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# Re: Sensors and Zooplankton

From: aebrain@a...
Date: Wed, 30 May 101 05:58:41 GMT
Subject: Re: Sensors and Zooplankton
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>  >Are those Not So Wee Beasties wandering
>  >around all quiet like or are they really really chatty?

> Well it'd be harder to find whales if they clammed up (which they
> periodically do anyway), but I was actually talking about zooplankton
(max
> about 5cm long). That's why it seemed weird that we can follow just
one of
> those around (or pay attention to whole school or whatever you want to
do
> in between), but can't figure out what kind of ship that is over
there. But
> like I always say I'm NO engineer!!!!

A few Sonar principles:
Long wavelength (Low frequency) sounds travel further than shortwave
(High
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.

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