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Seismic reflection lineIn September 2004, geophysicists and geologists from Stanford University conducted a seismic experiment in northwest Nevada. One component of that experiment was a high-resolution seismic reflection line across the Surprise Valley near Cedarville, California.
Why conduct a seismic experiment?Geologists are able to infer a lot about what is going on below the surface by looking at the geology that is exposed. In the Warner Range region, we were able to map a large fault on the east side of the range. What if we want to find out how much motion has occurred along that fault? There are several ways to do so, but one way is to look at the subsurface. By sending sound waves down into the earth and measuring them when they return, we can get a "look" at the rocks below the surface. How do you conduct a seismic experiment?On a basic level, a seismic experiment requires a source of seismic waves and a lot of strategically placed receivers. For this high-resoution profile across the Surprise Valley, we used a tri-axial Vibroseis truck called T-Rex (see photos below), on loan from the University of Texas-Austin. "Tri-axial" means that it is capable of shaking in three directions: vertically, horizontally in line with the length of the truck, and horizontally across the truck.
The receivers, or geophones, were buried in the ground every 40 m across the valley and into the mountain range. Each geophone is attached by a cable to a recorder, which was programmed at our staging grounds in Cedarville to turn on, record data, and turn off. The recorders are called "Texans" in honor of their development by the Texas Universities Seismic Instrumentation Alliance, and were loaned from PASSCAL/IRIS (see photos below).
The truck was driven for 16 km across the valley, stopping every 10 m to shake. Although this meant that the truck only travelled a few km per day, it used approximately 90 gallons of diesel every day. The geophones and receivers were then dug up and returned to their cases. Once back in Cedarville, they were connected to computers and the data from them was downloaded before the batteries went dead.
What were the results?The data collected in the field is initially processed by PASSCAL and were further processed by Derek Lerch here at Stanford. They combine data for each "shake" of the truck from all of the recorders, in order to get something like this:
The small flag near the middle of the graph at the top represents the location of the T-Rex truck, and the numbers along the top are meters of offset, or distance away from the truck. The vertical axis is not depth, but travel time of the seismic waves, which is roughly related to distance. The dark areas represent strong reflecting surfaces. Data from all of the shaking events are then combined to form something more like this:
Again, the vertical axis is two-way travel time in milliseconds, and the horizontal axis is distance across the valley. Based on these combined data, we can begin to make some interpretations of the data:
The red line is a very strong reflector, and probably represents the subsurface expression of the range-bounding normal fault. THe blue line probably represents the tilted basalt flows that are exposed on the eastern side of the Surprise Valley and continue at depth, but it is unclear if these units are cut by the fault. The flatter reflectors in the central portion of the image are probably a series of lake sediments that have repeatedly filled the Surprise Valley over the past few million years. |
