Sensors and Zooplankton

G'day Alan,

> In order to see a greeblie that's 5cm long,

Well like I said I'm not an engineer so I don't know if we've done something
that breaks the laws of physics or what but the stuff we're looking at uses in
excess of 150kHz to see it and its about 1.5 km
down....
so something doesn't gel here;)

Cheers

Beth

From: "Beth Fulton" <beth.fulton@marine.csiro.au>

> From: "Beth Fulton" <beth.fulton@marine.csiro.au>

Or the transducer is on a cable or a towfish... and there probably aren't very
many snapping shrimp around.

From: "Tony Christney" <tchristney@home.com>

> >Pushing the performance to the limit? Yes. You must have some really

G'day guys,

> Breaking? No. Pushing the performance to

You're talking about science funded by the Aussie government so I hardly

think so;)

> Or the transducer is on a cable or a

Well I went and checked up and while there are plenty of snapping shrimp it
turns out Tony was partly on the right track. They use a set of sensors some
ship board some that are lowered to depth and between them they get

stuff sorted....so I was only partially on track vs sensing stuff - told

you I was no engineer!!! Still begs the question of what sensors will be

capable of in space where you've only got back ground radiation and no
snapping shrimp or thermoclines to deal with;)

Cheers

From: "Beth Fulton" <beth.fulton@marine.csiro.au>

> >Breaking? No. Pushing the performance to

Right. Scratch that idea.

> >Or the transducer is on a cable or a

> >Are those Not So Wee Beasties wandering

> Well it'd be harder to find whales if they clammed up (which they

A few Sonar principles: Long wavelength (Low frequency) sounds travel further
than shortwave (High frequency) sounds. HF sounds carry more information, but
get absorbed very quickly. And there's a limit to how much energy you can pump
into the water before it starts boiling ( in fact, most military active sonars
heat water to something over 90c in their near vicinity, and would do more if
it wasn't for the fact that cold water is continually rushing past, but I
digress)

In order to see a greeblie that's 5cm long, you need a wavelength of 10cm or
less, 2.5cm or less by choice. The speed of sound in water is on the order of
1
km/sec, so a wavelength on the order of 1cm means a frequency on the
order of 100 KHz. At these wavelengths, there is essentially total absorbtion
after a few hundred metres. But at really short ranges, using such UHF (Ultra
High Frequency) active sonars, you can do neat things like detect mines,
figure out their shape etc. For those really interested, the Sonar Bible is
"Principles of Underwater Sound" by Robert J. Urick, ISBN: 0932146627,
referred to by sonar mavens as just plain "Urick".

Anyway... ships radiate at a number of frequencies, including the line
frequency of AC ( 50-300 Hz ), the frequency that any turbines move at,
the
usual 1-20 KHz of audible sound due to people talking, klaxons etc.
These travel further, but still get absorbed. Stuff moving through the water
creates turbulence, which also makes noise.

Whalesong, at really low frequencies, a few Hz, on the other hand, can travel
intercontinental distances. And a whale in a Deep Sound Channel, which
focusses sound and keeps it in the channel much like a fibre optic line
transmits light, such a beastie can hear an echo of its own song after it's
gone all round the world.

Anyway... you can sometimes hear ships at great distances, if you have a

receiver capable of tuning into the metre- or even 10-metre wavelengths.
But you won't get much data from it, basically a "there's something in about
that direction, could be a whale, an underwater volcano, or a supertanker,
anywhere from 10 to 10,000 km away, depending."

But get near a snapping shrimp, and you can't help detecting it by passive
means. And normal shrimp and other zooplankton turn up beautifully on UHF
active sensors, but only within half a km.