The ideal solution is a shallow water seismic profiling system. These units profile the subsea or river bed and give you precise variations in density of material in the river/seabed. Saves a lot geotechnical core sample time and money.
Find a specialised marine survey company who offers this...
Hi,
There are small compact inertial navigation systems available that can be used for such applications. They are the size of a small shoe box and can be flushed(using water) down the pipe at a fairly constant speak. They will measure X,Y,Z position along the pipe at user specified interval...
Hi,
I am a Kiwi surveyor with 11 years experience currently working in Singapore.
I suggest you contact Otago University in New Zealand who have in the past had to resolve such precise survey problems in their industrial engineering department which specialises in such high accuracy projects...