resting on the bottom

[gonzosts]

Since it is not part of the sim, no matter what science there is, your still SOL.
As far as how sonar works (at least in todays Navy) active sonar sends out a sound wave that bounces off solid objects. The return is what is messured by the sonar equipment. For a sub resiting on the bottom to show up depends on the bottom type. With a bottom of hard sand or rock a sub will be hidden as the return echo will be the same for the entire return unless the distance is short enough ( in case of shallow water) that the sub is higher than anything on the bottom. In the case of a sandy or muddy bottom, the transmitted sound wave will be absorbed into the bottom except where the sub is and the sonar equipment will show a return off the sub so a sub sitting in this type of environment is F*#Ked. With the use of side scan sonar used today on some platforms, no matter where you try to hide, the complete profile if the sub is outlined and it will even show which way you are going.

The only way to save your self (in the sim or in real life) is to stay out of the active sonar's cone....