WASHINGTON -- How badly hurt is the Iraqi army after 28 days of unremitting bombardment from the air?
The answer lies, literally, in the Arabian sand -- in what military analysts say is an almost continuous honeycomb of trenches, revetments, bunkers and foxholes spread across the desert wastes of Kuwait and southern Iraq.
There are deep, winding channels carved into the desert floor; sand-walled craters, sandbagged and netted with camouflage; and underground depots with ceilings of wood-buttressed tin or concrete. Together they represent the veins and arteries of Iraqi President Saddam Hussein's occupation force.
The shelters, experts say, are essential to the survival of the estimated 540,000 men, 5,000-odd tanks, 3,000 artillery pieces and 7,000 troop carriers that have to weather the airborne storm as they lie in wait for an allied land offensive.
As a result, they are the focus of intense scrutiny by hundreds of U.S. photo interpreters, battle analysts and intelligence officials whose task it is to assess the residual fighting capacity of the 28 divisions that make up Iraq's main fighting force.
Their findings, based on thousands of aerial reconnaissance photographs and a combination of guesswork and science, will ultimately form the matrix on which President Bush and his advisers will decide when and how to engage the Iraqis on the ground.
"We won't be going in blind -- we'll know a hell of a lot about [the Iraqi army's] strengths and weaknesses by then," said a military official. "But there's always that nagging doubt that you've missed something important -- something that could make the difference between success and disaster."
Much of the analysis will have to be based on assumption rather than confirmed damage, officials and analysts said. Many bombs are designed to penetrate underground bunkers, armed with delayed-action fuses to trigger an explosion after the heavy projectile has gone several feet into the ground.
U.S. pilots have reported secondary explosions during bombing raids, indicating successful strikes on often-hidden depots of fuel or weapons. The most spectacular of these was likened by Gen. H. Norman Schwarzkopf, head of U.S. forces in the gulf, to a volcano eruption.
Bomb damage assessment in aggregate, however, relies on smaller, more subtle signs: smoky fire, scattered debris, heat from camouflaged trucks or battle tanks that can be detected with heat-sensitive film, and the sudden interruption of electronic signals.
Then there is the problem of decoys -- fake targets, painted "bomb craters" and plastic vehicle shells -- that U.S. military officials acknowledge are an integral and effective Iraqi war tactic.
"The only way you can be sure of destroying a truck or a tank that has been well dug-in is to detonate a cluster bomb directly, or nearly directly, overhead," said John E. Pike of the Federation of American Scientists in Washington.
Cluster bombs, which spread hundreds of smaller bombs when they explode, have played a major part in the allied campaign.
A major shortcoming of aerial bombing, Mr. Pike said, is that "you have to put craters over an area the size of Rhode Island, and that's an extremely difficult task -- especially when you've got dugouts and foxholes spread out all over the country."