Seismological experiments come in two flavors -- passive, and active. Passive experiments are conducted by placing instruments in the field and measuring/recording the naturally occurring tremors. Active source experiments use artificial sources of energy to make the ground shake. The source can be anything from a hammer pounding the ground, to large explosions. This experiment was an active source one. Below is a picture of the source that was used -- a .223 rifle. The idea is pretty simple. The rifle is fired into the ground, the shock waves travel out and rebound off of the various layers of rock and sand below the rifle, and the recording instruments record the returning waves.

One of the recording instruments used for this experiment is pictured below. This is a Reftek 125 "Texan". These are self-contained, battery powered instruments that can be programmed to, for example, come on at 10:00am two days from now, record the electrical signals coming from a vibration measuring device called a geophone for three seconds, go off, and then repeat the whole thing 30 seconds later for the rest of the day. The idea is to cause an earthquake (using the gun) at the same time that the Texans are recording. Before they are placed into the field the Texans must be programmed, and have their fairly accurate internal clocks synchronized to a clock signal coming from a Global Positioning System (GPS) receiver. Once the Texans have been deployed, the shooter of the gun is ready, and the clock on the wall says that it is time to start the experiment, a laptop computer is connected to a GPS receiver, and a timer program that knows when the Texans will be recording indicates when the gun should be fired. The operator in the photograph below relays to the shooter when to fire the gun using a two-way radio. The shooting team, helpers, and a data recorder are off in the background. The operator is Galen Kaip of the University of Texas at El Paso (UTEP).

There were usually six people in the shooting team. One person shot the rifle, one person picked up the spent shell when it was ejected from the breech, another person loaded in a new cartridge, a forth person handled the cable that connected a switch that was on the rifle to the shot computer so the computer would know when the gun was fired, a fifth person recorded all of the information, and the sixth person supervised.

Below is a photograph of a Texan and its connected geophone. The geophone has about a ten centimeter long spike that is stuck into the ground. A shot hole is in the right-hand portion of the photograph with a white plastic bag stuffed into it to indicate its position.

This experiment used about 600 Texans. They were laid out in six rows at a time with each row having about 100 Texans. About 100 shots were fired between two rows from one side of the field to the other (left to right in the photograph above). During an hour break between shooting sessions one row of Texans and geophones would be picked up and moved, or "rolled", from the far side of the group of Texans to the near side of the group (towards the camera in the photograph above). Five shooting sessions, and four "rolls"were conducted each day.

Below is another view of the array of geophones and recorders.

Each Texan has its own internal clock. The daytime temperatures in Utah during the experiment were around 100 degrees Fahrenheit. After only a few minutes of laying in the sun the Texans became too hot to handle without gloves. It was thought that the heat may cause the clocks to drift, so each Texan was placed inside a brown paper bag to help keep it cool.

The way that the gun was shot was not just a straight line between the rows, but a series of diagonal lines. This can be seen in the photograph below. Each of the white spots on the ground were locations of shot holes. Each shot location was recorded, prior to shooting with a surveyor's transit, and during shooting by the person with the clipboard. Processing the data requires knowing how far each of the 600 Texans were from each shot hole. The operator at the laptop would inform the shooter after each shot if it was made within the window of time that the Texans were recording. If the window was missed then the shot for that hole was repeated.