"So what do you use? GPR? Seismics?"

"The short answer is yes." A.J. grinned. "Ground Penetrating Radar is just fine, for some things. But for others, some acoustic signals are good. Seismic shock is related to acoustics, of course, but I can induce different signal characteristics with acoustics than with a simple seismic signal. I can also sometimes get results with powerful magnetic fields. They react with the metals in the ground and bones, and bones are often packed with metal compared to their surroundings. I also use radiation detection-as I'm sure you know, sometimes fossils accumulate significant radioactives."

Helen nodded.

"There have been times I've used radiation directly in imaging, but that's not really practical in this setting, so I'll have to settle for whatever I get on the passives. Straight centimeter-scale radio waves on as high power as I can manage is another thing I'm going to try. While that wouldn't normally penetrate very far, a lot of your fossils here aren't all that far below the surface. I also try to use digital pulses where possible."

"Does that make them penetrate farther?" Helen asked. It didn't seem likely to her.

A.J. shook his head, smiling in acknowledgement of her doubtful tone. "Not directly, no. But what it does do is make it much, much easier for me to pick up the return signal from the noise, because I can listen for a specific pattern. I know what I'm looking for, in essence, and that really increases the chances of picking it up. Where the motes come in is in registering the returns from all different modes in thousands of closely related vectors, which the sensor net can coordinate and extract as precise survey points in spacetime. The motes construct their own ad hoc network and then derive their own relative positions with very high accuracy. Between time-of-flight, multiple triangulation, and a few other tricks like performing interference patterns, the network characterizes itself to within very small fractions of an inch. This means that the combined received signals are known to an extremely high degree of accuracy. That takes some processing time-that's what it's doing now, since I've stopped playing Tinkerbell.