How to find black boxes underwater absent any pinger signals? If CVR/FDR cannot be seen visually on the seafloor (because covered by a thick layer of mud/sediments) and no pinger signals are detected, how can they be found? Metal detectors? Any other methods?

case in point: The FDR of crashed Lion Air flight 610 was recovered on 1 Nov 2018, but 2 months later now the CVR still has not been found (while the pinger batteries are rated for 30 days only):


  « The search for the CVR is hampered by thick mud. The signal source is difficult to ascertain its position considering the sea floor is mud with a depth of more than 1 meter. The ping signal from the CVR has not been received for 2 days now (5 Nov). There are other means to find the CVR however. » (source)


Which "other means" are this ?





update 18 Dec 2018: Lion Air contracts MCS to locate and recover the CVR of the 737MAX.

update 21 Dec 2018: an expert answered the question for me. I'll paraphrase his explanation as an answer below.
The CVR on a Boeing 737 is located in the aft cargo compartment. The CVR itself is not built of the same materials as the aircraft, which is mostly aluminium and composites. By mapping the location of other components of the aircraft found in the sea bottom it should be possible to make an educated guess of the general location of the CVR (assuming the aircraft did not disintegrate, sending everything in different directions). One common way of tracking down the CVR is by using an underwater magnetometer to detect ferrous metal objects. The problem is that something as small as a CVR will only be detectable up to about 20' under the ocean floor. If it is buried deeper than that it will be difficult to find. That's why they have pingers, but those stop working when the battery runs out.