Although this paper is quite old, it provides a good overview of mine detection technologies including x-ray backscatter imaging. These technologies take a long time to develop, so many of are only recently seeing the 'light of day'.
It notes x-ray backscatter has several advantages, but is also limited to mines above 10cm depth. It doesn't mention speed/efficiency - probably because the technology was too infantile at the time to know.
My take is that it would require a combination of technologies and techniques. Perhaps one or more tech can be used to scan a large area for potential mines, then another to focus on potential targets to firm up probability before eliminating.
