# Event Coverage Detection and Event Source Determination in Underwater Wireless Sensor Networks
## 3 Event Detection and Sensory Data Aggregation
energy consumption becomes smaller when the transmission radius is set to a relatively large value, since a smaller number of hops are required when routing data packets to _SN_. Besides, our technique requires the maintenance of a routing tree, which induces some energy consumption. Generally, our technique is more energy efficient, especially when the transmission radius is a relatively small value. Figure 13 shows the energy consumption for (i) our technique and (ii) CARP as the routing protocol. The transmission radius \(r\) is set to 0.1 and 0.15, respectively. The number of sensor nodes, which drift with the water dynamics and change their coordinates, is set to 10, 20, 30, 40 or 50, respectively, where the \(x\)- and \(y\)-coordinates of these sensor nodes change no more than 3-5 m and their \(z\)-coordinate changes no more than 0-3 m, per time slot. The other parameters are the same as those in Figure 4:3 Figure 13 shows that our technique requires consuming more energy when the number of moving sensor nodes increases, while that for CARP is relatively steady (although large relatively). In fact, our technique may require one to re-select the parent nodes for sensory

Figure 4:4 Comparison of energy consumption for our technique (SDA) with respect to CARP as the routing protocol, when the number of time slots is set to 10 and 30, respectively. This figure shows that the energy consumption for our technique is less than that of CARP, especially when the communication radius \(r\) is set to a relatively small value. Figure 4:5 Comparison of energy consumption for our technique (SDA) with respect to CARP as the routing protocol, when the transmission radius \(r\) is set to 0.1 and 0.15, respectively, and the number of sensor nodes, which drift with the water dynamics and change their coordinates, is set to various values. This figure shows that the energy consumption for our gathered strategy is much smaller than that of CARP, especially when the number of moving sensor nodes is relatively small. data gathering and routing to _SN_, while it can hardly reuse the parents determined in previous time slot(s), when a larger number of sensor nodes changes their coordinates frequently. Generally, the larger the number of parent nodes is to be reselected, the larger the number of \(HeartB_{evt}\)-EPONG control packets to be transmitted and the more the energy is to be consumed during the sensory data routing procedure. Figure 13 shows that the energy consumption increases quickly along the increase of the number of moving sensor nodes. On the other hand, CARP requires one to select the parent nodes whenever a data packet is to be routed. This strategy suggests that the number of moving sensor nodes may not impact the energy consumption to an extent. Consequently, our technique is impacted by the number of moving sensor nodes and is more energy efficient, especially when the network topology is relatively steady. ## 6 Related Work and Comparison

Underwater WSNs are becoming a more pressing research topic, due to the rapid development of sensing technologies and the urgent requirement for studying the vast under-explored volume of ocean. Since the underwater environment is harsh and the communication cost is high, energy efficiency is a factor of core importance when detecting events in underwater WSNs [8]. Traditional techniques have explored the event detection and coverage determination in the underwater environment. In [17], the authors proposed to determine event locations through a sensor visualization approach. Intuitively, there may be multiple events occurring concurrently; however, these events may be located relatively sparsely in a relatively large network space. Since the water current is usually small in velocity in most situations, sensor nodes can be localized accurately. The drawback is the fact that this may cause relatively large energy consumption, especially when few events occur and quite a few sensor nodes are not involved in these events.