This trip took me to the big cities, as well as the little villages of the South American country of Venezuela in July 2001.
The original intent of this active source project was to determine the characteristics of the sub terrain along a 300 kilometer line running from Barcelona, Venezuela, south to the end of the road in the interior of the country. Thanks to super-human effort, and a lot of hair pulling by the folks in Venezuela, we not only did that, but we also did a whole lot more.
The physical work for this project actually began in April 2001 when one of the Principal Investigators (PIs) for the experiment, Michael Schmitz from Venezuela, paid a visit to the PASSCAL Instrument Center in Socorro. The timing was such that we just happened to have all of the equipment necessary to conduct a mini-experiment like the one that would be conducted in Venezuela. Normally everything is out in the field.
We programmed up, and deployed a "line" of fifteen Reftek 125 "Texan" recorders in our side yard next to the instrument center. The experiment in Venezuela used about 190 Texans. The Texan recorders, when connected to a geophone sensor that is stuck into the ground, can be programmed to record the vibrations caused by whatever vibration/energy, or shockwave, source that an experiment uses.
The source of energy for our experiment was a Betsy M3 Seisgun which is basically a shotgun pointing straight down that fires a 8 gauge shotgun slug into the ground. We had two shot windows. At the specified time that the Texans were programmed to start recording we fired the gun once at one end of the line, moved the gun to the other end of the line, and two minutes later fired one shot there when the Texans came on again. Each of the shot windows were only a few seconds long.
After the field work was complete the Texans were brought back inside, and the data that they recorded during the two shot events was downloaded and processed. It took about four hours of work to finish processing the data -- and this was a really small experiment.
The desired result of these active source experiments is a wiggle plot like the one below. The signal recorded by each Texan is represented by each vertical line. The lines are arranged in order from the Texan that was closest to the source (where the gun was fired) on the left, to the Texan that was farthest away from the source on the right. The wiggles represent the echoes of the shock waves generated by the gun bouncing back from different layers in the ground. In general, the plot is upside down. The further up the plot you go from the bottom of the picture the deeper the feature an echo is coming back from. With a plot like this, and in some cases I suspect after a lot of beer drinking and eye squinting, a scientist can determine what the ground looks like beneath the area being studied. You can see that the 9th Texan, or the sensor connected to it had a problem. Instead of recording a changing signal it just recorded pretty much continuous noise.