A Brief History of the Use of Sound in Ocean Exploration
But First, A Few Facts and Definitions
Sound waves are vibrations that are caused by a source - for example, a beaten drum, or any one of our sources - air guns, transducers, listed in the table of contents to the right. The vibrations cause a disturbance in the particles that compose the surrounding medium, in our case, seawater. These particles are disturbed in a sequential fashion, one disturbs its neighbor, etc. This disturbance is patterned as a wave that moves outward from the source in all directions- like the ones pictured in the visible pattern of sound waves above; thus moving the sound energy through water, or any other medium.
|Waveform showing wavelength and amplitude.|
The waves are described in terms of their wavelength - the distance between like points on consecutive waves; for example, between the peaks or troughs of two waves. These are points of what is termed corresponding phase of two waves. Waves are also characterized by their height (from peak to trough), also known as the amplitude.
The number of vibrations that occur in one second is known as the frequency of the wave, and is measured in cycles per second or Hertz (Hz); the higher the frequency of the wave, the shorter the wavelength. The frequency of the wave is constant, so that is why the sound waves are described in terms of frequency. Most of our sources are characterized as operating at a specific frequency or within a well-defined range of frequencies (Lurton, 2002).
The amplitude correlates to the amount of energy in a sound wave, the higher the amplitude, the more energy it carries. As the waves propagate through a medium, energy is transferred and ultimately lost due to the transfer of the disturbance from one particle to its neighbor. This depletion of energy is termed attenuation, and correlates to a loss of wave amplitude (McQuillin et al., 1979). High-frequency sound attenuates more rapidly than low-frequency sound, thus energy from lower frequency sources can propagate greater distances than that from high-frequency sources.
When the sound energy moving through water hits the sea floor, some of it is reflected, some is transmitted to the sea floor, some is refracted, and some is scattered.
The relative amount of energy that is transmitted vs. reflected is a function of the impedance (sound velocity x density) contrast between the seawater and the sea floor, the angle at which the sound hits the sea floor (incidence angle), and the roughness of the sea floor.
A Brief History
Exploration of the sea floor surface and shallow subsurface has fascinated mankind for centuries. The early explorers employed a multitude of ingenious mechanical devices and even some use of various energy forms - light and sound - in their investigations. The H.M.S. Challenger expeditions from 1872 to 1876 are the most extensively documented of the first large-scale studies of the oceans to provide some characterization of the sea floor.
|H.M.S. Challenger in New Mole (from H.M.S. Challenger Narrative, vol. I, p. 47; Courtesy of The Royal Society, London).|
They traveled under sail nearly 115,000 km through all the oceans, except the Arctic, performing soundings and dredging, among other scientific operations and observations. They carried aboard 144 miles of rope and 12 miles of piano wire to accomplish these measurements (Bailey, 1953).
|H.M.S. Challenger dredging and sampling configuration (from H.M.S. Challenger Narrative, vol. I, p. 57; Courtesy of The Royal Society, London).|
As the physical sciences evolved, various attempts were made to use advances in electromagnetic, light, and acoustic wave energy to probe the ocean environment, expanding the capabilities of expeditions. However, due to the high conductivity of salt water, light and electromagnetic energy were found to rapidly attenuate. Alternatively, acoustic energy was found to propagate well in sea water, offering an efficient transfer of energy underwater. Sound waves are mechanical vibrations of the medium (air, water) in which they travel, i.e. compressional waves - pressure fronts -- generated by vibrations from the sound source. (Knight, 1960)
|A visible pattern of sound waves. Sound waves are emitted from the horn at the extreme left and focused with the acoustical lens near gentleman's right hand. (Knight, 1960, p. 80 and Bell Telephone Laboratories, now www.lucent.com)|
The turn of the twentieth century saw the practical application of underwater sound, for example the submarine bell, used by ships for offshore navigation (Urick, 1983). The Titanic tragedy in 1912 prompted efforts to use echo ranging (see echosounder in tree to the right for a description of the method) to detect underwater objects.
In 1913, Canadian inventor, Dr. Reginald Fessenden, chief physicist for the Submarine Signal Co. of Boston, used sound waves to measure water depths and to detect icebergs. These seismic instruments he invented were used to record both reflections and refractions through geologic formations near Framingham, Massachusetts (Seitz, 1999).
The outbreak of WW1 in 1914 spurred on more developments using underwater sound for echo ranging and signaling. These developments lead to our modern echosounders. Between the two world wars, efforts were directed at better defining the characteristics and principles of underwater sound, as one facet of the burgeoning fields of oceanography and geophysics.
The onset of WW1, brought on another dramatic effort to understand both the concepts and practical applications of underwater sound for submarine, surface ship, weaponry, and target location needs (Urick, 1983). The term 'SONAR', SOund Navigation And Ranging, was first used during this time period. These wartime developments in underwater acoustics were applied to oceanographic research as well, as notable individuals and several nascent oceanographic institutions began large-scale investigations of the ocean basins. Echosounders became more sophisticated to detail bathymetry, while seismic and sidescan-sonar systems evolved to map the sea floor and subsurface.
The data gathered in these broad studies served as the basis for plate tectonic theory, the context in which most marine geologic/geophysical research is conducted.
This brief history is not intended to be a complete one. It is an overview from which readers may further explore the topic in libraries or on the web, using the topics presented above as starting points.
Bailey, Herbert S., 1953, The Voyage of the Challenger, Scientific American, reprinted in J. Robert Moore, Editor, 1971, Oceanography Some Perspectives, W.H. Freeman and Company, San Francisco, CA, p. 20 24.
Knight, David C., 1960, The First Book of Sound: A Basic Guide to the Science of Acoustics, Franklin Watts, Inc. New York, p. 80.
Lurton, X., 2002, An Introduction to Underwater Acoustics, Principles and Applications, Springer in association with Praxis Publishing, p.3.
McQuillin, R., Bacon, M., and Barclay, W., 1979, An introduction to seismic interpretation: Graham and Trotman, Limited, London, 199 p.
Report on the Scientific Results of the Voyage of the H.M.S. Challenger During the Years 1873-76, Narrative, vol. I, 1880, London:H.M.S.O., 1890.
Seitz, Frederick, 1999, The Cosmic Inventor, Reginald Aubrey Fessenden, Transactions of the American Philosophical Society, vol. 89, pt. 6, 69 pp.
Sengbush, R.L., 1983, Seismic exploration Methods, IHRDC, Boston, 296pp.
Urick, Robert J., 1983, Principles of underwater sound: New York, McGraw-Hill Book Company.