Spatial Distribution of Changes in Rockwall Surface at Yoshimi-Hyakuana Cave, Central Japan, Revealed by Repeated Terrestrial Laser Scanning

Abstract

For the understanding of weathering processes, quantitative detection and evaluation, including volume estimation, of surficial changes in rocks are fundamental issues. To assess morphological changes in a rockwall, we perform repeated measurement of wall surface using terrestrial laser scanning (TLS) at a test site of Yoshimi-Hyakuana cave in Saitama Prefecture, central Japan. The alignment of time series of point clouds was carried out by tie-point registration method at an initial step, and further refined by cloud-based registration method using the iterative closest point (ICP) algorithm for points on stable surfaces, resulting in registration accuracies of a couple of millimeters. The distances between the point clouds were then computed, and scan errors derived from the laser scanner device itself was found to be at several millimeter scales. Digital elevation models (DEMs) on a vertical plane were then generated from the point clouds, and analyses on the difference raster's of the DEMs successively revealed portions of detachment on the wall surface. The detachment areas have typical depths of 5 to 12mm and most frequent around 0.6–0.9 m above the ground. Supposed that the detachment is induced by salt weathering, the weathering processes including salt accumulations likely form vulnerable layers at such certain depths on the rock surface. The spatial distribution of the surficial changes will be further assessed by continuous measurements together with monitoring of amount and composition of materials detached from the wall surface.