The beauty of grouting is that you don't need to spend years mapping the voids and allowing them to get bigger, you just find where they are approximately and pump it in until they're full. It's the unsupported top bit of a void that migrates upwards. Once that bit is filled the hill becomes completely solid and stable. So EH's compulsion to precisely map the voids is misplaced, it seems to me.

As for the 3D imaging technique, that also is new and pretty much untried and the computer analysis has to be done in America (!) and has been mysteriously delayed a long time. I suspect it has proved insufficiently accurate for EH's requirements (accurate to within only 2 metres, I read) but the damnable thing is that in the real world of consolidation by grouting such accuracy is not needed or relevant. In Dudley we have this dopey idea that if you use conventional scanning to locate the voids then pump grout in until they're full then there's no space left so further collapses or upward migrations are totally stopped and the whole thing becomes 100% stable. But then, filling unstable limestone mining caverns totalling many thousand times greater volume than Silbury with complete success for the past 20 years tends to breed a rather offhand view of such matters.