as you have correctly pointed out the signal passes through solids quicker than liquids.
the link to Mike Thompsons Side scan sonar is tried and tested and to the best of my experience and knowledge working with sonar particulants in the water dont effect the signal too much - apart from tea, i once worked a shipwreck salvage project, we freed some tea which had been carried in the hold. That stuff really is a sonar killer.
The principle of ultrasonic ranging are quite simple and straight forward.
as it happens i have found some more juicy bits.http://www.fiz-ix.com/2012/01/how-to-configure-arduino-timer-2-registers-to-drive-an-ultrasonic-transducer-with-a-square-wave/
written by Jeff ?
this provides the starting block for the trigger pulse for the ultrasonic transducer creates 40 KHZ and creates signal bursts
TCCR2A = _BV(COM2A0) | _BV(WGM21) | _BV(WGM20);
TCCR2B = _BV(WGM22) | _BV(CS20);
OCR2A = B11000111; // 199, so timer2 counts from 0 to 199 (200 cycles at 16 MHz)
TCCR2A = 0;
TCCR2B = 0; // I think this is all that is needed since setting the CS bits to zero stops the timer.
delayMicroseconds(400); // wait for 400 microseconds
OK one small proble i dont appear to be able to get it working - or at least i cant tell until i get back to work and test pin 11 with a scope.
i have put a led on it and expected to see a fairly well, continuesly lit led - No.
so an ideas or suggestions would be helpful.
my plan would be to use this pulse/burst to drive a fet amplifier stage which in turn would drive the transducer with 110v at 40Khz - ok there is more circuitry than that ( like a line matched transformer etc) but you get the general idea.
if i can get this working im almost half way there (the other half is the receiver, what: amps filters FFT perhaps - ending up with a 0-5v output).
Im sure this project most be of huge value and interest, i have read several forums where people are asking if an underwater sonar is possible and cant find any real work that has been done on it.
the whole idea of building such a thing is cloaked in mystery even the online patents are non existant.
so any input please shout