Seagoing Tools of Oceanography
Since the ocean itself is not a convenient place to conduct research, scientists collect samples of water, sediment, flora, and fauna for study in laboratories aboard ship or ashore. Another approach is to create instruments or even automated laboratories to operate autonomously on the seafloor or in the water column. Oceanographic tools must carry out their missions in corrosive seawater and under high pressure in the deep sea. They must mesh with ships' winches and cranes and be of a size and weight manageable on the wet, unsteady, windswept deck of a research vessel. At the same time, they must be very precise. Some of the tools oceanographers employ are described here.
PLANT AND ANIMAL COLLECTING DEVICES
Collecting nets come in a wide array of sizes. The smaller ones, perhaps a meter long, may be towed briefly in near-surface waters. The largest multiple opening-closing variety consists of a great metal frame carrying as many as 20 nets and an environmental sensing array that sends information back to the ship's laboratory, where biologists signal the nets to open in sequence as they observe temperature, depth, salinity, and other characteristics of the water column. Such a net tow may last many hours.
When the samples come aboard the ship, some may be examined immediately under microscopes in the ship's lab, animals may be dissected or analyzed for clues to their food sources or exposure to pollutants, and other samples may be preserved for further work in shore-based laboratories. In other techniques, a series of samples may be collected by a device at the small end of a single net, or a silhouette photographic system may take pictures of the animals collected. Some animals, such as gelatinous zooplankton, are collected by scuba divers, who gently place specimens in glass jars to avoid damaging their fragile bodies. These are maintained in aquaria in the ship's laboratory and studied at sea, as they cannot be grown or preserved intact. Many techniques developed by divers to handle fragile plankton have been redesigned for use with submersibles. Bottom trawls, dredges, and coring devices are used to collect animals that live in or on the sediments and rock bottom. Another technique involves placement of trays of sterilized sediment back on the deep-sea floor to study colonization rates and animal distribution.
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. A standard rosette/CTD cast, depending on water depth, requires two to five hours of station time. New methods for this kind of sampling are being developed in order to reduce station time. 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.
The CTD is one kind of profiler - that is, it descends through the water column making continuous measurements. Another is the expendable BathyThermograph (XBT), a temperature probe released on a weighted copper wire that unreels to record temperature and depth while the ship is underway. At the instrument's maximum depth of 1,000 to 6,500 feet (300 to 2,000 meters), the wire breaks.
Data from the 10,000 XBTs U.S. oceanographers launch annually are a major source of physical oceanographic information. Other profilers, including some that measure sound velocity and microturbulence, are dropped from a ship to free-fall to the seafloor, adjust ballast, and return to the surface for retrieval. They provide data from both descent and ascent. Ship-wire-deployed and free-drifting water sampling and incubation instruments measure plant nutrient uptake, bacterial incorporation of dissolved organics, and plankton feeding rates. Specially designed water bottles enclose a sample, and then dispense chemical labels useful for a variety of measurements.
FLOATS AND DRIFTERS
Floats can be weighted to be neutrally buoyant at a particular depth, where they drift in the current while emitting periodic sounds. Such floats have been tracked for years by moored sound receivers to provide a long-term look at ocean currents. Trajectories of individual floats show how the water moves horizontally, and trajectories of groups of floats show how the water is mixed by eddies. This information is important for understanding how water tracers and pollutants are transported by the ocean. More recently, the sound sources have been moored while the floats act as receivers, surfacing at the end of an approximately two-year lifetime to report their data via satellite to a shore station. Other floats drift for two months, surface to transmit data to a satellite, and descend again for another two months of data collection. They can repeat this process for up to five years. Other combinations of these techniques are under development. Drogued surface drifters used for current studies also report position and data periodically via satellite transmission. Drifting sediment traps are used to study surface layer sedimentation, and such instruments as an acoustic backscattering device for collecting long-term data on plankton distribution are mounted on drifting buoys.
Instruments can be moored in the ocean for months or years to collect samples or data. Anchors are connected to holding lines with acoustic couplings that are released to recall the instruments. Flotation holds the instruments and their tether line upright in the water column and brings them to the surface on release. Currents meters, which may employ rotors, electric fields, acoustic/electromagnetic techniques, or acoustic Doppler profiling to track water motion, are often deployed on moorings. As scientists try to understand movement of materials through the oceanic system, they moor sediment traps at various depths for many months to collect samples of particles sinking through the water column. ubsurface moorings are also used to suspend settling plates above the deep-sea floor to collect larval forms of benthic animals.
For many years, current meters have been attached to moorings as described above, left for a time to collect data, and then retrieved. In a more recent technique, an acoustic Doppler profiler measures currents while a ship is underway. Sound signals sent from the moving ship bounce back to receivers aboard the ship for processing to give a vertical profile of horizontal water motion relative to the ship. Precise modern navigation allows ship motion to be subtracted from the data. These devices can also be used on moorings and on profilers. The acoustic Doppler technique and acoustic backscattering also have potential for measuring the biomass of animals that reflect the sound signals. Other uses of sound capitalize on the fact that low-frequency sound can be recorded after travelling great distances in water. Sophisticated data processing reveals the effects of temperature, density, and currents on its travel time. Acoustic tomography employs the effect of temperature on the speed of sound in seawater (sound travels faster in warmer water) to record temperature profiles over long ocean transects. Sound is also used for geophysical exploration of the seafloor and the layers beneath it. Sidescan sonar offers profiles of rock outcroppings and sediment surfaces at ranges up to several kilometers. Recently developed dual-frequency sidescan sonar can even be used to distinguish rock types. Multi-channel seismic profiling penetrates several kilometers into the seafloor and the reflected or refracted sound gives pictures of various layers as the speed and direction of the sound waves are altered by the density, elasticity, and flow properties of the material they pass through. Multibeam bathymetric systems available on some of the larger UNOLS vessels generally consist of two instrument arrays attached to the ship's hull. One array transmits a succession of sonic pulses in a 60° , 90°, or 120° swath. The sound reflects from the seafloor and is received by the second array of instruments. Numeric and graphic displays of data in the ship's laboratory provide maps of the seafloor topography. Acoustic systems also allow remote estimates of organism biomass, individual sizes, and numbers of animals present in the water column. Operating in conjunction with pumping systems, these instruments can yield organism samples as well as readings of the water's physical, chemical, and optical properties.
While not exactly "seagoing" instruments, Earth-orbiting satellites play an important role in modern oceanography. Their use for relaying data from instruments at sea to ship or shore is expected to increase with time. Receipt of data in "real time," that is, as it is being taken, enables researchers to monitor the performance of equipment in the field (and send a ship to service it, if necessary), to make decisions about an experiment underway, and to distribute data quickly. Two-way communication via satellite allows control of remote instruments. In addition, receipt of satellite data useful for directing experiments at sea allows oceanographers to make efficient use of precious ship time. Cruises designed to study biological productivity may be guided by information from ocean color scanners that detect reflected light and interpret it in terms of chlorophyll content, which denotes phytoplankton production. Satellite-based instruments useful to oceanographers studying ocean circulation include altimeters, which register variations in sea level slope that indicate current flow, and infrared sensors that show currents, eddies, and other circulation features.
Seafloor rocks can be gathered by towing a dredge consisting of a steel box and chain bag. More precise sampling may be accomplished using a submersible with robotic arms or a remotely operated vehicle equipped with television so that the area where the rock was found can be described in detail. Sediment sampling devices include a box corer that drops into the mud and brings back a block of near-surface 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 (six meters) long with little core-top disturbance.
Exchanges across the air-sea interface, including heat and fresh water, couple the ocean and atmosphere and are of major interest in studies of global climate. A collaborative effort of scientists from several oceanographic institutions aims to provide accurate measurements at this interface particularly for the World Ocean Circulation Experiment (WOCE). Sensors being designed or upgraded for use either on research vessels or buoys include those to measure surface temperature, air temperature, wind speed and direction, barometric pressure, solar and long- wave radiation, humidity, and precipitation. From these measurements, accurate estimates of air-sea fluxes can be made. The sensor package includes the capability to telemeter some data on a regular basis via satellite to a central data facility.
These "free vehicles" may be described as a way to take the laboratory to the seafloor. Miniaturization in electronics now allows scientists to put computers of increasing sophistication in pressure cases and send them down several thousand meters to control a variety of instruments. One such lander is released from the research vessel at the sea surface. It checks its sensors as it descends and reports their status to the ship. It then adjusts buoyancy as it approaches the bottom to insure a soft landing for the delicate instruments it carries and to avoid disturbing the sediment surface. The lander makes a second buoyancy adjustment to insure stability once it has landed. Chambers are then inserted into the sediment and sealed to isolate portions of the bottom from surrounding seawater. As the preset series of experiments on the seawater/sediment surface boundary begin, the lander confirms its activity to the ship, which then steams away to return several weeks later for recovery of the lander.
REMOTELY OPERATED VEHICLES
These robots, used for many years for simple oceanographic tasks, are increasingly sophisticated. Research vessels transport remotely operated vehicles (ROVs) to study sites and provide staging, servicing, and monitoring platforms for them. In the most sophisticated systems, the ROV is lowered on a cable alone or in a protective vehicle and then operates on a slack tether that decouples it from the ship's surface motion. Television cameras serve as "eyes" for the shipboard researchers, who receive television signals and control the vehicle via a fiber-optic cable. An ROV can explore, take photographs, collect samples, or handle instruments, operating around the clock for many consecutive days.
The heart of any research voyage is the "main lab," where plans are made (and changed), samples are prepared and analyzed, data is received and processed and the occasional party takes place. The laboratory is active day and night. It is seldom a tidy place - the lab configuration usually changes completely from one cruise to the next, and wires, sampling containers, tools, computers, and analytical instruments cram every available square centimeter. For some work, especially that requiring an extremely clean environment or specialized suites of equipment, portable vans may be outfitted and lifted aboard the research vessel to provide additional laboratory space. Scientists find it is increasingly important to analyze samples aboard ship rather than preserve them for processing ashore. Marine chemists, for example, often want to analyze many samples as soon as possible and to base tomorrow's work on today's sophisticated analysis.
An excerpt from The Research Fleet, prepared by the University-National Oceanographic Laboratory System (UNOLS), University of Rhode Island.