Calculated Versus Observed Underwater Sound Speed - Essay Example

Calculated Versus Observed Underwater Sound Speed Word count: 2564 with glossary Aim The aim of this paper is to discover differences, if any, by comparison, of calculations of underwater sound speed that are made with calculations and with measurements. Objectives 1. Define sound speed 2. Discuss formulae used for sound speed. 3. Discuss observed methods of calculating sound speed. 4. Discover differences between calculated and observed sound speed. 5. Discuss the possible error that can occur. 6. Discuss the effects salinity and temperature have on the velocity of sound. Abstract Sound velocity or speed is very important to anyone using sonar equipment in areas where the salinity level, as well as other factors, changes. Sound is the most preferred energy to be used by underwater warfare, communications and navigation because its range of transmission is greater in comparison to other forms of energy, such as light or radio waves. When sound speed is calculated by formulae, how is it different than when observed with measuring devices What effect does the salinity or temperature of water have on the calculations for sound speed Introduction The report will present information that was ascertained in other research such as the studies at Dart Observatory at the Brittania Royal Naval College. It will analyze the influences on sound velocity, of salinity and temperature, and show how researchers find results and make determinations based on those analyses and calculations. The speed of sound in air is approximately figured out by the formula . . . speed of sound (m/s) = 331.5 + 0.60 T(C). The speed or velocity, at which sound travels through water was first researched by Sir Isaac Newton in 1687 when he found that measurements of sound in fluids relied only on the physical properties of the fluid, such as elasticity and density (Funk & Wagnalls). The speed of sound in water is about four times greater than that in air. Although this seems to contradict the physical law that the denser the gas, the slower the speed of sound, the sound speed is actually determined more by the elasticity of the medium (Urick, 1983). In 1822, Daniel Colloden used an underwater bell in an attempt to calculate the speed of sound underwater in Lake Geneva, Switzerland. His attempts resulted in figures remarkably close to today's accepted values (Acoustics . . . 2006). But sound speed cannot be discussed without mention of Jaque Sturm, French mathematician, who made the first accurate measurements of sound velocity in water in 1826. World War I created a great necessity to study the propagation of sound under water, with more progress in World War II and increased understanding from current research (Funk & Wagnalls). The fact that sound moves in a straight line in a medium of equal density (ibid.) led to studies of water variables. Sonar's accuracy depends upon: 1. The reflection of sounds propagated in water. 2. Whether sound is reflecting or refracting. 3. Levels of salinity, while generally constant in the open ocean, greatly changes how sound travels through shallow water. 4. Temperature, a foremost factor in sound speed calculations, usually becomes lower at greater depths of water, decreasing sound speed at about 3 m/sec per degree Celsius. Below 1000m, though, temperature becomes generally constant and pressure is the predominant consideration. But a depth change of about 165m can cause the same change in sound speed as a one-degree temperature drop. Acoustic Tomography (a type of underwater CT scan) and Sofar Floats are examples of technologies and instruments that measure the movement of large scale ocean water mass. A unique feature of the ocean is the Sofar Channel in the upper regions of the deep ocean. In this layer of the ocean, at about 1250 meters below the surface in the northwest Atlantic, the temperature and pressure act to provide a "long range acoustic path or channel"(Acoustic . . . 2006). The SOFAR float is an instrument designed to be neutrally buoyant at a certain depth and transmits timed acoustic pulses within this sound channel. The ALACE (Autonomous Lagrangian Circulation Explorer) Float, developed by Doug Webb, drifts for a month or more measuring the ocean environment and then rises to the surface and transmits its present position and findings to a satellite, which then sends data to the researchers. The ALACE can do this repeatedly for four years (Acoustic . . . 2006). Measuring equipment includes using a fixed buoy that has been equipped with several kinds of measurement devices, such as the Valeport 108 MKIII - a direct reading current meter that measures speed, direction, conductivity, temperature and pressure. With this device, researchers at Dart Observatory have found tidal measurement errors of less than 2cm. Previous studies could not rely on such accurate data. (Thain 1998) Another measuring device is a bathythermograph , BT. Older BT systems used a mechanical device that was lowered into the water on a cable. The temperature was then inscribed on a smoked piece of glass (Urick, 1983), which had some inherent problems. The expendable XBT does not require retrieval of the sensing unit, however; the sensing wire snaps off and recording of temperatures stops. When XBT temperature vs. depth trace calculations is converted to sound speed vs. depth, a sound speed profile is created similar to that from a sound velocimeter. Using an acoustic Doppler current profiler, ADCP, vertically fitted over the bow of a survey boat gives accurate data but has a one metre area at the top of the water column that does not allow data. To verify the ADCP measurements, an InterOcean S4 Buoy, a current meter, is used prior to a survey. Both instruments agree to within 5cm S-1 (Thain 1998). Comparisons with the CTD probes' calculations show negligible variances. The focus of the Dart Observatory projects (Priestley, Thain, Davidson) has been to discover the levels of erosion of sediments, to record the stratification of the estuary, to measure salinity and temperature, and to profile sound speed in the fronts as they occur. These studies have relied on Chen and Millero formulae as well as observed measurements to calculate sound speed. Conditions during neap tide cycles are radically different from spring tide cycles in their levels of mixing, salinity and temperature gradations, and cause sound speed to refract considerably differently than might be expected by other calculations. Chen and Millero (1977) explain that lake-water is not pure water, especially when calculations for density or pressure, temperature and other variables are considered. The properties of lake water can be determined from the equation for sea water provided that the total mass fraction of dissolved salts in sea water and lake water are equated. The international standard algorithm, known as the UNESCO algorithm, is attributed to Chen and Millero (1977), and has a more complicated form than the MacKenzie or Medwin equations. It also uses pressure instead of density as a factor. You must use another equation to convert data from pressure to density. There is a lot of debate about the accuracy or range of applicability with any of these equations. The choice of equation depends on the project and need for accuracy for which its being used. Because the time it takes to convert pressure statistics to depth is almost half the computation time, researchers look for a better equation to use (Wright 1996). Mellor (1991) found an accurate formula that relates in situ density to potential temperature, salinity and pressure, and uses about a third of the computation time, with smaller differences. In a study by Desiree Batton, the complexity of multi-beam sonar as it refracts sound requires the use of sound speed formula comparisons that use formulae of Chen and Millero, MacKenzie, and Medwin (Batton 2004). The Chen and Millero forumulae, which has over 30 terms, was used in Batton's study to graph the differences between calculations, because it has the median prediction for sound speed of the three formulae used (ibid). ). Differences between their formulae have been as large as 0.28m/s and as small as 0m/s for calculations of sound speed in a water column, although other studies have found even higher levels of difference (Dinn et al, 1997). A consistent finding in the barograph comparisons is that the Medwin formula predicts faster sound speeds than the Chen and Millero formula but then "gradually reduces speed with depth" (Batton 2004). The Mackenzie formula, on the other hand, predicts slower sound speeds than the Chen and Millero formula as a rule. One study of sound refraction has shown that the different sound speed formulae errors/differences escalate as the beam angle increases (ibid). The MacKenzie model does not seem to change in difference from one angle to another, while the Chen and Millero model varies from 1/2 metre at 35 degrees to as much as 1.5 metres at 55 degrees. Batton asserts that to have accurate depth data, the study must have local real-time sound speed profiles in the area under study. Significant mistakes can be made by the changes in temperature and salinity. Because taking continuous profiles every 0.7km is not realistic, Batton suggests reducing the maximum beam angle to 50-55 degrees, an angle not as affected by refraction. According to this study, the formulae are not particularly wrong, but have consistent errors for every project. Using direct reading sound velocity probes, for calculating refraction on oblique angles, would have no errors, by contrast. Also noted in this Batton study are the errors associated with equipment readings, such as the instrument that measures sound speed parameters and velocity at the face of the transducer. "The refracted horizontal distance error is +/-0.25m/s. If the instrument and sound velocity formula introduce errors in excess of 0.1m before measurements have even started" (Batton 2004), then it is difficult to meet the standards of the International Hydrographic Organisation. The dimensions of an estuary, the depth of the water, and the level of sedimentary mixing that occurs, as well as which type of tidal action is involved, will have effects on the propagation of sound, also. Research at the Tamar estuary created sound speed profiles using salinity, temperature and pressure data given by instruments. A tidal intrusion front is one in which the stratification of the estuary has produced a buoyant top layer that causes the influx of seawater to move under it, thus causing varying salinity levels and temperatures throughout the front. Even though fronts are generally transient events, studying the fronts has given researchers a more accurate assessment of the variables of sound speed (Lewis). Using formulae according to Tucker and Gazey, 1966, Applied Underwater Acoustics, profiles were divided into regions where the sound speed gradient was constant for ray tracing computations. The horizontal distance that a sound ray refracts while traveling through seawater is the result of this computation. In a detailed survey at the mouth of the Dart estuary it was found that the data on sound speed was very closely tied to the data on salinity at those same points. "At all stages of the tide, salinity is shown to be the dominant influence on sound speed" (Thain & Priestley), which shows how important the study and measurement of salinity levels is to the functioning of underwater equipment, such as the range of sonar. Along with measuring salinity, temperature variations must be included into the statistics to clearly find sound velocity patterns. But velocity is also influenced heavily and the refraction of sound and/or acoustic absorption occurs more often with dense levels of particulate matter in the water, so stratification/de-stratification is an important consideration as well. Altogether, data will show that observed measurements provide a valuable sound speed profile and are extremely necessary when trying to use formulae to calculate accurate assessments (ibid.). References Acoustics Primer, www.Institute For Marine Acoustics Sonar Primer_filesInstitute For Marine Acoustics Sonar Primer.htm, Accessed 01/06 Batton, Desiree. 2004, The Effect of Refraction on Oblique Angles of Multibeam Sonar, The Hydrographic Journal. No. 113 C-T Chen and F.J. Millero, 1977, The use and misuse of pure water PVT properties for lake waters, Nature Vol 266, 21 April 1977, pp 707-708. Dinn, D. F., Costello, G. and Loncarevic, B. D.,1995, The Effect of Sound Velocity Errors on Multi-Beam Sonar Depth Accuracy. Proceedings of Oceans 1995 Challenges of Our Changing Environment, San Diego, 1001-1010. Funk & Wagnall Encyclopedia, (1979) Vol. 22 Lewis, R., Uncles, R., Stephens, J., Riddle, A., Lewis, J., The Formation of Gravity Currents in the Tamar Estuary. www.mba.ac.uk/SWMSF/ECSA_abstracts/Lewis.doc Accessed 1/06. Medwin, H. and Clay, C., 1998, Fundamentals of Acoustical Oceanography. Academic Press, London. Mellor, G. L., 1991, An equation of state for numerical models of oceans and estuaries, Journal of Atmospheric Oceanic Technology, Vol. 8, 609-611. Priestley, A. D. and Street, P. R.,. 1998, Continuous Monitoring of the Dart Estuary: Initial Results. Oceanology International 1998 (Conference Proceedings Vol. 3), The Dart Observatory. Priestley, A. D. and Street, P. R., 1998, The Dart Observatory: an estuarine monitoring system. International Ocean Systems Design Vol. 1, Jan/Feb 1998 Speed of sound, en.wikipedia.org/wiki, Accessed 01/06 Thain, R. H. and Priestley, 1998, Sound Speed Variability Across An Estuarine Front. A. D., Dept. of Marine Science, Britannia Royal Naval College 6pp Thain, R. H., Priestley, A. D., and Davidson, M. A., The formation of a tidal intrusion front at the mouth of a macrotidal, partially mixed estuary: a field study of Dart estuary, UK. 2004 www.sciencedirect.com Accessed 1/06 Urick, R. J., 1983, Principles of Underwater Sound. McGraw-Hill, USA. 3rd ed. Wright, D., 1996 An Equation of State for Use in Ocean Models: Eckart's Formula Revisited, Journal of Atmospheric and Oceanic Technology: Vol. 14, No. 3, pp. 735-740. Nomenclature 1. Estuary - that part of a river's mouth or lower course where the river's current meets the sea's tide 2. Front - tidal intrusion; buoyant outflow at an estuary mouth becomes blocked by a strong tidal inflow of dense(r) seawater, the inflowing water plunges below the estuarine water forming a distinctive V-shape of foam and debris at the surface. 3. Halocline - a well defined vertical salinity gradient in ocean or other saline water. 4. Macrotidal estuary - has tidal range that reaches a maximum of 5.2m at springs. 5. Neap - designating tides midway between spring tides that achieve the least height. 6. Propagation - to breed; to transmit 7. Refraction - change of direction (of heat, light, sound, etc.) caused by passing from one medium into another where the wave velocity is different. 8. Ria - a long narrow inlet of a river that decreases in depth from mouth to head, drowned river valley. 9. Salinity - The measurement of dissolved salts in water. Usually given as (x) grams salts in 1,000g water. 10. Seasonal estuary - an estuary in which salinity at any one geographic point changes seasonally. 11. Sonic - pertaining to a speed equal to that of sound at the same height above sea level 12. Sound - transmitted energy; also narrow inlet of water between oceanfront land and mainland. 13. Spring tides - Fortnightly tides occurring when the vertical tidal range is maximal. 14. Stratification (SPM) - the presence of different infaunal species at distinct respective horizons below the sediment-water interface in parts per million. 15. Tide gauge - for measuring the level of a tide; usually equipped with a marigraph 16. Valeport 108 MKIII - direct reading current meter that measures speed, direction, conductivity, temperature and pressure. Tidal measurement errors of less than 2cm. Read More

 

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