"Seeing" a Laramide Age Mountain Front

June 4th (Tuesday), 6 p.m., Teton Co. Library Auditorium – Open to Public. Presentation: “Seeing” a Laramide Age Mountain Front”, Presented by Morgan Brown, Tenax Geophysical.

The Wind River Range, the Gros Ventre Mountains, the Big Horn Mountains, and other Laramide Age mountain uplifts are, along with their adjacent broad basins, what define most  of what we see as we drive across Wyoming. But these are complex features, and it is a major challenge to study and understand what the transition from mountain range to basin (the “mountain front”) looks like in the subsurface  –  an understanding that is important from both a scientific and economic perspective. The tool that allows us to “see” & understand these features is seismic surveying & data processing. In this talk Dr. Morgan Brown will use an important example from the Front Range of Colorado to explain how such a tool is used, and as a result, what a mountain front looks like.

In 1983, the US Department of Energy (DOE) acquired a two dimensional (2D) seismic reflection survey near the Rocky Flats Nuclear facility, south of Boulder, Colorado.  The study aimed to gain a clearer picture of shallow faults that might have connected buried contaminants to the groundwater aquifer.  Luckily for scientists & industry, the seismic line extends west of the Rocky Flats facility, providing perhaps the clearest view of the fascinating Front Range compressional tectonics. 

Seismic reflection surveys are reflection soundings.  Seismic waves (created using dynamite or vibrating vehicles) are propagated into the subsurface.  The waves reflect from buried rock layer boundaries and return to the surface, where they are recorded by arrays of seismometers.  We record the reflection in time, but for most applications (e.g., oil & gas drilling), we must know the actual depth to the target.  If the geology of an area is simple, the speed of seismic waves tends to vary in a simple manner – & we can convert the measured times into a depth if we have data on rock velocities.  However, in complex geology, the seismic velocity may vary radically over a short distance.  This means that a seismic time image may be a horribly distorted, inaccurate depiction of the true geology.

In order to recover an accurate image in depth, the author applied state-of-the-art seismic imaging technology.  Particular modeling challenges included: 1)) understanding where the reflection data came from in the subsurface; 2)) choosing appropriate rock velocities as input; 3)) utilization of geologically sound possible models; 4)) iterating with other experts to generate a product that was constrained by data and represented a realistic possible outcome. This process was complex, challenging, time consuming. But it resulted in a much better “picture” of what the mountain front looked like, and if a very expensive exploration well were to be drilled where and how it should be drilled.

This presentation will provide the audience with an understanding of the powerful technology used, and how it can provide critical insights into the geology along a mountain front, insights which help us to understand how they formed across Wyoming, as well as greatly assisting in exploration for petroleum in basins such as those in Wyoming (and elsewhere).