Abstract
Abstract. Terrestrial Laser Scanning (TLS) greatly facilitates the acquisition of detailed and accurate 3D measurements of remote rock outcrops, at an operational range from several meters to a few kilometres. Reliable, quantitative measures of rock discontinuity roughness are necessary to characterize and evaluate the mechanical and hydraulic behavior of the rock mass. The aim of this research is to investigate the TLS potential and limitations for a reliable estimation of small scale roughness. TLS data noise and resolution define the level of extractable morphological detail, and therefore need to be known and associated with roughness value. The stationary variant of Discrete Wavelet Transform (SWT) was applied to estimate TLS noise level and perform wavelet denoising in direction of range measurements. Denoised TLS data were compared to reference surfaces of decreasing resolution (reference grids) in order to define the size of extractable surface detail. Noise and resolution effect on rock surface roughness, wavelet denoising success and extractable roughness scale were investigated with comparative analyses of TLS and reference surfaces. The developed methodology enabled reasonable TLS noise estimation, improved capabilities of TLS for modelling fine features of an irregular rock surface, and indicated the surface scale that can be reliably extracted from the TLS data.
Highlights
Advantages of Terrestrial Laser Scanning (TLS) are acknowledged in many fields of geological engineering, since it permits an in-situ acquisition of a large and remote surface in a short period of time, and represents the 3D surface structure with a relatively dense and precise point cloud (e.g. Buckley et al, 2008)
While decent results have been obtained in quantifying waviness (Fardin et al, 2004), finer details of unevenness have been hindered by TLS data precision and resolution
The terrestrial laser scanner Riegl VZ400 and the optical 3D coordinate measuring sensor GOM ATOS I were employed for data collection (Fig. 1)
Summary
Advantages of Terrestrial Laser Scanning (TLS) are acknowledged in many fields of geological engineering, since it permits an in-situ acquisition of a large and remote surface in a short period of time, and represents the 3D surface structure with a relatively dense and precise point cloud (e.g. Buckley et al, 2008). Roughness can have a prevailing influence on the shear strength. The parameterization of roughness, to fully capture the influence of roughness on shear strength, remains a challenge; it needs to consider that roughness is direction and scale dependent (Rengers, 1970). Measurements need to performed in the anticipated shear direction and at the engineering scale of interest. Larger scale roughness features are referred to as waviness and represent surface irregularities with a wavelength greater than about 10 cm (Priest, 1993). Smaller scale features are referred to as unevenness and include finer features that are superimposed on the waviness. While decent results have been obtained in quantifying waviness (Fardin et al, 2004), finer details of unevenness have been hindered by TLS data precision and resolution
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