Just about the time the light at the end of the project showed up things started to unravel. Everything had gone suspiciously smoothly the first three days.
How many technicians does it take to diagnose a problem with the thumper? While there is only four of them in the picture below, Robert, Al, Bruce Engler, of the Geophysical Technology Department, and Russ, we all got involved. About one and a half hours into the fourth and final day the thumper would thump, but the recorders, linked by a long cable to the thumper, would not start recording. The "trigger" had been lost. It turns out that the Gisco unit uses an accelerometer, in the picture on the right, attached to the top of the ram to detect when the ram has hit the ground. That signal is sent to an electronics control box and a contact is closed which signals to the recorders to do their thing. The accelerometer went bad. Fortunately, we had a replacement.
Once the accelerometer was replaced we found, much to our dismay, that things were still not quite right. Some trigger signals were coming through and some were not, which technically speaking WAS and improvement, but was still not close enough for this government work.
Attention turned to the long cable that was connected between the thumper and the Geometrics units. Every time the thumper was moved the trigger cable had to be moved too. They do break, but after a bit of testing it looked like the cable was not the culprit. Robert usually took care of moving the cable around and was pretty careful about it.
We switched from using a physical cable to transmit the trigger signal to using a radio trigger. The 'talk' circuitry of a small walkie-talkie was connected to the electronics control box on the thumper.
The 'listen' circuitry of another walkie-talkie was connected to a small control box, and the trigger cable from the Geometrics recorders was connected to that. As the ram was lifted the two control boxes would establish a link through the two walkie-talkies and generate a trigger signal when the ram hit the ground. It worked quite well, except that there was still a problem with getting all of the thumps to generate a trigger. We managed to finish up the project by just staying in a spot and thumping as many times as it took to get ten good triggers, or until Greg lost his patience and told us to move on to the next shot location.
There was a bit of rain, and quite a bit of lightening the evening of the third day. It's possible that either a bit of moisture got into the electronics control box on the thumper, or that a nearby lightening strike scrambled a bit of the circuitry in the control box and caused all of the trouble that we had. Every time a thump failed to generate a trigger, Al would disconnect and reconnect the power to the control box on the thumper. That usually got us a few more triggers before it had to be done again. And again. And again.
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