I don't understand your question. A problem for what? For the geophysics methods or for the TBM?
Geophysics methods give a measure of density based on wave reflection/refraction and are therefore at the mercy of energy absorption of a given material. Resolution is a function of energy and so the ground smears in your results. The variability of the ground may or may not pose a problem to the geophysics methods used, it depends on how they do it.
For the TBM, inconsistency in the ground can obviously pose problems. Based on what I know about this project (which is not much), Bertha has mixed face tooling on its cutter head but is primarily a soil machine. This means that large boulders can pose a problem as the cutter head isn't designed to deal with massive, hard objects most likely. The way most of these soil machines work is that they have a cutter head with tooling that essentially scrapes the soil and there are windows in the head that open to an area behind the cutter head which the soil moves through before being spit out as muck from a conveyor or screw jack. This area behind the cutter head generally has a system to crush large rocks that enter this area into sizes that are fit for the muck system. The problem arises when you have a rock too large to fit through the window as the cutter head isn't really equipped to bust up rocks (for multiple reasons).
So what is the typical procedure when encountering a large boulder? Get people there and just drill through it manually? Or would they have to completely remove the entire boulder? (how?)
