# Event Coverage Detection and Event Source Determination in Underwater Wireless Sensor Networks
## 1 Introduction  
The Earth's surface is mostly covered by oceans, which impact our life extensively [1]. Although knowledge about the oceans is of core importance to our life, only around 1% of the whole ocean has been explored, due to various factors, including vast volume, high pressure and the harshness of underwater environments [2; 3]. The importance and hardness of underwater exploration is evident through the recent search-and-rescue effort for the Malaysia flight MH370 in the Pacific Ocean [4]. In this context, exploring the vast ocean volume has been a critical and urgent task for the last few decades. With the advance of communication and sensing technologies, sensor nodes can be deployed in the underwater environment, and underwater wireless sensor networks (WSNs) have recently attracted significant attention and been considered as a promising alternative to exploring underwater environments [5]. Specifically, smart things, also called underwater sensor nodes,sense and record current (maybe historical, as well) information about underwater environments. These underwater smart things interconnect with each other and establish a network for gathering and routing sensory data to the sink node(s), which is (are) typically sonobuoys deployed on the ocean surface [6]. World-wide underwater WSNs establish the Internet of Underwater Things (IoUT) [7], which aims to collaboratively explore the vast ocean volume. IoUT supports wide-spread applications in scientific, industrial, military and other domains. Sensor nodes in the underwater environment are typically powered by batteries, which are hard, if not impossible, to replace or recharge nowadays [5]. Ambient energy harvesting and batteries with super-capacitors are promising to be adopted in the future [8]. In this setting, energy efficiency is a key factor to be considered, when mechanisms are to be proposed for underwater environment monitoring and event coverage detection [9]. Unlike terrestrial WSNs [10], where sensor nodes can be localized through global positioning systems or other localization techniques [11], sensor nodes underwater are localized through limited communication with anchor nodes or reference nodes, leveraging specific localization technologies [12, 13]. Sensory data are gathered and routed to sink node(s) in a pro- or re-active fashion [14, 15, 16]. Underwater WSNs are mostly responsible for monitoring the oceanic environment and detecting whether events have occurred or not [17]. Generally, an event can be anything that reflects an improper situation according to collective sensory data with respect to certain criteria. An example is presented in [8] about marine shellfish monitoring, where the attributes, like pH value, biotoxin value, water temperature, salinity, _etc._, are monitored. When an event like pollution is detected, sensory data for multiple kinds of attributes should be routed to sink node(s) immediately for quick event coverage detection and proper response enactment. It is worth mentioning that no event may occur in underwater WSNs for most time durations. This means that sensor nodes may not need to report their sensory data to sink node(s), since sensory data may be within the range of healthy. On the other hand, when an improper situation, which may indicate the occurrence of an event, is detected, sensory data of relevant sensor nodes should be gathered and routed to sink node(s) immediately for the identification of the location, coverage and sources for this event. Therefore, energy-efficient techniques for supporting sensory data routing, event location and coverage detection are fundamental in underwater WSNs. Event coverage detection and location determination have been explored relatively extensively for two- and three-dimensional terrestrial WSNs [18, 19, 20]. Generally, sensor nodes are either static or moving along pre-determined itineraries and can be localized through global positioning systems or other localization techniques [11].