Mapping surface fractures on the Sierra Nevada, using GIS
Practical technique for mapping surface fractures at regional scales is shown. On ArcView, modified Digital Elevation Model (DEM) dataset generate fascinating images that reveals structural features in western side of the Sierra Nevada. Nine wells, drilled on the inferred fractures, produced a satisfactory water discharge.
Site area
A digital elevation model (DEM) is a digital representation of ground surface topography or terrain. This DEM shows an area located on the western side of Sierra Nevada, Madera County, California. The DEM has a 10 meter-horizontal resolution, and 0.1 meter on vertical scale. Prepared by the U.S. Geological Survey, this data set was established back in 1978, which is better because the area was less populated therein less man made features.
DEM analyses
To enhance the topographic features from DEM, I prepared ‘shaded relief’ images using ArcView 3.2 with Spatial Analyst extension. These ‘shaded relief’ images were adjusted to the sun position from different azimuth. Also, the images are modified for different colors to intensify the structural relief/features.
Figures below are showing the results of these modifications. On each figure, numbers on the right top corner refer to the sun position for Spatial Analyst extension on ArcView 3.2. For instance, Figure 1 is a shaded relief where the sun positioned at azimuth 315˚N on 45˚ angle/altitude from the land surface (default values on ArcView 3.2). The color ramp for colored images is Sea Floor Elevation with the darkest blue represents lowest relief (value). Figure 3 illustrates shaded relief where the color code is reversed, thus the low relief area appears high and vice versa. (click image for larger view).
Fracture analyses
DEM analysis identified three dominant fracture orientations. They are mostly in the northwest-southeast (N295E) and northeast-southwest (N235E) directions, and a few on relatively north-south (N342E) direction. These are on agreement with previous research by UC Berkeley near the area (Karasaki, et. al., 2000).
Logically, predicting which tectonic stress that generates a 100 to 1000 feet fracture is unfounded because the Sierra Nevada experienced a number of tectonic activities, even until today. As we know, one tectonic stress direction can creates relatively short fractures at numerous orientations.
Wells drilled on fractures
Nine wells (A to I) that located at and near the inferred surface fractures yield a good amount of water. Well yields on Table 1 below are based on airlift-tests that run from 1 to 2 hours. Long-term capacity is mostly lower than the airlift-test result, but the number represent general information of the pumping capacity of the well.
Conclusion
By modifying a DEM, we try to intensify the topographic relief appearance of an area. On ‘shaded relief’ image, adjusting the sun position azimuth and changing the color code will expose more of surface features.
Digital Elevation Model is a powerful tool for regional fracture mapping, at least up to 1 inch = 1,000 feet scale. Modifying the DEM image can reveal interesting structural patterns in the area, and their potential relation to ground water.
Reference
K. Karasaki, B. Freifeld, A. Cohen, K. Grossenbacher, P. Cook and D. Vasco. 2000. A multidisciplinary fractured rock characterization study at Raymond field site, Raymond, CA. Journal of Hydrology, vol. 236, issue 1-2, pp. 17-34.
I’ve got to say, this is a great post and a great blog alltogether. wty1d8