Have you ever went metal detecting? While the rewards can be worth it - it's a terribly inefficient and physically exhausting exercise. You move your arm left-right-left-right-left... While handheld detectors can be helpful for surveying small areas, if you're looking to cover a large area (a beach for instance), you'll be begging for a more streamlined approach that requires less manual effort and turns up more reliable results.
Let's combine a bunch of technologies to brainstorm a more reasonable approach to metal detecting.
1. Let's replace your feet. The metal detector should be motorized.
2. Let's replace your memory. Often, especially as a beginner, you'll scan an area more than once. The detector should be aware of it's position with at least centimeter precision. This wouldn't be precise enough with GPS, but very possible using your own localized triangulation system. The detector would be programmed to scan a geographical area without visiting the same location twice.
3. Let's replace the coil. The detector should be capable of providing high-resolution images of the area it is inspecting. These images can be created by using an X-Ray machine. The images should be saved to an onboard computer.
4. When the images are saved to the computer, an image algorithm properly stitches them together and takes note of any interesting object's coordinates within the localized triangulated zone.
5. The motorized detector should be remote-controllable. This feature not only protects the user's health from dangerous X-Rays, but, allows the detector to enter dangerous zones, such as, minefields.
6. Once the detector has completed it's survey of the area, or, in real-time, from a distance, the user can inspect the detector's output (by logging into the machine) and decide whether particular objects are of interest, or just junk. Because the detector has marked the coordinates of each point, it can be reprogrammed to revist these points (with the X-Ray off) and guide the user to the objects one-by-one.
Some other features: The detector could drop flags, it could even implement a digger.
I'm far from an expert on X-Ray photography and I'm not sure what the exposure time is for these devices.
Would work well only on flat areas (fields or beaches)...