Boundary 1: Boundary Read Online Free Page B

rule, penetration increases with increasing wavelength—but the level of detail that can be detected decreases with increasing wavelength. If I want a shorter wavelength to give me a readable return, then I need a lot of power."

    Helen nodded along with Jackie, as A.J. continued to carefully sift his Fairy Dust onto the ground in the area of the fossils and the rock still left to be removed.

    "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.

    "So once the network's fully characterized, I start setting off the signal pulses. I let the network know"—he tapped his glasses and the virtual control interface that only he could see—"exactly what signal I'm about to send, then trigger it. The net records all the responses it can, I hit it with another pulse; maybe change modes, it starts building up a rough picture. I examine it, see if I've got something coming up. Maybe I go back, do a few more GPR or radio shots, or try another acoustic signal, or shift frequencies. Eventually, I've got all the data I think will be useful. Then I can really go to town on this stuff; sensor fusion, bandpass filtering, synthetic aperture, Kalman and Weiner filters, all that kind of thing, plus some tricks of my own.

    "With a handful of these motes and no special signal generators, I can use the ambient sound to locate and determine the number, direction, and general composition of your tents—without any of my dust motes actually touching the tents. Heck, with equipment twenty years older, I could send any two of you off to have a conversation, and not only locate you, but pick out your entire conversation, whispered, on the other side of a hill three hundred meters off. These motes have access to my own neural net code, expert systems, fuzzy logic structures, all sorts of stuff in the control unit and local