When I was retiring from Raytheon we had a concept using existing sensors and towed sensors, that used bottom mounted, continuously radiating low freq transducers, and bi-static or muti-static processing to track undersea objects. The navy rejected it since it would also light up blue forces. Today's signal capture and retransmit stealth technology might negate even the best active SONAR directly, but event-driven multi-static passive may still be our best alternative to Russian surge deployment. Pretty old technology though... probably obsolete in the UUSV world of today.