I wouldn't know where to start for a passive sonar, except that my bat detector emits a weird sound if I move the mic closer to something or farther away. This happens even if I move it fairly slowly, although it's more noticeable at higher speeds. It also happens if the mic is stationary and the target is moving (I tried that to eliminate the possibility that the sound was wind on the mic).
So I don't know whether this effect could somehow be used to determine the distance to something. I think I could get enough information from the weird sound to indicate how fast something was approaching or receding, but not how far away it is. Also, telling the difference between approaching receding might be tricky without DSP, which I've never done.
Why do you want a "passive" sonar? You don't just mean an ultrasonic listener, do you?
Torben