memyself: Do some research perhaps?
Magnetic anomaly detection (MAD) is not some magic that just "finds" stuff. I presume that is what you are implying?
Any reasonable sized structure (including plastic boats believe it or not) moving through water will create a magnetic signature that can be detected - Russia built a class of subs from Titanium in the misplaced belief that MAD detectors would not "see" them - but it is the motion of the structure, through a conductive medium (sea water) that creates electronic currents on the surface of the structure that can be detected, in addition to any ferrous metals.
However, once such a non-ferrous structure stops moving, the magnetic signature does too. No more MAD detection.
The second problem of acoustic detection, is that these systems have been highly "tuned" to detect the targets of choice - i.e. submarines. LOFAR buoys and the system used to interpret the signals received, are adapted to focus on the low frequency range (where a subs noise signature differs most from the normal background), and not the 37.5kHz tone of the data recorders. In fact this is near the upper range of LOFAR reception, which is 40kHz.
VLAD is no good the audio range is limited to 2400Hz, way below the pinger frequency.
Basically, the problem is way harder than it might seem from behind the keyboard. I believe the French tried really hard and even had support from the Sonar system manufacturer in an attempt to reprogram the software, but the depth of the pinger was very different from those the system was intended to detect signals from. An R&D project in other words on a very, very short, 30-day schedule (based on battery life of the pinger).
I already posted proof that acoustic detection is difficult...