Just as the job was finished, a roaring squall sent everyone dashing for cover.
“We’re going to set off a man-made earthquake in a moment, Sandy,” Donovan said when the dripping boy climbed into the instrument truck. “Watch carefully. When I give the word, Ralph will explode the dynamite. The shock will send vibrations down to the rock layers beneath us. Those vibrations will bounce back to the line of geophones and be relayed to the seismograph here. Since shock waves travel through the ground at different speeds and on different paths, depending on the strata that they strike, they will trace different kinds of lines on this strip of sensitized paper. I can interpret those lines and get a pretty good picture of what the situation is down below.”
“You mean you can make an earthquake with dynamite?” Sandy cried.
“A mighty little one. But it will be big enough for our purposes. This seismograph measures changes of one millionth of an inch in the position of the earth’s surface.” He started the wide tape rolling, and picked up a field telephone that connected the three trucks.
“All ready, Ralph?” he asked. “Fine! I’ll give you a ten-second countdown. Ten. Nine. Eight. Seven. Six. Five. Four. Three. Two. One. Shoot!”
There was a subdued roar deep underground. A geyser of earth and splintered rock spouted from the shot hole. The seismograph pens, which had been tracing steady parallel lines on the paper, began tracing jagged lines instead.
“All right, Ralph,” Donovan spoke into the phone. “If the rain lets up, have the boys string another line of geophones and we’ll cross-check.”
They got in one more shot before the increasing thunderstorm made further work impossible. Then Ralph and Hall sprinted over from the shooting truck and spent the next hour listening while Donovan explained the squiggles on the graph.
“So you’re not too happy about the situation, Don?” the producer asked at last.
“I hate to say so, John,” the geologist answered, “but things don’t look too good. We’ve found a dome, all right, but I’m afraid it has a crack in its top. Look at this.” He put away his magnifying glass, lighted up, and pointed his pipe stem at a sharp break in the inked lines. “I can’t take the responsibility for telling you to spend a hundred thousand dollars or so drilling five thousand feet into a cockeyed formation like that.”