History of Remote Sensing: A Landsat Image - Lecture Material - Completely Remote Sensing tutorial, GPS, and GIS - facegis.com
Remote Sensing of the Ocean Floor

About 70% of the Earth's surface is covered by water - mostly marine (oceanic). The shallowest water is adjacent to continents and islands; the deepest (down to 12000 meters; 37000 feet) is in trenches near subduction zones. The first illustration shows the main features on ocean floors:

Features on the ocean floor.

This is a topographic sketch map of the Pacific Ocean, the largest continuous expanse of submarine topographic which contains these features. Especially prominent are the trenches, the volcanic island chains, and the ridges cut by transverse faults:

Map of the Pacific Ocean floor.

Even prior to modern times it was well known that the ocean floor near the continents was teeming with life. Here are two examples of life forms attached to the sea floor in shallow seas.

Typical invertebrate marine life.
Life forms around a seamount.

But, despite some knowledge of the ocean floor in offshore environments, surprisingly little detail is known about the vast majority of the ocean floors across the globe. Only about 3% has been explored to any extent. Exploration is twofold: instruments such as cameras have imaged small areas on the floor or much larger areas by sending probing waves from transmitters to the floor and measuring the return signals, and direct observation by humans either in shallow waters (scuba divers) or deep waters (in submersible vessels). Most subaerial remote sensing devices that peer at the Earth, as from airplanes or satellites, do not work in water. Sonar (described below) is the customary remote sensing device; cameras (photographic; digital; television) also are used. And, direct sampling of the floor is possible by drilling into ocean beds from platforms or from ships;

Drilling from a ship.

The principal way in which the ocean floor is mapped and studied is through the use of acoustic waves. This diagram depicts the main techniques for using sound waves to map the floor, locate objects on the floor, and search for manmade and animal objects within the ocean.

Various acoustical techniques used to study the ocean floor.

The most widely used technique for ocean floor mapping is Sonar (acronym for Sound Navigation and Ranging). For an overview of the principles behind Sonar, consult the Wikipedia and LEI websites:Sonar Overview How Sonar Works.

The Sonar technique dates back to the 1930s and was much advanced by naval military needs in World War II. The principle behind Sonar is similar to that involved in radar and in making sonograms of the human body's organs. A sound pulse (either single frequency or multi-frequencies) is sent by a transmitter in a narrow direction or more spread out. The rate at which the pulse travels is generally well known for water; it varies with density and with temperature. The pulse can hit an object - ocean floor, a boat, or a school of fish - and is reflected from same as an echo. The reflection can move off in various directions but one such direction goes directly back to the transmitter source and is picked out by a receiver. Since travel times out and back are known for the water medium, the delay between outward pulse and return pulse gives a time that leads to the distance to the reflection object. This diagram shows the concept for a signal sent from a submarine;

Schematic showing operation of sonar.

When pulses are sent in various directions at once and their return signals are likewise received, the small differences in travel time translate (in modern systems with the aid of a computer) into indications of variations in the shape and distances of surfaces that are usually varied in form, such as those characteristic of topography. This diagram depicts a general case:

In one mode of data gathering, a Sonar device is towed behind a ship, as shown here:

Ship-towed sonar device.

Here is a close up showing one of the smaller Side Scan Sonar units as it is being lowered on cable from its ship:

Sonar equipment.

One of the most sophisticated Side Scan Sonar devices is GLORIA (GLORIA stands for Geological Long Range Inclined Asdic), mounted on the side of a ship:

The GLORIA 'fish'.

Sonar is frequently used by navies to search for, then examine, sunken ships. Here is a Sonar view of a submarine resting on the sea floor.

Sonar view of German submarine U-166.

Sonar has been used to look at ships lost at sea. Two merchant vessels, the Frank Palmer and the Louise Craig, collided in December of 1962 and both sank. Here is a Sonar image of the two resting side by side.

Sonar image of two merchant vessels on the sea floor.

Parts of the sea floor have been mapped using Sonar to obtain images that display the topography. Three examples appear here; their captions provide explanatory information:

The Taney seamounts near San Francisco.
A mosaic of the seafloor made from multiple sonar images.
A part of the Mid-Atlantic Ridge mapped by sonar.

Here are two maps of the seafloor off the Australian coast that show the end product of a Sonar scan of the topography:

As mentioned above, Sonar can detect objects in the water above the ocean floor. A common use is to locate schools of fish. Here is a sequence of images that show how one such school was monitored as it grew in numbers of individual herring:

Growth of a school of herring.

The size of a Sonar unit has now diminished to one that can fit in a hand. These fishing Sonars sell for about $60 and thus are affordable for the average sports fisherman.

A portable handheld Sonar unit.

One might think that underwater cameras or televisions would be useful in obtaining images of the sea floor. They have been used but suffer from one severe limitation. Most of the ocean water below a few hundred meters is totally dark; illumination associated with the camera would be restricted to a small area. Thus most photography and image making is done from light provided by a submersible craft.

Submersibles, some unmanned and others with one to several humans, have been lowered into the ocean on cables but a few have been operated as "free" craft that control their movements including return to the mother ship. Submersibles have been employed for sea-oriented exploration since the end of World War II. They can be operated much like submarines, from whose technology they sprang. Most are small and have been likened to midget submarines.

Some of the submersibles mounted on cables are called bathyspheres (although some are spherical, today they normally are elongate). The first dive made from a manned bathysphere was to a depth of about 300 meters off Bermuda. Its occupant was the famed oceanographer William Beebe.

William Beebe inside his bathysphere.

Here is a cutaway schematic of a typical bathysphere:

Schematic diagram of a bathysphere.

Two oceanographers associated with bathyspheres are the brothers Auguste and Jacques Picard.

Some unmanned submersibles are simply open platform frames within which are the instruments that make the observations and measurements. These ROVs (Remotely Operational Vehicle) are lowered on cables to the sea floor to observe in place. These are then raised off the floor and moved to other locations. Examples of these ROVs are NOAA's Hercules and Argus, which sometimes were lowered together but performed different functions:

The Hercules ROV.
Argus 1.

True midget subs used in oceanographic studies were first deployed in the 1960s. The best known of these is Alvin, first used in 1964. It takes its name from its designer, Allyn Vine.

The submersible Alvin.

Made of structural strong and re-enforced metals, these submersibles were usually employed to make truly deep dives. They became the main vehicles for exploring below 3000 meters. The deepest dive on record was to the bottom of the Marianas Trench in the western Pacific. A depth of 37000 feet was attained by the Trieste on January 23, 1960, with Jacques Picard onboard.

The Trieste.
Schematic of the Trieste.

One of the most intrepid and versatile of the deep sea explorers is Dr. Robert Ballard. He has made many dives in manned submersibles and has made major discoveries with unmanned submersibles. His fame was firmed up when he discovered the wreckage of the Titanic, which sank on April 15, 1912 during its maiden voyage after striking an iceberg about 523 km east of Newfoundland. The ship lies at a depth of 3780 m; 12600 feet. Artifacts have since been recovered from the vessel. (This notorious maritime incident is immortalized in two great movies, both entitled "Titanic".)

The railing of the Titanic.
Overview of the Titanic.

Ballard has discovered many wrecks including cargo sailing ships hundreds of years old, even older boats lost in Roman times, and some modern vessels (the German battleship Bismark, for instance). This is a view of the U.S.S. Yorktown lost during the battle of Midway in 1942 in the Pacific in which 4 Japanese carriers were sunk in what many consider the greatest sea battle in history.

The U.S.S. Yorktown.

Ballard has financed his oceanographic studies in part through these spectacular underseas finds. But his main goals have always been scientific knowledge. He is one of the first to have discovered 'black smokers' - vent structures on mid-oceanic ridges in which heated waters build up pillars from precipitation of dissolved chemicals. Here are two view of black smokers, followed by a diagram that shows their nature in detail:

A black smoker.
Another black smoker.
The chemistry of a black smoker.

The black consists in part of manganese oxides, but many other metal elements are enriched in the particulates that make up the "smoke". Manganese and other elements also form away from the ridges as nodules produced by precipitation of dissolved elements in seawater. These nodules occur in huge clusters in ocean beds and represent a major source of valuable metal elements (Mn, Ni, Co, V, Cr, others). Schemes to mine these nodules by sucking them up have been proposed and demonstrations of feasibility implemented.

Manganese nodules on the seafloor.

But the discovery of black smokers led to an associated discovery that has major implications for the origin of life. Tube worms and other creatures are encrusted on and near the smokers:

Tube worms.

These animals develop at great depths where sunlight never reaches. Unlike most animals they do not utilize sunlight directly or indirectly to maintain life. They derive their energy from the heat associated with the smoker fluids. One hypothesis, based on this observation, is that life may have originated in ocean deeps where thermal energy brought about its synthesis.

Oceanography as a science has matured in the 20th century and remains a major field of research as the still largely unexplored ocean floors are being mapped, sampled, and documented visually. The variety of methods involved in the study range from emplacement of instrument as platforms on the floors, such as the geophysical seismometers shown in the next illustration, to voyages of research vessels that have their own onboard instruments and can lower submersibles.

Geophysical instrumentation on the seafloor.

One of the best known oceanographers who has popularized the study of the oceans is Jacque-Yves Costeau whose famous boat, the Calypso 2 (a converted World War II minesweeper), is shown here:

The Calypso 2.

Nowadays individuals without any scientific background can enjoy exploring the ocean floor. In the last half century scuba diving has opened up the sea bottom in shallow waters off islands and continents. Submersible cameras allow photography to document sea life and other features:

Scuba diver with camera.

We will encounter illustrations that deal with the surfaces of the oceans in several Sections of this Tutorial. Treatment of remote sensing of the marine surface is assigned it own pages in Section 14, pages 14-11 through 14-14.

Source: http://rst.gsfc.nasa.gov