ENVI Helps Geologists Comprehend the Mystery of Wheeler Ridge

Imagine watching a multistory parking garage collapse into a pile of rubble as you try to stay standing, the earth shaking below you. Or, try to envision an apartment complex falling in on itself, chance determining which of the people inside live or die. On January 17, 1994, the people of Northridge, California experienced these terrors as a result of a magnitude 6.7 earthquake. By the time it was over, 57 people had perished and property damage totaled more than $10 billion. This relatively moderate-sized earthquake went down as the second most significant disaster in United States history, surpassed only by Hurricane Andrew. Even in an area as closely studied and monitored as the Los Angeles Basin, the fault that caused the earthquake was unknown prior to the event. Karl Mueller, a structural geologist and assistant professor at the University of Colorado Department of Geological Sciences, and Adam Bielecki, graduate student, are using ENVI (the Environment for Visualizing Images) to study the morphology of Wheeler Ridge, an active fold produced by crustal shortening above an active thrust-fault northeast of Los Angeles. Analyzing the movement of Wheeler Ridge's "young" surface (between 7,000 and 125,000 years old), should contribute to more accurate long-term forecasts of thrust-fault behavior. This will lead to the development of better models that try to forecast the future occurrence of damaging earthquakes in densely populated areas.

An aspect of Wheeler Ridge that has long intrigued geologists is a series of step-like terraces that extend for more than 40 kilometers along its northern flank. Explaining the origin of these terraced hillsides may help to understand the surface changes wrought by past thrust-type earthquakes, and lead to the ability to predict the style and magnitude of ground-deformation produced in future ones.

High-Resolution Data Analysis in ENVI

In addition to information compiled during on-location field studies, Mueller and Bielecki's research incorporates remote sensing data gathered by NASA's RASCAL instrument (RAster SCanning Airborne Laser), a scanning laser altimeter mounted on a T-39 jet aircraft. Using a nominal straight-and-level flight pattern, RASCAL maps the surface of the earth in 100 meter wide swaths by firing a laser 5000 times a second and recording the time the laser pulses take to bounce off the ground and return to the aircraft. This time-of-flight data is then corrected for sensor pointing attitude and a global positioning system (GPS) derived trajectory of the T-39 in order to produce accurate, geo-referenced values for surface elevation and the spatial position of each footprint. With flight speed nominally maintained at 100 meters/second, the topographic data has 1.67 meter along-track and across-track resolution between each measurement footprint, along with +/-5 centimeter vertical resolution. This high-resolution remote sensing produces massive data sets that do not lend themselves to interpretation using conventional software packages.

Although RASCAL is unusual in that it provides high resolution, highly-accurate information and, in turn, large datasets, many researchers will be demanding the level of precision laser-based remote sensing delivers. "ENVI is the wave of the future," says Bielecki, referring to the ease in which ENVI processes large datasets of digital terrain data. "Another useful feature is that it is compatible with widely-used image formats such as JPEG, GIF, TIFF and PICT."

Formatting Data in IDL

The first step in processing the data is to parse the flight-corrected RASCAL data into reasonably-sized portions which can be handled by the CPU. This unformatted data is then converted into three vectors (LON, LAT, ALT) which, in turn, are triangulated and interpolated with IDL's TRIANGULATE and TRIGRID routines using a grid spacing calculated by applying a function Bielecki wrote using IDL. This produces a 2-dimensional grid, which is referred to as a digital elevation model (DEM). From there, the data can be refined in IDL to remove noise and other unwanted artifacts. The ability to write batch files in IDL saved a great deal of time, since you can automate all of these tasks for each parsed section of the data set," says Bielecki. The data is then exported into ENVI, where the TOPOGRAPHIC MODELING features are used to produce graphics.

Visualizing Fold Growth

According to Mueller, "All the algorithms that we thought we were going to have to come up with ourselves are in ENVI. It really helped us make the jump to analyzing high-resolution data." The data are represented in several different forms. Color-coded contour maps, 3D grayscale and color renderings, 3D shaded relief maps, and long, mosaicked images each provide insight. "ENVI's topographic modeling is great. It allows the creation of shaded relief, aspect, and slope images, which are all key elements in testing all the viable hypotheses for terrace development based on morphology and attitude," Bielecki adds. Contouring routines included in ENVI enable the contour map to be overlaid on 3D images, with a range of values displayed and attributed with different colors and line weights while being annotated for easy identification.

Applying Earthquake Science in the Real World

Builders, city planners, land use managers and federal agencies such as FEMA who plan responses to future earthquakes all can benefit from Mueller's research. "Knowing how fast a fault is moving, which faults are active and which are inactive, and how big the earthquakes are along a fault are critical factors," Mueller says, referring to determining where an earthquake will occur and its potential severity.

More than 400 faults exist in the Los Angeles area alone, many of which are only expressed at the surface as active folds. Wheeler Ridge presents an ideal natural laboratory for developing geomorphic models that can be applied to similar features in dense urban areas like Los Angeles.

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