Energy Efficient Designs of an Underwater Acoustic Sensor Network for Ocean Current Monitoring

Providing energy to an underwater sensor network has always been a challenge due to the rough condition at sea as well as the lack of access to deployed equipment for battery replacement. Moreover, the lack of solar energy excludes the use of solar cells in cold oceanic regions. In such harsh conditions, maximizing sensors’ life time is an essential goal. In the area of ocean current measurement, some of the existing methods are mostly limited to measure only the surface current and not the shallow water current, while some other methods measure the speed of water in a vertical column only at one location. There are other systems that measure and store the current data of different locations and depths over time (few days) so the current data saved in them are not real-time. This study aims to overcome some of these limitations and proposes a real-time measurement method for wide area averaged current. Thus, in this paper, novel underwater sensor network topologies and architectures have been designed and proposed. These new proposed architecture designs specifically aim to maximize the network lifetime by minimizing the energy demand of the whole network. For this purpose, two types of network topologies, Hexagonal and Square, with two different configurations of with- and without-centre node for each type, have been designed and offered. The method used in the current measurement networks is based on transit time method and could be considered a modified version. Using the new modified measurement method, these novel architecture designs unravel the limitations of the existing current measurement methods. In this paper, the proposed architecture designs’ performance has been compared to each other and also their pros and cons have been discussed

Real-time measurement of wide-area near-surface ocean current

Of all of the physical parameters of the ocean realm, the speed and direction of the movement of ocean water, otherwise referred to as ocean “current,” is one of the most problematic to characterize. Currents influence the global climate, used for producing power, are crucial in determining the oil spill trajectories and ocean contaminant control, can either work against or with the movement of ships at sea and govern the movements of icebergs. Icebergs are a threat to offshore industries and marine transportations, particularly in places like the Northwest Atlantic, because of damages they can cause once they strike the oil platforms or ship hulls. They are steered by the near-surface current and not the surface current. Therefore, measurment of the real-time ocean currents at desired depths is valuable for the industries or researchers who are dealing with or studying the oceanographic data. Ocean current measurment methods that are currently being employed for ocean monitorings, are not able to measure the real-time current at certain desired depths over a larg area of the ocean. Thus, the existing current measurement methods need improvements. Limitations of the existing methods are as follows. Acoustic dopler current profilers (ADCP), are one of the most popular methods employed by most of the industries dealing with the oceanograghy. ADCPs are capable of measuring the current at any desired depth; however, their measurement method is of a point nature and they cannot measure an area averaged current data. Other techniques such as high frequency radio detecting and ranging systems (HF-RADAR) are also used to measure the surface currents (down to 15 m). These shore-based current meters with radio antenna, follow the same premise of the ADCP. In other words their measurement is dependant on the Doppler effect to determine the direction and velocity of the currents; however, they are capable of evaluating only the surface currents and not the near-surface currents (70-100 meter of depth is considered in this thesis as this is the depth oil structures are deployed in the Northwest Atlantic Ocean). Another group of instruments used for current measurement are floats and drifters which report their data to a centre device which is usually a satelite. The current data obtained with these instruments are fed into modeling systems, e.g. in (Chassignet, Hurlburt et al. 2006), for the ocean forcasting. The problems that exist with the available real-time current data from the satelite is that it is the very shallow current data (down to 15m that can be called surface). The data from other devices like floats is very sparse to include the horizontal information. Hence, Chassignet et al. use data assimilation of the past knowledge and ocean dynamics in order to predict the ocean features. Therefore, it is important to develop a method by which adequate data could be provided for the ocean prediction and modeling system. Thus, the focus of this thesis is on designing a method which is real-time and measures the near-surface current. On the other hand, energy suplies to the instruments in open water is limited as they work mainly rely on batteries and it is difficult to access the instruments in harsh condition to replace the batteries. Moreover, in cold regions the solar power is very limitted and thus using solar cells is not practical. Therefore, in order to measure the ocean current in real time, a novel method along with a sustainable architechture design is being proposed in this dissertation. The new method is based on transit time with the difference that in transit time method waves need to travel in both directions; up- and down-stream. But with a modification in the newly designed architecture; which is adding an extra node in the center of the network’s cells, sound waves need to travel on only one direction. This helps with saving a great amount of energy and covering a larger area in comparison with the networks which are developed using transit time method. Experimental results as well as simulations verify that the new proposed method is both efficient and practical