I've dredged sand and pea gravel for road construction and have ran into black sand "rivers" a few times over the years.  The "rivers" have always been between 5-10 ft from the surface after removing the overburden.  I've worked 9 to15" dredges and left 5 to 10ft sheer face walls where Black Sand deposits can be viewed vertically.  We always pumped the material because we could not determine how far the "Black Sand River" would meander.  There have always been Gold flakes throughout the Back Sand.

            Now my question, Is there some kind of equipment that can detect the magnetic sand at 10 feet down?  and determine the depth of the back sand deposit?

Any an all responses would greatly be appreciated.