4 research outputs found
Design Criteria and Practical Insights into an Underwater Current Measurement System Along With Simulation Results of a Real-Case Scenario in the Northwest Atlantic Ocean
Acoustics have been used in underwater communication and environmental sensing for a
century. Sound waves propagate well in water; however, the marine environment poses many
challenges to this phenomenon. Designing and deploying an underwater acoustic sensor network
has always been a challenge due to the inhomogeneity of the propagation medium. In this paper,
a background theory of the underwater sound propagation is provided followed by practical
observations and insights into innovative ideas achieved in a lab-scale prototype which assisted
in overcoming these challenges. These observations are used to propose a large-scale deployment
strategy in the Northwest Atlantic region. Bellhop simulation results provide evidence of the
effectiveness of a large-scale system design. This work is focused on finding optimal positioning
of the acoustic sensors in the sea while minimizing the multipath effect at the receiver. In
addition, the process for precise current speed measurement in a laboratory environment has
been explained which elaborates on the practical aspects of a large-scale network deployment in
the ocean. The Doppler effect, caused by the motion of the transducers due to wave motion in
the sea, is also considered and analyzed for signal processing needs
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