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I. Introduction

In the seawater, the transmissions of electromagnetic waves and light waves with different wavelengths are almost prevented, and the use of lasers to determine the depth of the seabed is also very limited. The use of sound waves for communication, detection of targets, mapping of the bottom, monitoring of the movement of enemy submarines, tracking the attack of torpedoes, determination of the distance of resonated sound objects, looking for the ‘dark sound’ areas as a safe haven for submarines… have been conducted since the early twentieth century, but the accuracy and effectiveness of this method have just been improved for a few decades, in addition to the completion of the research equipment has been raised. The research projects on the speed of sound in the central region of Vietnam Sea has currently been limited, the marine scientific problems, in general, relating to the speed of sound have almost not been studied, reviewed and evaluated fully (I did not understand this sentence. Please check grammar).

Studying on the distribution of the temperature, salt and velocity aims to contribute to the general knowledge of marine science, also to solve the essential issues because of the domestic requirements. Da Nang and Quy Nhon are significant situations for both the water transportation activity and the national security. Therefore, studying on the spatial distribution of the speed of sound has a great meaning to support economic development and national security.

II. Materials and Methods

 To calculate the speed of sound in Da Nang and Quy Nhon sea areas, we used the data of water temperature, salinity from a survey conducted in (01/07/2010-07/07/2010) under the cooperation between the Institute of Marine Environment and Resources (Vietnam Academy of Science and Technology) with the Hydrosphere Atmospheric Research Centre (Nagoya University, Japan) about "Drifting buoy observation along Vietnam coast in summer 2010”. The temperature and salinity data with depth in Da Nang - Quy Nhon were measured by using a compact CTD (Conductivity/ Temperature/ Depth), the temperature and salinity were observed with 0.1 m depth at six points in each section.

The speed of sound in the seawater depends on the temperature (T), salinity (S), and hydrostatic pressure (P) of sea water. The intimate relationship is expressed as the following expression:

C = Co +CT +CS +CP +CTSP. With Co: standard sound speed; CT, CS, CP: the correction due to the effects of temperature, salinity, hydrostatic pressure. CTSP: the correction due to the coincidental effect of temperature, salinity, hydrostatic pressure. Hydrostatic, however, is a function of depth, so we can calculate the sound speed in water through the parameters of temperature, salinity and depth.

There are several methods to calculate and determine the speed of the sound wave in the sea. In this study, we used the equation given by Mackenzie (1981) who represents the speed of sound in seawater as a function of salinity, temperature and depth.

C  =1448.96+4.591T-5.304x10-2T2+2.347x10-4T3+1.340(S-35)+1.630x10-2D+

1.675x10-7D2-1.025x10-2T(S-35)-7.139x10-13TD3

       - T: Sea water temperature (oC)

       - S: Sea water salinity (%o) 

       -  D: the depth of water layer (m)

The above equation gave accurate results in temperature range of 2 - 30°C, salinity of  25 - 40‰, depth of 0 - 8,000m.

III. Results and discussion

III.1. Temperature and salt structure

The sea in Centre of Vietnam is a deep sea area with a large transparency, open sea, and the water mass is exchanged directly with the offshore. So the T-S structure (T-S structure means density structure?) and salinity structure has the same attributes with the Ocean [2, 3](I did not understand this sentence). In Da Nang section, thickness of surface mixed layer was approximately 5 m. The salinity from 5 to 20m depth in coastal areas was well mixed.. In Quy Nhon section, the temperature tends to rise gradually from the nearshore  to offshore and to decrease from the surface to the bottom (I think what water temperarute decrease from surface to the bottom is common sense). The homogeneous layer of surface temperature offshore area  is down(?) to a depth of 35-40m while this exists only at a depth of about 5-10m in coastal areas. The homogeneous layer is followed by the mutant layer (What is mutant layer? thermocline?)  of the temperature with a trend to decrease gradually in depth. The homogeneous salinity layer has also the distributon rules similar to that of the homogeneous temperature layer. The mixed layer in salinity, however, was quite complicated (especially in coastal areas, stations QN-3 and QN-2), with the dramatic fluctuation in depth of 30-40m and 70-90m. Owing to the period of the prevailing Southwest Monsoon in July on the Southest coast of Vietnam, the water mass transport of the southwest surface water invades the southern sea on the latitude of 9o North. There is a presence of the modified winter - summer water block (what is winter-summer water block?)  in the offshore water at this latitude that creases the disturbance and the rise of the maximum salinity in Quy Nhon coast (I did not understand this sentence).  

III.2. Acoustic velocity distribution

As we know, there are four types of sound rays: plus refraction (type I), minus refraction    (type II), the transition of minus refraction from the upper layer to the plus refraction in the underlying layer (type III) and the underground (type IV) [1, 4].

Through the chart of the acoustic velocity distribution in Da Nang section (Figure 4), sound velocity reaches its maximum in the surface layer (from surface to a depth of about 17m) and fluctuates in the range from 1544 to 1545 m/s. The vertical gradient of sound velocity in the surface layer is small ,and averaged vertical gradient of sound is  about 0.03 at the staion DN-2 and about 0.17 at the station DN-6.      From this depth (I did not know this depth), the sound velocity experiences a significant reduction and this is called the mixed layer of sound velocity, at about 5-10m depth in the coastal area (station DN-1, DN-4), and at the depths from 10 to 15 m in the offshore (station DN-5, DN-6). In this layer, the sound velocity declines sharply, the vertical gradient of velocity reaches the average value of about 1.31 at the station DN-2 and about 0.96 at the station DN-6. Below this layer, the sound velocity decreases gradually with depth, attains the average value of 0.21 at the station DN-2 and 0.25 at the station DN-6.

The contours of sound velocity in the surface layer has the distribution similar to the distribution of the sine function (Fig.5), this can be explained by the reflection machanism of the rays: when the sound beam passed down the layers to the mixed heat-salt layer, it  would be reflected to the sea surface where it would be reflected back again to the water environtment. In general, the sound velocity tends to be declined with the depth and to be homogeneous horizontally. However, in offshore area, the mixed layer of the sound velocity is deeper than that in the coastal areas.

In Quy Nhon section, it is clearly seen that the sound velocity tends to fall gradually from the surface to the bottom and descends from the offshore to the near shore corresponding to depth. At the depth of about 40m (station QN-1, Figure 6) in the offshore area, there is a zone in which the sound velocity reaches the maximum value (1,545.23 m/s), the minimum value of sound velocity across the cross-section is 1,515.19 m/s at a depth of 103m at station QN-5. The maximum velocity exists from the sea surface to a depth of about 30 m in coastal areas, and from the surface to a depth of about    40 m in the offshore. In this layer, the sound velocity is relatively uniform but below this layer it is the mixed layer which has the rapid decline of sound velocity. From the surface to the depth of the mixed layer, the sound velocity tends to increase slightly with vertical gradient value (0.017). On the other hand, it begins to collapse with vertical gradient value (0.35) from the mixed layer to the bottom. From the sea laws about changing the vertical sound velocity, the refraction of the rays on this cross-section belongs to the refracting type III (a type of refraction which occurs when the sound velocity increases in the surface layer and decreases from the lower boundary of this layer to the bottom).

IV. Conclusion

With the current sate of data, the T-S field with depth is still limited in Vietnam. In the partnership with the NagoyaUniversity (Japan), analyzing and calculating the monitoring data has initially given the picture of T-S structure and the distribution of sound velocity by depth and longitude. The distribution characteristics of T-S structure and the sound velocity in July, 2010 are affected by the Southwest Monsoon and water circulation. In the studied zone, Da Nang - Quy Nhon, the maximum of the sound velocity exists from the surface layer to a depth of about 30m and 40m in the coastal and offshore area, respectively. The sound speed is quite homogeneous in this layer and would be declined rapidly in the underlying layer.

References

•          Pham Van Thuc, 2004. The sound velocity field in the upwelling zone in the south central of Vietnam. Journal of Marine Science and Technology (Vietnam), volume 1 (number 1), pages 23-24

•          Nguyen Kim Vinh, Vo Van Lanh, 2001. Seasonal change of the thermal salt structure along the coast of Vietnam. Journal of Marine Science and Technology (Vietnam), Volume 4 (No.4), pages 1-

•          Nguyen Ba Xuan, 2003. Distribution characteristics and variation of the temperature and salinity factors in the Central Coast of Vietnam in the southwest monsoon period. Collection of Marine Research, (No.8), pages 47-54.

•          Nguyen Ba Xuan, 2008. The structure and distribution characteristics of the seasonal average sound velocity in sea areas of Vietnam. Final report of projects of the Institute of Science and Technology of Vietnam.

(1)Tran Anh Tu, (1)Le Duc Cuong, (1)Do Trong Binh,

(2)Akihiko Morimoto, (3) Tetsuo Yanagi

(1)Institute of Marine Environment and Resources, Vietnam

(2)Hydrosphere Atmospheric ResearchCenter, Nagoya University, Japan

(3)Research Institute for Applied Mechanics, Kyushu University, Japan