Acoustic Remote Sensing Applications
This book, which is divided into three parts, gives a state-of-the-art report on technical developments in instrumentation and on theoretical advancements in acoustic remote sensing. It explains the utilization of acoustic techniques in studies related to
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Springer-Verlag Berlin Heidelberg GmbH
Foreword Acoustic remote sensing is one of the most useful methods in our technological kit of tools for research and modification of our environment. In the ocean sonars provide information on depths and bottom profiles, locate shoals of fish, trace motions of pollutants in the benthic boundary layer, and measure the spectrum of wind-driven waves in the oceanic surface layer. In the atmospheric boundary layer where we (other than "frequent fliers" and astronauts) live most of the time, sodars (or echosondes) provide data on wind shears, turbulence parameters needed for pollution control, and inversion heights. The earliest form of acoustic remote sensing--rangefinding--antedates the use by humans. Some species of bats emit sound pulses with frequencies up to 200 kHz, and determine distances to ground, cave walls, or prey from the time delay of the echoes. Similarly, some dolphin species find ranges to shores, colleagues, or shoals of.fish in the same way. For man, the use of sound for remote sensing could not begin until the speed of sound was known. The first measurement of the speed of sound in air probably was that of Mersenne 1 using rather primitive methods: eyes and ears to detect a musket shot at night, with the delay between the two timed by his own pulse or, later, by the swings of a pendulum. A few decades after Mersenne's work, William Derham 2 invented an improved pendulum timer that, together with longer paths, allowed more accurate measurements of sound speed. These results, presented by Derham at the Royal Society, induced the President of the Society, Isaac Newton, to try to improve his incorrect prediction (from assuming an isothermal process) by unconvincing mathematics. The discrepancy remained for almost a hundred years until Laplace 3 obtained the correct value by assuming an adiabatic process. With a reasonable estimate of ambient sound speed in hand, Derham was able to pursue answers to a number of questions about the nature of sound. Among other results, he proved that wind along the sound path linearly increased or decreased the measured apparent speed, depending on whether the wind was blowing toward or away from the observer. Values of wind were obtained by another of his inventions, throwing up chaff (from grain-not radar chaff) and I F.M. Mersenne, Harmonicorum Liber., Paris (1636). z W. Derham, Experimenta et observationes de soni motu, Phil. Trans. Roy. Soc. London, 26, 2-32 (1708). 3 P.S. Laplace, M6chanique C61este, Paris (1799).
vi Foreword measuring the distance it traveled before hitting the ground. Recently, Delaney 4 plotted Derham's data on sound speed vs wind, clearly showing the linear dependence as claimed. Finally, Derham advanced and explained the echo-delay method for range finding. For example, using a building on the far bank as a reflector, he obtained reasonably accurate measurements of the width of the Thames near London. Plans to make a similar measurement of the width of the English Channel were aborted by the outbreak
