Quantifying Fold and Fracture Attributes Using RTK GPS and Laser-Scanning
In areas of folding there is generally assumed to be a correlation between fold curvature and the orientation, distribution and density of related fault and fracture systems. The ability to analyse curvature magnitudes and directions over small-scale fold structures offers a powerful new method of investigating the relationships between folding and fracturing. Case studies of well-exposed natural folds can provide essential information regarding fold geometry and the distribution of fracture networks over folded surfaces. However, conventional methods of field-based data collection are largely incapable of capturing, preserving and quantifying spatial variation over folded surfaces with adequate precision. Real-Time Kinematic (RTK) GPS and terrestrial laser- scanning (LiDAR) can be used to collect dense networks of georeferenced points across folds exposed in the field. The spatial precision inherent in both techniques permits a rigorous and quantitative analysis of fold and fracture properties.
We present two associated datasets collected over single folded bedding surfaces using RTK GPS, and use these to quantify the spatial relationships between surface curvature and fracturing: 1) Data points collected with regular spacing (30-50cm) across the fold surfaces were used to construct gridded Digital Elevation Models, which form the basis for curvature analysis using matrix algebra. Variations of curvature magnitude are used to identify regions of high and low curvature, corresponding to invariant fold properties such as hinge- and inflection-lines. 2) Data points representing fracture locations across the folded surfaces are used to analyse attributes such as fracture spacing and fracture linkage. The spatial precision of RTK GPS allows the curvature models and fracture datasets to be combined and compared. Fracture attributes in 1-D and 2-D can be compared with principle curvature magnitudes whilst the orientation of open fractures can be compared with variations in principal curvature directions. We are currently using laser- scanning to analyse a series of outcrop-scale folds exposed at Scremerston, Northumberland, UK. The x,y,z coordinates of the laser-scan point data can be used as direct input for curvature analysis, and interactive point-picking allows us to rapidly generate fracture networks from the laser scans.