Application of LIDAR to resolving bedrock structure in areas of poor exposure: An example from the STEEP study area, southern Alaska

An airborne, LIDAR (light detection and ranging) data set is used to map bedrock structure in an area with poor outcrop and indistinct stratigraphy. A 1-m, bare-earth filtered digital elevation model (DEM) is used to construct multiple shaded-relief images that show subtle topographic features suggestive of folded strata. Preliminary geologic interpretations exported to a field GIS (geographic information system) with GPS (global positioning system) identified the features as resistant beds beneath vegetation, and allowed construction of a regional map of bedding and fault traces. Bedding traces and faults were then analyzed with several 3D software applications. The ability to precisely map these features in 3D allowed projection onto cross-section lines, which helped define the structure as detachment folds cut by younger high-angle faults. Comparison between aerial photography and shaded-relief images of the bare-earth DEM indicates that structures beneath open forest and shrub are better revealed by shaded-relief images. The ability to create hillshade images using different sun angles supplements structural mapping with aerial photography, even in open tundra with no significant vegetation canopy. Aerial photography would generally be more informative in areas of extensive outcrop, and draping aerial photography on a LIDAR DEM would produce spectacular visualization capabilities. These techniques should yield good results where well-bedded rocks lie beneath thin soil cover and open canopy forest but may not be successful in deeply weathered tropical forest or glaciated areas where glacial sculpting and deposition obscure bedrock structure. We propose a mapping workflow that includes image processing of hillshade images, together with 3D visualization and mapping methods.