Over the last few decades combat engineers have had to acquire a new set of skills for finding tunnels and caves. That’s because many actual and potential foes are increasingly using tunnels and caves as part of their combat tactics. For Western armed forces this has resulted in a proliferation of techniques and equipment for detecting their underground dangers.
The most recent area where tunnel hunting became essential was in Afghanistan. Here tunnels and caves were long used for refuge in peace and war. Finding tunnels in Afghanistan turned out to be quite difficult. Entire mountain ranges were largely composed of limestone, the ideal type of rock for natural caves to form and for artificial tunnels to be dug and then improved over centuries of use. In addition to the many natural limestone caves, in many fertile parts of Afghanistan there are ancient Iranian ("karez") irrigation systems that consist of covered trenches with a well every hundred meters or so. Some of these covered trenches are centuries old, well known by the locals, and provide both a traditional hiding place as well as a means of moving around without any enemy on the surface knowing what you are up to. The karez were particularly dangerous because they made it easier for hostile Afghans to ambush foreign troops or escape when at a disadvantage.
There are a lot of techniques for finding caves and tunnels, and you often have to use two or more to make sure you find all that you need to know about what is really down there. The most effective method is to ask locals. The United States did this early on and compiled a large database of “known caves and tunnels” which was added to and modified by the use of technical means. There are lots of these technology solutions.
The most “high tech” (literally) are the use of geological patterns that can be seen in satellite or aerial photos. This technique has become more useful in the past decade as UAVs, using high resolution digital cameras and automated analysis of digital pictures, became available and more powerful geological patterns have become easier to identify and more likely to spot tunnels and caves.
Satellites and aerial recon also use heat detection. Infrared (heat sensing) cameras have gotten smaller, cheaper, and more effective in the last decade and can more easily spot the heat coming from cave entrances or vents. The increasing sensitivity of infrared instruments has defeated deception methods that use baffles and other methods to reduce heat coming from caves. Some caves are so deep that very little heat reached the surface. But even these caves give off some heat and a powerful enough sensor will pick it up.
An increasingly useful technique (again because of cheaper and more powerful instruments) is seismic (ground vibrations) detection. This uses a form of sonar. To make it work you have to put some of these metal listening devices in the ground and then use explosives or a large hammer to send a signal that will bounce back and show where there are voids (areas where there is nothing). A variation on seismic detection is using larger networks of microphones listening for the tell-tale signs of people digging or moving around and talking underground. This technique has become more effective with the growing availability of cheaper and more powerful sensors.
Ground-penetrating radar can be used from ground vehicles or aircraft. Similar to seismic detection, current equipment can detect something close to the surface (less than a hundred meters/three-hundred and twenty feet) down. Airborne equipment can cover larger areas more quickly but does not go down as deep.
There are some older techniques that can still, under the right conditions, prove useful. This depends on finding a mountain stream that disappears into a cave and pour dye, especially one that can be detected using electronic instruments, into it. Then point those sensors downstream to see where the dye shows up. A variation on this is to literally blow smoke into a cave lower down on a hill or mountain and see where, if anywhere, it appears further up the hill.
Another high-tech method, made more useful with better instruments that have shown up in the past decade, uses very tiny changes in gravity. This works because the gravity over a cave will be a tiny bit different than the gravity over nearby rock. Another methode made possible by better technology is earth resistance tomography. Electrodes are deployed in an area and a current is sent into the ground to map the differences in electrical resistance. This can detect tunnels up to a hundred meters down and less if used from low flying aircraft or helicopters. It works best over flat ground.
Taken together, all these techniques have made caves and tunnels a lot less useful for Islamic terrorists and, according to interrogations and captured documents, this has hurt morale in a big way and made life a lot more nerve wracking for those seeking safety underground.