> Very similar to the familiar "metal detector" design

The detector doesn't have to go in the ground. It can sit on
top, perhaps under a plank, which would also be the spacing
for the poles, so the whole thing could be moved intact from
point to point

Silicon Chip had this as a project a couple of years ago

http://www.siliconchip.com.au/cms/A_102998/article.html

Available as kit KC5402 from www.jaycar.com.au

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