Our investigations in the tunnels were very limited because of safety. We had a structural engineer on site to evaluate the stability of the one tunnel that we entered. There was no excavation and only a few modern artifacts recovered. 

The city was a bit disappointed that we didn't use cameras mounted on robots or some other high tech approach, but the budget did not allow it. Certainly remote sensing is a legitimate approach, especially for mapping the extent of a network of tunnels, although any number of obstacles could prevent access of a robotic vehicle.