Final answer:
Seismic or geophone sensors are used in exterior Intrusion Detection Systems to detect digging and tunneling activities, as they can sense ground vibrations while filtering out normal background noise.
Step-by-step explanation:
The type of exterior Intrusion Detection System (IDS) sensor that is effective for detecting intrusion by digging and tunneling is a seismic or geophone sensor. These sensors are specifically designed to detect vibrations or movements in the ground, making them suitable for identifying potential intruders who are attempting to breach a perimeter by digging underneath it. The seismic sensors can be programmed to ignore ambient noise or regular background vibrations while being sensitive enough to pick up on the anomalies caused by digging or tunneling activities.