Renowned US military tech bureau DARPA has issued a request for an ultrasensitive gravity-measuring instrument which could be mounted in a low flying aircraft and detect underground tunnels.

The project, known as Gravity Anomaly for Tunnel Exposure (GATE), was formally announced on Friday. In essence, it will detect an absence of underground mass - dirt, rock or whatever - by picking out the tiny drop in the local gravity caused by the missing stuff:

The explicit action of digging tunnels introduces a void into the subsurface geology. Gravity gradiometers measure tiny spatial variations in the pull of gravity caused by these underground voids. The GATE objective is to develop, integrate, and demonstrate a prototype airborne gravity gradiometer system which is capable of detecting the mass deficit of a void in the presence of geological and topological variability.

A nothingness detector, in other words, able to perceive the hole rather than the doughnut. But in this case the wacky research agency seems to believe that the current state of the art in gradiometer systems is close to being discriminating enough to do the job already. What's needed is the ability to tune out all the noise and clutter from natural geological features, the carrying aircraft's vibration and so forth.

"This is not expected to be a sensor technology development activity, but rather focus on adapting the existing technology to GATE application," says the solicitation document (pdf).

DARPA thinks that it should be possible to fit a ground-penetrating grav spyeye and all necessary processing on board the carrying aircraft, which can be manned or unmanned. There shouldn't be any need to pipe massive amounts of sensor data back to a ground station for analysis, either. The final prototype, according to the solicitation, should offer "the ability to detect the presence of a tunnel" and "the ability to image skeletal outline of a tunnel network".

As to what you might use the GATE tunnel-sniffer gear for, DARPA does offer a hint:

To address the more immediate underground tactical threats, the Gravity Anomaly for Tunnel Exposure program will focus on border and perimeter breaching tunnels.

There have been a few border-breaching tunnels in the news lately, as it happens. ®