3.1: Difficulties of Studying the Ocean Environment

Why did it take so long to discover the most fundamental secrets of the oceans? Why did we not know of the existence of the immense mountain chains passing through all the oceans until we were already looking beyond the Earth and launching satellites and humans into space? The answer lies in the hostility of the oceans to oceanographers and to their instruments. In many ways, the ocean depths are more difficult to explore than the surface of the moon or Mars.

A principal focus of oceanographers in developing techniques and instruments to study the oceans has always been, and still is, to overcome the many problems unique to the oceans. The most significant of these problems are the following:

• Visiting the ocean depths is difficult because we cannot breathe in water
• Water absorbs light and other electromagnetic radiation, such as radar and radio waves, severely limiting their use for remote sensing in the oceans
• The oceans are extremely deep
• Pressure in the ocean depths is extremely high
• Seawater is corrosive
• The sea surface is dynamic

“Seeing” through Ocean Water

Compared to the atmosphere, water is a much more efficient absorber of electromagnetic radiation, including radio and radar waves and ultraviolet, infrared, and visible light. In all but the shallowest areas, the seafloor cannot be seen by the naked eye or with any type of optical telescope. Even in the clearest ocean water, we see at best a distorted image of the seafloor, and only where the maximum depth is a few tens of meters at most. Because we cannot see the seafloor, mapping the ocean floor was more difficult than mapping the surface of the moon. Only in the 1920s did oceanographers discover that sound waves could be used as their “eyes” to see the seafloor. Oceanographers also discovered that the magnetic and gravity fields of the seafloor could be sensed through the depths of ocean water. Even so, our ability to study the deep ocean is still limited by its lack of transparency to electromagnetic radiation. For example, radar and other instruments carried on satellites can produce extraordinarily detailed maps of the planet’s land surface in a matter of days, but they cannot map the seafloor directly because most electromagnetic radiation cannot penetrate the depths of the oceans. However, satellite sensors can map the seafloor topography indirectly by making very precise measurements of sea surface height. Satellite instruments can also be used to produce excellent maps of ocean surface features, including wave patterns, sea surface temperatures, and the abundance of photosynthetic life in the near-surface waters.

Inaccessibility

The average depth of the oceans is 3800 m, and the greatest depth is 11,040 m. These depths are farther below sea level than the average and greatest elevations of the land are above sea level. The average land elevation is 840 m, and the maximum elevation, at Mount Everest, is 8848 m. Most of the ocean floor is as remote from sea level as the highest mountain peaks are. Put another way, most commercial airplanes fly roughly as high above the land as the deepest parts of the ocean are below the sea surface.

Until the recent development of autonomous underwater vehicles, oceanographers had to lower instruments or samplers, usually on a wire, and then haul them back up to the ship to take a sample of the deep-ocean waters or sediment. Because the oceans are so deep, the process of lowering an instrument or sampler, probing or sampling the water column or seafloor, and retrieving the instrument is extremely time-consuming. Getting a single sample of mud or bottom water at one place on the ocean floor usually takes many hours. In contrast, a scientist studying the land can collect many samples of rock, soil, plants, and animals much more efficiently. AUVs can now perform some of these sampling missions, but they too take a lot of time to travel from surface to seafloor and back.

Research vessels, most of which travel at only about 20 km•h^–1, consume large amounts of time and fuel going to and returning from sampling locations far from land. Until the development of the satellite-based Global Positioning System (GPS) for civilian use in the 1980s navigation far from land was difficult. Determining the location of a specific sampling site, or its relocation to resample, was much more difficult than on land. Because research vessels operating in the open ocean can cost tens of thousands of dollars a day to operate, the large amount of time needed to sample the deep oceans means that few samples can be collected during any oceanographic cruise and that each sample is very expensive to obtain. Therefore, samples of the seafloor, oceanic waters, and organisms living in the oceans have been obtained only at intervals of tens of kilometers throughout most areas of the oceans, especially the deep oceans. The advent of AUVs lowers the cost and allows much greater sampling frequency, especially sampling of some water properties, and drastically lowers costs. 

Pressure

The pressure of the atmosphere at sea level is about 1.03 kg•cm^–2, or 1 atmosphere (atm). On a journey to space, a space capsule is subject to a 1-atm pressure change because the atmospheric pressure in outer space is effectively zero. Therefore, manned spacecraft must have hulls that can withstand a 1-atm pressure difference. Because most electronic equipment can operate without any problem at zero atmospheric pressure, unmanned satellites need no protection against pressure differences. In contrast, on a journey into the oceans, the pressure increases by 1.03 kg•cm^–2 (or an additional 1 atm) for each 10 m of depth. Hence, the pressure at 100 m is 11 times as high as the pressure at sea level (1 atm of air pressure plus 10 atm of water pressure).

In the deepest part of the oceans, at 11,000 m, the pressure is a truly astounding 1101 times as high as atmospheric pressure, or over 1100 kg•cm^–2, which is more than a tonne of pressure per square centimeter. Therefore, manned submersibles designed to dive to the greatest depths of the oceans need hulls capable of withstanding a greater than 1000-atm pressure difference. Most submersibles are not designed to dive that deep, but even shallow dives to 1000 m require hulls that can withstand a greater than 100-atm pressure difference. Submarines and submersibles must have hulls of thick metal, and viewing ports of thick, durable glass or plastic. Deep-diving manned submersibles must be massive, even when made of strong, light materials, such as titanium. In addition, submersible hulls must withstand the metal-fatiguing stresses of repetitive pressurization and depressurization. These requirements make deep-diving submersibles almost prohibitively expensive. 

Conductivity, Corrosion, and Fouling

Seawater poses a problem for unmanned instrument packages because most of these rely on electrical components. Such components will not work if immersed in seawater, because seawater conducts electricity and causes short-circuiting. Oceanographic instruments must be placed inside watertight containers called “housings” that must be able to withstand oceanic pressures, because the interiors of the housings normally remain at atmospheric pressure.

Seawater is extremely corrosive, as divers and other water sports enthusiasts quickly discover when they forget to wash their equipment with freshwater after use. Therefore, all wires, cables, sampling devices, and instrument housings must be protected. Iron and most steels corrode quickly in seawater, so special marine-grade steel or other materials must be used to minimize corrosion. These materials were not available to early oceanographers, who used more expensive and heavier materials, such as brass and bronze. Steel is still the best material available for wires to lower and raise most instrument packages or samples. Even so, the most corrosion-resistant steel wires must usually be further protected by a coating of grease or plastic. Most measurements of trace metals and organic compounds dissolved in seawater were useless until the past several decades because of contaminants from corroding wire and metal sampler parts, and from the grease.

In addition to corrosion problems, a variety of marine organisms foul instruments that are left in the ocean to record data for days, weeks, or months, as is necessary for some studies. Some marine organisms, such as barnacles, quickly adhere to and colonize the surface of virtually any solid material. Instruments that rely on freely moving parts or on a clean surface-to-seawater contact can quickly be rendered inoperable by such biological fouling.

Wave Motion

Perhaps the most obvious difficulty faced by oceanographers is that the ocean surface is dynamic, and research vessels therefore cannot provide a stable platform on which to work, especially in bad weather. The perils of working on a rolling and pitching vessel are many. First and foremost, oceanographers must battle seasickness. In addition, they suffer mental and physical fatigue and disorientation caused by working long hours at odd times of day on an unstable platform. (Many shipboard research activities are continuous 24 hours a day due to the cost of operating research vessels.) Oceanographers treasure the rare days of calm seas. Satellites now allow some scientists to remain on land and direct scientific studies remotely using fast data and video communications. However, except for those limited tasks that can now be performed by autonomous vehicles, vessels must still be manned by skilled technicians and crews. Besides the personal hardships, there are many dangers and difficulties associated with the deployment and retrieval of often extremely heavy instrument packages over the side of a research vessel. The sight of heavy equipment swinging wildly on a wire from a crane over the deck of a ship when seas are rough is indeed frightening. Hanging over the side of a ship in a storm to clamp instruments that must be attached at certain intervals to a heaving wire is an experience few people would relish.

Less obvious than seasickness and the perils of equipment deployment and retrieval, but just as difficult, are the problems associated with using scientific instruments and performing scientific experiments in shipboard laboratories. Most scientific instruments are delicate and made to be used in a normal vibration-free and motion-free laboratory environment. The lurching, pounding, and vibrating to which such equipment is subjected at sea quickly expose any weaknesses. Equipment must be specially designed or modified to operate reliably at sea. In addition, all equipment must be clamped or tied down in bad weather.

Logistics

A profusion of other, lesser problems is associated with studying the oceans. For example, on a research vessel hundreds of miles, and therefore days and tens of thousands of dollars, away from port, broken equipment cannot be taken to a repair shop, a technician cannot be called in, and spare parts cannot be picked up at a store. Oceanographers and research vessel crews have become skilled and ingenious at using available materials to fix equipment at sea. Nevertheless, even the greatest ingenuity sometimes fails, and research efforts must be postponed until the next cruise to the appropriate location, which may be several years later. Such postponements can also be caused by bad weather that slows or prevents work at sea, although most ocean research cruises are planned to allow some leeway for bad-weather days.

In the rest of this chapter, just a few of the many and varied techniques, instruments, and samplers used by oceanographers of yesteryear and today are briefly reviewed. As you study this material, keep in mind the difficulties of working in and on the oceans.