Case Study:
How do we know what the seafloor looks like?

The vastness of the ocean and the secrets of the submerged landscape contribute to its mystery. Seawater is opaque to light beyond a few hundred meters' penetration. Hence, there are no mountaintops one can scale to directly gaze at vast expanses of the abyssal seafloor.
William B. F. Ryan (1992) "An Introduction - Down to the Sea in a Ship" Oceanus, Woods Hole Oceanographic Institution, MA

Consider yourself a new student of marine geology and that you want to understand the technology used on ships for deriving data about the sea floor. Use the following questions as a guide.

Why might a historical perspective be valuable?
How has the technology changed? How has it stayed the same?
How have the images of the seafloor changed within the past 50 years?

Begin by examining historical documents. Except when noted, text and images courtesy of The National Oceanic and Atmospheric Administration, U.S. Department of Commerce

Early History: Non-European or Western Culture

Early navigation was done by keeping in constant view of land.
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Alexander the Great founded the first marine science library in 3rd century BC at Alexandria, Egypt.

Cartographers produced charts and maps representing a spherical earth on a flat surface with latitude (parallel to the equator) and longitude lines (running from pole to pole) dividing the surface of the earth. (NOAA Photo Library)

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The library became a warehouse of all ancient writings and of all ships' logs that entered the harbor remained a maritime studies center for 600 years. In 127 BC Hipparchus oriented these charts by placing east to the right and north to the top and coming up with the division of degrees into minutes and seconds his system is still used today in our latitude and longitude system (e.g., 56°36'15N, 78°10'06E)

Other scholars working at the library invented the astronomical, geometric, and mathematical basis for celestial navigation (finding one's position on the earth by using the positions of celestial bodies)

Christianity identified Alexandria's library with paganism and burned it in 415 AD; western intellectual development in ocean sciences declined during Dark Ages. Eastern peoples: Arabs, Chinese, and Polynesians pursued voyages and invented navigational compasses during this time.
Between 300 and 600 AD, Polynesians colonized nearly every inhabitable island in southern Pacific Ocean.
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Chinese invented the rudder, watertight hulls, and sophisticated sails on multiple masts, all of which were critical to sailing with large vessels.
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The 1700's

The British Longitude Act of 1714, in the reign of Queen Anne, promised a prize of 20,000 English pounds for a solution to the longitude problem to anyone that could provide longitude to an accuracy of 1/2 degree. It was an immense amount of money at the time, the equivalent of millions of dollars today. Read Dava Sobel's explanation:
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"To know one's longitude at sea, one needs to know what time it is aboard ship and also the time at the home port or another place of known longitude-at that very same moment. The two clock times enable the navigator to convert the hour difference into geographical separation. Since the earth takes 24 hours to revolve 360 degrees, one hour marks 1/24 of a revolution or 15 degrees. And so each hour's time difference between the ship and starting point marks a progress of fifteen degrees of longitude to the east or west. Every day at sea, when the navigator resets his ships clock to local noon when the sun reaches its highest point in the sky, and then consults the home port clock, every hour's discrepancy between them translates into another fifteen degrees of longitude. One degree of longitude equals four minutes of time the world over, although in terms of distance, one degree shrinks from 60 nautical miles at the Equator to virtually nothing at the poles. Precise knowledge of the hour in two different places at once - a longitude prerequisite so easily accessible today from any pair of cheap wristwatches - was utterly unattainable up to and including the era of pendulum clocks. On the deck of a rolling ship such clocks would slow down, or speed up, or stop running altogether. Normal changes of temperature encountered en route from a cold country of origin to a tropical trade zone thinned or thickened a clocks lubricating oil and made its metal parts expand or contract with equally disastrous results. A rise or fall in barometer pressure, or the subtle variations in the Earth's gravity from one latitude to another, could also cause a clock to gain or lose time." Longitude by Dava Sobel, Published by Walker & Co New York, N.Y.; retrieved August 2005
The British Longitude Act challenged many men, both of wealth and education, such as Astronomers, and men who worked with their hands, such as mechanics, to create an accurate way to tell time at sea. John Harrison a carpenter turned clock maker, born in 1693, proved to be the right man for the job, but it took him over 25 years, and he had to contend with Royal Society.
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It turned out ultimately to be a battle between the 'establishment' led by the 5th Astronomer Royal, the Rev. Neil Maskelyne who had attended Westminster School and Cambridge (described by a contemporary as "rather a swot" and "a bit of a prig"), and John Harrison a carpenter turned clock maker, born in 1693.
Harrison had built his first pendulum clock entirely of wood when he was twenty. He later published his "Equation of Time" tables 'enabling a clock user to rectify the difference between solar or true time as shown on a sundial to the more regular "mean time" as measured by a clock'. The sun is actually a poor timekeeper as the difference between 'solar' and 'mean' time, widens and narrows, as the seasons change.
In retrospect, a pendulum clock by Harrison for Brocklesby Park Stables was his first step towards making a sea clock. This clock never needed oiling, has been running for 274 years, is friction free as parts are carved out of Lignum Vitae, a tropical hardwood that provides its own grease. Eliminate the need for oil in a clock, and you solve a major problem as oil gets thicker/thinner as temperature changes.
Harrison took five years to build his first sea clock.

This information was retrieved from http://www.sailtexas.com/long.html on August 14, 2005
A story from 1725 recalls how fishermen could not find the sea floor. "Myth and the Sea: The Bottomless Abyss."

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"The fishermen working on that slope where they are in the habit of finding coral at 150 and 200 fathoms (1 fathom = 6 feet), and their lines not allowing soundings in greater depths, imagine that the bottom cannot be found, and call it in their exaggerated jargon a bottomless abyss, impossible to be sounded. This idea entertained by people of experience in marine matters, as well as by the simple fishers, appears to be absurd, and founded merely on the fact that nobody has yet cared to undertake the trouble and expense required for such soundings, which according to all appearances will never be made unless some Prince orders for that purpose special vessels with suitable instruments." In Histoire Physique de la Mer (1725) by Count Luigi Marsigli. Published by Doubleday & Company, Inc. Garden City, New York. p. 177.

In 1768 H.M.S. Endeavor sailed out of Plymouth Harbor under the command of James Cook of the British Royal Navy.
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Cook reported on marine life, geological formations, ocean floor types, and produced highly accurate charts that were used as late as W.W.II to invade Pacific islands. Improvements in temperature measurement are made in the 1780s, but still fall short and produce inaccurate results; emphasis shifts from salinity being a major contributor to internal circulation of the oceans (Marsigli's original idea) to temperature being the major contributor -- not until 1819 that Marcet shows both contribute to water's density and that they can counteract each other.

The 1800's

First attempt at a bathymetric map by Matthew Fontaine Maury. Published in The Physical Geography of the Sea. 1855. (NOAA Photo Library)

1807 President Thomas Jefferson signed a law authorizing the formation of an agency to survey the coast.
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This agency, which became known as the United States Coast Survey, was NOAA's earliest "ancestor." Under Ferdinand Hassler, its early surveys hugged the coasts and harbors but began inching seaward. Its initial research efforts included embryonic studies of tides and tidal currents, the collection of bottom samples to determine sea-floor characteristics for the anchoring of vessels, and soundings to establish the depth and physical features of near-shore waters.
In 1831, on a voyage in the H.M.S. Beagle, Charles Darwin deduced how atolls and reefs were formed around and upon volcanic islands.
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In 1832 Charles Lyell Comments on Future Possibilities for Marine Archaeology.
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Track of the flagship Vincennes first telegraph cable is laid across the Straits of Dover in 1851 and stimulates new technologies to protect, raise and lower cables to the sea floor.
In 1840 The First Modern (successful) Deep Ocean Sounding was taken with a line of 2,425 fathoms (1 fathom = 6 ft = 2 meters).
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"On the 3rd of January, in Latitude 27 degrees, 26 minutes S., longitude 17 degrees, 29 minutes W, the weather and all other circumstances being propitious, we succeeded in obtaining soundings with two thousand four hundred and twenty-five fathoms of line, a depression of the bed of the ocean beneath its surface very little short of the elevation of Mount Blanc above it." Sir James Clark Ross, in command of Her Majesty's ships Erebus and Terror, en route to the Antarctic continent. This was the first successful deep-sea sounding ever taken (Jan. 3, 1840). As quoted in The Sea Around Us (1951) by R. Carson. Published by Oxford University Press, New York. p. 56.
In 1855 M.F. Maury visualizes the seafloor and writes an account of it in his book "The Physical Geography"
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"Could the waters of the Atlantic be drawn off, so as to expose to view this great sea-gash, which separates continents, and extends from the Arctic to the Antarctic, it would present a scene the most rugged, grand, and imposing. The very ribs of the solid earth, with the foundations of the sea, would be brought to light, and we should have presented to us at one view the empty cradle of the ocean ...." In The Physical Geography of the Sea (1855) by M. F. Maury. Published by Harper and Brothers, New York. p. 209.
On May 29, 1867, a 270-fathom dredge haul from a few miles north of Cuba yielded a basketful of living creatures which disproved that the sea floor was void of living organisms.
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Dredging and sounding equipment on the HMS Challenger (NOAA Photo Library). Four months later, Wyville Thomson, on the HMS Lightning, dredged sea life from 650 fathoms southwest of the Faeroe Islands in the North Atlantic. He went on to dredge the following year at 2,400 fathoms, finding life there and ending forever the concept of a lifeless (azoic) zone in the deep seas. He built on his successes and went on to lead the great Challenger Expedition, which dredged and sounded its way around the world from late 1872 to 1876.
The first modern bathymetric map was created from soundings made in the Gulf of Mexico. (NOAA Photo Library)
The First Accurate Bathymetric Chart was rendered in 1888.
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"Sir William Thomson would find it difficult indeed to recognize his original machine as now used on board the Blake, with the modifications introduced by Lieutenant Commander Sigsbee.
Diagram of the piano wire Sigsbee Sounding Machine from 1875. NOAA Photo Library
During the four years of his command, the latter ran no less than 12,766 nautical miles of sounding-lines, with the necessary serial lines of temperatures. As the result of his magnificent work, the Coast Survey is publishing a hydrographic chart of the Gulf of Mexico, not merely unequaled for its accuracy, but unique as the first chart of any extent which carries the littoral hydrography to great depths." In Three Cruises of the Blake, (1888) by A. Agassiz, Volume I, p. 15.
Alexander Agassiz comments on the first use of steel cable by an oceanographic ship in 1888. "My familiarity with the successful use of very long steel ropes for mining purposes naturally suggested their adaptation to the new purpose of deep sea work." In Three Cruises of the Blake (1888) by A. Agassiz. Published by Houghton, Mifflin, and Company, Boston. p.28.
A steam winch with steel wire was first used for deepsea dredging on the Coast Survey Steamer Blake. Source: Deep Sea Sounding and Dredging (1880) by C. Sigsbee. (Courtesy of NOAA Photo Library.) All dredging cruises previous to the first cruise of the Blake had used hemp rope for dredging.

The 1900's

The role of echo sounding in exploring the sea was first considered a supplementary device.
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"Depth determinations by means of reflected sound from the bottom are reported to be feasible; and though it may never supplant the standard method of sounding, there is a possibility of it becoming a valuable supplementary device." In "Mapping the Pacific - Survey of Shoreline and Coastal Waters" by Commander J.T. Watkins, U. S. Coast and Geodetic Survey. Published in The Proceedings of the First Pan-Pacific Scientific Conference, August 2-20, 1920 (1921). Special Publication Number 7, Part 1, 1921 by the Bernice P. Bishop Museum.
Looking for Atlantis: A Suggestion to Survey the Mid-Atlantic Ridge in 1940.
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"Several years ago Professor Richard M. Field, Chairman of the Commission on Continental and Oceanic Structure of the International Union of Geodesy and Geophysics, suggested to the author the desirability of a hydrographic survey of the Atlantic Ridge and inquired as to its feasibility. It is the purpose of this article to propose a modern hydrographic survey of the Ridge and to describe from the standpoint of a practical hydrographer tentative plans, including methods and technique, adequate for a survey as well controlled as those made in recent years over the continental shelves off the North American coasts." In "A Survey of Atlantis" by (1940) Captain G. T. Rude of the U. S. Coast and Geodetic Survey in United States Naval Institute Proceedings, Vol. 66, No. 8, Whole No. 450, pp. 1105-1123. August 1940.
While exploring the Marianas Trench in 1960 humankind reached it's deepest depth.

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On January 23, 1960, the bathyscaph Trieste reached the greatest oceanic depth existing on our planet. Jacques Piccard and Lieutenant Don Walsh of the United States Navy, piloted the Trieste to the sea floor in the deepest part of the Marianas Trench, known as the Challenger Deep. Although their onboard depth indicator registered 37,800 feet, this was later corrected to 35,800 feet as the result of calculations by Dr. John Knauss (a future administrator of NOAA) and Dr. John Lyman. This is Jacques Piccard's description of what he saw on this dive to the deepest part of the sea.
"The bottom appeared light and clear, a waste of snuff-colored ooze. We were landing on a nice, flat bottom of firm diatomaceous ooze. Indifferent to the nearly 200,000 tons of pressure clamped on her metal sphere, the Trieste balanced herself delicately on the few pounds of guide rope that lay on the bottom, making token claim, in the name of science and humanity, to the ultimate depths in all our oceans - the Challenger Deep. "The depth gauge read 6,300 fathoms - 37,800 feet. The time - 1306 hours.
A major development of the 1950s was the invention of the Precision Depth Recorder (PDR) at the Lamont Geological Observatory of Columbia University.
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This machine measured depths with errors of less than 1 percent of total water depth. The PDR discovered abyssal plains -- the flattest places on Earth. The Pioneer was equipped with a PDR for its Pacific coast bathymetric survey, and the PDR also helped Bruce Heezen, Marie Tharpe, and Maurice Ewing of Lamont discover the Mid-Atlantic Ridge Rift Valley in 1959.
This was, perhaps, the single most significant bathymetric discovery made since the beginning of deep ocean exploration. Like the recognition of sea-floor magnetic striping, and the earlier seismological work of C&GS scientist Nicholas Heck, which established the correlation of many intra-ocean earthquake epicenters with the location of mid-ocean ridges and rises, this discovery of the median rift valley of what we now know are active oceanic ridges was a major stepping-stone on the road to the Theory of Plate Tectonics.
The discovery of a rift valley in the center of mid-ocean ridges was one of the most significant bathymetric discoveries ever made, but the significance was not realized right away. The following is an account from: "The Mid-Atlantic Rift Valley as Girl Talk"

Show me the sea-floor rendering by Marie Tharp.

In 1952, Marie Tharp was a draughtsman for Bruce Heezen and others working at Lamont Geological Observatory in New York. Bruce Heezen related the following story concerning the realization that a rift valley existed in the middle of the Mid-Atlantic Ridge: "Marie's job for me was to decide what a structure was --- whether a rise in the echo soundings represented a hill or something longer like a ridge --- and to map it. In three of the transatlantic profiles she noticed an unmistakable notch in the Mid-Atlantic Ridge, and she decided they were a continuous rift valley and told me. I discounted it as girl talk and didn't believe it for a year." As quoted in The Floor of the Sea (1974) by William Wertenbaker. p. 144.

Three Great Tools of the 1960s: Deep Tow instrument systems, multibeam sounding instruments, and manned research submersible.

The first was the Deep Tow instrument system, built and operated by the Scripps Institution of Oceanography.
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This tethered system was lowered from a surface vessel to just above the sea floor. The system was originally designed to obtain sea-floor slope information in the deep sea. Ultimately, it evolved into a system with multiple sensors for characterizing the deep-sea environment including a downward-looking, narrow-beam sounding system, a sidescan sonar system, television and still-camera systems, and a variety of other sensors and sampling devices. Bottom-mounted acoustic transponder navigation instruments were developed to support the relative navigation of the Deep Tow instrument within a study area. Virtually all deep-towed instrument packages ultimately trace their lineage to the Scripps system.
Multibeam sounding instruments were the second major development of this period.
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The first multibeam sounding system, known as the Sonar Array Sounding System (SASS), was installed on the U.S. Navy Ship Compass Island in 1963. Multibeam sounding systems obtain depths over a swath of bottom perpendicular to the heading of the survey ship, as well as directly below the ship (as in single-beam sounding systems). Such sounding arrays, coupled with accurate navigation, allow the immediate generation of accurate sea-floor maps. Multibeam sounding systems are now the de facto instrument for most modern-day bathymetric mapping efforts.
The third major instrument developed during this period was the manned research submersible.
Show me the Alvin.

The U.S. Navy acquired the bathyscaph Trieste from the Piccard family -- the Swiss engineers who designed and built it -- and used it to dive to the deepest spot in the ocean in 1960. Nevertheless, it was ill-suited to the needs of research scientists. Consequently, the Navy funded Woods Hole Oceanographic Institution to procure a small research submersible. In 1964, the now-famous Alvin was launched and began its career of unparalleled oceanic research and exploration. Alvin has transported scientists to the edge of creation while studying processes on the ocean ridge systems; its viewing ports have been a window to the unsuspected chemosynthetic communities discovered in the vicinity of hydrothermal activity and cold seeps in many areas of the ocean; and from its interior scientists have observed, for the first time, deep-sea sedimentary and biological processes. Helping to publicize its use as an ocean observation tool was its discovery in 1966 of a hydrogen bomb, lost off Palomares, Spain, from a U.S. Air Force B-52 that had collided with a tanker plane during refueling operations.