Computers use digital data to transmit and receive information including e-mail messages and Internet web pages. Is it possible to transmit this kind of data underwater? Can a submerged submarine send and receive e-mail? Submarines do not have telephone or cable connections, and radio signals do not propagate underwater, so a submarine relies on sound to send and receive digital data.

The preceding section explained how underwater telephone systems use sound to communicate underwater. These systems produce very low quality signals, similar to poor telephone connections that cellular phones often experience. Telephone modems allow computers to transmit and receive information over telephone lines; however, they do not work very well with poor connections. Special acoustic modems that can successfully transmit digital data underwater have been developed. These modems convert digital data into special underwater sound signals that can be transmitted between two submerged submarines or between a submerged submarine and a surface ship. These digital signals can represent words and pictures, just as on land, allowing submarines to send and receive e-mail. Underwater acoustic modems are relatively slow compared to telephone or cable modems on land. Nonetheless, this technology is very important because it provides an accurate and efficient means to send and receive data underwater.

Basic Acoustic Communication Model. Diagram courtesy of Benthos, Inc.

In addition to submariners, researchers also need to send and receive data underwater. Oceanographers also use acoustics to control underwater instruments and acquire the data that they collect remotely.

Acoustic links are used to control underwater instruments and acquire the data remotely. Diagram courtesy of Benthos, Inc.

This technology can also be used to control small, unmanned submarines, called Autonomous Undersea Vehicles (AUV's), and get data back from them in real-time. These vehicles are currently under intensive development and are beginning to be widely used for oceanographic research and other purposes.

Autonomous vehicles working under the ice can be controlled and their data can be transmitted to a topside station using underwater acoustic links. Diagram courtesy of Benthos, Inc.

Underwater data links can also be combined with satellite data links to provide data in real-time from instruments on the seafloor to scientists ashore. One application of this technique is to provide early warnings of tsunamis generated by undersea earthquakes. Tsunamis are large waves that are generated when an earthquake causes the seafloor to move. They can cause great damage when they come ashore in populated areas, such as Hawaii or Japan. Tsunamis can be detected by pressure sensors that are deployed on the seafloor. The U.S. National Oceanic and Atmospheric Administration (NOAA) Deep-ocean Assessment and Reporting of Tsunamis (DART) program has installed bottom pressure sensors on the seafloor north of Hawaii. The pressure data are transmitted to a near-by surface buoy via an acoustic data link using underwater modems. The data are then relayed to researchers on land in real-time via satellite, to provide early warnings of tsunamis before they come ashore.

Real-time tsunami warning system. Pressure data collected by sensors on the seafloor are transmitted to the surface buoy via an acoustic link that uses underwater modems. The data are then relayed to researchers on land via satellite. Photo courtesy of NOAA PMEL Deep-ocean Assessment and Reporting of Tsunamis (DART) Program.