Differences in Sound Velocity Variability between the Dart and Tamar Estuaries - Essay Example

Comparison in Sound Velo Variability over Neap and Spring Cycle in Dart and Tamar Estuaries, UK, and Its Impacts on Sonar Performances Aim The aim of this paper is to examine the differences in sound velocity variability between the Dart and Tamar estuaries using data from an oceanographic study. To investigate the temporal variability in sound velocity from one location in the Dart and Tamar estuaries with time. Objectives 1. Plan and conduct an oceanographic survey. 2. Present the data visually using appropriate techniques. 3. Analyse the variability in sound velocity. 4. Present the results and analysis in a formal scientific report. 5. Determine variations in sound velocity profiles. 6. Determine variations in sound velocity gradients. 7. Determine variations in surface sound velocity. 8. Determine variations in bottom sound velocity. Abstract Sound velocity or speed varies considerably in different conditions underwater. One of the biggest differences is the level of salinity in the water, so where seawater meets freshwater there is a dramatic change in sound velocity. This information is very important to anyone using sonar equipment in areas where the salinity levels, as well as other factors, changes. Sound is the most preferable energy to be used by underwater warfare, communications and navigation because of its range of transmission in comparison to other forms of energy, such as light or radio waves. Studies of the variables of sound transmission aid those who need to know how well their equipment will function given different environments. Introduction This project entails the planning and execution of an oceanographic survey to determine the differences in sound velocity variables over the neap and spring tide cycles in the Dart and Tamar estuaries, and their impact on sonar-related performances. This study is largely based on research provided by Desiree Batton in conjunction with the Hydrographic Society, The Dart Observatory at the Britannia Royal Naval College and Graham Tattersall of the CEFAS Lowestoft Laboratory, as well as additional research. The tide cycles at the mouths of these two estuaries differ in some factors that influence the movement of sound. This paper plans to show what those factors are and how they affect sonar usage. The report will present data that was ascertained by using the current appropriate techniques and tools. It will analyze the variability of sound velocity, present the results and make determinations based on those analyses and calculations. Several determinations will be necessary to conclude this paper, such as the variations in bottom sound velocity as opposed to the variations in surface sound velocity. For instance the calculations for bottom sound velocity will take the higher level of mixing into account, while the calculations for surface sound velocity will need to look at temperature variables more, especially spring tide. Differing types of measuring equipment will be needed to gain the information required. In addition, time of day and the varying points of tidal activity are considerations that will add into the data. Limitations of this report may be found in the inaccuracy of some instruments that measure salinity or other factors, and . . . Undertaking this project is an effort to coalesce the information previously put forth by researchers of these two estuaries and discover causes for the variables between the them. This project is designed to correlate with other projects that focus on sound speed or sound velocity. The need to understand how sound travels through tidal waters, in particular the Dart and Tamar estuaries, is of great significance to ongoing research. The research of A. D. Priestley and R. H. C. Thain, as well as M. A. Davidson and others is reflected in this project. Their work, which has increased the understanding of estuarine stratification, “fronts” and hydraulic dynamics, lay much of the groundwork for future projects. The focus of the Dart Observatory projects have been to discover the levels of erosion of sediments, to record the stratification of the estuary, to measure salinity and temperature, and to profile the “fronts” as they occur. The speed, 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. The first accurate measurements of the speed of sound in water were made in 1826 by the French mathematician Jacque Sturm. Studies of the generation and propagation of underwater sound arose from the military necessity of World War I, with great progress in World War II and increasing attention in current times (Funk & Wagnalls). Learning more about how sound refracts as it moves through varying tidal conditions in shallow water will aid those in the fields of minewarfare and sonar operations (Thain & Priestley). Sound moves in a straight line in a medium of equal density. (Funk & Wagnall) Sonar depends on the reflection of sounds propagated in water. Whether sound is reflecting or refracting is of great importance to sonar applications. Levels of stratification and levels of salinity greatly change how sound travels through shallow water. Accurate calculations and research will give sonar operations more accurate and reliable results. Development of sound speed profiles will also aid in further research and the development of sonar devices. The complexity of the estuarine tidal variables requires the use of sound speed formula comparisons based on formulae of Chen and Millero, and MacKenzie and Medwin (Battonn 2004). Differences between their formulae have been as large as 0.28m/s and as small as 0m/s for calculations of sound speed in the water column, although other studies have found even higher levels of difference (Dinn et al, 1997). One study of sound refraction has shown that sound speed formulae errors escalate as the beam angle increases (Batton 2004). This shows the challenges researchers face when trying to accurately create a profile. David Prandle of the Proudman Oceanographic Laboratory in Liverpool, states in his Evolution of Estuaries – Past and Future, that studying estuaries: can provide perspective on future impacts of global climate change, can provide a “framework” for comparative research into predicted changes, and can “predict which estuaries are likely to be most sensitive to change”. In the Dart estuary, factors that have influenced the research into sound velocity have been: the changes in salinity as the “fronts” move, the temperature differences between upper layers and lower layers, and the stratification of sediments and pollution that occurs in varying degrees depending on the time of year. These three things alone will affect the levels of sound velocity greatly. This project will show that conditions during neap tide cycles are radically different from spring tide cycles in their levels of mixing, salinity and temperature gradations, which would cause sound speed to refract considerably differently than might be expected. The Dart Observatory through the Britannia Royal Naval College’s Marine Environmental Science Department has done extensive research of the Dart estuary since 1997 to study the temporal variability and try to discover trends within the data. From this research, this paper will develop conclusions, and, along with additional data gathering, try to find out what the comparisons are of the sound velocity variables during neap and spring tides, and compare these findings to those of the Tamar estuarian studies. Snell’s Law of Refraction is a practical result of ray theory (a tracing out of sound paths) that states that for a ray incident on the interface of two media, the sine of the angle of incidence times the index of refraction of the first medium is equal to the sine of the angle of refraction times the index of refraction of the second medium. (1626). A sound wave, which enters another medium or a layer of the same medium that has different characteristics, will change direction and/or speed. “A sound ray always bends toward the region of slower sound speed” (textbook not named). Using this law, conclusions will be drawn as to the refraction of sound in shallow water, as well as seawater. Sound velocity propagation in shallow waters have variables that include: the dimensions of the estuary, the depth of the water, and the day and night temperatures of the water. Variables also include the level of mixing that occurs dependent upon which type of “front” is dominant, as well as which type of tidal action is involved. Unlike the research at Dart estuary, research at Tamar estuary created sound speed profiles using salinity, temperature and pressure data given by instruments. Profiles were then divided into regions where the sound speed gradient was constant for ray tracing computations, using formulae according to Tucker and Gazey, 1966, Applied Underwater Acoustics. This computation yields the horizontal distance that a sound ray refracts while traveling through seawater. The Dart estuary fluctuates from well-mixed to partially mixed depending on freshwater discharges and tidal mixing. The spring neap transition produces a marked de-stratification/stratification level as compared to the autumn and winter levels (Priestley & Street, 1997). There are different types of estuaries. The Dart estuary has tidal currents of 2.5 knots during mid-ebb spring tides, and reaches a maximum of 5.2m at spring tides, which gives it the classification of macro-tidal. The Tamar estuary is meso-tidal, a ria that lies on the border of Devon and Cornwall. This estuary system includes the Tamar, Lynher and Tavy rivers that drain into the English channel (Tattersall 2000). Graham Tattersall, a physical oceanographer and hydrodynamic modeller, works with CEFAS Lowestoft Laboratory to examine sediment erosion and transport in these waters. His observations, documented in his thesis for the University of Wales, show that “suspended sediment concentrations increase on the late ebb at spring tides, and return to background level on the flood tide” (p289). These increased concentrations at spring tide are similar to those at Dart estuary. In a report by Hydro Tasmania, Scientific Report on the Trevallyn Elver (Glass Eels) Passage, concerning movements of fish near the Trevallyn Dam, the water discharge into the Tamar estuary was researched. The result of those studies were that variable flow velocities and water chemistry from this unnatural discharge did not affect the water levels of the Tamar estuary very much and that the Tamar’s water levels are “mainly determined by tidal conditions”. Using a fixed buoy that has been equipped with several kinds of measurement devices, including the Valeport 108 MKIII – a direct reading current meter that measures speed, direction, conductivity, temperature and pressure, the researchers at Dart have gotten tidal measurement errors of less than 2cm. Such accuracy gives their research much more validity than previous studies. A front is the area where the ocean meets the mouth of an estuary. There are several types of fronts, such as plume fronts, tidal mixing fronts, shear fronts and tidal intrusion fronts. The research at Dart estuary studies these fronts. While the neap tide does not exhibit a front, the spring tide brings a tidal intrusion front into the estuary. 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. This occurs one hour after low water (LW+1), and results in a supercritical flow condition, then a seabed depression, and finally baratropy causes the front to be arrested over the seabed depression, which decays the front at LW+3 (Thain et al., 2004). Tidal intrusion fronts are important criteria when determining the patterns of estuarine circulation and mixing (Simpson & Turrell, 1986). Some of the influences caused by tidal fronts are: buoyant pollutants that mix into the water column (Simpson & Turrell, 1986), persistent anoxic pools of water that form in parts of the estuary, increases in biological activity, the movement of sediment and modified sonar propagation patterns (Priestley & Thain, 2003). Even though fronts are generally transient events, studying the fronts will give a more accurate assessment of the variables of sound speed. Research has found that the strongest flows in the surface layer, as well as the vertical stratification, along with the weakest tidal flows happen at neap tides (LW-1). There is a definite variation in surface particulate matter (SPM) concentrations between neap and spring tides that can range from 0mg/L at neap to 60mg/L at spring tide. Bed SPM concentrations can be even more dramatic, ranging from 20mg/L at neap to 120mg/L at spring tide. In studying tidal patterns, it was also discovered that the variability of sound speed structure affected sonar propagation patterns, and the converging currents may cause acoustic absorption (Thain & Priestley). The structure of a front, and its flow dynamics, characterize a front and define the boundaries that will cause marked sound speed variables. Fronts travel in landward directions and decay after a few hours. A detailed survey at the mouth of the Dart estuary was taken March, 2002, in which flow, temperature, salinity, density and sound speed were evaluated. It was found that the data on sound speed at LW-1 and LW+1 was very closely tied to the data on salinity at those same points. Between LW-1 and LW+1 there were marked changes in both salinity and sound speed. According to A. D. Heathershaw, high SPM concentrations present in the frontal zone are likely to lead to marked attenuation of high frequency sonar by absorption processes. In a study by Plymouth Marine Laboratory (Lewis et al) at the Tamar estuary, the determination of current and salinity profiles on two low range tides resulted in findings that a “landward directed near-bed flow developed shortly before predicted low water on both days” of the study. The saline intrusion was part of a “marked halocline that persisted throughout the tidal period” (Lewis et al). “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, as well as predicting the range of sonar. At the same time, the levels of stratification between neap and spring tides are a factor that requires serious examination and inclusion into the overall data. Because there is virtually no front at neap tide, the estuary stratifies completely (Thain & Priestley) for a short time before the spring tide de-stratifies the estuary. 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, too, in determining sound speed. In addition, temperature variations must be included into the statistics to clearly find sound velocity patterns. Altogether, this information should provide a valuable sound speed profile. References Batton, Desiree. The Effect of Refraction on Oblique Angles of Multibeam . . . The Hydrographic Journal. No. 113 2004 Dinn, D. F., Costello, G. and Loncarevic, B. D., 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. 1995 Funk & Wagnall Encyclopedia, (1979) Vol. 22 Heathershaw, A. D., Richards, S. D. and Thorne, P. D. Acoustic absorption and scattering by suspended sediments in turbid coastal waters. Journal of Defence Science. Vol. 1, No. 2. 1996 Lewis, R. E., Dispersion in Estuaries and Coastal Waters. John Wiley & Sons, Ltd., Chichester, U.K. 312 pp. 1997 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., Fundamentals of Acoustical Oceanography. Academic Press, London. 1998 Prandle, D., Evolution of Estuaries – Past and Future, www.mba.ac.uk/SWMSF/ECSA_abstracts/Prandle.doc Accessed 1/06. Priestley, A. D. and Street, P. R.,. The Dart Observatory. Continuous Monitoring of the Dart Estuary: Initial Results. Oceanology International 1998 (Conference Proceedings Vol. 3) 1997 Priestley, A. D. and Street, P. R., The Dart Observatory: an estuarine monitoring system. International Ocean Systems Design Vol. 1, Jan/Feb 1998 Simpson & Turrell., Referenced in Priestly & Thain’s paper: The Physical Oceanography of the Lower Dart Estuary: Temporal Trends and Frontal Dynamics. 1996 me.lab@virgin.net Accessed 1/06 Tasmania, H., South Esk – Great Lake Water Management Review, Scientific Report on the Trevallyn Elver Passage, p. 2, Aug. 2003, www.tamar-trac.com/research/trevally_eels.pdf Accessed 1/06 Tattersall, G. R., Tamar Estuary Sediment Dynamics, PhD Thesis, University of Wales, Bangor, 289p. 2004 Thain, R. H. and Priestley, A. D., Dept. of Marine Science, Britannia Royal Naval College: Sound Speed Variability Across An Estuarine Front. 6pp 1998 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., Principles of Underwater Sound. McGraw-Hill, USA. 3rd ed., 1983 1. Barotropy – a state of fluid stratification in which surfaces of constant pressure and others of constant density do not intersect but are parallel. 2. Estuary – that part of a river’s mouth or lower course where the river’s current meets the sea’s tide 3. 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 (Simpson & Nunes, 1981; Dyer, 1997). 4. Halocline – a well defined vertical salinity gradient in ocean or other saline water. 5. Macrotidal estuary – has tidal range that reaches a maximum of 5.2m at springs. 6. Neap – designating tides midway between spring tides that achieve the least height. 7. Propagation – to breed; to transmit 8. Refraction – change of direction (of heat, light, sound, etc.) caused by passing from one medium into another where the wave velocity is different. 9. Ria – a long narrow inlet of a river that decreases in depth from mouth to head, drowned river valley. 10. Sonic – pertaining to a speed equal to that of sound at the same height above sea level 11. Sound – a relatively narrow passage of water between larger bodies of water, or between mainland and island; also, transmitted energy, topic of this paper. 12. Tide gauge – for measuring the level of a tide; usually equipped with a marigraph 13. Valeport 800 series– electromagnetic current meter that measures current speed and direction. A fixed system fitted to the base of a buoy, one metre below SWL. 14. Valeport 108 MKIII – direct reading current meter that measures speed, direction, conductivity, temperature and pressure. Tidal measurement errors of less than 2cm. 15. 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 (Simpson & Nunes, 1981; Dyer, 1997). 16. Refraction – change of direction (of heat, light, sound, etc.) caused by passing from one medium into another where the wave velocity is different. 17. Salinity – The measurement of dissolved salts in water. Usually given as (x) grams salts in 1,000g water. 18. Seasonal estuary – an estuary in which salinity at any one geographic point changes seasonally. 19. Spring tides – Fortnightly tides occurring when the vertical tidal range is maximal. 20. Stratification – the presence of different infaunal species at distinct respective horizons below the sediment-water interface. Read More