Application of semi-automated methods for extraction of geological surface orientations: A case study from the outer Garhwal Himalaya.

The application of Unmanned Aerial Vehicles (UAV) is becoming increasingly common in geological mapping. The acquired UAV images help in building a 3D virtual outcrop model after processing. In particular, the use of UAV provides a rapid and low-cost method for estimating planar discontinuity orientation in inaccessible outcrops. Several open-access software packages are now available for automatic extraction of the orientation of planar structures using point cloud data generated from UAV images. This study demonstrates software applications for extracting geological discontinuities and compares the results with direct field observations in the outer part of the Garhwal Lesser Himalaya. Two types of geological surfaces, namely, the bedding surface and the fracture surface, are tested by processing point clouds in the discontinuity set extractor (DSE) and the FACETS (cloudcompare) software. Both the DSE and the FACETS require the availability of distinct 3-D exposures and clean point clouds on virtual Outcrop models. Results from both the software deviate from the field observations by a few degrees. Between the two software, FACETS gives relatively more accurate results than the DSE. The compass tool is an additional advantage in the cloudcompare (FACETS). These techniques have been successfully demonstrated in rock mass characterization and slope stability studies. In general, the semi-automatic methods are useful in studies requiring the orientation of a well-exposed single surface or several surfaces in a small area that is inaccessible. These techniques, however, become time-intensive due to noisy data in geologically complex areas where the rocks are cut by multiple sets of discontinuities. [ABSTRACT FROM AUTHOR]