CHAPTER 1 INTRODUCTION

Transcription

1 CHAPTER 1 INTRODUCTION Water is an efficient medium for the transmission of the sound. Sound travels more rapidly and with much less attenuation of energy through water than air. This characteristic resulted in development of submarine acoustic methods that are of tremendous value in navigation (Sverdrup et al., 1961). Sound speed varies considerably from the ocean surface to the ocean floor and this gradient determines the direction that sound waves travel in the ocean. A sound speed profile (SSP) graphically depicts these changes. SSPs vary both in time and space. The largest temporal fluctuations are in the upper ocean, mainly on account of diurnal and seasonal variations of temperature and salinity (T/S) (Brekhovskikh and Lysanov, 2003). Conventionally, SSPs are either obtained by a velocimeter that measures sound speed in the ocean directly or by using the in situ T/S profiles (Lii et al., 2003). Nevertheless, these observations have limitations in both temporal and spatial resolutions. Estimation of SSPs using surface parameters obtainable from remote sensing platforms overcomes this problem and provides an ample opportunity to monitor operationally the changes in SSPs more frequently at a finer spatial resolution over larger spatial scales. The main objective of this work is to estimate the SSP from surface parameters alone, which are also obtainable from remote sensing platforms, using artificial neural network approaches. 1.1 IMPORTANCE OF SOUND SPEED PROFILES IN THE OCEAN Acoustic systems play an essential role in many civil and military applications. The speed of sound in the ocean is the most important oceanographic parameter that determines many of the characters of sound transmission in the ocean. Sound speed is commonly measured by a velocimeter directly, or by hydrological observations of temperature, salinity and depth. Because the sound speed in the ocean increases with temperature, salinity and pressure, it varies significantly with time of day, season and depth. The horizontal variations of sound speed in ocean are usually slight, but 1

2 are quite strong in particular zones and cannot be neglected. Due to the stratification of temperature and salinity, the sound speed is approximately horizontally stratified. In typical situations, the stratified ocean may create convergent zones and shadow zones, depending on the variation of sound speed with depth (Clay and Medwin, 1977). Spatial variation of sound speed cause acoustic rays to bend according to Snell's law (Urick, 1983) and sound is partially reflected as the sound speed varies sharply. Hence, an important requirement of navy for strategic applications is the accurate determination of sound speed structure in the ocean. Acoustical properties of the ocean environment largely determine the submarine operations and their characteristics. SSP is having potential applications for the anti-submarine warfare (ASW). The sound speed structure is of particular importance since it has a direct impact on the way sound energy travels through the ocean. This is a critical issue for the navy since it employs sound energy as the primary method of detecting and locating submarines and mines. In "active" sonar systems, a ship, plane, or submarine will use sonar to create a pulse of sound in the water and will then listen for echoes bouncing off the target submarine or mine. In "passive" sonar systems, the searcher listens for the weak acoustic signals generated by the target submarine itself. In either case, the structure of the sound speed environment radically affects the propagation path and mode of the sound energy. Therefore, knowledge of the sound speed structure is required to correctly tune the sonar systems to the environment and to choose the appropriate set of tactics to locate the target. Variations in sound speed refract the acoustic energy and can focus it much as lenses do with light. These variations occur in both the horizontal and vertical planes due to both dynamic oceanographic processes and hydrostatic effects due to depth. A submarine can take advantage of these variations by positioning itself to be "acoustically invisible" to ships and planes that may be searching it. On the other hand, if those ships and planes know the sound speed structure in an area, they can use that information to improve their chances of detection by sending the signals from a different location. SSPs in the ocean are also important for global underwater communications. Sound in the sea can often be trapped and effectively carried very long distances by the 2

3 deep sound channel that exists in the ocean. These sound channels can be used for the underwater communications. Sound channels can be detected by knowing the sound speed structure of the ocean. 1.2 SOUND SPEED PROFILE IN THE OCEAN Acoustic signals can travel great distances in the ocean and for that reason a wide range of scientific applications use them. The speed of sound varies with the characteristics of the medium through which it travels. Sound travels about four times faster in water than in air because of differences in the properties of the two media. The most characteristic feature of the oceanic medium is its heterogeneous nature. Sound speed varies with the depth of the ocean and its variation is directly proportional to the changes in temperature, pressure and salinity. Graphical representation of change in sound speed with changing ocean depth is called an SSP and is shown in Figure Sound Speed (m/s) ?200.c 300 Q Figure 1.2.1: Sound speed profile in the ocean Factors Affecting Sound Speed in the Ocean The ocean is divided into three main layers according to its subsurface thermal structure (Figure 1.2.2). The first layer is isothermal layer or the mixed layer in 3

4 which temperature remains almost constant. The second layer is the thermocline where temperature decreases rapidly with depth and the third is the deep layer in which temperature decreases gradually. Primarily, temperature, salinity, and pressure are the three factors, which affect sound speed in the ocean (Kuperman, 1997). Within the mixed layer, the pressure, which increases with depth, plays the most important role and results in increasing sound speed with depth. On the other hand, in the thermocline region, effect of decreasing temperature dominates over the effect of increasing pressure. In this region temperature decreases rapidly with increasing depth, hence sound speed decreases even though the pressure increases. In the deep layer as the temperature changes are less dominant; the increasing pressure with depth primarily dominates and sound speed continuously increases with depth up to the bottom of the ocean. The effects of salinity on sound speed propagation are extremely complex though sound speed increases with increase in salinity. Thus, temperature, pressure and salinity profiles in the ocean determine the speed of sound. \ i '. \ \MIXED LAYER E J3 O -* Salinity - Pressure - Temperature Figure 1.2.2: Subsurface temperature, salinity and pressure profiles Some of the surface parameters affecting the T/S profiles in the ocean and in turn affecting the SSP also are sea surface height (SSH), sea surface temperature (SST), surface winds, net heat flux (NHF), net radiation, fresh water flux. Similarly, the subsurface parameters that affect sound speed are internal waves (Rudnick, 2004), 4

5 frontal zones, meso-scale synoptic eddies and subsurface currents. Since the main aim of this study is to estimate SSPs from surface parameters alone, a brief description of some of these parameters is given below Sea surface height and meso-scale eddies SSH represents the entire T/S profiles of the water column with respect to bottom at that location. Any change in SSH affects the T/S profiles and so the sound speed. Meso-scale eddies that are reflected in SSH have the shape of rings. The parameters of eddies vary over a wide range of about km (Brekhovskikh and Lysanov, 2003). In the eddy zone sound speed has a complicated structure. In Figure 1.2.3, the distribution of the speed of sound is shown in the vertical plane crossing a cyclonic eddy separated from the Gulf Stream. An elevation of the isovels (lines of constant sound speed) as high as 700 m is observed in the central potion of the eddy. As a result, the vertical gradient of the sound speed markedly increases towards the centre of the eddy (Brekhovskikh and Lysanov, 2003). Range I km J i 1 1 i" i 'W 35 39'N 64 42'W 36 34'N Figure 1.2.3: Sound isovel section in the vertical plane passing through a cyclonic eddy separated from the Gulf Stream. The number near the curve denotes the sound speed in m/s. (Source: Brekhovskikh and Lysanov, 2003} 5

6 Sea surface temperature Due to the diurnal heating and cooling, usually, the near-surface diurnal temperature variation averages about 0.5 C; seasonally, it may vary as much as 3 C. As the sea surface heats during the day, a conditional thermocline often develops within a few meters of the surface changing the vertical temperature profile and hence the sound speed in upper layers of the ocean. Similarly, seasonal variations in SST also change the sound speed structure. For example, winter cooling causes convective mixing and mixed layer deepens Surface winds Surface wind blowing over the ocean surface cools the SST increasing the density of upper layers. Since the denser water is formed above the less dense water, static stability of water reduces. Denser water sinks down and mixes with the low density water. This process changes the vertical temperature profile and causes the change in sound speed also. More the surface winds, more the mixing in the upper layers (Gopalan, 2000). More mixing in the ocean deepens the mixed layer and causes more changes in SSP Net heat flux and net radiation Net heat gain at the ocean surface is a balance between absorbed solar radiation, latent and sensible heat fluxes and the net outgoing long wave radiation. Input of heat or fresh water at the sea surface makes the seawater less dense and reinforces the tendency for that water to remain at the surface. Loss of heat and the residue of high salinity water due to evaporation make the surface water denser. This causes the surface water to sink up to the depths where it mixes with water of same density (Gopalan, 2000). Strong winds mix this water and affects the SSPs by lowering the mean temperature of the layer. 1.3 ACOUSTIC PROPAGATION IN THE OCEAN Sound waves propagate in the ocean according to the sound speed structure of the ocean. These sound waves can be classified in following categories. 6

7 1.3.1 Direct Path Sound waves that travel directly from the source to the receiver without interacting with the sea surface or bottom are direct path sound waves. This propagation path occurs when sound travels in a nearly straight-line direction between the source and the receiver, with no reflection from the sea surface but with only one change in direction due to refraction. The ray that becomes tangent at (or parallel to) the sonic layer is called the limiting ray. The near surface depth at which first maxima of the sound speed occurs is called sonic layer depth (SLD) (Figure 1.2.1). Zero layer depth is a special case where SLD occurs at the surface, which happens frequently in tropical and subtropical regions where surface winds are insufficient to cause mixing (website 1 ). In Figure 1.3.1, direct path propagation occurs out to the point where the limiting ray refracts upward and reflects from the sea surface. SOUND SPEED RA NG E 9- \ * \ \ \ D E P T "7 H \ I 1 J Figure 1.3.1: Direct path sound propagation (Source: Website ) Surface Duct and Shadow Zones A sound ray leaving the source in a positive gradient bends upward to the surface. When it reaches the surface, the ray reflects downward toward the SLD and continuously refracts back toward the sea surface. The reflection and refraction of sound rays within a surface duct result in a region where little sound energy penetrates. This is shadow zone region and is always present in a surface duct situation (Figure 1.3.2). 7

8 The limiting ray forms a shadow zone. A source placed in the sonic layer cannot detect any object like a submarine placed in the shadow zone. The next ray beneath the surface duct penetrates the sonic layer and refracts downward toward the slower sound speed. This ray is the limiting ray for the portion of the shadow zone in the thermocline region below the sonic layer. The top of the surface duct is the sea surface, and the bottom of the duct is the sonic layer. RANGE - Figure 1.3.2: Surface duct propagation and shadow zone formation (Source: Website 2 ) Sound Channels Sound channel is a region in the water column where sound speed first decreases with depth to a minimum value and then increases (Figure 1.3.3). For a sound channel to exist, there must be a negative sound speed gradient, over a positive one. The depth of minimum speed resulting from this gradient is the sound channel axis (Website 2 ). Above this axis, the sound speed increases mainly due to temperature increase, below this increase in hydrostatic pressure is mainly responsible for increasing sound speed (Brekhovskikh and Lysanov, 2003). The lower boundary of the channel is the depth of maximum sound speed below the sound channel axis. The upper boundary is the depth above the sound channel axis where the sound speed is equal to the maximum sound speed of the lower boundary. 8

9 Sound rays above the sound channel axis refract downward, and rays below the sound channel axis refract upward, thus trapping sound within the sound channel (assuming the source is in the channel). Velocity SLD- TSOUND SPEED UPPER J EQUAL TO BOUNDARY] LOWER BOUNDARY LS0UND SPEED a Q f DEPTH OF AXIS A MINIMUM L VELOCITY f DEPTH OF MAXIMUM LOWER J VELOCITY BELOW BOUNDARY! SOUND CHANNEL L AXIS Figure 1.3.3: Sound speed profile and the sound channel (Source: Website 2 ) Shallow sound channel Shallow sound channels occur in the main thermocline. Although shallow sound channels occur frequently in the open ocean, they normally do not last long. They are usually associated with fronts and eddies and are, therefore, transitory (Website ). Occurrence of the shallow sound channel is shown in Figure Figure 1.3.4: Shallow sound channel (Source: Website ) 9

10 Deep sound channel The deep sound channel is a permanent feature of the deep ocean. The upper boundary of the deep sound channel forms at SLD, where the decrease in sound speed begins to produce a negative sound speed gradient. At the base of the thermocline, the sound speed begins to increase (because of an increase in pressure) forming the axis of the deep sound channel. The lower boundary of the deep sound channel forms at the depth where the sound speed below the axis is equal to the sound speed at SLD. Figure 1.3.5: Deep sound channel (Source: Website 2 ) Normally, the deep sound channels are too deep to access, except in high latitudes (Website ). The propagation of sound rays in the deep sound channel is shown in Figure In case of zero layer depth upper boundary of the deep sound channel occurs at the surface and the lower boundary of the deep sound channel forms at the depth where the sound speed below the axis is equal to the sound speed at the surface (Brekhovskikh and Lysanov, 2003). 1.4 STATE OF THE ART Traditionally, SSPs have been computed from T/S profiles (Roden, 1979, Kumar et al., 1993, Ltt et al, 2003, Baques et al and St Pierre and Wage, 2005) in absence of direct measurements from velocimeter (Kroebel and Mahrt, 1974; Hay, 10

11 1991; Voulgaris and Trowbridge, 1998; Snyder and Castro, 1999; Zhang, 2001; and Sweeney, 2006). Since the in situ measurements of these two parameters are sparse both in space and time, it is worth attempting estimation of SSPs from surface parameters alone, particularly, those obtainable from remote sensing platforms. Many studies have been carried out to estimate T/S profiles from surface parameters through several models and techniques (Kao, 1987; Wunsch and Gaposchkin, 1980; Khedouri and Szczechowski, 1983; Fiedler, 1988; dewitt, 1987; Vossepoel et al, 1999; Ali et al., 2004; and Guinehut et al., 2004). Since, estimation of SSPs from the estimated T/S profiles may lead to larger errors it is worth attempting estimation of SSPs directly from surface parameters. Park and Kennedy (1996) developed a method to estimate SSPs from remotely sensed SST. They obtained a mean error of m/s in sound speed prediction. For the first time, estimation of SSPs from all surface parameters besides SST that are obtainable from remote sensing platforms has been attempted in this thesis. 1.5 IMPORTANCE OF THE STUDY The sound speed in the ocean determines the characteristics of the sound transmission that has civil and military applications. The locations and the extent of shadow zones and the sound channels depend upon the sound speed structure. The vertical gradients of sound speed in most of the regions of the ocean is about a thousand times the horizontal ones, except in areas of convergence of cold and warm currents (Brekhovskikh and Lysanov, 2003). Limited availability of direct observations of the vertical profiles of velocimeters or temperature and salinity, from which sound speed can be calculated, confines specifications and investigation of temporal and spatial variability of the three dimensional structure of the sound speed in the oceans. Hence, it is of importance to find suitable methods of estimating the SSP from surface parameters alone that are available from remote sensing platforms. 1.6 STUDY AREA AND THE DATA USED The study area for the present work has been discussed in this section along with a brief description of the datasets used. 11

12 1.6.1 Study Area The North Indian Ocean spanning from 0 N to 25 N latitude and 40 E and 100 E longitude covering the Bay of Bengal and the Arabian Sea was selected for the study (Figure 1.6.1). Reason for selecting this area for studying its acoustics is because Indian Ocean plays a key role in the international affairs. It is situated at the junction of the main commercial and strategic maritime routes. It is an important area for international exchanges. The last decade has shown an increase in the presence of extra regional forces in the Indian Ocean, in terms of both numbers and capability. This trend is likely to continue in the foreseeable future. Maritime security environment has become more complex, and significantly more challenging. It is required to research on the acoustics of the Indian Ocean in order to have an effective Indian security environment. Figure 1.6.1: Study area This region is land-locked in the north and is an area of intense air-sea interaction. In situ observations are very sparse over the Indian Ocean. The methodology of estimating SSPs by using the data from remote sensing platforms presents an opportunity to study the SSPs in greater detail both temporally and spatially Data Used For the present study, two types of data sets were used. First data set contains the in situ observations of the ocean surface and subsurface parameters from special campaigns and the second data set comprises of ocean surface parameters from the remote sensing platforms. Detailed explanations of the all the datasets used along with the accuracies are given in the corresponding chapters. 12