Working with the radar operator, I'd dug a test pit outside the search area to al ow assessment of density, moisture content, layer changes, and other soil conditions. We had refil ed the hole, burying in it a length of metal rebar. The operator had then scanned the pit for control data.

He was completing the final tuning of his equipment when Frog got out of the Jeep and sidled over to me, fol owed closely by his guard. It was one of several forays he'd made, the sniper-free morning having al ayed his anxiety.

"What the fuck is that?" he asked, indicating a set of devices that looked like a contraption from Back to the Future. Just then Claudel joined us.

"Frog, you could benefit from some new adjectives. Maybe get one of those calendars that shows you a different word every day."

"Fuck you."

In a way I appreciated the English expletives. They were like sounds of home in a foreign land.

I looked to see if Frog was merely cracking wise, but the pale green eyes suggested a genuine interest. O.K. Where he was going Frog wouldn't be having a lot of scientifical y broadening experiences.

"It's a GPR system."

He looked blank.

"Ground-penetrating radar."

I pointed to a terminal plugged into the cigarette lighter of a four-wheel-drive vehicle.

"That's the GPR machine. It evaluates signals sent from an antenna, and produces a pattern on that screen."

I indicated a sledlike structure with an upright handle and a long, thick cable connecting it to the GPR box. "That's the antenna."

"Looks like a lawn mower.

"Yeah." I wondered what Frog knew about lawn care. "When an operator pul s the antenna across the ground it transmits a penetrating signal, then sends data to the GPR machine. The radar machine evaluates the strength and rebound time of the signal."

He looked as if he was with me. Though pretending disinterest, Claudel was also listening.

"If there is something in the soil, the signal is distorted. Its strength is affected by the size of the underground disturbance, and by the electrical properties at the upper and lower boundaries. The depth of the feature determines how long the signal takes to go down and back."

"So this thing can tel you where you've got a stiff?"

"Not a body specifical y But it can tel you there's a subsurface disturbance, and it can provide information about its size and location."

Frog looked blank.

"When you dig a hole and put something in it, the spot is never the same as it was before. The fil may have less density, a different mix, or different electrical properties from the surrounding matrix."

True. But I doubted that would be the case here. Ten years of water seepage has a way of obliterating soil differences.

"And the thing that's been buried, whether it's a cable, unexploded ordnance, or a human body, wil not send the same signal as the soil around it."