Structural Geology with Structure-from-Motion: Multi-view Photogrammetry at the Whaleback Anticline, Bear Valley, PA
Juilet G. Crider, University of Washington
Keith R. Hodson, University of Washington
Mary Beth Gray, Bucknell University
Arlo Weil, Bryn Mawr College
"Structure-from-motion" is a photogrammetric technique to produce high-resolution digital topographic models from standard digital photographs. Rooted in the principles of traditional photogrammetry, SfM offers three major advances: 1) SfM uses automated point matching from photographs at varying distances and view angles ('scale invariant feature transform'), eliminating laborious manual identification of common points and permitting the use of photographs from arbitrary positions. 2) Relative position of the common features is determined by simultaneous inversion of many points from many images. The camera positions and lens geometry are outputs of the inversion rather than required input. 3) Multiview "stereo" is used to densify the initial point cloud to produce highly detailed 3D models, with results equivalent to terrestrial laser scanning. Furthermore, because the model is produced from photographs, the resulting form can be "textured" with the same photos to produce a photorealistic (color) model. Optionally, models are georeferenced with independently surveyed (e.g. by differential GPS) ground control points identified in the model. There are abundant opportunities to leverage this technique for structural geology.
The Whaleback Anticline in Bear Valley, near Shamokin Pennsylvania, is an iconic structure and a popular field trip destination for Structural Geology. The anticline is on the southern limb of the Western Middle-Anthracite-Field Synclinorium in the Alleghanian Valley and Ridge Province. It was excavated during coal mining, revealing the full 3D form of the folded sandstone surface, including a variety of secondary structures. The outstanding exposure permits collection of both the complete 3D form of the surface and a highly detailed record of strain across the fold at a variety of scales. It is an deal site to test the relationship between fold form and the secondary deformation that results from folding.
At the Whaleback, we are using SfM to: 1) Generate base images for mapping. We use images acquired by remote-controlled aircraft to produce a custom, vertical orthophoto of the field area. 2) Create orthorectified photomosaics of outcrops. SfM can mosaic photos from different distances and perspectives into a single image. Orthorectification removes distortion so that images can be used directly for geometric analyses. 3) Produce a 3D digital surface model for analysis of fold form. At the Whaleback, we have an unusual opportunity to capture the 3D geometry of a large folded surface (wavelength ~35 m). We use SfM to produce both a visual representation and a quantitative description of the fold for further analysis of strain.