I think radar is mostly impractical due to poor signal penetration into the sorts of material (and at the sorts of ranges) that would be useful. Sonar or other

I think radar is mostly impractical due to poor signal penetration into the sorts of material (and at the sorts of ranges) that would be useful. Sonar or other acoustic-type seismic sensing (drill holes, place microphone/transducer array, fire small explosive shot, and analyse shockwave propagation) is more practical, but still slow (needs a lot of boreholes), not amazingly high resolution, and probably disruptive to the neighbours if you're in an urban environment. Thinking aloud here (and for all I know, it already happens), something like using oilwell-drilling tech to push a narrow horizontal bore along the tunnel path, and then sending a semi-autonomous sensor robot to do these sorts of scans at regular intervals might be better than drilling the inspection bores from the surface. There's still a bunch of issues if you drill into a pressurised water or mud pocket, but hopefully less than if your 10m-dia main cutter hits same.