Hmmmm? Well, you may want to post this in the DW section here. There are a couple of former Sonar techs who could answer this for certain. I was an A-ganger so my knowledge is limited with regards to sonar.
However, I do know that HF works best for navigational purposes like detecting Ice bergs, Mines and operating under the polar caps. A rock being submerged? Well I would think the rock would have to be part of a land mass because rocks generally do not float or have neutral bouyancy, so again I think either they did not have a fathometer or it was not turned on in order to avoid this situation. The signal bounced back from active sonar will be dependant upon signal strength, target distance and density and thermal condition etc.. so I cannot say yea or nay but someone here will I am sure.
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