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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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