High-Speed Acoustic Communication For Seafloor Observatories
DOEI Project Funded: 2001
Scientists studying the interior of the earth use earthquake data from widely separated sensors to map the structure of the mantle, core and the continental plates. This is done by tomography, a process that allows visualization of the interior of a solid structure by analyzing the nature of signals that have passed through it. Mapping the earth’s structure with any degree of resolution requires many earthquake measurements, which are made using seismometers placed all around the world. However, since much of the world is covered by water, many seismometers must be placed on the seabed to collect the desired data. Providing access the data remains a significant challenge, and is one of the most pressing technology aspects of deep-ocean observatory research and development.
A Network of Sensors
Modern ocean bottom seismometers are based on their terrestrial counterparts that have evolved from large instruments with drums of paper that record the movement of a needle in response to the shaking of the earth called helicorders, to small, stand-alone network appliances, which are now deployed by the thousands in the United States alone. The seismometers allow scientists to find the source of an earthquake and determine its magnitude as soon as the energy is recorded and sent across the Internet. The United States Geological Survey maintains a web site that shows current seismic activity and contains links to descriptions of US and world-wide seismic networks (http://earthquake.usgs.gov). The challenge for scientists whose focus is on those portions of the earth that are under the ocean is to not only place their sensors deep underwater, but to access the data within hours, not a year after an earthquake occurs when the instrument is brought back to the laboratory after recovery by a ship.
Communications for Ocean Bottom Sensors
Land seismic networks depend upon the Internet to allow scientists immediate access to data from around the world. No similar infrastructure exists within the ocean, and thus the technology being developed for seafloor observatories includes not just sensors such as seismometers, but data communication systems too. Because radio waves cannot be used under water, and because cables are expensive and difficult to install, acoustic signals are often used for data transmission through the ocean, in this case from the ocean bottom to a surface buoy with a satellite data link to shore. Unfortunately, the data rates of acoustic links are much slower than those available on the Ethernet (even slower than typical home telephone modems!), and the time it takes for acoustic signals to travel through the water (1500 meters in 1 second) means that it would take a full minute to get a response from a computer located just 25 miles away. Computer programs used to receiving answers in a fraction of a second cannot tolerate long delays, and thus a special approach is necessary to get data from the ocean bottom seismometer to the buoy and then back to shore for the scientist.
Network Appliance Talks to Network Appliance
The DOEI-funded project was development of a method to connect the seismometer network appliance to the acoustic modem. How was this done? After debating custom hardware and Internet interface software we decided to add another network appliance to bridge the gap between the network-ready seismometer and the acoustic modem. This appliance is really just a small computer that has an Ethernet connection for the seismometer, and a serial port for the acoustic modem (just like a home computer with a telephone modem). For this computer we used something called the Bitsy (http://www.applieddata.net), and as the photo shows, it is much smaller than a standard PC, and thus easy to install into a glass ball with the seismometer and the modem to go on the ocean floor.
Software engineers worked with a WHOI seismologist to understand how best to retrieve data from the seismometer, and developed programs that could be executed with very simple commands sent over the acoustic link. The data from the seismometer was then placed in what we called the outbox (like mail ready to be picked up) on the Bitsy computer to await an acoustic connection from the buoy.
The System Goes to Sea
This DOEI project was done in conjunction with a large NSF project which funded the buoy, satellite link, and other sensors. The entire system was taken to sea and deployed in November of 2003, then retrieved in January 2004. Over 50 Mbytes of data were sent over the acoustic link, and the seismometer network appliances worked flawlessly. The complete seismometer system includes two glass balls full of electronics and batteries, and several more that are simply full of air to float it back to the surface once a weight is released.
The final deployment will be in May of 2004 near the Nootka Fault off of the coast of Washington. There the system will be used to demonstrate the capabilities of acoustically-linked network sensor appliances, and provide scientists with much-needed observations of seismic events recorded on the ocean floor.
The work described here was done by Ken Peal, James Doutt, Matthew Grund, and Lee Freitag. Dr. John Collins of the Geology and Geophysics department was instrumental in helping us to understand how data is stored and retrieved by the seismometer. The work was coordinated with two National Science Foundation projects: Development of a New Generation of Ocean Bottom Seismic Instruments for Marine Seismic Studies, and Development and Testing of a Deep-Water, Acoustically-Linked, Moored-Buoy Seafloor Observatory.
Originally published: January 1, 2001