We generally use a combination of methods appropriate for the geological problem that we're dealing with. Traditionally we use some kind of airborne geophysics--like the gravity system that was described--to identify the major geological features that we can then plan our work around. If there are major features, we'll go in and identify what those are geologically. Then we can use the remote information to track them for some distance, for example, so that we know where that feature is going on the ground.
On November 16th, 2011. See this statement in context.