Extracting Fold Form for Scientific Investigations and Education

Mattathias D. Needle, University of Washington
Juliet G. Crider, University of Washington

Structural geology is an inherently three-dimensional and visual sub-discipline of the Earth sciences. Recent technological advances, such as Structure-from-Motion (SfM) and 3D-printing, enable the 3D visualization of geologic features as a tool for both scientific inquiry and education. We report on a workflow for capturing the geometry of structures in the field, preparing the geometry for scientific investigation, and 3D-printing scaled models as an educational tool.

The Whaleback Anticline is an exceptionally well-exposed fold in the Alleghanian Valley-and-Ridge Province of Pennsylvania. Excavated by strip mining along most of its ~100-m extent, the 3D geometry and associated meso- and micro-scale structures are directly observable, which makes the fold an ideal locale for understanding the relationship among fold-related structures of various scales. By using the Whaleback's 3D geometry as a constraint, we intend to explore the capabilities of analytical and numerical fold models for the prediction of strain distribution and rheology. To extract geometric data describing fold form from the Whaleback, we generated a digital model using SfM photogrammetry.

With UAV photography, GPS-surveyed ground control, and Agisoft Photoscan (photogrammetry software) we produced a 3D point cloud and digital model of the Whaleback and surroundings. To best represent the form of the anticline for further analysis, we cleaned the point cloud of vegetation and soil cover, leaving only the folded sandstone surface. We fit non-uniform rational basis splines (NURBS) to the point cloud using a standard CAD program (Rhino) and a third-party, NURBS-generating extension. The product of this process is a smoothed surface represented by splines, which can be operated on mathematically.

To share this excellent example of a fold with those unable to visit and to provide a tactile model of the structure for classroom instruction, we created a 3D-printed model of the Whaleback Anticline. Using a polygonal mesh constructed within Agisoft to intersect a plain digital box in Rhino, we built a "watertight" model for 3D printing. The product is an object featuring the Whaleback surface model and a rectangular base that reproduces detailed features of interest, such as the network of secondary faults along the north limb of the Whaleback. We have successfully printed this object and intend to explore the capabilities of this workflow for the reproduction of even finer-scale features from the Whaleback.