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.
REFERENCES
Adams,
K.T. 1942, Hydrographic Manual,
Government Printing Office, Washington.
Agassiz, A. 1888, Three Cruises
of the United States Coast and Geodetic Survey Steamer BLAKE,
Houghton, Miffin and Company, Boston.
Bates, C.C., Gaskell, T.F., and Rice, R.B. 1982, Geophysics
in the Affairs of Man, Pergamon Press, Oxford, UK.
Blewitt,
M. 1957, Surveys of the Seas,
MacGibbon and Kee, London.
Deacon, G.E.R. editor 1962, Seas,
Maps, and Men, Doubleday and Company, Inc., Garden City, New
York.
Hersey, J.B. 1977, A Chronicle of Man's Use of Ocean Acoustics: Oceanus,
Vol. 20, No.s, pp. 8-21.
International Hydrographic Organization 1987, IHO Information Paper,
No. 8, (Revised July 1987), General
Bathymetric Chart of the Oceans, IHO, Monaco.
Maury M.F. 1859, The Physical
Geography of the Sea, Sampson Low, Son, and Co., London.
Menard, H.W. 1986, The Ocean
of Truth, Princeton University Press, Princeton, New Jersey.
Morison,
S.E. 1978, The Great Explorers,
Oxford University Press, New York. Morison,
S.E. 1971, The European Discovery
of America, The Northern Voyages A.D. 500-1600, Oxford University
Press 1971, New York.
Murray,
and Hjort, J. 1912, The Depths
of the Ocean, MacMillan and Co., Limited, London.
Needham, J., Ling, W. and Guei-Djen, L. 1971, Science
and Civilization in China, Vol. 4, Parts 2 & 3, Cambridge
University Press, Cambridge.
Nelson,
S.B. 1982, Oceanographic Ships
Fore and Aft, Government Printing Office, Washington, D.C.
Soule, G.
1976, Men Who Dared the Sea,
Thomas Y. Crowell Company, New York. Spiess,
F.N. 1967, Deep Tow Workbook,
University of California, San Diego.
Spiess, F.N. and Lonsdale, P.F. 1982, Deep Tow Rise Crest Exploration
Techniques: Marine Technology
Society Journal, Vol.16, No. 3, pp. 67-75.
Stanton, W. 1975, The Great
United States Exploring Expedition of 1838-1842, University of
California Press, Berkeley.
Tanner,
Z.L. 1897, Deep-Sea Exploration,
Government Printing Office, Washington, D.C.
U.S. Navy
Hydrographic Office 1962, American
Practical Navigator,
Government Printing Office, Washington, D.C.
Veatch, A.C. and Smith, P.A. 1939, Atlantic
Submarine Valleys of the United States and the Congo Submarine Valley,
Special Papers Number 7, Geological Society of America, Boulder, CO.
Vogt, P.R. 1986, The Geology of North America The
Western North Atlantic Region, Geological Society of America,
Boulder, CO.
Wertenbaker, W. 1974, The Floor
of the Sea - Maurice Ewing and the Search to Understand the Earth,
Little, Brown and Company, Boston.
White, D. 1989, Personal Communication, General Instruments Corporation,
Massachusetts.
