Oceanography (2+1)

Variation of sound velocity in the ocean

Sound velocity in the ocean ranges from about 1,400 to 1,570 m/sec. it increases with increasing temperature, salinity and pressure or depth in the ocean. The increase in sound velocity is about 4.5m/sec for 1^oC increase in temperature; about 1.3 m/sec for one-thousandth part increase in salinity and about 1.7 m/sec for 100m increase in depth. In the oceans, horizontal variations of sound velocity are smaller compared to vertical variations of sound velocity, because horizontal variations of temperature and salinity are smaller than the vertical variations.

The vertical variation of sound velocity in the deep waters of the ocean can be divided into three layer. In the upper layer of the ocean, which extends from surface to 100-150m depth, temperature and salinity variations are negligible as a result of mixing of waters due to wind and waves. The sound velocity increases as depth increases in this layer due to the pressure effect. In the middle layer or the thermocline layer, which extends from 100-150m depth to 1500m depth in the ocean, temperature decreases very rapidly. Sound velocity decreases very rapidly in this layer due to temperature effect. In the bottom layer i.e., below the thermocline layer and extending to the bottom of the ocean, temperature and salinity variations are again very negligible. In this layer, sound velocity increases with depth due to the effect of pressure.

In the shallow waters of coastal regions and on the continental shelves, the sound velocity profile tends to be irregular and unpredictable. It is greatly influenced by surface heating and cooling, changes in salinity and water currents in these regions. The shallow water sound velocity profile is complicated by the effect of salinity variations caused by nearby sources of fresh water and contains several layers of different sound velocity gradients which have little temporal or spatial stability. the transmission of sound in the ocean is dependent on several environmental factors, the most important of which are the sound velocity distribution in the ocean, the depth and configuration of the bottom and the sediment characteristics of the bottom. In addition to temperature, salinity and pressure, the characteristics of the bottom also influence the propagation of sound in shallow water through absorption, reflection, etc.

It is produced in the upper parts of the ocean. The positive gradient is due to the isothermal layer with the velocity of sound increasing with depth because of increasing pressure.