(Admittedly without knowing much about it) I'll throw Ground Penetrating Radar onto this armchair metaphorical ideas whiteboard. The industrial CT scanner idea

(Admittedly without knowing much about it) I'll throw Ground Penetrating Radar onto this armchair metaphorical ideas whiteboard. The industrial CT scanner idea uses x-rays, while GPR is more in the UHF/VHF frequencies which probably means cheaper/easier? The tech seems to have some tunability for the specific application: > Thus operating frequency is always a trade-off between resolution and penetration. Optimal depth of subsurface penetration is achieved in ice where the depth of penetration can achieve several thousand metres (to bedrock in Greenland) at low GPR frequencies. Dry sandy soils or massive dry materials such as granite, limestone, and concrete tend to be resistive rather than conductive, and the depth of penetration could be up to 15 metres (49 ft). However, in moist or clay-laden soils and materials with high electrical conductivity, penetration may be as little as a few centimetres. https://en.wikipedia.org/wiki/Ground-penetrating_radar https://en.wikipedia.org/wiki/Ground-penetrating_radar