In article ,
Frnak McKenney wrote:

On Mon, 22 Mar 2010 09:20:59 -0400, Joseph Gwinn wrote:
In article ,
"Ed Huntress" wrote:

[...]

What Dan is talking about is measuring it across oceans. It may
well be possible to a high degree of accuracy. And it very well
may tell you all kinds of useful things about the relationships
of ocean temperatures in certain regions to weather. It probably
also will tell you about a *mean* temperature trend along a
specific Great Circle line.

Measuring across oceans is precisely what is done. And, acoustic
computerized tomography gives you the voxel-by-voxel temperature
of the ocean, not just the mean temperature of a great-circle path.

The method resembles that used with X-rays (in CT scanners), where
the X-ray attenuation of a very large number of independent paths
and directions is combined mathematically to yield the 2D
attenuation function (which we see as the image).

Circling back to acoustics, what is measured is the transit times
along a large number of paths through the ocean of interest.
These transit-time measurements are combined to compute the speed
of sound in each and every voxel, yielding the speed image. From
this (and independent salinity measurements) one can compute
temperature.

Not just salinity. From the Science Observer column of the
March-April 2010 issue of American Scientist, pp. 121-122:

Amplifying with Acid: More carbon dioxide in the atmosphere means a
noisier ocean

"...[CO2 absorption] also has a secondary consequence: it
decreases the ocean's ability to absorb low-frequency sound.

"Oceanographers Tatia Ilyna and Richard Zeebe at the University of
Hawaii, along with geochemist Peter Brewer of the Monterey Bay
Aquarium Research Institute in California, report in the December
20 issue of Nature Geoscience that lowering the pH of the ocean by
0.6 units could decrease underwater sound absorption by more than
60 percent."

I read this too, but they don't claim a significant effect on the speed of
sound, only reduced sound attenuation, which actually makes speed measurements
easier.


"... Changes in pH can impact the deep ocean because at about 1
kilometer down, the properties of temperature and pressure combine
to produce a 'channel' of water in which sound can propagate for
many thousands of kilometers. Whales and other marine life make
use of this channel for long-range communication. Most human-made
noise forms at the surface, but it can reflect and refract down
into this channel as well. ..."

This channel is very effective, allowing communications across the entire
Pacific Ocean basin, for instance. This works because confining the sound to
spread in a 2D channel (versus 3D space) changes the inverse-square-of-distance
path attenuation law into a simple inverse-of-distance path attenuation law,
which makes huge difference at large distances.


You may now return to your regularly-scheduled discussion, dialogue,
and/or debate. grin

Yep.

Joe Gwinn