[Nexus7]

If between you and your contact there's a seamount, and you have a good guess of the range of your contact, you can use the seamount to have a raw contact's depth indication, changing your own depth. When the contact disappears from your sonar, mark your depth and then it's trigonometry...

depth(target) = depth(ownship) - distance(target)/distance(seamount)*(depth(ownship)-depth(seamount))

example:
depth(ownship)=800 feet
depth(seamount)=700 feet
distance(target)=7 nm
distance(mount)=5 nm

---> depth(target) = 633 feet