Nevertheless, it will be very difficult to smoke him out of his underground lair, as the president has promised.
The dugouts where al Qaeda is holed up are about 10 to 30 feet (3 to 9 meters) deep, Bearden said. "You have to drop bombs down sheer walls and rock faces. Most were built to be safe from any air or missile attack.
Finding Cavities Would be Difficult, Expert Says
"All you're doing is bouncing rubble around. If you hit a guy, you're lucky."
Even finding the underground cavities presents a difficult technical challenge, said Mats Lagmanson, a geophysicist and president of Texas-based Advanced Geosciences Inc.
All three techniques that geologists use require that the searchers and their equipment be on the ground near the cavity.
Lagmanson's company has used electrical resistivity to find a previously unknown cavern, Sting Cave, north of Austin, Texas. In this method, he said, an electrical current is injected into the ground and its resistance measured at several points. He likens it to a CT scan.
Researchers also measure the gravity at ground level, looking for tiny variations. A lessening of gravity may indicate a large underground void.
The third method, ground-penetrating radar, has been used to locate underground archaeological sites. "It's beautiful when it works," Lagmanson said. But it works best at shallow depths and certain kinds of soils block the signal.
Those searching for bin Laden and his caves have their work cut out for them.
"Most people think it should be an easy task," Lagmanson said. "But it's not that easy to find."
Copyright 2001, The Dallas Morning News