im not sure your going to see a signature of a nickel deposit because nickel alloy steels are not magnetic. this probably explains why they were looking for magnetic lows previously. the signature of magnetics usually means iron bearig rock or an iron formation. which in most cases is an indicator of mineral presents.i know in my area you look for rusty rocks to indicate a good area for gold associated with an iron presents.most of the mineral types in a formation will be found around the same area depending on the sulfides present with the volcanic event. most base metal occurances are searched for in an area of low sodium content rocks because the forming of the vms deposit almost always was formed in a heated underwater condition.