Terms and Definitions

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Classification Project: Terms and Definitions

Term	Definition(s)
AUVs	, or Autonomous Underwater Vehicles, are underwater robotic vehicles that do not have a tether to the surface. They are preprogrammed to operate over a particular course or to respond to sensor data or perhaps acoustic commands. Applications include surveying, scientific data collection and mine-hunting.
Batteries	: All types of batteries used topside (out of water) can also be used undersea. There are a few factors unique to using batteries underwater. One is that some batteries can produce gases that are potentially explosive. In the open air this is usually not a problem because the small volume of off-gases is quickly diluted. However, in sealed pressure housing, gas build-up can cause problems ranging from water leaks to explosive failure. Pressure relief valves can safely vent gas to eliminate these problems. Some batteries and other subsea electronics use pressure-balanced, oil-filled packaging. In this method, all air spaces and voids in a housing are filled with oil or hydraulic fluid. A flexible membrane on the housing transmits ambient water pressure through the housing. Because the pressure inside the housing is the same as the outside water pressure, it is said to be equalized. Since there is no pressure gradient, a heavy pressure-proof housing is not required. ... (more at Batteries)
Bottom Samplers	are sediment sampling devices. There are many different types of bottom samplers. A box corer can be dropped into the mud to bring back a block of nearsurface sediment. A piston corer can return a cylinder of sediment up to 100 feet (33 meters) long that may encompass several million years of sedimentary history. For researchers especially interested in the seawater-seafloor interface, a gravity corer can return cores up to 20 feet (6 meters) long with little core-top disturbance. Vibracorers use a motor that produces a high-frequency oscillatory vibration to help propel a long core barrel into unconsolidated sediments.
Buoys
CTDs	(conductivity temperature depth) measure how the electrical conductivity and temperature of the water column changes relative to depth. Conductivity and temperature information is valuable, because the speed of sound in seawater can be derived from these variables. This is used to correct devices that use sound underwater. CTD data can also be used to calculate water density. Oceanographers use CTDs to study the physical properties of water, which can help them understand currents, mixing, biological processes and other phenomena. For this reason they are one of the most common instruments used by marine scientists. A CTD may be deployed by itself, attached to a submersible, or as part of a larger metal water sampling array known as a rosette, or carousel. Multiple water sampling bottles are often attached to the rosette to collect water at different depths of the cast.
Cables	can be electrical, fiber optic, or a combination of the two. Underwater electrical cables date back to the advent of the telegraph the mid 1840s. Hemp, tar and jute were used around conductors. The advent of plastics and other synthetic materials brought about a variety of new cable designs. The components of a typical underwater electrical cable are: § Conductors —the metallic component of cables through which electrical power or electrical signals are transmitted. ... (more at Cables)
Cameras	are used underwater for all sorts of inspections and documentation. Most types of cameras that can be found topside can be housed and used underwater. They are used by divers, on subsea vehicles and as stand-alone devices. Film cameras are rarely used any more because of the limited amount of images that can be stored before the camera must be brought to the surface so the film can be changed. Thus, for still images, digital cameras are usually preferred. Video camera types include color, black and white, and low light. For low light, imaged-intensified CCD cameras are used, as are tube-type cameras known as SITs (silicone intensified targets). Dome port optics are used in some cameras to correct the distortions caused by looking through a flat window underwater. Features to consider when selecting an underwater camera include, fixed vs. zoom lens, field of view, depth capability, resolution and sensitivity.
Chemical Sensors	are commonly used to detect hydrocarbons, nutrients (nitrate, nitrite, ammonia, urea, phosphate, silicate, and iron) as well as pH and dissolved oxygen in and on water. A variety of sensing techniques are employed including optical methods.
Compasses
Connectors	are used to join a cable to a device such as an underwater vehicle, instrument or battery; or to join two cables together. They can be used for electrical conductors or optical fibers, or both. There are many types of underwater connectors, some can be mated when wet, others must be dry. Some of the common types are: § Cable Glands —actually not a connector at all but a rubber grommet in a housing that seals around a cable. They are inexpensive and easy to install but are not reliable for long-term immersions or in dynamic situations where the cable will be subject to flexing. ... (more at Connectors)
Corrosion	results when two dissimilar metals are in contact with each other and are immersed in a conductive solution (i.e., seawater). Corrosion can cause structural failure of materials or cause sealing surfaces to leak; thus, it is a very important engineering concern. The rate of corrosion is determined by several factors, including: § Valence of the metals ... (more at Corrosion)
Current Meters	are devices that measure water velocity and direction. Measuring currents is a fundamental practice of physical oceanographers. By determining how ocean waters move, scientists can determine how organisms, nutrients and other biological and chemical constituents are transported throughout the ocean. There are several common types of current meters: § Mechanical current meters are like underwater pin wheels. The rate at which the wheel spins is used to calculate the current flow. A drawback to mechanical meters is that they do not work well in weak currents. ... (more at Current Meters)
Data Loggers	are used to acquire and store information from sensors. The entire spectrum of data storage devices used topside have and can be housed in pressure vessels. These include hard disk drives, flash memory, video tape, etc. When brought to the surface, the logged data can be transferred to a computer or the memory can be removed for archiving. Factors to consider when selecting a data logger include how many sensors will be measured, for how long at what sampling rate, how much power is available, how much space is available, etc. Environmental sensors and transducers often measure things such as temperature, pressure, sound or light, as voltages or current levels. These analog signals must be converted to digital ones for use by computers. The conversion, known as A/D conversion, is done by electronic sampling of the analog signals. ... (more at Data Loggers)
Data Multiplexing	is the practice of sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. In analog transmission, signals are commonly multiplexed using frequency-division multiplexing (FDM) in which the carrier bandwidth is divided into sub channels of different frequency widths, each carrying a signal at the same time in parallel. ... (more at Data Multiplexing)
Data Telemetry	is the process of measuring data at the source and transmitting it automatically. There are many ways to telemeter data back to shore such as by cell phone, radio transmitter, microwave and satellite. There are several satellite services available (ARGOS, Inmarsat, Iridium, GOES, Orbcomm) that can receive data from most anywhere on the planet. Data can be transmitted from a variety of platforms, including moored buoys , drifting floats, and even marine animals. Acoustic modems are used to telemeter data through the water column from a source to a receiver. They rely on coded pulses of sound. Acoustic modems are often used with seafloor sensors to telemeter data to a surface buoy or surface ship. From there the data can be relayed to shore using one of the previously mentioned methods of telemetry.
Dynamic Positioning	is a method to keep a ship, platform or subsea vehicle stationary over a fixed spot on the seafloor. Such station keeping is desirable for many tasks including coring, drilling, lifting, sampling and diver support. DP systems use thrusters in the bow and stern to maintain position. A computer is used to direct power to each thruster when necessary based upon input from GPS, compass , motion sensors, wind sensors, current meters , acoustic navigation equipment and other systems. More information on this subject may be found at http://www.imcaint.com/divisions/marine/reference/intro.html
Echo Sounders	are used to measure water depth by sending an acoustic pulse to the seafloor and measuring how long it takes to be reflected back to the surface. Single beam echo sounders have a single transducer to transmit and receive sound. Multi-beam echo sounders have many transmit/receive hydrophones and thus cover a much larger area on the seafloor. This greater coverage makes it much quicker to survey a given area. Interferometry-based swath bathymetry survey systems offer wide-area, high-data-density coverage and are particularly suited to surveys in shallow coastal regions The term “interferometry” is generally used to describe swath-sounding sonar techniques that use the phase content of the sonar signal to measure the angle of a wave front returned from a sonar target. This technique may be contrasted with multibeam (and single beam) systems, which look for an amplitude peak on each beam in order to detect the sea-bed, or other targets, across the swath.
Flotation	: In marine technology, flotation can be divided into materials intended for use on the surface and those for use underwater. For surface flotation, buoys are fabricated out of steel or other metals, molded plastic or plastic foams. Ionomer foam is a popular material because of its reduced weight and high durability. Underwater, subsea flotation is often needed to achieve neutral buoyancy for subsea vehicles and platforms or to provide a positively buoyant upward force. Because of the compressive force of water pressure, subsea flotation is a more challenging problem then it is on the surface. ... (more at Flotation)
GPS	(Global Positioning System): Users of a global positioning system can calculate their location anywhere on the earth. Two “public” GPS systems are The NAVSTAR system, owned by the United States and managed by the Department of Defense, and the GLONASS system, owned by the Russian Federation. Global Positioning Systems are space-based radio positioning systems that provide 24-hour, three-dimensional position, velocity and time information to suitably equipped users anywhere on the surface of the Earth. The GPS signals do not penetrate through water, so subsea vehicles must surface to get a GPS fix. The system works by a constellation of satellites that transmit timing information, satellite location information and satellite health information. The user requires a special radio receiver – a GPS receiver – to receive the transmissions from the satellites. The GPS receiver contains a specialized computer that makes calculations based on the satellite signals. The user gets 24-hour, three-dimensional position, velocity and time information and does not have to transmit anything to the satellite. ... (more at GPS)
Geophysical Instruments	are used by scientists and those seeking hard minerals and hydrocarbon deposits to study those parts of the earth hidden from direct view. Instruments used underwater include magnetometers (which are described in this glossary under their own listing), gravimeters, and seismometers. Seismic devices are another class of underwater geophysical equipment used to generate shock waves so reflected signals can be used to detect structures under the sea floor. Dynamite was originally used to generate shock waves. Today mechanical devices such as air guns, bubble pulsers, boomers and sparkers are utilized. The reflected signals are detected by hydrophones that can either be placed on the seafloor or towed behind a ship. Streamer cables are arrays of hydrophones that are wired together and contained in oil-filled plastic hose. ... (more at Geophysical Instruments)
Hydrophones	: Underwater hydrophones detect acoustic signals in the ocean just as microphones collect sound in the air. Most hydrophones are based on a special property (piezoelecticity) of certain ceramics that produce a small electrical current when subjected to pressure changes. When submerged in water, a ceramic hydrophone produces smallvoltage signals over a wide range of frequencies as it is exposed to underwater sounds propagating from any direction. By amplifying and recording the electrical signals produced by a hydrophone, sound in the sea can be measured with great precision. Although a single hydrophone records sound arriving from any direction, several hydrophones can be simultaneously deployed in an array, and the resulting signals can then be manipulated to “listen” in any direction with even greater sensitivity than a single hydrophone element. Whether within an array or as a single element, the hydrophone is the basic sensor of underwater acoustics. Things to consider when selecting a hydrophone include receiving response, beam width, and depth rating.
Inertial Navigation Systems	When you spin a gyroscope, its axle wants to keep pointing in the same direction. If you mount the gyroscope in a set of gimbals so that it can continue pointing in the same direction, it will. This is the basis of the gyrocompass . If you mount two gyroscopes with their axles at right angles to one another on a platform, and place the platform inside a set of gimbals, the platform will remain completely rigid as the gimbals rotate in any way they please. This is this basis of inertial navigation systems (INS). In an INS, sensors on the gimbals’ axles detect when the platform rotates. The INS uses those signals to understand the vehicle’s rotations relative to the platform. If you add to the platform a set of three sensitive accelerometers, you can tell exactly where the vehicle is heading and how its motion is changing in all three directions. With this information, a ship’s autopilot or an AUV’s guidance system can keep the vehicle on its intended course.
Lighting	is used underwater to provide illumination for divers, for submersible pilots and passengers, and for cameras . Three general classes of lights exist: incandescent, arc, and LEDs. Incandescent lights work by heating a filament to the point where it radiates light. Quartz halogen lights are the most commonly used incandescent. The name derives from the quartz glass bulb used that is filled with a halogen gas which is used to redeposit evaporated filament materials back on the filament. Incandescent lights are inexpensive but bulbs are fragile and have relatively short life times. ... (more at Lighting)
Magnetometers	are used to measure anomalies in the earth’s magnetic field. These can be due to local geologic features or manmade objects in the area (i.e., ship wrecks). Underwater magnetometers are typically towed behind a ship. Proton magnetometers operate on the principal that the protons in all atoms are spinning on an axis aligned with the magnetic field. Ordinarily, protons tend to line up with the earth’s magnetic field. When subjected to an artificially-induced magnetic field, the protons will align themselves with the new field. When this new field is interrupted, the protons return to their original alignment with the earth’s magnetic field. As they change their alignment, the spinning protons precess, or wobble, much as a spinning top does as it slows down. The frequency at which the protons precess is directly proportional to the strength of the earth’s magnetic field. ... (more at Magnetometers)
Optical Oceanographic Sensors	measure the optical properties of water such as absorption, attenuation, scattering, fluorescence and volume scattering function. The instruments used to measure these parameters include: Transmissometers are devices for measuring transmission or beam attenuation. They work by shining a narrow, collimated beam of light through the water. A receiver with a narrow field of view measures how much light arrives at the other end of a set distance. Light that is lost to absorption or is scattered will not be detected. ... (more at Optical Oceanographic Sensors)
Pan and Tilts
Physical Oceanographic Sensors	are used to measure the basic physical properties of seawater. The most common are conductivity, temperature, and pressure (depth). Together these three sensors form a CTD , a common oceanographic instrument (described under its own listing in this glossary). Conductivity is the measurement of the water’s ability to conduct electric current. It is often used as a proxy for measuring salinity. The basic unit of conductance is Siemens (represented by an S), formally called the mho (omh spelled backwards). The principle by which instruments measure conductivity is simple. After an electrode consisting of two conductive plates is placed in the water, a sinewave voltage is applied across the plates and the current is measured. Electrodes are prone to corrosion , and the pumps that supply them with water can foul. ... (more at Physical Oceanographic Sensors)
Pressure Housings	are used to enclose electronics underwater. There are two primary types of housings. Pressure compensated housings are fluid filled and are maintained at ambient pressure. A flexible diaphragm is typically used to transmit the external pressure to inside the housing. Benign substances such as mineral oil or silicone oil are used for the compensating fluid which must completely fill the inside of the housing leaving no air pockets. Components that are air or gas filled cannot be used, because they will implode under pressure. ... (more at Pressure Housings)
ROVs	or Remotely Operated Vehicles are underwater robots used for a wide variety of tasks, ranging from simple inspection to maintenance and repair work. These tasks are performed on offshore oil rigs, ship hulls, docks, mooring buoys, dams, bridges and power plants, and in water tanks. ROVs are lowered on a cable either alone or in a protective cage and then operated on a slack tether that decouples it from the ship’s surface motion. Video cameras serve as “eyes” for the operator who controls the vehicle via the cable. An ROV can explore, take photographs, collect samples, or handle instruments, operating around the clock for many consecutive days. Job-specific tool skids are sometimes mounted under larger vehicles. A good write up on ROVs and their history may be found at the MTS ROV committee Web site http://www.rov.org/info.cfm .
Releases
Remote Sensing	is ocean data collected by sensors on satellites or aircraft. The term implies that the sensor is placed at some considerable distance from the sensed target, in contrast to close-in measurements made by “in situ” sensing. Commonly measured parameters are ocean color, sea surface temperature and sea surface altitude. Ropes and Tension Members are used to carry loads on moorings, towed sleds and devices lowered over the sides of ships. Fiber ropes, wire ropes and chains are examples of flexible tension members. Synthetic fiber ropes are made from Nylon, Dacron, Polypropylene, Kevlar, Vectran and Spectra. Considerations in selecting a material are strength, weight/buoyancy, and stretch. Synthetic fiber ropes can be terminated by eye splicing or by socketing with epoxy. ... (more at Remote Sensing)
Scour	is the destructive effect that flowing water has on a submerged object. When a manmade object such as a bridge pier, pipeline, cable, etc. is submerged in flowing water, the local effect is to increase the current velocity around the object. This increased velocity has the tendency to remove or scour away the bottom material that supports the structure.
Seals	: The type of seals discussed here are not marine mammals or navy commandos. Seals are a means for keeping water out of pressure housings . Types of seals used in the marine environment include gaskets, shaft seals, lip seals, stuffing tubes and gland seals. The o-ring is the most commonly-used type of seal underwater. It is a solid piece of elastomeric material shaped like a doughnut or torus. When pressed between mating surfaces, an o-ring blocks the passage of liquids or gases. The oring is the most widely used seal due to its simplicity, low cost, ease of installation, and small space requirements without supporting structures. An O-ring can be considered an incompressible viscous fluid with very high surface tension. This “fluid” is forced by mechanical or hydraulic pressure to flow into the sealing cavity, blocking the flow of the lessviscous fluid being sealed. Properly installed, the O-ring is squeezed about 10% to 15% of its original cross-sectional diameter. The compression absorbs the tolerance stack up between mating surfaces (or between shaft and gland in dynamic applications) and forces the elastomer into microscopic surface grooves on mating parts. Successful use of o-rings depends upon proper groove dimensions and selection of the right elastomeric compound. Compounds are chosen for their resistance to chemicals and temperatures. Common materials utilized include Nitrile, neoprene, flurocarbon (Viton), silicone, fluorosilicone, and urethane. ... (more at Seals)
Sediment Traps	are used to collect samples of particles sinking through the water column. Scientists can learn the rate of sedimentationand about the organisms that populate the water column using sediment traps.
Slip Rings	are electromechanical devices that allow the transmission of power and electrical signals from a stationary to a rotating structure. Also called a rotary electrical joint, collector or electric swivel, a slip ring can be used in any electromechanical system that requires unrestrained, intermittent or continuous rotation while transmitting power and/or data. They are commonly used on cable winches . For fiber optic conductors the Fiber Optic Rotary Joints (FORJs) are used in place of slip rings.
Sonar	(SOund NAvigation and Ranging) is used for many purposes. Passive sonars detect noise from marine objects, such as submarines, ships and marine animals. Active sonars emit a pulse of sound or “ping,” into the water and then listen for an echo when the signal is reflected off an object. To measure the distance to an object, one measures the time from emission of a pulse to reception. Some common sonar devices are: Side Scan Sonar —as the name implies, is a sonar that looks out sideways. It is used to map seafloor topography or to locate objects on the bottom or in the water column. It consists of a tow fish with a transducer running along each side. Acoustic pulses are transmitted orthogonal to the axis of the tow fish. The receiver measures the time it takes for the pulses to return and their strength. These are combined together to create a “shadow picture” that depicts the shape and texture of the seafloor and any objects laying on it. A good review of side scan sonars can be found at http://inventors.about.com/gi/dynamic/offsite.htm?site=http://www.instituteformarineacoustics.org/SonarPrimer/SideScanSonar.htm ... (more at Sonar)
Sub-Bottom Profilers	use low-frequency sonar (2 to12kHz) to penetrate the seafloor. Sound pulses are reflected from the boundaries between sediment layers. This information can be used for geological studies and to find buried objects such as shipwrecks, mines and lost equipment. In side scan sonar surveying, it is desirable to cover the seabed quicker and at higher resolutions. This causes great difficulty for conventional side scan technology. There are only two choices—using higher frequencies (to achieve narrower beams and thus higher resolutions) or using longer apertures to achieve the same effect. The downside to this approach is either shorter operational ranges with higher frequencies (compromising coverage rates) or problems associated with handling large tow fish. Similarly, as the imagery is tied to the sonar beam width, the resolution of conventional side scan degrades with range. Two techniques have been developed to get around these obstacles. Multi-beam side scan sonars use several transducers along each side of the tow fish. With beam steering and focusing techniques, several simultaneous adjacent parallel beams are generated per side. This allows for 100 % coverage at very high tow speeds with extraordinary resolution and image clarity. Another method, Synthetic Aperture Sonar (SAS) synthesises a long aperture image by adding successive sonar returns, which are processed to compensate for the movement between returns and other factors. The results are much improved resolutions.
Submersibles	: Manned submersibles are noncombatant craft capable of independent operation on and under the water’s surface. Manned submersibles have their own propulsion power and a means for direct viewing for the occupants who are in a dry one-atmosphere cabin. Manned submersibles have a long history going back hundreds of years and now employ many of the technologies described in this glossary. Today they are primarily used forscientific research, exploration and tourism Major subsystems include the pressure hull, viewing ports, life support systems, exostructure, ballast/trim, propulsion, batteries and fairings/sail. Operational instrumentation typically includes lights, cameras , sonar , thruwater communications, and often a manipulator. Navigation equipment consists of a pressure sensor, tilt sensor, gyrocompass , doppler velocity log (DVL), underwater tracking system, altimeter and perhaps an inertial navigation system that would integrate the data from all the other sensors. In addition, missionspecific scientific equipment is often added. The MTS Manned Submersibles Committee web site at http://www.mtsociety.org/pro_committees/mannedsubmersibles/default.html has a suggested reading list and links to many valuable resources. Also, the out-of-print classic “Manned Submersibles” by Frank Busby is available on-line at http://busby.psubs.org/html/page-001.html . A comprehensive compilation of American subs by Will Forman is The History of American Deep Submersible Operations .
Surface-Supplied Air Diving	: Divers tethered to a surface-supplied-air-diving system can achieve dives up to 190 feet of seawater (FSW) in air diving and 300 FSW in mixed gas diving. Air is supplied via an air compressor or high-pressure air flasks. A surface-supplied-air diving system can permit a diver to stay down for a longer period of time than a SCUBA system would allow.
Tide Gauges	are used to measure changes in sea level. This is important to know for many reasons, including the correction echo sounder data used in making hydrographic charts. The first known tide measuring devices were used by the Egyptians 5,000 years ago, making tide gauges the first known marine technology. Wooden or reed sticks and later marble columns planted in the river bottom were sufficiently long enough to extend above the waterline where the fluctuating level of the Nile River could be measured against markings. The hydrological information collected is one the longest scientific time-series data sets ever collected. Today, tide staffs are still widely used. ... (more at Tide Gauges)
Towed Vehicles	: Many types of underwater towed vehicles have been developed for oceanographic real-time surveys. They offer the capability of moving in wide area with stable attitude at speeds much faster then ROVs and AUVs . Towed vehicles are used for such tasks as monitoring the oceanographic environment, surveying the seabed and inspecting pipelines on the seabed. Some types can be steered and others can be winched in and out.
Underwater Acoustic Imaging	is used to produce high-quality images in water where visibility is too poor to use optical methods. In underwater acoustic imaging, electrical energy (electricity) is sent to a crystal, which causes the crystal to vibrate. The sound of the vibration (mechanical energy) travels out into the water column until it hits an object and is reflected back to the crystal. After the energy returns to the crystal, a computer converts it to an image.
Underwater Positioning and Tracking Systems
Water sampling	devices range from a bucket dropped over the side of a ship to large water bottles sent thousands of meters toward the seafloor on a wire. Probably the most commonly used water sampler is known as a CTD /rosette: it is a framework designed to carry 12 to 36 sampling bottles (typically ranging from 1.2- to 30-liter capacity) and a conductivity / temperature/depth sensor that sends information to the laboratory so that the water bottles can be closed selectively as the instrument ascends. Alternatively a single water bottle can be equipped with a spring-loaded release and lowered on a line. When the desired depth is reached, a messenger weight is slid down the line and trips the release allowing, the end caps to snap close on the bottle. The largest water bottles, called Gerard barrels, collect 250 liters. Particles in the water samples may be quantified with a transmissometer sent down the wire or attached to a CTD /rosette. Aboard the ship, a flow cytometer may be used to analyze particles in the form of single-celled organisms for optical properties indicative of their physiology and structure.
Wave Gauges	are made in several ways. Electric wave staffs measure wave height by changes in resistance. More common is using a pressure sensor with a current meters in a technique called PUV. Wave height is measured as changes in pressure. With an array of three or more sensors, wave direction can be determined. Another method uses Doppler current meters to form a virtual wave array. This is done by bottom mounting an upward-looking, multiplebeamed Doppler so it can measure the range to the surface as well as the orbital wave velocities in a series of bins extending away from the instrument. These mani fold measurements allow one to differentiate among multiple sources of waves—something that a single point sensor cannot do. ... (more at Wave Gauges)
Winches	are devices used to pay out, pull in, and store cable. They consist of a movable drum around which a cable is wound so that rotation of the drum produces a drawing force at the end of the cable. Winches can be powered by diesel, hydraulic or electrical power. A reel is a handpowered winch. Winches are selected by cable capacity, line speed, strength (line pull) and material (typically steel or aluminum). Options include remote controls, level winds, variable speed drives, brakes, clutches (for free spooling), cable counters and heave compensation. There are many special types of winches for specific applications, including those for ROV umbilicals, diver hoses, and CTDs . Slip rings are used with winches to transfer electrical power. Other related devices include: § Traction Winches are used for tensioning the cable so there will be no spooling problems due to slack line. ... (more at Winches)