Marine Seismic Sources – Part XII: Sound in the Sea

Aristotle (384–322 BC) was among the first to note that sound could be heard in water. Nearly 2,000 years later, Leonardo da Vinci (1452-1519) made the observation quoted above that ships at great distances away could be heard underwater. In 1743, Abbé Nollet conducted a series of experiments to settle a dispute about whether sounds could travel through water. With his head underwater, he reported hearing a pistol shot, bell, and shouts. He also noted that an alarm clock clanging in water could be heard easily underwater, but not in air, clearly demonstrating that sound travels through water.

The name of Willebrord Snellius (1580-1626), a Dutch astronomer and mathematician, has for several centuries been attached in English-speaking countries to the law of refraction of light. But it is now known that this law was first described by the Arabian optics engineer Ibn Sahl (940- 1000) working in Baghdad, when in 984 he used the law to derive lens shapes that focus light with no geometric aberrations.

Development of Acoustics

Reproduction of a page of Ibn Sahl’s treatise on Burning Instruments from 984 showing his discovery of the law of refraction. In the treatise, he set out his understanding of how curved mirrors and lenses bend and focus light. Source: From R. Rashed, 1990Lord Rayleigh’s first researches were mainly mathematical, concerning optics and vibrating systems, but his later work ranged over almost the whole field of physics, sound and wave theory, electrodynamics, hydrodynamics and photography. He was awarded the Nobel prize in Physics in 1904 for his work on gases.In 1878 Lord Rayleigh published The Theory of Sound, a work marking the beginning of the modern study of acoustics. Lord Rayleigh was the first to formulate the wave equation, a mathematical means of describing sound waves that is the basis for all work on acoustics. His work set the stage for the development of the science and application of underwater acoustics in the twentieth century.

The sinking of Titanic in 1912 and the start of World War I provided the impetus for the next wave of progress in underwater acoustics. Anti-submarine listening systems were developed and in 1914, Reginald A. Fessenden developed the echo-ranger. The development of both active ASDIC and passive sonar (SOund Navigation And Ranging) proceeded apace during the war, driven by the first large scale deployments of submarines. Other advances in underwater acoustics included the development of acoustic mines.

The period between the two world wars was a time of increased discovery about underwater acoustics. Scientists were beginning to understand fundamental concepts about sound propagation, and underwater sound was being used to explore the ocean and its inhabitants. For example, shortly after WWI, the German scientist H. Lichte developed a theory on the bending, or refraction, of sound waves in sea water. Building on work by Lord Rayleigh and Snell, Lichte predicted in 1919 that, just as light is refracted when it passes from one medium to another, sound waves are refracted when they encounter slight changes in temperature, salinity, and/or pressure. He also suggested that ocean currents and seasonal changes affect how sound moves in water.

Scientists also discovered that low frequency sound could penetrate the seafloor, and that sound is reflected differently from individual layers in the subsurface sediment. For the first time, using sound, scientists could create a picture of what was beneath the seafloor. This provided clues to the history of the earth and a means for prospecting for oil and gas beneath the seafloor.  Pioneering work was done by Maurice Ewing, A. Vine, B. Hersey, and S. (“Bud”) Knott. In 1936-37, Ewing, Vine, and Worzel produced one of the earliest seismic recorders designed to receive sound signals on the seafloor. The need to generate high-energy, low-frequency sound that could penetrate deep into the seafloor led to the use of explosives and eventually to the development of air guns and high-voltage discharges (sparkers).

The development of ocean bottom seismic stations continued sporadically until the early 1960s when nuclear monitoring became important. Then, new generations of seafloor seismometers resulted from Vela Uniform, a U.S. project that was set up to develop seismic methods for detecting underground nuclear testing. In the seismic industry, Eivind Berg and coworkers were the first to develop 4-component ocean bottom sensors, more than 60 years after Ewing’s first trials.

Rapid Advancement

A German U boat: the study of acoustics was spurred by the use of submarines during World War I. Source: NumeriusThe passenger ship Titanic’s tragic sinking on April 14, 1912, encouraged developments of underwater detection systems. The French physicist Paul Langevin developed what he termed a “hydrophone” as a mechanism for ships to more readily detect icebergs. Similar systems were put to immediate use as an aid to underwater navigation by submarines.The beginning of WWII marked the start of extensive research in underwater acoustics. Nearly all the established methods of studying submarine geology were found to have military application, so progress in underwater acoustics, as in areas like radar and weapons, was shrouded in secrecy. At the end of the war, the U.S. National Defense Research Committee published a Summary Technical Report that included four volumes on research discoveries, but much of the work done during the war was not published until many years later, if at all.

The WWII effort focused on making careful measurements of factors that affected the performance of echo ranging systems. Things that affect the performance of sonar systems are described by what is now called the “sonar equation” which includes the source level, sound spreading, sound absorption, reflection losses, ambient noise, and receiver characteristics.

The rapid advancement of underwater acoustics continued after WWII, with wartime developments leading to large-scale investigations of the ocean’s basins. Coupled with advancements in technology (e.g. computers), underwater acoustics became an important tool for uses such as weather and climate research, underwater communication, and not the least, seismic exploration.

Lasse Amundsen is adjunct professor at the Norwegian University of Science and Technology (NTNU) and at the University of Houston, Texas.Martin Landrø is professor in Applied Geophysics at NTNU, Trondheim, Norway.
References:

www.dosits.org History of underwater acoustics

M Ewing and A C Vine 1938 Deep Sea Measurements without Wires and Cables, Trans. Amer. Geophys. Union, Part 1, 248–251

M Ewing, J L Worzel and C L Perkeris 1948 Propagation of sound in the ocean: The Geological Society of America, Memoir 27 

R. Rashid, “A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses,” Isis, 81, pp 464-491, 1990.)

Kirkwood, J.D. and Bethe, H. A., 1942, Office of Scientific Research and Development Report No. 588.

Keller, J.B. and Kolodner, I.I., 1956, Damping of underwater explosion bubble oscillations, Journal of Applied Physics, 27, 1152-1161.