thanks for the suggestions. good idea that of using resistivity. the electric behaviour of the tunnel gives infinite resistivity which would be easily picked up by the intrument. the only drawback is that for each Vertical Electric Sounding (VES), the rods have to be spread apart from the source and voltohmeter at least 7 times to make the 7 recordings. that's only for one point on the surface. imagine the amount of work to to scan, say 1 hectare, every 0.5 m ?. that's 40000 measurements !... let alone the interpretation and so on ...

on the other hand, GPR takes a lot more effort and bear in mind that's a very uneven surface ground, i.e., karstic area with a lot of outcrops and vegetation.

I thought one could use a simpler instrument, ie electromagnetic.

appreciate your thoughts tough...