Geological structure measurement by LiDAR technology is a remote sensing method applied in structural geology. It enables monitoring and characterisation of rock bodies. This method's typical use is to acquire high resolution structural and deformational data for identifying geological hazards risk, such as assessing rockfall risks or studying pre-earthquake deformation signs. Geological structures are the results of tectonic deformations, which control landform distribution patterns. These structures include folds, fault planes, size, persistence, spatial variations, and numbers of the rock discontinuities in a particular region. These discontinuity features significantly impact slope stability, causing slope failures or separating a rock mass into intact rock blocks (rockfall). Some displaced blocks along faults are signs of earthquakes. Conventionally, geotechnical engineers carried out rock discontinuity studies manually. In post geological hazards studies, such as rockfall, the rockfall source areas are dangerous and are difficult to access, severely hindering the ability to carry out detailed structural measurements and volumetric calculations necessary for hazard assessment. By using LiDAR, geological structures can be evaluated remotely, enabling a 3-D investigation of slopes with virtual outcrops. LiDAR technology (Light Detection and Ranging) is a remote sensing technique that obtains precise 3-D information and distance. The laser receptor calculates the distance by the travelling time between emitting and receiving laser pulses. LiDAR produces topographic maps, and it is useful for assessing the natural environment.
Importance of measuring geological structures by LiDAR
Geological structures are responsible for providing distinct physical properties to rock masses. Discontinuous properties and plate tectonic forces may alter rock masses and their geometries. These structures contain joints, fractures, bedding planes, shear zones, mechanical breaks, or any other features ranging from microscopic (<1 cm, foliation development by metamorphism) to macroscopic scale (>100m, mid-oceanic ridges). Geological structures are typically elongated, their orientations are often described as "strike". If a rock body is extensively tilted, taking its slope resistivity into account, it may have a high potential to cause rockfalls. The use of LiDAR in the structural analysis allows measuring landform features from a single outcrop scale to a terrestrial scale. Some geological structures measurement and their importance are listed below:
Rock plane orientation measurement and rockfall risk assessment Rock plane orientations are the natural inclinations that occurred on a rock plane. Some examples of rock planes are bedding planes, fault planes. The planes' orientations are measured by dip and dip direction with a clinometer and compass, where dip represents the maximum inclination of a plane to the horizontal, dip direction is the direction of the intersection line between horizontal and the inclined plane. A stereonet can visualise the distribution of dip and dip directions to analyse the kinematics of a slope. Kinematics represents the motion of a rock body without external forces that cause them to move. Kinematics analysis concentrates on the possibility of translational failures due to planes sliding although other types of failure modes, such as wedge and toppling failures, can also be recognized.
Faults behaviours measurement and earthquakes predictions Faults behaviours can be used to measure the rate of sediment transportation and predict earthquakes. An earthquake can contribute to the formation of faults scraps. One side of a block will be relatively upthrown, causing vertical displacements. Therefore, given the parameters of fault scraps, structural geologists are able to trance the age of it and deduce the time involved to form such features. Earthquakes are initiated by slow slips. Slips are the displaced blocks along two sides of a fault. However, these slips are undetectable by seismometers (maximum 5mm/day). When the slipping blocks reach a critical rupture velocity, the faults would gradually evolve into a final quake size by linear acceleration along fault planes. The critical displacement of faults is proportional to the initial rupture velocities. After collecting LiDAR data from pre-earthquake and post-earthquake landforms, by constructing 3-D digital terrain models, the displacement and deformations can be derived. Thus, scientists can predict the final earthquake scale in the future by determining faults and slips' characteristics and areas' size, and short-term earthquake predictions are possible.
Surface processes and geological mapping When carrying out geological mapping, interpretations through aerial photographs and satellite imagery are often used, but forest vegetation remained the major challenge for mapping. For example, characterising physical landform features at ridges and valleys are somehow complicated, many of these features are forest-covered. The topographic maps are then constructed by obtaining data manually. LiDAR provides the full-waveform system, it enables the laser pulse to penetrate canopies and vegetations. This system allows obtaining bare-ground geological data points. Webster et al. have discovered new craters in Northern Canada by harnessing LiDAR data and digital terrain models. A digital terrain model has to be constructed to measure structural parameters (tilt angles, river incision depths). With the precise bedrock and surficial lithology mapping by LiDAR, structural geologists can reconstruct surficial processes involved.
Traditional structural measurement Traditional structural orientations can only be assessed on reachable exposed rock mass manually. Conventionally, engineering geologists investigated rock discontinuities studies with only a limited number at a time. The discontinuities may not represent the whole outcrop. Thus, the traditional rock plane orientation measurement may contain bias. Geotechnical studies also investigate other geomechanical parameters, such as persistence, block size and rock joint spacing.
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