3.7: Scuba, Manned, and Unmanned Submersibles

Direct observation is often the only or the best way to sample or measure many ocean processes. For example, a seafloor area may be characterized by jumbled rock formations and highly variable assemblages of organisms. In such areas, geological oceanographers can best identify and collect rocks and sediment of interest if they can visually inspect and select samples. Similarly, biologists can ensure that they are collecting the important species if they are able to see and select the organisms to be sampled. Direct observation in their natural habitat is also important for those species that do not survive transfer to an aquarium or laboratory, for biologists to study.

Scuba and Habitats

In shallow waters, sampling is often performed by scuba divers. However, because of the dangers of the bends and nitrogen narcosis, scuba divers cannot descend safely below about 90 m and they can remain underwater for only a short time.

Scuba equipment supplies a diver with air at the pressure that corresponds to the diver’s depth. Thus, the pressure of the air breathed increases with depth. Divers are susceptible to two dangerous syndromes. The first of these, nitrogen narcosis, occurs when high-pressure air causes high concentrations of nitrogen to build up in the bloodstream. This causes symptoms very similar to those of alcohol intoxication—with possible consequences similar to drinking and driving. The second ailment, the bends, is a different life-threatening medical problem. It is caused by breathing high-pressure air at depth for too long and then returning to the surface too quickly. As the depth and immersion time of a dive increase, nitrogen continues to dissolve into the diver’s bloodstream and then transfers into other body tissues. If the diver returns to the surface too rapidly, the excess nitrogen in the blood and tissues cannot escape quickly enough and forms damaging gas bubbles in the body.

One way of extending the length of time that scuba divers can remain at depth safely is to use an underwater habitat (Fig. 3-22). Scientists can live for days or weeks in such habitats, which are pressurized and anchored on the seafloor. The scientists can safely make multiple scuba excursions to research sites at approximately the same depth as the habitat, as long as they return to the habitat and not to the surface. Although underwater habitats have many valuable uses, particularly for behavioral and other biological studies, their utility is limited. They are expensive to maintain and operate, they cannot readily be moved to new research sites, and they cannot significantly increase the maximum depth at which scientists can work using scuba. In addition, to avoid the bends, scientists who live in habitats for a week or more must spend several days in a decompression chamber at the end of their stay, even if the habitat is only a few meters deep.

Figure 3-22. Underwater habitats, such as Aquarius, shown here in the Florida Keys, enable scientists to live and work up to several tens of meters underwater for several days without incurring decompression sickness. However, at the end of their stay they must undergo very long decompression periods in a decompression chamber.

Manned Submersibles

Most of the ocean floor and almost all of the ocean volume are too deep for scuba divers to reach. Marine scientists have used a variety of research submersibles to visit and work at greater depths. Manned oceanographic research submersibles are small submarines usually designed to carry no more than two or three scientists. They enable the scientists to observe and photograph organisms and the seafloor. Equipment is often attached to the outside of the submersible to collect organisms, rocks, sediment, and water samples that can be selected visually by the submersible’s occupants.

The first recorded successful use of a submarine was in 1620. The vessel, built by a Dutchman, Cornelius Drebbel, had a waterproof outer skin of leather and was propelled by 12 oarsmen. It was reported to be able to stay as deep as 4 or 5 m underwater for several hours. Between 1620 and the 1930s, many different submarines were developed, but they were used primarily as warships. Today, the vast majority of submarines are still warships that are not suited to, or used for, most oceanographic research.

During their early development, research submersibles had very limited capabilities because they were built primarily to transport explorers who sought to dive to ever-greater depths. Such explorers usually performed scientific observations as only a secondary interest. Early submersibles, called “bathyscaphes” or “bathyspheres,” provided little more than windows and lights for their occupants to view the oceans and seafloor, and most were not equipped to collect samples. In addition, most early submersibles could only sink to the seafloor and then drift with the currents or move short distances before being hauled back onto a ship. Therefore, positioning a submersible precisely at a previously selected site on the seafloor was not possible.

The exploration phase of research submersibles climaxed in 1960 when the bathyscaphe Trieste visited the deepest part of the ocean, the Mariana Trench, 10,850 m below the ocean surface. Since 1960, many new submersibles have been designed, but very few of these are capable of reaching the depths achieved by the Trieste. Most are designed for much shallower dives. Recent advancements in submersible design have centered on improving the submersible’s ability to find precise locations on the seafloor, travel across the seafloor during a single dive, and collect samples at selected locations. Modern submersibles are strange-looking vessels with one or more protruding robot arms and a variety of baskets, other sample-collecting devices, video cameras, and measuring instruments that hang from the hull within reach of a robot arm (Fig. 3-23). They also carry powerful lights because sunlight does not penetrate more than a few hundred meters of seawater. Submersibles are now used for a variety of undersea observations, particularly observations of underwater volcanic features and of hydrothermal vents and their unique biological communities (Chaps. 6, 15).

Figure 3-23. A typical submersible, such as the Johnson Sea Link shown in this photograph, consists of a glass or plastic sphere inside a hull. The scientists descend within the sphere, which is provided with air and heat. Motors, articulated arms, and many different types of samplers and cameras are mounted outside the sphere. These are used to provide propulsion and collect samples.

Unfortunately, submersibles have disadvantages that limit their usefulness. Most submersibles must have a large surface vessel to carry or tow them to the research site, with a large crew to launch, retrieve, maintain, and repair them. Because submersibles are very expensive, the cost of each dive is extremely high in relation to the cost of other oceanographic research efforts. Submersibles have very small interior crew spaces, so dives of several hours are an uncomfortable ordeal for the pilot and passengers. The discomfort, the need to carry air-recycling systems for the crew compartment, and the limited battery power available to operate the motors, life-support system, and scientific equipment are all factors that limit the maximum duration of each dive. Limited dive duration restricts the area that can be studied on each dive, particularly in deep waters where the submersible may spend several hours descending and ascending.

Remotely Operated Vehicles

Many of the limitations of submersibles can be overcome by use of a remotely operated vehicle (ROV). The basic ROV consists of a television camera mounted on a frame or sled. An electric motor, which drives a propeller, and electronically controllable steering devices are also mounted on the sled. The sled is attached to a surface ship by a cable through which power is supplied from the ship, video images are transmitted to the shipboard laboratory, and steering signals are sent to the sled. ROVs are much less expensive than submersibles and can spend many more hours underwater. They can be deployed from much smaller research vessels than the ones that carry submersibles; they do not require the difficult, expensive, and time-consuming procedures and equipment needed to protect the lives of submersible crews and passengers; and they can allow several scientists at the same time, rather than the one or two in a submersible, to view the seafloor and to direct sampling activities.

The sampling and measurement operations performed with submersibles involve remote manipulation of robot arms and other equipment located outside the submersible. Because nearly identical equipment can be mounted on ROV sleds, they can perform the same tasks as submersibles, but they are operated remotely by a pilot sitting comfortably in front of a video monitor in a ship’s laboratory. In fact, if steerable video cameras are mounted on the ROV, the shipboard ROV operator has a better view of the environment than a submersible operator has, because the submersible operator’s vision is often restricted to several small, fixed window ports. In addition, an ROV enables several scientists of different disciplines to take part in each “dive” and to participate, either from the research vessel or remotely by video link, in deciding when and where samples should be collected. Although they are less glamorous than research submersibles, ROVs may eventually replace almost all submersibles for ocean science.

Perhaps the most famous, although scientifically not very useful, achievement of modern research submersibles was the successful exploration of the wreck of the Titanic and the recovery of some of its artifacts. What is not widely known is that the manned submersible dives to the Titanic were made possible by the work of an advanced ROV, the Argo. It performed the lengthy and difficult search for the wreck and the video exploration of the wreck and surrounding debris field. This preliminary work made it possible to send submersibles some months later to explore the wreck further and collect artifacts. Another ROV, Jason Jr., was used in the subsequent exploration of the Titanic by the submersible Alvin. Jason Jr. was attached to the Alvin rather than to a surface ship.

The Jason II/Medea system, developed since the exploration of the Titanic, is an example of sophisticated tandem ROV systems now used (Fig. 3-24). Medea is attached to a surface research vessel by a long cable. It carries lights, a video camera, and other instruments to survey the seafloor as it is positioned just above it. Medea can operate to depths of about 6500 m. Jason II is an ROV attached to Medea by a cable 35 m long. Once Medea has located an interesting area of seafloor, Jason II can be sent out to make a much more detailed survey of the area surrounding Medea’s location. Jason II can be maneuvered very precisely because it is not connected directly to the cable attached to the ship above. Consequently, Jason II is isolated from the sometimes substantial ship motions that are partially transferred down the cable to Medea, which enables Jason II to more easily locate and collect samples on the seafloor. The Jason II/Medea system can remain at work continuously on the seafloor for a week or more. Such systems can locate or relocate themselves on the seafloor with a precision of a few meters by analysis of the travel times of sound pulses transmitted from the surface vessel, and from transponders mounted on the vehicles and anchored to the seafloor in a triangle surrounding the study area.

Figure 3-24. (a) The Jason II/Medea ROV system. The many motors, instruments, sampling arms, lights, and cameras that are mounted on Jason II and on Medea are operated remotely from a surface vessel. Jason II is connected to Medea by a 35-m-long neutrally buoyant cable and is thus isolated from any ship motions that are transmitted down the cable to Medea. (b) A typical ROV (Hercules) can do all the sampling tasks that can be performed by humans in a submersible but at a much lower cost and from smaller vessels.

Autonomous Floats and Gliders

Even ROVs may eventually become obsolete as the development of autonomous underwater vehicles (AUVs) is progressing rapidly. An AUV is a vehicle that moves through the ocean with no human occupants and with no direct connection to surface vessels or submersibles. It performs exploration and sampling tasks according to preprogrammed instructions or through limited communication of data and instructions to and from remote operators. The simplest and most widely used example of an AUV may be Argo (not the same as the ROV Argo mentioned earlier) (Fig. 3-25) and other similar floats. These are autonomous instrument packages programmed to alternately descend and ascend through the water column collecting data on the properties of water masses. They transmit their data to shore by radio each time they reach the surface. The floats are part of the Global Climate Observing System/Global Ocean Observing System (GCOS/GOOS), which now has about 4,000 of these floats deployed throughout the world’s oceans. Some of these floats are now being equipped with additional sensors to measure parameters such as dissolved oxygen concentration, pH, nitrate concentration, chlorophyll-a concentrations, suspended particle concentration, and ambient light levels. These additional sensors are not yet proven to be as accurate and durable as the basic CDT but are expected to yield useful data and to be improved over time.

Figure 3-25. An Argo float system is deployed from a research vessel to begin its long unattended journey through the ocean depths.

One particularly interesting development of AUVs is the development of gliders. These look somewhat similar to the unmanned aerial drones now used by the military (Fig. 3-26). The most interesting feature of some of these gliders is that they can generate their own power by diving repeatedly between the warm surface water and colder water below and exploiting this temperature difference to generate electricity. This gives gliders the potential for deployments of virtually unlimited length of time and enables them to cover very large distances in the oceans in a single deployment. To become more useful to science, future AUVs, including gliders, will require the development of sophisticated new subsea robotics, artificial intelligence, and other technologies for underwater observation, sampling, and communications that can be incorporated in the AUV.

Figure 3-26. A Slocum Glider about to be deployed from a research vessel. Basic gliders can be small and easily deployed, while larger gliders can support longer missions and more complex instrumentation.

Accurate navigation and position fixing of data collected by AUV is a major problem. While there are a number of sophisticated technologies, such as inertial navigation that are adapted for AUV use, none of these systems currently provides the high degree of accuracy of positioning required for many current and potential uses of AUVs, such as returning to an exact location to resample and determining precise location for regional scale bathymetric maps. However, a relatively new and expensive technology based on a permanent or semi-permanent array of sonar beacons placed on the ocean floor (in an array similar to that in Figure 3-18a) can provide very accurate positioning. To use such systems, AUVs must be equipped to sense or interrogate such beacon arrays. This technology is, at present, used primarily for military AUVs used by a small number of nations that have built sonar beacon arrays in their coastal operating areas. However, this technology is expected to become less expensive and to be used extensively for scientific study purposes in future.