Sound travels farther and about five times faster below rather than above the ocean's surface. The average speed of sound in the ocean is 1450 meters (4752 feet) per second. Actual speed depends on pressure (depth), temperature and density; it ranges from 1200-1520 m (3900 - 5050 feet) per second. Understanding the physics of sound makes it possible to map the ocean floor and to navigate its inky darkness. Both are critical for successful vent research.
Mapping The Ocean Floor
Background:
Side-scan sonar, or an echo sounder, towed behind a ship sends
sound waves through the water to bounce off the ocean floor and
return. The instrument records, then maps the ocean floor. It is
possible to map 3 km of ocean in a single "ping" using advanced
models of this technology.
At a far simpler level, we can map the contours of the ocean bottom using an easy formula. For our activity, we will assume that depth, temperature and density make the sound travel at 1524 m (5000 feet) per second. The formula is:
(We divide by two because the time recorded is the time it takes for the round trip journey to go down and come back up again.)
Problem:
Use this formula to determine the missing depths based on
readings taken 10 miles apart, then plot them to represent an
imaginary section of ocean floor.
Horizontal Distance (miles)
--------------
Time (seconds)
--------------
Depth (feet)
--------------------- 0 ------------------------------------------------------ 3 ---------------------------------- ?
-------------------- 10 ---------------------------------------------------- 4.0 -------------------------------- ?
-------------------- 20 ---------------------------------------------------- 4.2 --------------------------- 10,500
-------------------- 30 ---------------------------------------------------- 4.3 --------------------------- 10,750
-------------------- 40 ---------------------------------------------------- 4.5 --------------------------- 11,250
-------------------- 50 ---------------------------------------------------- 4.2 --------------------------- 10,500
-------------------- 60 ---------------------------------------------------- 4.0 -------------------------------- ?
-------------------- 70 ---------------------------------------------------- 3.6 ----------------------------- 9,000
-------------------- 80 ---------------------------------------------------- 3.8 ----------------------------- 9,500
-------------------- 90 ---------------------------------------------------- 4.0 -------------------------------- ?
-------------------
100
---------------------------------------------------
3.6
-----------------------------
9,000
Solution:
Before you plot, check the depths of the missing depths. At 3
seconds, the depth is 7500 feet; at 4 seconds, 10,000.
Now plot the points. To check your computation, click here. The graph you draw should show, from left to right, a depression and a rise followed by a smaller depression and rise.
What is a fathom?
It is a nautical measure. One fathom equals 1.83 meters, or 6
feet.. A point 2,743 meters (9,000 feet) below the surface could also
be said to be at a depth of 1500 fathoms. In English, to fathom means
to understand thoroughly, to get to the bottom of a matter.
Oceanographers fathom the ocean's depths.
More on Mapping:
Scientists now use a combination of sophisticated equipment to
map the ocean floor and obtain three-dimensional models of its
mountains, valleys, plains, and other geological features. Much
remains to be done; an estimated 95 percent of the ocean floor is yet
unmapped.
This bathymetric map of the
Northeast Pacific shows the relative heights of the Juan De Fuca
Ridge, Blanco Fracture Zone, and Gorda Ridge.
If you would like to learn more about the advanced mapping techniques used by US National Oceanic and Atmospheric Administration scientists, click here to link with their Web page. http://www.pmel.noaa.gov/vents/geology/methods.html