USING 3D DIGITAL MODELS TO PERFORM OUTCROP-BASED STRAIN ANALYSIS: A CASE STUDY FROM THE STILLWELL ANTICLINE, WEST TEXAS

The construction of high-resolution 3D outcrop models is an important new supplement to classic field-based geologic data collection. We applied this new and rapidly-evolving technique to our field site at the Stillwell anticline, located in the Trans-Pecos region of west Texas. The anticline was formed during the Laramide Orogeny, when contractional deformation created the impressive fault-propagation fold. Since that time, weathering and erosion have exposed excellent cross sectional views of the rocks affected by folding. Previous researchers have shown that rock deformation is concentrated within the hinge and forelimb region of the anticline, but much of the outcrop exposure across those sections of the fold is difficult to document in the field. To address this problem, we used Structure from Motion (SfM), which uses an advanced algorithm to match points in overlapping pictures to construct accurate 3D models. To capture photos for analysis, we used an unmanned aerial vehicle (UAV), a quad copter with an attached GoPro camera. We removed lens distortion and cropped the images in order to produce the most accurate visual representation of the site. We then applied Agisoft Photoscan Professional, which uses SfM algorithms to process the photos and create high-resolution 3D models of our outcrops. We exported the completed models to ArcScene by ESRI, georeferenced the models, and drew bedding, faults, and fracture lines directly onto the model surfaces, thus creating accurate annotated models of Stillwell anticline outcrops. The models clearly reveal that deformation in the hinge zone and forelimb consists of ramp-flat faulting, with interlayer slip also accommodating a portion of total strain. This method of digitally reconstructing the geologic structures provides an innovative tool that can be used to supplement and enhance classic field-based techniques.