What made this project a whole lot more difficult than it needed to be were the sensors that PASSCAL supplied. They were standard geophones that did their best work measuring vibrations in the 4.5Hz range. The thing that made it all difficult was that instead of there just being one geophone on a wire there were six! This meant that planting the geophones took five times longer than was necessary! The reason that they showed up at this experiment was that PASSCAL doesn't have any single 4.5Hz geophones. If we were working in the fertile dirt of the state of Iowa it wouldn't have been a problem, but the soil isn't as kind as that where we were working.
There were 17 lines of about 20 stations each which made for about 320 stations to cover the whole site. The ground was not particularly soft anywhere on the site. On top of that, no pun intended, the whole place was covered with gravel. Greg Elbring, of Sandia's Geophysical Technology Department, is standing on the left. He was the one we all pointed to when anyone asked who was in charge. Neil Symons, from the same department, is bent over apparently drilling holes in the earth??
By the end of the project we figured out a pretty good system for planting the geophones -- when we had enough people. One or two people would go out first with garden hoes and scrape away the gravel at each site. The sites had been pre-marked with small pink flags and paint before the seismic work began. One person then came in with an electric hand drill and drilled six holes in the dirt. That's what Neil was doing above. Then up to three people came in after the holes had been drilled and planted the geophones. It worked out quite well. It took about 4 hours to move all of the cables and geophones.
Below is the six planted geophones and the long cable that connects them all. The cable had a penchant for turning into a rat's nest of knots whenever you moved it or stared at it for too long. Untangling all of the knotted cables was almost as much fun as planting the geophones. In the lower right-hand corner is the main data cable that carried up to 60 stations worth of data to the recorders.
If you build them, they will get in the way. Some of the sites just happened to be in areas that were under some of the buildings, trailers, and antennas. Some of those sites didn't get installed. Others that did required a bit of work.
The base of one of the antennas just happened to be in the way of two sites. The spikes on the bottoms of the geophones were removed and small holes were drilled in the cement to accommodate the small nub that was left on the bottoms of the sensors. It was a good thing that the nubs were only about 1 centimeter long, because the cement used to hold the antenna up, in fine U.S. Government paid-for style, must have been about 6000 psi concrete, if there even is such a thing. It was NOT soft, and was definitely harder than your average public sidewalk. We let Russ Keefe, of the Geophysical Technology Department, do the drilling here.
Playdoh, from a Playdoh Party Pack no less, was used to stick the sensors to the cement. If they had just been left sitting on the cement without the Playdoh some of the energy from the vibrations that we created would have been lost. This is what is known as coupling. You want the sensors to be as firmly attached to the ground, or whatever else they may be on, so they will pick up as much of the generated signal as possible. Below is Greg creating a work of art that even his kindergarten teacher would have been proud of.
To help with the coupling some rocks were also placed on top of the geophones that were squished down in the Playdoh.
DIRECT LINKS TO PAGES:
A QUICKIE IN KIRTLAND
THAT'S A FACT
SENSORS FROM HELL
THIS WASN'T A BAMBI MOVIE...
THUMP AND STACK
CAN'T WIN 'EM ALL
THE QUICK AND THE DONE