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 published 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. Tis 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. Te 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 airguns and high-voltage discharges (sparkers). Te development of ocean-bottom seismic stations


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


3.6.2 Rapid Advancement


Te 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 applications, 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. Te WWII effort focused on making careful measurements


of factors that affected the performance of echo ranging systems. Tings that affect the performance of sonar systems are described by what is now called the ‘sonar equation’, and includes the source level, sound spreading, sound absorption, reflection losses, ambient noise, and receiver characteristics. Te rapid advancement of underwater acoustics continued


after WWII, with wartime developments lead ing to large- scale investi gations of the ocean’s basins. Coupled with advancements in technology (e.g. com puters), underwater acoustics became an important tool for uses such as weather and climate research, underwater communication, and not least, seismic exploration.


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Figure 3.43: Extract from Ibn Sahl’s treatise On Burning Mirrors and Lenses 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.


3.6.3 Some Masters of Underwater Sound


Te word ‘science’ is derived from the Latin word ‘scientia’ which means ‘knowledge’. Here we would like to digress and present some major scientific theories and discoveries, spanning many years, which eventually led to developments in acoustics. As inventions and discoveries added to one another, technology and science advanced and evolved. It is important to realise that all successes require a long journey with many failures and failed experiments along the way. We hope that the little stories that follow inspire your curiosity and whet your appetite for discovery.


Aristotle (384–322 BC), the Greek philosopher, in his treatise On the Soul, wrote that “sound is a particular movement of air.” He understood that sound consisted of compressions and rarefactions of air which “falls upon and strikes the air which is next to it…” – a very good expression of the nature of wave motion. A notable quote, true on so many levels, is: “Knowing yourself is the beginning of all wisdom.”


Figure 3.42: Aristotle.


Ibn Sahl (940–1000) was a mathematician, physicist and optic engineer at the Abbasid Court in Baghdad. Under the Abbasid caliphate (750–1258), the focal point of Islamic political and cultural life shifted eastward from Syria to Iraq. In 762, Baghdad, the circular City of Peace, was founded as the new capital. Te first three centuries of Abbasid rule were a golden age when Baghdad was one of the cultural and commercial capitals of the Islamic world. In 1990 the Egyptian science


historian, Professor Roshdi Rashed, credited Ibn Sahl with developing the first law of refraction, also known as Snell’s Law, named after Willebrord Snellius (1580–1626). Ibn Sahl used the law of refraction to derive lens shapes that focus light without geometric aberrations.


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