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Historical C&GS | Nautical Charting | Soundings

SOUNDING POLE TO SEA BEAM

Captain Albert E. Theberge, NOAA Corps (ret.)

Published in: Technical Papers 1989 ASPRS/ACSM Annual Convention

Surveying and Cartography Volume 5, 1989. Pp. 334-346.

NOAA Central Library Call No. TA501. A638 1989 Vol. 5.

ABSTRACT

Man's ability to determine the character of the seafloor has been evolving for over 3500 years. Depictions of Egyptians using sounding poles and line and sinker sounding methods date to 1800 B.C. These hand-manipulated methods allowed plumbing the depths to ten's of fathoms. By the first century B.C., this method was sufficiently advanced that a depth of over one mile was measured in the Mediterranean. Although various investigators attempted to make deep-sea soundings, it was not until the 19th century that the advent of mechanical sounding systems made deep-sea soundings feasible on a systematic basis. The electronic echo-sounder was developed in the early 1920's and in its various versions it has obtained soundings over millions of miles of survey lines beginning with early investigations off the California coast. In the 1960's, a revolution in our ability to depict the seafloor began with the advent of multi-beam swath sounding systems. Today's bathymetrist has the ability to obtain unprecedented resolution and coverage of seafloor features with these hull-mounted systems.

SOUNDING POLE AND LEADLINE The Classical Period

The first evidence that man was measuring the depths is found in tomb paintings of ancient Egypt dating from 1800 B.C. The bas-relief carvings of Deir al-Bahri were ordered by Queen Hatshepsut to commemorate a voyage to the land of Punt in approximately 1500 B.C. One of these carvings shows a man using a long slender pole as a sounding pole on the bow of a large vessel under oar and sail (Soule 1976). There are also paintings of men taking deeper soundings by means of a weight attached to a line dating from this period.

However, the first written account of a line with a weight attached for sounding did not occur for another millennium. The Greek historian Herodotus writes of a sounding in 66 feet of water, far offshore from the mouth of the Nile River, bringing up yellow mud similar to that deposited on land by the annual flood of that great river. This brief passage shows that hydrographic knowledge had evolved to an awareness of regional depths as well as seafloor characteristics by the 4th century B.C. About 100 B.C., Posidonious lowered a line into the Mediterranean somewhere between Rome and present day Sardinia. Strabo, a Greek geographer is quoted: "The sea of Sardinia, than which a deeper sea has never been sounded, measuring, as it does, according to Posidonius, about 1,000 fathoms (Soule 1976)." This isolated incident marks the only successful recorded deep sea sounding in the entire history of mankind up to that time and remained so for the next 1900 years.

Approximately 150 years later Acts 27, Verses 27-44 of the New Testament recounts the shipwreck of Paul on the island of Malta:

"....as we were drifting across the sea of A'dria, about midnight the sailors suspected that they were nearing land. 28 So they sounded and found twenty fathoms; a little farther on they sounded again and found fifteen fathoms. 29 And fearing that we might run on the rocks, they let out four anchors from the stern...."

It appears that by the First Century A.D. soundings were taken regularly while in proximity to the shore. This is also perhaps the first recorded sounding line. Continuing Acts 27:

...39 Now when it was day, they did not recognize the land, but they noticed a bay with a beach, on which they planned if possible to bring the ship ashore... then hoisting the foresail to the wind they made for the beach. 41 But striking a shoal they ran the vessel aground; the bow stuck and remained immovable, and the stern was broken up by the surf...

"That they did not recognize the land" shows that early sailors relied on the memory of their pilot to effect safe trips. Knowledge of local coastal configuration resided nowhere but in a pilot's memory although sailing directions called a periplus did exist by the First Century A.D. giving general coastal configurations. Depth information was limited to personal knowledge of a given area or what the cast of the lead showed as a ship approached shore.

Going to the fourth century A.D. there was a famous Indian pilot named Suparaga who "knew the course of the stars and could always readily orient himself; he also had a deep knowledge of the value of signs, whether regular, accidental, or abnormal, of good and bad weather. He distinguished the regions of the ocean by the fish, by the color of the water, by the nature of the bottom, by the birds, the mountains [land marks] and other indications" (Needham 1971). It would seem that the ways of the sailor in the East where the same as those in the West.

The Awakening

For the next thousand years little changed for the mariners of the world with respect to depth-finding, navigation, and charting. Technology slowly moved ahead with the introduction of the magnetic compass in the 12th century to western seafarers, the development of the portolano pilot guides with the accompanying portolano charts in the thirteenth century, and improvements to navigational instruments. In the early fifteenth century, Prince Henry the Navigator founded the first school for navigation at Sagres, Portugal, on the southwest promontory of Europe. This resulted in the development of the lateen-rigged caravelles (a new type of vessel which allowed sailing closer to the wind than the old square-rigged vessels) which literally explored the world and fostered many other new developments in navigation and technology. Most importantly, Prince Henry brought scientific methodology and the concept of systematic mapping and exploration to the trades of the mariner and chartmaker.

Africa was rounded and the Americas discovered but the mariner`s primary sounding tool remained the leadline. Routers or rutters of the sea (similar to today's Coast Pilots or sailing guides), such as the famous Hastings Manuscript of the late 15th Century, were much improved in quality from the early portolanos and by 1584 Lucas Jans Waghenaer of Holland had published his famous atlas Spiegel der Zeevaert which included some of the earliest charts showing actual depths. His name was commemorated by the term "waggoner" which was synonymous with pilot guide for centuries.

The state of knowledge of the coastal waters of Western Europe at this time is epitomized by the account of Master Jackman on Sir Martin Frobisher's return voyage to England in 1578. Using an armed lead (these have a socket filled with wax or tallow to pick up bottom samples), he 'sounded and had 70 faddems, oosy sand, whereby we judged us to be northwards of Scilly, and afterward sailed south east all that night'. The ship rounded Lands End safely, and in three days 'had sight of the Start, 5 leags off, God be praysed!" (Morison 1971).

During this time many rutters were being published such as William Bourne's, who wrote in 1574 "Also it behoueth him to be a good coaster, that is to say, to knowe every place by the sight thereof" (Morison 1971). Rutters also included information (sometimes ad nauseam) on the character of the bottom. A typical passage in the earliest English sailing directions refer to "sandy wose and black fishey stonys .... redd sande and black stonys and white shellis .... grete stremy grounde with white shellis ... the grounde is redd sonde and white shellis amonge .... the grounde is white sonde and white shellis". That there were fine arguments as to which white shellis were which as suggested in Survey of the Seas (Blewitt 1957) is corroborated by an account of the "Ship Hopewell from Newfoundland bound for London in August 1587 `drawing neere the coast of England' sounded and found seventy fathom, but nobody could agree on interpreting what the lead brought up; so through `evil marinership were fain to dance the hay foure days together' running northeast, southeast, east, and east-northeast" (to dance the hay refers to a folk dance in which the participants moved in circles) until finally sighting a known point on land (Morison 1971).

Although sounding technology did not significantly improve, advances in surveying and charting the depths continued over the next two hundred years. In 1647 Robert Dudley's atlas, 'Dell Arcano del Mare (Secrets of the Sea), was published posthumously. This work was well ahead of its time with all of its charts being constructed with the Mercator projection as well as being the first charts with printed depths on the east coast of North America. Numerous navigation instruments were designed and with the development of the chronometer, longitude was now within the grasp of the surveyor and navigator. The principles of triangulation were being applied to the problem of surveying at sea beginning in 1747 with Murdoch MacKenzie (Senior) working on the coast of the British Isles. James Cook independently discovered this method and used triangulation extensively for land control in his great survey of Newfoundland. Indeed, Cook's work is considered to be the first scientific large-scale hydrographic survey ever to be carried out (Blewitt 1957).

Interest in deep-sea soundings began during this period. On September 4, 1773, Captain Constantine John Phipps lowered a line with a 150-pound weight attached from H.M.S. RACEHORSE into the Norwegian Sea. He sounded and found 683 fathoms with a very fine blue soft clay bottom (Deacon 1962). It is noted that Ferdinand Magellan attempted to sound off the Unfortunate Islands (present-day Puka-Puka in the Tuamotu Archipelago) on January 24, 1521, and found no bottom (Morison 1978). Various accounts state that he put out anywhere from 200 to 750 fathoms of line before giving up.

Improved Mechanical Sounding Devices

Following Phipps' sounding, the next milestones in deep-ocean sounding occurred six decades later. Charles Wilkes, while leading the United States Exploring Expedition (1838-1842), was the first to attempt using wire as opposed to heavy rope for sounding. Because Wilkes used copper wire, the tendency of the line to break, kink, and snarl far overcame any speed of sounding increase. As a consequence Wilkes gave up, although his first sounding on the Antarctic shelf in 320 fathoms was with copper wire (Stanton 1975). In 1840 Sir James Clark Ross conducted the first open ocean deep-water sounding in 2425 fathoms in the South Atlantic at Latitude 27o 26'S, Longitude 17o 29'W (Deacon 1962). Deep-ocean sounding was given added impetus by the desire to lay a Trans-Atlantic cable. By the mid 1850's sufficient depth information had been acquired that Lieutenant Matthew Fontaine Maury, the "Pathfinder of the Seas", was able to publish the first bathymetric map of the North Atlantic Ocean basin. However, many of the soundings on this map were erroneous and, coupled with the paucity of data, led to missing of some major features and the delineation of some features that did not exist.

Mechanical sounding instruments took a leap forward in 1872 with the invention of a sounding machine using small diameter pianoforte wire. This machine was introduced by Sir William Thomson (later Lord Kelvin). In 1872, Sir William made a test of this instrument from his private yacht the LALLA ROOKH, and described this first successful wireline sounding: "When from two thousand to twenty-five hundred fathoms were running off the wheel, I began to have some misgivings of my estimation of weight and application of resistance to the sounding-wheel. But after a minute or two more during which I was feeling more and more anxious, the wheel suddenly stopped revolving, as I had expected it to do a good deal sooner. The impression on the men engaged was that something had broken, and nobody on board, except myself had, I believe, the slightest faith that the bottom had been reached.... until the brass tube with valve was unscrewed from the sinker and showed an abundant specimen of soft gray ooze.... That one trial was quite enough to show that the difficulties which had seemed to make the idea of sounding by wire a mere impracticable piece of theory have been altogether got over" (Agassiz 1888).

Sir William Thomson also invented the pressure tube method of sounding which became known as "self-acting sounding" (U.S. Naval Hydrographic Office 1962). This method allowed the ship to continue steaming while dropping the sounding tube over the side. It used a long sinker equipped with tubes lined with silver chromate. The compression of the air in the tubes indicated by the white line of the silver chromate, was a measure of water depth. By 1888 the steamer BRITANNIC had sounded using this method in one hundred and thirty fathoms over the Newfoundland Banks while cruising at sixteen knots (Agassiz 1888).

Although Sir William invented the pianoforte wireline sounding instrument and provided the great CHALLENGER expedition with his instrument, the British naturalists chose to use the Baillie sounding machine, a fibre-line machine using hemp No.1 line weighing 200 pounds to the nautical mile, for the 363 soundings taken during the four-year expedition. It remained to the Americans to perfect the use of wire sounding (Tanner 1897).

The U.S.S. TUSCARORA, sailing a few months after the CHALLENGER, undertook a cable survey from California to Japan. At the direction of Commodore Ammen, Chief of the Bureau of Navigation, the TUSCARORA was outfitted with a Thomson machine. Captain George E. Belknap, commanding officer of the TUSCARORA, successfully used the wire sounding machine for this survey. After this success, a Thomson machine was installed on the Coast Survey Steamer BLAKE which was used to delineate the Gulf of Mexico, much of our Atlantic continental shelf and slope, and much of the Caribbean. Lieutenant Commander Charles D. Sigsbee significantly improved and modified the Thomson machine (sufficiently to have the machine christened the Sigsbee sounding machine) and also directed the survey of the Gulf of Mexico, which resulted in the first truly modern bathymetric map.

One other notable name in deep-sea sounding during this period is that of Commander Zera Luther Tanner, USN. He was commanding officer of the United States Fish Commission Steamer ALBATROSS for eleven and a half years. During this period, the ALBATROSS worked from the U.S. East Coast to the Bering Sea doing fisheries surveys, deep sea dredging, and cable surveys such as one from California to Hawaii. Commander Tanner invented a shallow water wire sounding machine called the Tanner sounding machine and many other oceanographic instruments.

Many variations of the wireline sounding machine were developed over the next 50 years, most notably the Lucas Sounding Machine which was primarily used on British ships and the LL type and Deep-sea sounding machines used by the Coast and Geodetic Survey (C&GS). The wireline machines delineated the major features of the ocean basins of the world including many of the trenches, rises, and continental slopes we know today. Notable successes included: further delineation of Maury's "Telegraphic Plateau"; the surveys of the Caribbean Sea and Gulf of Mexico by the BLAKE and ALBATROSS; valid soundings of 9636 meters by the U.S.S. NERO in the Challenger Deep (Mariana Trench); 8525 meters in the Nares Deep (taken in Puerto Rico Trench) taken by the U.S.S. DOLPHIN in 1902; and 8513 meters in the Tuscarora Deep (Kurile Trench) taken by the U.S.S. TUSCARORA in 1874. As impressive as these successes were, it is well to remember that Sir John Murray, one of the outstanding oceanographers of the late Nineteenth and early Twentieth centuries, compiled only 5969 soundings in depths greater than 1000 fathoms by 1912 (Murray 1912). Deep ocean sounding by mechanical means remained a painfully slow process. Fortunately for the oceanographic community, a new method was being developed.

ECHO SOUNDING

Early Sound Methods

Aristotle was one of the first, if not the first, to recognize that sound could be heard in water as well as air. Two millennia later, Leonardo da Vinci observed that by placing a long tube in the water and the other end to the ear one could hear ships from afar.  Francis Bacon was another early observer who discovered that sound can travel through water.

Beginning in the mid-eighteenth century, scientists began experimenting with sound in water. In 1807, Dominique Francois Jean Arago made the specific suggestion that water depths might be measured by sound propagation, although, unfortunately he did not act on this proposal (Adams 1942). In 1826 Daniel Colladon and Charles Strum made measurements of the speed of sound in Lake Geneva that averaged 1435 meters per second and reported on the work of Francois Sulpice Buedant, who measured an average sound velocity of 1500 meters per second in the sea off Marseilles in 1820 (Hersey 1977). In 1859, Matthew Fontaine Maury wrote of various methods and suggestions for using sound to measure ocean depth such as "exploding petards, or ringing bells" and even timing the descent of an explosive-rigged harpoon and then listening for the one way signature of the explosion after impact (Maury 1859). Those methods tried did not work, most probably because the listening device was employed above the water-air interface instead of following da Vinci's earlier observations.

For the fifty years following Maury, most sound work was devoted to horizontal propagation including such innovations as ship-to-ship signaling, shore-to-ship signaling, and the detection of objects in the water (icebergs, in response to the TITANIC disaster and submarines in WWI).

In 1901, the Submarine Signal Company was formed and provided underwater signaling devices to the United States Lighthouse Service. In 1910, the brilliant Reginald Fessenden joined the company. He invented an oscillator in 1911 that he steadily improved. Within a few years, his massive 250kg transceiver went to sea on the U.S. Coast Guard Cutter MIAMI, and on April 27, 1914 he was able to detect an iceberg over 20km away. While conducting this experiment, Fessenden, who was quite seasick, and his co-workers, Robert F. Blake and William Gunn, serendipitously noted an echo that returned about two seconds after the outgoing pulse. This turned out to be a return from the bottom. "Thus, on just one cruise.... Fessenden demonstrated that both horizontal and vertical echoes could be generated within the sea..." (Bates et al. 1987). This breakthrough paved the way for rapid advances in sounding technology over the next few years (as well as in submarine detection).

Paul Langevin, a French physicist, and Constantin Chilowsky, a Russian electrical engineer in Switzerland, collaborated to produce transceivers using the piezo-electric effect of quartz crystals. By mid-1916 the British, under Robert Boyle, were also working on the problem of ultrasonics and developed quartz oscillators for use primarily in submarine warfare. In 1919, the French obtained sonic soundings in 60 meters depth from an underway vessel at 10 knots. They followed this success with 4000 meter echo soundings from the cable ship CHARENTE in the Bay of Biscay. In 1920 the French Centre d'Etudes de Toulon ran the first line of higher frequency soundings (outside audible range). In 1922, the French surveyed a cable route from Marseilles to Philippeville, Algeria, which is claimed to be the first practical application of echo sounding. 1922 also saw the United States Navy installing Dr. Harvey Hayes' Sonic Depth Finder on the U.S.S. STEWART which sounded in the Atlantic and Mediterranean. The U.S.S. CORRY and U.S.S. HULL also were equipped with a Hayes Sonic Depth Finder at this time and produced the first bathymetric map based solely on echo sounding. This map covered the area of what is now known as the Southern California Continental Borderland (Nelson 1982).

Sound in the Coast and Geodetic Survey

At this time many surveying organizations were adopting sonic sounding devices. For brevity, the history of the Coast and Geodetic Survey and its descendant organizations will be emphasized in this section. In 1923 a Hayes Sonic Depth Finder was installed on the Coast and Geodetic Survey Ship GUIDE and was first used to take deep water soundings during a voyage from Norfolk, Virginia to San Diego, California. An operator with earphones listening for the return signal transmitted a sound signal through the water at the precise instant the return echo was heard. The operator varied the interval between transmit and receive until both the echo and transmit pulse were heard simultaneously. A dial on a variable speed mechanism manipulated by the operator served to indicate the depth. The following year a similar instrument was installed on the USC&GS Ship PIONEER.

Because of varying skill levels of the operators, inability to sound in less than 100 fathoms, and inherent instrumental errors, the Sonic Depth Finder was inadequate for precision surveying needs. In answer to the need for a more accurate depth registering device, Dr. Herbert Grove Dorsey, who later joined the C&GS, devised a visual indicating device for measuring relatively short time intervals and by which shoal and deep depths could be registered. In 1925, the C&GS obtained the very first Fathometer, designed and built by the Submarine Signal Company. (Fathometer is now a product name for Raytheon Corp.) This was the 312 Fathometer which was used primarily for deep-water soundings. With this system, depths were read by noting the position of a continuously rotating white light at the instant the echo was heard in the operator's headphone. This method was replaced by the red-light method which utilized a rotating neon tube that flashed adjacent to the depth scale at the arrival time of the echo. Although the French had devised a paper recording device in 1919 and the Europeans had been using such paper copy devices for years, it was not until the late 1939 that the C&GS installed a Hughes-Veslekari graphic recording device on the ship OCEANOGRAPHER, followed a year later by installation on the EXPLORER.

Concurrent with improvements in recording devices were improvements in the sound projectors and receivers used for echo sounding. Early devices tended to use acoustic waves in the audible human range and were described as sonic. Later models increased frequency past audible range and were termed supersonic, or ultrasonic. The transmitting units evolved from hammer or striker units to electromagnetic, magnetostrictive, or piezoelectric. For early sonic-type receivers either a carbon button or electromagnetic-type element was employed while for supersonic frequencies, echoes were detected on magneto-strictive or piezoelectric receivers.

Following the introduction of the 312 Fathometer, C&GS either built in-house or procured a number of sounding instruments. The 412 Fathometer which was installed on the NATOMA and other vessels in 1928, was a striker type instrument which proved unreliable. The Dorsey Fathometer No.1 for shoal water work was installed on the LYDONIA in 1934; the Dorsey Fathometer No. 2 for depths greater than 20 fathoms was first installed on the OCEANOGRAPHER and HYDROGRAPHER in 1937; and the Dorsey Fathometer No. 3, which was an all-depth instrument was first installed on the WESTDAHL in 1938. The Dorsey Fathometer No. 3 became the C&GS standard by 1941. In 1940, the 808 Fathometer, which was portable and equipped with a graphic recording device, became the C&GS standard on launches and small boats (Adams 1942).

The New Understanding In 1939 A.C. Veatch and P.A. Smith of the United States Coast and Geodetic Survey published Geological Society of America Special Papers Number 7 ATLANTIC SUBMARINE VALLEYS OF THE UNITED STATES AND THE CONGO VALLEY. This paper was a milestone in the explosion of investigation and understanding of the nature of the seafloor that has continued to the present day. Although other investigators were aware that the seafloor was not a smooth featureless plain, this paper brought that knowledge into the collective consciousness of oceanographers and earth scientists worldwide. No longer was the world constrained to the view of Alexander Agassiz that "The monotony, dreariness, and desolation of the deeper parts of this submarine scenery can scarcely be realized".

Prior to the Veatch and Smith paper, notable advances had been made beginning with the Mid-Atlantic Ridge studies in the South Atlantic by the German research vessel METEOR and the early work of Francis P. Shepard, the "Father of Marine Geology", on submarine canyons. By 1939, P.A. Smith had made the first description of a submarine volcano with his description of the submarine topography of Bogoslof Volcano. Upon naming of Davidson Seamount in 1938, the U.S. Board on Geographic Names added the note, "The Generic term 'seamount' is here used for the first time, and is applied to submarine elevations of mountain form whose character and depth are such that the existing terms bank, shoal, pinnacle, etc., are not appropriate". It is fitting that this seamount, off the California coast, was named for the great George Davidson of the C&GS, who devoted much of his professional life to surveying the waters of the United States West Coast.

During World War II, Dr. Harry Hess of Princeton University, was commander of a troop transport in the Pacific. By monitoring his echo-sounder he discovered many flat-topped seamounts at varying depths which he named guyots in honor of a nineteenth century swiss scientist. In a series of expeditions beginning with MIDPAC in 1950, H.W. Menard and Robert S. Dietz, then of the Naval Electronics Laboratory, and Harris B. Stewart, then a graduate student at Scripps Institution of Oceanography, established the continuity of the extraordinary fracture zones of the North Pacific Ocean, including the Mendocino fracture zone, the most prominent fault scarp on earth (Menard 1986). Instrumentation kept pace during this period with the development of the Precision Depth Recorder by Bernard Luskin and Maurice Ewing's group at Lamont Geological Observatory. This instrument used a frequency regulator and an expanded depth scale to obtain unprecedented sounding accuracies. This led directly to the verification and more precise definition of vast abyssal plains by Maurice Ewing on the VEMA in 1953 (Wertenbaker 1974). These developments were followed up in the 1950's and early 1960's by the discovery of the continuity of the globe-encompassing mid-ocean ridge system and the correct hypothesis of Bruce Heezen and Marie Tharp of Lamont that this was in fact the site of a great mid-ocean rift system.

All of these discoveries, coupled with the systematic ocean surveys of the North Pacific Ocean carried out by C&GS vessels in the late 1950's and 1960's, were instrumental in the formulation of the theory of seafloor spreading and plate tectonics. The addition of a towed magnetometer that was developed by the Scripps Institution of Oceanography to the West Coast cruises of the C&GS ship PIONEER led to the discovery of the remarkable pattern of magnetic striping that is the key to our understanding of the evolution of the oceanic basins. The PIONEER survey was "one of the most significant geophysical surveys ever made" (Menard 1986).

TODAY'S SYSTEMS

In the late 1950's and early 1960's a number of evolutionary concepts were advanced that have fundamentally changed how we look at and map the seafloor. Sidescan technology was developed as a qualitative means of obtaining the sonar equivalent of an aerial photograph. Quantitative means improved rapidly with the development of improved single beam sounding systems and multi-beam swath systems.

During this period, the Scripps Institution of Oceanography began developing the Deep Tow vehicle under Fred Spiess of the Marine Physical Laboratory. This instrument was developed in response to a Defense requirement to understand the micro-topography of the seafloor, in particular bottom slopes averaged over 30 meters or so. (Spiess 1967). This vehicle evolved into one of the great research tools of ocean science as narrow beam echo sounder, magnetometer, sidescan sonar, bottom penetration sonar, photographic capability, and other sensors were added in response to varying research requirements (Spiess 1982). Spiess's group also developed acoustic transponder navigation to allow pinpoint positioning of this instrument relative to the seafloor. Some of the most precise profiles of the deep sea floor ever observed have been obtained with this machine.

Another major advance in seafloor imaging occurred as the result of a failed attempt by General Instruments Corporation (GI) to win a government contract for aerial radar mapping systems using what is known as the Mills Cross Technique. The engineers involved in this system contacted Harold Farr, Paul Frelich, and Richard Curtis of GI's newly-formed sonar group to inquire if they had any use for the concept. To use a cliche, the rest is history. By 1963, GI had developed and installed on the USNS COMPASS ISLAND the first operational Sonar Array Sounding System (SASS) using fan beam technology (White 1989). The SASS mapped a swath of seafloor by using beam-forming techniques to obtain up to 61 individual depths for each emission of the sonar system and, by so doing, developed a high resolution contour map of the seafloor. SASS has been used exclusively for defense purposes, although some data sets have been released for civil scientific use. However, using similar technology, Narrow Beam Echo Sounders (NBES) of two and two third degree beam width were developed by GI and installed first on the Coast and Geodetic Survey Ship SURVEYOR (now NOAA ship) and eight additional vessels.

In 1968, GI proposed a commercial swath-mapping system which came to be known as Sea Beam. The first delivery was to the Australian vessel HMAS COOK in 1975. However, the first operational system was on the French research vessel JEAN CHARCOT. The first United States vessel equipped with Sea Beam was the NOAA Ship SURVEYOR, which became operational in early 1980, while the first U.S. academic vessel equipped with this system was the Scripps vessel THOMAS WASHINGTON. Many vessels world-wide are now equipped with Sea Beam or similar systems. Sea Beam has mapped much of the East Pacific Rise, defined some of the world's great trenches, traced the sinuous courses of many offshore canyon systems, defined the tectonic fabric of many oceanic transform faults, discovered a whole new class of non-transform offsets of ridge axes called overlapping spreading centers, and mapped numerous cratered sea mounts in the Pacific and potentially economically significant Gulf Coast salt domes.

Swath-mapping technology has given earth scientists and engineers a new look at much of our planet's surface that was hidden prior to the development of these wonderful tools. Paralleling the development of quantitative methods of observing the seafloor has been the development of a wide range of qualitative sidescan imaging tools which are capable of producing sonar derived "pictures" of the seafloor comparable to terrestrial aerial photography. There are numerous shallow water systems available today but the most widely used deep-water systems have been the British GLORIA system which has imaged literally millions of square nautical miles of the world ocean since its introduction in the early 1970's and the sea MARC series of vehicles which had its roots in the search for the TITANIC. (For a description of types of systems available and principles used for seafloor imaging see Vogt 1986).

The Future

The first printing of the "General Bathymetric Chart of the Oceans (GEBCO)" which was organized and financed by His Serene Highness Prince Albert I of Monaco, was in 1904. In a prophetic statement, Professor Julien Thoulet of the group of geographers producing this map series, foresaw the continuing efforts of succeeding generations of sea surveyors and mappers:

"The work is completed... Here then is everything which is known today about the relief of the ocean floor. For many years to come, mariners, telegraphists, engineers, oceanographers and scientists will continue their soundings, for now we must proceed to fill in the details; no point of any sea on the globe will escape our investigations. The incessant and untiring efforts of succeeding generations are the glory of mankind..." (International Hydrographic Organization 1987)

With these new systems we are continuing "to fill in the details". Although it is tempting to believe that the final word in defining the seafloor is occurring with swath mapping technology, it is more probable that succeeding surveyors and cartographers will have access to ever more advanced technology of higher resolution. Faster techniques such as airborne laser hydrography for the nearshore area and high resolution systems such as interferometric side scan sonar systems, providing both imagery and bathymetry, are already emerging as operational tools for the modern hydrographic surveyor.

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