The nature of the ocean environment and its vast size has necessitated the development of sophisticated equipment and techniques for various underwater applications including diver-to-diver communications, ROV/AUV docking, communications and oil and gas explorations. To facilitate scientific exploration a wide variety of systems and vehicles have been developed to operate within the shallow continental shelf region or in deep oceans. For successful underwater electromagnetic (EM) wave operation, knowledge is required of the wave transmission properties of seawater over all distances both short and long. This information is required for such activities such as: sensor systems, imaging, position fixing, measurement of speed, obstacle detection and avoidance, guidance, communication of data/voice and remote control. This paper presents a new approach of EM wave propagation through seawater. The experimental results conducted in the laboratory and the real environment of seawater is presented.

Propagation of electromagnetic waves at MHz frequencies through seawater

It is often preferable to avoid using divers to undertake sub-sea activities, the alternatives being autonomous or remotely operated robotic vehicles and manipulators. This will only be achievable if robust communications can be established through seawater. Presently for such sub-sea activities it is necessary to use acoustic modems, which are only capable of operating with data rates of up to 50kbs?1. Optical sensors can also be used but these rely on clear water and in many sea conditions propagation beyond 10m is not possible. This paper will present new experimental results for electromagnetic wave propagation through seawater at MHz frequencies. These frequencies would enable the use of high speed data rates, suitable for a wide range of sub-sea activities.

Experimental Investigations of Electromagnetic Wave Propagation in Seawater

There are actually three different types of data telemetry in the aquatic environment: acoustic, optic and electromagnetic. Table I shows the pros and cons of the different technologies. This paper will concentrate on the use of electromagnetic waves as a carrier through-water, through-ice and through-ground.

Underwater radio frequency communications

A fundamental impediment to the use of dense underwater sensor networks is an inexpensive acoustic modem. Commercial underwater modems that do exist were designed for sparse, long range, applications rather than for small, dense, sensor nets. Thus, we are building an underwater acoustic modem starting with the most critical component from a cost perspective - the transducer. The design substitutes a commercial transducer with a homemade transducer using cheap piezo-ceramic material and builds the rest of the modem's components around the properties of the transducer to extract as much performance as possible. This paper presents the design considerations, implementation details, and initial experimental results of our modem.

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Design of a low-cost, underwater acoustic modem for short-range sensor networks

In subsea applications, there is a growing demand for high-speed wireless communication links for transmitting data between different equipment. Radio communication is constrained by the high attenuation in seawater. Only a very short range is achievable, even at low frequencies. In this thesis an independent, battery-driven radio frequency transmitter is developed and tested to investigate the properties of, and prove the concept of underwater radio communication. The transmitter is made on the basis of a cNODE Research Midi system from Kongsberg Maritime. A software-defined radio receiver is used to measure and store the received signal for post-processing. BPSK modulated data is transmitted through seawater with a 5 MHz carrier with several data rates from 237 bps to 46.598 kbps. Two tests with approximately 50 m and 11 m depths in Trondheimsfjorden have been conducted. Maximum range was measured to be close to 5 m at 9 m depth with 1 W (+30 dBm) output power. Although, this is believed to be in an area where there might be a lot of freshwater mixed in the seawater, which would in theory improve the range compared to for pure seawater. Data from the software-defined radio has been successfully demodulated with a custom made MATLAB program for all data rates, hence proven the underwater radio communication concept.

Underwater Radio Communication

Experiments are described for determining the distributions of current, the driving-point impedance, and the electromagnetic fields characteristic of dipole antennas immersed in the sea near the surface. Measured and theoretical results are compared at two frequencies with good agreement.

R. King

Papers

Electromagnetic propagation between antennas submerged in the ocean