Stepping backward.. Lower tech.

[ArmySGT.]

Originally Posted by StainlessSteelCynic

I don't think you'd need to produce older tech sonar for harbour defence and so on because I think there would still be many commercial fathometers and civilian fishfinders/fishsounders (which work on the same principle and technology as the fathometer) available to use for that purpose.

By the early 1990s both devices were using LCD screens for their displays and they become much more widely available to the recreational fishing & boating community and commercial marine industry. They are both a type of sonar and probably more recognizable by the name "echo sounder". The commercial marine industries (fishing, cargo, passenger etc. etc.) have been using echo sounding for decades for navigation and Western maritime safety regulations typically require every large vessel (100+ tons) operating in restricted waters to have a fathometer (of the constant recording type).
Older fathometers (e.g. the strip chart recording types) used transistors so would be more resistant to EMP as well.

I reckon there would be plenty of opportunities to plunder fathometers and fishfinders from commercial vessels simply because many of those vessels would no longer be operating. I also think for the 1990s period, the number of recreational fishing boats carrying fishfinders/fishsounders would be large enough to make it worthwhile to recover and use those units for harbour defence purposes and so on.

I agree in that it would work in only the most basic way....... Something is down there... Something large is down there. I just question the utility of something like this due to the limited range and narrow projection.

It might be my understanding is wrong, but any sonar from the 70's should give depth, speed, and an indication of mass (displacement). A system from the 90s can distinguish a whale from a school or fish, from a attack submarine.... comparing recorded acoustic profiles.