Comment by defrost

> some kind of metal detector sellotaped to the underside

Been a thing since the 1970s, physics dictates dimensions though - you can go wrap or dangle:

* https://csegrecorder.com/articles/view/the-megatem-fixed-win...

* https://cseg.ca/airborne-electromagnetic-systems-state-of-th...

* https://www.ga.gov.au/scientific-topics/disciplines/geophysi...