Detection of subtle tectonic–geomorphic features in densely forested mountains by very high-resolution airborne LiDAR survey

Abstract

Despite successes elsewhere in the world, 2m resolution DEMs from a standard airborne LiDAR survey failed to detect small tectonic–geomorphic features in densely-forested high-relief mountains of central Japan. Our new 0.5-m DEMs from an unprecedentedly high-resolution LiDAR survey along the Neodani Fault now reveal a number of previously unknown fault scarps as well as other hidden geomorphic features. The survey achieves a ground-return density of 6.2m−2 out of a total shot density of as much as ~12.7m−2. The main factor to gain sufficient ground returns in unfavorable conditions is a large side lap of ~70% between flight swaths, which means that any specific area in the target zone is scanned three or more times from different angles. Evaluation of DEMs with resolution from 0.25 to 10m assures that a 0.5m resolution LiDAR DEM is necessary for the detection of subtle tectonic breaks. Another key factor for complete detection of small tectonic–geomorphic features is the application of a recently developed DEM visualization “Red Relief Image Map (RRIM)”, which allows mapping of all the small features with various sizes, orientations and morphology, overcoming major drawbacks of classic DEM visualizations. A very high-resolution LiDAR survey aided with RRIM visualization as used in this study provides a more reliable approach for the detailed mapping of slight active fault traces hidden under dense vegetation.