Project 3:
Seafloor Mapping with Sonar

Brandon Schmoyer




Single Beam Active Sonar
Dive and Discover

 
Brief History of Sonar
The first roots of sonar can be dated back to 1822. Daniel Colloden used an underwater bell to calculate the speed of sound underwater in Lake Geneva, Switzerland. This early research led to the invention of dedicated sonar devices by numerous other inventors. In 1906, Lewis Nixon invented the very first sonar type listening device, as a way of detecting icebergs. The interest in sonar grew during World War I, with a need to be able to detect submarines. It was not until 1915, when Paul Langevin invented the first sonar type device for detecting submarines called an "echo location to detect submarines" using the piezoelectric properties of the quartz. Although too late to help the war, Lanevin's efforts heavily influenced future sonar devices. The current sonar devices were passive listening devices, lacking the ability to transmit signals. It was not until 1918 that both Britain and the United States had built avtice systems, enabling the tranmission and the reception of signals (About Inventors).
How Sonar Works
Sonar, an acronym for SOund Navigation And Ranging, a method of detecting and/or locating underwater objects, based on the reflection of underwater sound waves (MSN Encarta). There are two general classifications of sonar, passive and active. Passive sonar involves listening for sounds generated by a target. Therefore passive systems only receive sound waves from outside sources. They do not send out sound waves themselves. This application is used for stealth in military operations when silence is of utmost importance to avoid detection by the enemy. Unlike passive systems, active sonar systems not only receive sounds but they also transmit. Active sonar systems emit pulses of sound waves that travel through the water, reflect off the target and return back to the ship by way of “echo”. By knowing the speed of sound in water and the time of the received echo, computers can quickly calculate the distance and orientation of the target (Howstuffworks). The following portions of this paper will focus on active sonar systems and applications.

The major components used in echo sounding aboard ships are transducers, which both transmit and receive sound waves, and a computer processor. First, the transducer, acting as a transmitter, sends a cone of sound down to the ocean floor, which will then reflect back to the ship. The returning echo is received by the transducer, now acting as a receiver, amplified electronically, and then recorded on graphic recorders. The time taken for the sound to travel from transmission through reflection and to return to the transducer is then used to calculate water depths. The faster the sound waves return, the shallower the water depths and the higher the elevation of the seafloor. In accordance, the longer the sound waves take to return, the deeper the water depths and the lower the elevation of the seafloor. Commonly, in order to produce a continuous line showing ocean depths directly beneath the ship, Echo sounders repeatedly ping the seafloor as the ship moves across the surface (Dive and Discover).

“Echo sounders use different frequencies of sound to find out different things about the seafloor. Scientists typically use echo sounders that transmit sound at 12 kiloHertz (kHz) to determine how far down the seafloor lies. However, they use a lower frequency (3.5 kHz) sound, which penetrates the seafloor, if they want to “see” accumulated layers of sediments below it.” (Dive and Discover)


Hull-mounted multibeam sonar (left) and towed side scan sonar (right)
National Oceanic and Atmospheric Administration
Multibeam Sonar System

Multibeam sonar is an advancement on its predecessor, single-beam echo sounding. Multibeam Bathymetry is based on the fact that more beams are better that one. Similar to the Single-Beam system, the multibeam system measures water depth. However, the multibeam system uses multiple 12kHz transducers, sometimes up to 120, sending out many beams of sound, simultaneously, to obtain a series of water depth readings along the path of a moving ship. The transducers are arranged in a precise geometric pattern on the ships' hulls. The swath of sound they send out covers a distance on either side of the ship that is equal to 200% of the water depth. Due to the uneven seafloor, which causes continuously different water depths, the sound bounces off the seafloor at different angles and is received by the ship at slightly different times. All the signals are then brought together and processed by computers aboard the ship and converted into water depths. The water depths are then automatically plotted as a bathymetric map, as pictured below, with an accuracy of ten meters (Dive and Discover).


Multibeam Sonar
Diver and Discover


Gridded Multibeam Sonar Depths
National Oceanic and Atmospheric Administration
Sidebar Sonar System

Side scan sonar is a specialized sonar system for searching and detecting objects that rest on the seafloor. Similar to other sonar, a side scan transmits sound energy and analyzes the returning echo which has bounced off the seafloor or other objects. The basic components used in side scan sonar are a towed transducer, which collects the data, and a communication cable, which provides a means to transmit the newly acquired data to the computer processor. “In a side scan the transmitted energy is formed into the shape of a fan that sweeps the seafloor from directly under the towfish to either side, normally to a distance of 100 meters.” (National Oceanic and Atmospheric Administration) Certain frequencies work better than others. High frequencies, 500kHz to 1MHz, provide excellent resolutions but the acoustic energy only travels a short distance. Whereas lower frequencies such as 50kHz to 100kHz yield lower resolution but the distance that the energy travels is greatly improved. (Klein Associates Inc) The strength of the return echo is continuously recorded, rather than the time of travel of the echo as in the multibeam system, creating a “picture” of the seafloor, as pictured below, built up one line of data at a time, instead of water depths acquired in multibeam. Hard objects reflect more energy causing lighter signal on the image. Soft objects, that do not reflect energy as well, show up as darker signals. The absence of sound, such as shadows behind objects, show up as very dark areas on a sonar image (Klein Associates Inc).


Side Scan Sonar Image of the Monitor
National Oceanic and Atmospheric Administration

Side scan sonar systems are one of the most accurate systems for imaging large areas of the seafloor, capable of detecting an object on the sea floor that measures 1m x 1m x 1m from shadow length measurements (National Oceanic and Atmospheric Administration). There are several common uses for side scan sonar systems due to its increased accuracy over other methods. Surveying, locating pipeline routes and other features, target search operations for pinpointing smaller objects such as drowning victims in vehicles, shipwreck locations, downed aircrafts, and collecting geological information about the composition of the seafloor are just a few of the uses.
Mapping an Undersea Transcontintenal Fiber Optic Cable Route
Which technology, multibeam or side scan, would produce data most suitable for mapping an undersea trancontinental fiber optic cable route?

Effectively mapping an undersea transcontinental fiber optic cable route would require knowledge of not only the depth of the seafloor but also the composition of the seafloor, yielding a more cost effective and accurate placement of the cable. The collaboration of multibeam and side scan sonar systems, in a series, would provide the desired factors. The following describes the optimal aforementioned process. In the very beginning research and planning steps, multibeam sonar would be implemented in a “target” area. The multibeam sonar would essentially eliminate certain areas of the target area that were either too deep or too shallow, uneven, or based on many other factors accurate up to ten meters. Then, if more detail was thought to be needed, the side scan method would be implemented in the same target area and would reveal the composition of the seafloor. This method could also eliminate certain portions of the target area, based on circumstances relating to the contents of the floor, such as a sunken ship, etc. which are accurate to one meter. Then one would simply overlay the results from the two methods and whichever areas are “usable” within both projects would yield the easiest area to lay fiber optic cable. With the best path selected to lay cable, the possibility of unexpected complications is greatly reduced, therefore increasing the probability of keeping costs low.
Sources
About Inventors (2006). The History of Sonar. Retrieved December 7, 2006, from
http://inventors.about.com/od/sstartinventions/a/sonar_history.htm
Dive and Discover (2005). NADCON - Mapping the Ocean Floor with Echo Sounding. Retrieved December 5, 2006, from http://www.divediscover.whoi.edu/tools/sonar-singlebeam.html
Dive and Discover (2005). Multibeam Bathymetry - The Successor to the Single-Beam Echo Sounding. Retrieved December 5, 2006, from
http://www.divediscover.whoi.edu/tools/sonar-multibeam.html
Howstuffworks (2006). How Submarines Work. Retrieved December 5, 2006, from http://science.howstuffworks.com/submarine4.htm

Klein Associates Inc. (No Date). Side Scan Sonar a Description.
Retrieved December 4, 2006, from
http://www.l-3klein.com/ssdescription/ssdescription.html
MSN Encarta (2006). Sonar. Retrieved December 5, 2006, from
http://encarta.msn.com/encyclopedia_761575038/Sonar.html
National Oceanic and Atmospheric Administration (2006). Side Scan and Multibeam Sonar. Retrieved December 5, 2006, from http://chartmaker.ncd.noaa.gov/hsd/wrecks.htm

National Oceanic and Atmospheric Administration (No Date). Specifications and Deliverables. Retrieved December 5, 2006, from http://chartmaker.ncd.noaa.gov/hsd/specs/CHAPTER6.pdf
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