The ground penetrating radar couldn't determine exactly what was happening inside the biodome due to the interference, and it couldn't tell foliage from loose dirt and rubble, so the only real answer was to send the team down there and get some direct data.
The tunnel in the house was the obvious answer since it led down to the lower levels of the biodome's infrastructure and would let them infiltrate without damaging the dome itself.
Ideally, they would be able to leave the facility intact for the next person who came through to study this fallen civilization. There might not be anything left alive in there, but on the off chance that there was, it would be an incredible find and one of the longest-running biodomes in history.
