This section is divided into 3 parts; Who, How and Why

Introduction

This Chapter examines the transformation from Marine Studies into Marine Science, focussing on the changing perceptions, methods and pioneers of the study of the oceans.

History of Ocean Exploration: How they explored the oceans back then

About 4000 BC the Egyptians built the first sailing ships. The Phoenicians, Chinese, Greeks, Polynesians and the Vikings were all early explorers. However their voyagers were limited by their navigational skills. Navigation is essential when you are out in the open sea out of sight of land.

In about 150 AD the Greek, Ptolemy, compiled a map of the Roman world that showed latitude and longitude. Latitude is the north –south coordinate. The North Pole is a 90º North, the equator is at 0º and the South Pole is a 90º South. Longitude is the east-west coordinate. Greenwich England is at 0º - referred to as the Prime Meridian.

Measuring latitude happened fairly early on, as it is relatively easy to calculate. If you know the day of the year, you need only observe the elevation of the sun above the horizon at noon and correct for the tilt of the earth. About 900 AD the Vikings began to use the North Star to determine their latitude at night. The North Star (sometimes called the Pole Star or Polaris) does not change position during the night. The further south you go, the lower the star appears in the sky, until it disappears at the equator. Sailors used a quadrant – which is just simple device for calculating the angle of elevation of the sun or star – to determine their position. To go to a place of known latitude, they sailed their ships to that latitude and then sailed east or west along the latitude line until the destination was reached.

HOW FAST ARE WE GOING? Early mariners used a device called a chip log and reel for measuring their speed through the water. The chip log was made of a triangular piece of wood, weighted on one side and attached to a line with marked (knots) lengths. When thrown from the stern of a vessel the line was allowed to run out for a specified time. The number of knots that had come off the reel determined the vessel's speed. Today a ship's speed is still referred to in knots. 1 knot = 1 nautical mile per hour = approximately 50 cm per second 1 nautical mile = 1º of latitude at the equator = 1.853 km

Lines of latitude and longitude. Greenwich became the recognised site for the Prime Meridian after 25 countries reached agreement in 1884 at a conference in Washington, USA.
From http://chandra.harvard.edu/xray_astro/navigation8.html

Longitude on the other hand posed a much bigger problem. The Greeks were the first to notice that longitude was a function of time. The earth rotates 360 degrees of longitude in 24 hrs, therefore every 15º represents 1 hour. To determine longitude you have to know the local time and the time at the starting point (or at the Prime Meridian). That is, if a sailor knew the time (taken when the sun was at its highest point) and the time at the starting point, the difference would indicate ship's longitude relative to the starting longitude. Every four minutes of difference is equal to one degree of longitude. However to make this calculation work, the sailor had to have a reliable and accurate clock.

In 1714, England’s Board of Longitude offered a huge reward to anyone whose method of measuring longitude could be proven successful. The Board favoured a solution that computed longitude using the difference in the position of the moon, sun and stars from one location to another. The Board did not believe that a clock that could keep accurate time at sea could ever be built (the most accurate clocks at this time were pendulum clocks which do not work well at sea).

In 1735 John Harrison, a self-taught English clock maker, invented a chronometer that was not affected by changes in temperature or movement. Harrison’s clock utilised a counterbalanced mechanism controlled by springs, which ensured that any change in motion which affected one of the balances was compensated for by the other balance. He won the prize, but for political reasons didn’t receive all the prize money until 1773 (when he was over 80). You can read the story of John Harrison in the 1996 book by Dava Sobel called Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. You can also view his marine timekeeper at
http://www.rog.nmm.ac.uk/museum/harrison/h4.html

Early explorers went out looking for trade routes or new lands to colonise. It was not until Captain Cook that systematic scientific investigation of the oceans began. Although Cook’s objective was ostensibly to raise the British flag in the South Seas, during his voyages from 1768 to 1779, he made important scientific observations. He collected marine specimens and terrestrial flora and fauna from the places he visited. Another early contributor to marine science was the mid-nineteenth century director of the US Naval Depot of Charts and Instruments, Matthew Maury. Maury assembled observations on winds and currents collected by many different organisations around the world, into a book, the Physical Geography of the Seas. It is due to this publication that he is known as the "father of physical oceanography".

The first truly scientific expedition was the HMS Challenger in 1872. The expedition, entirely devoted to science, followed Charles Darwin's work on the HMS Beagle. On December 21 1872, it began its four year, 127,000 km journey around the world. The goal of the expedition was to gather as many observations of oceanic phenomena as possible. In addition, the cruise was to determine if there was life in the deep oceans.

HMS Challenger sailed from Sheerness with 20 Naval officers, 200 crew and 6 scientists.

While aboard the Challenger, scientists took readings on salinity, temperature and density, which contributed to the rapidly growing understanding of physical oceanography. Information about currents, sediment, and meteorology was also collected. Near the Philippines, it was proven that life did exist in the deep when samples were taken from depths of over 8000 meters. All the data were consolidated into a fifty-volume set, entitled the Challenger Report, which set the scene for future marine research and is still consulted today (eg. the global seabed data base developed here in the department by Chris Jenkins includes the sediment descriptions done by the Challenger scientists- in some locations these are still the only descriptions available).

Some of the ways that we explore the oceans now

During World War II many countries, including the US and Australia, recognised the strategic importance of understanding the marine environment. Following the war there was a major push, especially in the US, to study oceanogaphy and marine geology. Marine research has come along way since the days of the Challenger expedition. On that cruise scientists could only sample the surface sediments and depth readings were done using a hemp rope.

Today Harrison’s chronometer has been replaced by the Global Positioning System (GPS), a series of 24 earth orbiting satellites that can be used to determine latitude, longitude, elevation, velocity and precise time. A GPS receiver is used to find the receivers distance from 4 or more satellites (generally there are 8 or more satellites visable to a GPS receiver at any one time). The satellites send out radio signals which the receiver picks up. The receiver measures the time it takes for the signals to travel from the satellite, converts this to distance and calculates a position. Because the satellites are orbiting at a distance 35,000 km overhead, the radio signals are quite weak and you have to be outside in an open area for the GPS to receive the signals effectively.

GPS satellites orbit the earth every 12 hours. Signals received from the satellite can be used to calculate location within a fraction of a meter, time within a millionth of a second and velocity within a fraction of a km per hour (photo from US Naval Observatory).

Today we have replaced the hemp rope used by the crew of HMS Challenger to measure water depth with acoustic instruments that not only measure bathymetry but can also produce 2D and 3D images of the seafloor and subseafloor.

Listed in the table below are some of the intruments used today to map the oceans.

* High frequency instruments are usually referred to in kHz eg. a common echo sounder is 12 kHz.

Echo Sounders

Echosounders are used to measure water depth by sending acoustic pulses via a transducer. The acoustic pulses are reflected at the sea floor and the reflected echoes are received at the transducer. Echo sounders repeatedly "ping" the seafloor with a narrow cone of sound (<5º) as the ship moves along the surface, producing a continuous line showing depths directly beneath the ship. A conventional, single beam echo-sounder records the time taken for the acoustic pulse to travel from the transducer to the sea-bottom and back again. The water depth is then calculated from the two-way-travel-time and the assumed velocity of sound in water (1500 m/s).

In the early days of ocean exploration until as recently as 15 years ago, marine geologists wrote down individual readings from echo sounders, plotted them on navigation charts showing the ship’s position, and then drew contour lines joining points of equal depth (isobaths). In this way, they produced bathymetry maps of the sea floor. The Ocean Sciences Institute here at the University of Sydney, in conjunction with the Australian Navy, used this method to map the sea floor in the South Pacific during the early 1980’s.

One of the limitations with single beam echo sounders is that no information is gathered beween the survey lines. Recent advances in the technology have produced echo sounders with multiple beams. These systems employ sound waves propagating at angles which vary from vertical to nearly horizontal. A wide swath can be surveyed in a single transect. The multi beam systems not only provide an acoustic backscatter image, but also as many as ~120 spot measurements of water depth across the ships track. The spacing of the spot measuremnets and the width of the swath varies with water depth. (typcially 7 times depth).However, interpreting the signals from a multibean system is more complicated than for a single beam system. Ship motion (pitch, roll, heave and heading variations) cause short term variations in the transducers vertical position and in the orientation of the transmitted pulse. These motions must be recognised (by DGPS, heave-compensators and gyro compass) and compensated for in the data processing.

Echo sounders use different frequencies of sound to find out different things about the seafloor. Echo sounders that transmit sound at 12 (kHz) are used to determine water depth. However, using a lower frequency (3.5 kHz) sound, which penetrates the seafloor, scientists can view the surficial sediments, generlly to a maximum depth of 50 m, depending on the substrate.

Print out of an echo sounding profile. Note the vertical exaggeration.

Side Scan Sonar

Side scan sonar instruments transmit sound into the water at an angle rather than straight down. Unlike the echo sounder, where only the travel time of the signal is recorded, the side scan sonar also records the amplitude of the returned signal as a time series. The recorded amplitudes are a measure of the acoustic backscatter and specula reflection from the sea floor. The intensity of the returning acoustic signal is controlled primarily by the roughness and composition of the sea floor. A stronger return is received if the seafloor slopes toward the instrument. The strength of the sound recorded by the side-scan sonar instrument is converted to shades of gray. A very strong return is white whereas a very weak return is black (however this is dependant on how you set up the instrument- some people prefer the strong reflecters to be black and the weak to be white). The echo strengths that fall between these two extremes are converted to different shades of gray. Features that stick up above the surrounding seafloor will cast acoustic shadows, ie. no sound reaches this area. The sea floor is typically surveyed in swaths 100-500 meters wide which are mosaicked together to form a composite image of the survey area.

The USGS used the side scan sonar device known as GLORIA to map large areas of the US EEZ (Exclusive Economic Zone). GLORIA was developed in the U. K. at the Institute of Oceanographic Sciences specifically for maping the morphology and texture of the sea floor in the deep ocean. The system is operated at a frequency of about 6.5 kHz and can map a maximum swath up to 30 km on either side of the ship’s track (however it is generally less than this). Information on Gloria can be found at http://walrus.wr.usgs.gov/gloria/gloria.html.

Strong reflections (high backscatter) from boulders, gravel and vertical features facing the sonar transducers are white; weak reflections (low backscatter) from finer sediments or shadows behind positive topographic features are black (from the USGS).

Example of the side scan mosaic. This is the zone between East and West Sydney Airport Runways. Engineered craters, pockmarks, and coarse sediment aprons are easily seen, in addition to bubble-wakes from small boats (From Chris Jenkens.

http://www.es.usyd.edu.au/geology/centres/osi/scex/geophys.html)

Reflection Seismic

Reflection siesmic produces a profile of the subsurface geology. A towed seismic source emits acoustic low frequency (high penetration) energy at regular intervals. The transmitted acoustic energy is reflected from boundaries between the various layers of different acoustic impedances (i.e. the water-sediment interface or between geologic units). The boundaries that are visible in a seismic record are determined from the two way travel time – that is the time that it takes the transmitted acoustic energy to travel from the source to each boundary surface and back to the receiver. Once the speed of sound within the individual units is known, depth to each unit can be calculated.

When viewing seismic records you often see features that are described as "multipes".
These are multiple images of the seafloor reflector. Multiples are most often seen on sub-bottom profiles where the seafloor is characterised by a very strong reflector. When this occurs, the reflected acoustic wave from the seafloor travels upwards through the water column only to be reflected from the air-water interface, thus propagating downwards through the water column once again. Multiples are usually easy to recognise as they always occur at twice the water column travel time (or depth) and generally parallel the seafloor reflector.

(from USGS)

Refraction Seismic

Seismic refraction involves the observation of a seismic signal that has been refracted between layers of contrasting seismic velocity. Sound is generataed at the surface using a hammer, gun or explosive source and the shock wave data is recorded by a linear array of sensors.

Refraction seismic is more commonly used on land than in the marine environment. However, the technique is occassionally used in situations like the construction of a port or runway when it is necessary to locate the bedrock. Refraction sceismic can also be used to find buried archeological sites or assess subsurface geological hazzards.
A more detailed explanation of refraction seismic theory and application can be foiund at
http://www-geology.ucdavis.edu/~gel161/sp98_burgmann/cannon/refraction.html

Looking at the Sea Floor from Space

A topographic map of the worlds oceans with a horizontal resolution of 1 to 12 kilometers has been produced by combining depth soundings with high-resolution marine gravity information from the Geosat and ERS-1 spacecraft. You can find this map at the National Geophysical Data Centre
http://www.ngdc.noaa.gov/mgg/fliers/97mgg03.html

You think the ocean is smooth? Well it’s not. The surface of the ocean bulges outward and inward corresponding to the the topography of the ocean floor. The bumps, can be measured by a radar altimeter aboard a satellite. These bumps and dips in the ocean surface are caused by minute variations in the earth's gravitational field. For example the extra gravitational attraction due to a massive mountain on the ocean floor attracts water toward it causing a local dip in the ocean surface. When you map all these dips and bulges you can see that the ocean floor is covered with scars caused by the movement of the plates. All the undersea volcanoes greater than 1000m high are also visible.

More pages....

Exercises

People, Places, Things

Some of the terms, events, people and equipment associated with Marine Science.

Lets go!

Have a Go!.

In the three sections of this section, a myriad of instruments have been discussed. Obviously, the best way to become familiar with what can, can't and should be done with each one is best discovered by actual application. This, hopefully, is the next best thing - a hybrid of practical and theoretical application.

Lets go!

Crossword.

A bit of a no-brainer, but any exposure to 'nollige' is good exposure.

Lets go!

Additional Exercises

Additional information to allow you to test some of the ideas and concepts from this chapter.

- continental jigsaw?
- Challenger Path?
Sampling-type problems?

Lets go!

Helping Hints

We realise that not everyone has had experience with the intrinsic concepts of Marine Science. This 'computer' has numerous examples which will help you understand these fundamental facets! Very helpful, very colourful!

Lets go!

exit