This is a very interesting and worthwhile project. I am working on an anti-poaching UAS. I also was considering metal detection for weapons detection. I opted out for now as we think that humans near rhinos at night are probably not there for a good reason anyways. My concern with putting a metal detector on a UAS is that is a challenging radio environment to send and receive information or to control the craft in a variety of ranges and conditions. I have looked into millimeter wave radar (MMW radar) and combined with hyperspectal imaging shows promise for long range metal detection. It may be an avenue you consider as well considering the radio environment of a quadcopter. I think it is a great project and I wish you the best as you move forward.