skewness distributions in the network region. This figure shows that different sensor node deployment strategies may have a relatively large impact on the number of sensory data packets to be forwarded by certain relay nodes. of our sensory data gathering strategy than the traditional non-gathered one on reducing the number of sensory data packets. Figure 10 shows the energy consumption for the gathered or non-gathered scenarios. As presented in Figure 9, sensory data packets are much fewer for gathered cases when the transmission radius \(r\) is set to a relatively smaller value, and hence, energy consumption is also quite less in this situation, although sensory data packets should be larger in size for the gathered strategy than for the non-gathered strategy. Consequently, our gathered strategy is more energy efficient, especially when \(r\) is set to a relatively small value. ### Comparison with CARP for the Number of Control Packets and Energy Consumption

This section presents the result of our experiments for our sensory data gathering technique (denoted SDA in the following) with respect to CARP [27], where CARP serves as the protocol for routing sensory data to \(SN\). As mentioned before, the routing strategy adopted in our technique is developed through improving the mechanism of CARP. Figure 2:7 Comparison of the energy consumption when the transmission radius \(r\) is set to various values, for the strategies that sensory data are gathered, or not gathered, by relay nodes.