Chapter 3 - The Physical Setting
3.7 Important Concepts
- If the ocean was scaled down to a width of 8 inches
it would have depths about the same as the thickness of a piece
of paper. As a result, the velocity
field in the ocean is nearly 2-dimensional. Vertical velocities
are much smaller than horizontal velocities.
- There is only one ocean with three official parts (basins).
- The volume of ocean water exceeds
the capacity of the ocean basins, and the ocean overflow on to the
continents creating continental shelves.
- The depths of the ocean are mapped by echo sounders which measure the
time required for a sound pulse to travel from the surface to the bottom
and back. Depths measured by ship-based echo sounders have been used
to produce maps of the sea-floor. The maps have poor horizontal resolution
in some regions because the regions were seldom visited by ships and
ship tracks are far apart.
- The depths of the ocean are also measured by satellite altimeter systems
which profile the shape of the sea surface. The local shape of the surface
is influenced by changes in gravity due to sub-sea features. Recent maps
based on satellite altimeter measurements of the shape of the sea surface
combined with ship data have depth accuracy of ±100 m and horizontal resolutions
- Typical sound speed in the ocean is 1480 m/s. Speed depends primarily
on temperature, less on pressure, and very little on salinity. The variability
of sound speed as a function of pressure and temperature produces a horizontal
sound channel in the ocean. Sound in the channel can travel great distances.
Low-frequency sounds below 500 Hz can travel halfway around the world
provided the path is not interrupted by land.