543 10.26 Maximum peak-to-peak source levels of high-frequency clicks of marine mammals 546 10.27 Peak equivalent RMS and peak-to-peak source levels of low-frequency marine sounds.
WHAT IS SONAR?
Wee represents and imitates all articulated sounds and letters and voices and tones of animals and birds. Without light in their natural habitat, dolphins have developed a sophisticated form of sonar over millions of years, without which they would be nearly blind.
PURPOSE, SCOPE, AND INTENDED READERSHIP
The strength and character of the echo can also provide an indication of the soil type. Fishing sonar: Sonar equipment used by the fishing industry uses the same principle as the echo sounder, except that its purpose is to detect fish rather than the seabed.
STRUCTURE
It concludes with a speculative account of the possible future development of sonar performance modeling in the 21st century. It contains information about fish and their swim bladders that will be useful to a reader interested in the interaction of sound with fish or fish schools.
A BRIEF HISTORY OF SONAR
The ability to use the M-V tube in motion was a great advantage, but it came at a price - the noise of a ship en route. Although alternatives to quartz had been explored in the interwar years, particularly in the U.S.A., the asdic sets used by the Royal Navy still relied on the quartz technology developed during World War I.
1975) Investigation of the absorption of 1 kHz sound in seawater, PhD thesis, University of California, San Diego. 1967) Analytical Description of Low Frequency Attenuation, J. 1967) Principles of Underwater Sound for Engineers, McGraw-Hill, New York.
ESSENTIALS OF SONAR OCEANOGRAPHY
If the source is at depth z0 below the surface (see Figure 2.3), the contribution to the pressure field at the receiver due to the direct path is given by equation (2.12) with a source-receiver separation equal to . One of the factors that limit the performance of sonar is the presence of background noise in the sea.
ESSENTIALS OF SONAR SIGNAL PROCESSING
If the signal is well sampled in time (so that jð!!mÞ tj 1), the denominator of equation (2.66) can be approximated by the argument of a sine function. Specifically, full width at half maximum (fwhm), i.e. the spectral width between the half-power points is given by By exact analogy with the time-domain filter, if the origin is at the geometric center of the array, it follows.
Here, the distance Dx plays the role of D in the time domain filter and determines the resolution of the spatial filter in the wavenumber domain. The squared magnitude of the normalized array response for an incident plane wave, known as the array beam pattern, is. Because of its ability to selectively amplify sound waves coming from a narrow range of angles ("beam"), the surround filter is called abeamformer.
ESSENTIALS OF DETECTION THEORY
Dxcosm deg: ð2:94Þ This approximation for the beamwidth works best at angles close to the broadside direction. Due to statistical fluctuations in the noise, there is always a chance that the threshold will be exceeded when there is no signal, and conversely there is also a chance that the threshold will not be exceeded even when the target is present. Let the probability density function (pdf ) of the observable noise befNðxÞ, so that the mean and variance of the distribution are.
The probability that this will occur due to a single observation is equal to the area under the pdf curve to the right of the threshold. In each case, expressions are quoted for the false alarm probability pfa and detection probability pd as a function of the signal-to-noise ratio (SNR). For an amplitude threshold AT, and assuming a Rayleigh distribution for the signal as well as for the noise, the false alarm and detection probabilities are
IfRandMare are both large, no detailed analysis is usually necessary, because in this situation the detection probability is always close to unity. If you stop your ship, and place the head of a long tube in the water, and place the other end against your ear, you will hear ships at a great distance from you Leonardo da Vinci (15th century).
INTRODUCTION
Regardless of the application, sonar performance must depend on the probability of successful detection each time the sonar is used. Less obvious, but equally important, is the observation that the effectiveness of sonar also depends on the number of false alarms,3 due to the time and other resources spent on investigating them. Much of the modeling of sonar performance, and the main focus of this book, is concerned with calculating detection and false alarm probabilities for a given scenario or scenarios.
The above definitions of signal and noise are necessarily vague, as the distinction between them depends on details of the signal processing that have not yet been specified. The noise definition as ''all sound that is not part of the signal'' means this. The conversion to decibels turns the product of ratios into a sum of the logarithms of these ratios.
PASSIVE SONAR .1 Overview
This is partly because standards can and do change with time and circumstances, and partly because not all decibel users adhere to these standards.8 The only safe 3.2 Passive sonar 59 Sec. The range at which this occurs (2.45 km) is the detection range. The second crossing, at 3 km, has no special significance.). To answer part (iv), it is convenient to rearrange the sonar equation in the form (see Eq. 3.139).
The signal energy is the integral of the square of the pressure over the pulse duration t (Equation 3.150). The important noise is that in the processing bandwidth (ie the width of each Doppler binf). In other words, "in" indicates the acoustic field incident on the target, and "0" indicates the scattered field at small distances from the target.
LNf ¼NLf AG: ð3:222Þ With these definitions, the sonar equation becomes. the sonar using a pulse with a duration of 0.2 s. ii). Linear signal excess and double detection probability (Equation 3.68) for incoherent CW active sonar.Above: vs. range for array depth 100 m – the two curves intersect at the detection range, where ¼2pd¼1;lower:vs. array depth for target range 900 m. probability), it does not affect the predicted detection range here.
An important characteristic of the sea is its depth. In the oceans, water depth values between 2 km and 5 km are common, rising to about 10 km in the deepest ocean trenches. The continental shelves have a water depth of typically 20 m to 200 m. m. There is little seasonal variation, so only the annual average is shown. The Atlantic Ocean is about one degree warmer than the Indian and Pacific Oceans. This is a consequence of the oceanic thermohaline circulation, a global conveyor belt that begins its cycle as surface water in the North Atlantic.4. Annual mean temperature map at 3 km depth from the World Ocean Atlas (WOA, 1999). The deep water temperature in major non-polar oceans is between 1C and 3C. Green indicates land (or water depth less than 3000 m).
SðS;YÞ ¼0:924½1þ0:0018ðS35Þ YY^ expð2431=YYÞ^ nPa s: ð4:19Þ A related parameter is the bulk viscosity (also known as bulk or compression viscosity as volume or compression viscosity is the value given by viscosity 4 by viscosity 8). . The presence of bubbles can have a significant effect on sound attenuation.11 Also important, especially in coastal regions, is the possible presence of large numbers of fish. Discussion of the effects of bubbles and fish on attenuation is deferred to Chapter 5. The current focus is on the effects of S, T, z, and K. The applicable pH scale is not stated (the author was unaware at the time of the ambiguity), but it is the same scale as used by Francois and Garrison (1982b). In the absence of a statement to the contrary, it is assumed here that Francois and Garrison (1982b). Garrison used the same pH scale as Mellenet al. (1987), which, according to Brewer et al. (1995), is the NBS scale. Conversion from pH to K to obtain equation (4.32) was done using equation (4.14).
PROPERTIES OF BUBBLES AND MARINE LIFE
Fractional sensitivity of seawater attenuation (equation 4.35) to temperature (T), salinity (S), acidity (parameterized through K), and depthzÞ. The attenuation coefficient is calculated using equations (4.26) to (4.33). The last column of Table 4.3 shows this aspect ratio X, estimated for each species by replacing the volume V with that of the animal. The value of X varies between 0.11 (using m=L37 kg/m3, for the franciscana dolphin and sperm whale) and 0.26 (m=L337 kg/m3, for the northern sea lion, walrus and elephant seal). The elasticity of fish meat is determined by the (complex) shear modulus. The real part determines the pressure exerted by the bladder wall on the gas content (Andreeva, 1964).
The imaginary part of determined losses due to vibration of the flesh. The value of is subject to considerable uncertainty but a typical value, attributed by Love (1978) to 4.2 Properties of Bubbles and Marine Life 153 Sect.4.2]. Acoustic properties of whale tissue. Attenuation measurements are standardized for easy comparison by dividing by frequency and presented in units of dB/(m kHz). The actual measurement frequencies are 100 kHz (Miller and Potter, 2001) and 10 MHz (Jaffeet). al., 2007). The geographical distribution of two important North Sea species is shown in Figure 4.18, including an indication of the variation with season (summer vs. winter) and fish size (adults vs. juveniles). The data show that, during the 1980s, herring were common. in both summer and winter throughout the North Sea except in the 4.2 Characteristics of bubbles and marine life 157 Sec.4.2].
PROPERTIES OF THE SEA SURFACE
Wind speed scaling factors to convert from a reference height of 20 m to the standard reference height of 10 m (Dobson, 1981). The legend shows the temperature difference jTairTwaterjinC. Red curves indicate stable conditions (Tair>Twater); blue curves indicate unstable conditions (Tair For simplicity, it is assumed here that the difference between the wind speeds at 10 m and 12.5 m can be neglected. of square displacement from the mean surface) vs. surface wave frequency O, for wind speed between 0 m/s and 20 m/s, is of the form. Sea State Description Significant RMS Wind Speed of Estimated (WMO code wave roughness corresponding to Pierson–Beaufort . 3700) height Moskowitz spectrum power. Wind-generated bubbles near the ocean surface are mainly caused by burning waves. The number of whitecaps, and thus the bubble population density, is highly correlated with wind speed. Bubble density is highest near the sea surface and decreases with increasing depth away from the sea surface. Some measurements of bubble population density, from Trevorrow (2003), are shown in Figure 4.20.
