Thank you for your expert advice, Mr. Hoagland -- Sorry, I mean Kevin. :)
The problems I see with these detectors is a depth scan on only 12 inches and a width scan of slightly more than the coil width (typically under 12 inches). Of course, width scan is considered unimportant due to the recommended lateral sweeping performed manually.
Being a lazy mammal, I fantasize about attaching a detector to a fixed position on the back of a boat & gentlly floating down stream letting the current do the sweeping. Down anchor on a hit & life is good.
Current detectors seem to come up short in that regard, even the PI units.
However, the new emerging technology of Ground Penetrating Radar offers hope ... at a considerable price. The Detech SSP 5100 comes with a 1 meter X 1 meter coil! It can detect to a depth of 4 meters. All for the princely sum of $2,000.
What are your thoughts on this emerging technology?