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.