Characterizing Sedimentary Outcrops with Laser Scanning: Applied to Cretaceous deep marine limestone-marl sequences in the Vocontian Basin, France

The traditional approach to studying and characterizing exposed, sedimentary, or layered, rock outcrops has depended on both the experience of the geologist involved and on the accessibility of the rock surfaces themselves. Morphological characterization of an outcrop, such as its layered rock orientation and thicknesses, are features that can be measured by hand or with 3D instrumentation such as a total station or GNSS. However, these methods can be time consuming when the scale of the outcrop is tens of meters large and the tall outcrops themselves block GNSS signal. The terrestrial laser scanner is a device thatmeasures line-of-sight distances to create a 3D point cloud representation of the target, an outcrop in this case. It can collectmillions of points in just minutes with millimeter accuracy. If the same characterization features are to be measured, quantifiable orientation and thicknesses values need to be estimated from this 3D point cloud. A semi-automatic work flow has been developed to extract these values and to represent them in a 3D virtual environment. The work flow contains various algorithm steps for extracting sedimentary layers from an outcrop. The algorithms were developed in Matlab on a 5 x 5 m sample section of the La Charce outcrop, a sedimentary outcrop in the study region with well-defined bedding layers. The local surface normals of the points in this section were calculated, grouped, and then filtered to extract points only belonging to the tops of bedding layers. Planes were fit to these grouped ’top’ points and estimated values for the dip and dip direction (layer orientation) and exposed layer thicknesses were calculated from the planes. Based on hand measurements taken in the field of these same parameters, the algorithms estimated values fall within one standard deviation of orientation field measurements, and may actually be more precise considering a more complete outcrop containing unreachable surfaces was used in the algorithm calculations. Thickness estimates from the field varied greatly (20¡300cm) and are dependent on the exact locations the measurements were taken (algorithm estimates fall within this range, regardless). Close-range three dimensional measurement data from terrestrial laser scanning has proved to provide adequate information for estimating morphological features of rock outcrops. This means that, in addition to traditional field measurements, laser scanning be used to both validate and gain possibly more precise insight to the true geometry of a sedimentary outcrop. It is a relatively fast and simple method that ’brings’ the outcrop back into the laboratory for a repeatable, more detailed analysis. This developed method, fully functional for the La Charce sample outcrop, demonstrates a viable road map for extracting morphological features from 3D point clouds of sedimentary rock outcrops.Civil Engineering and GeosciencesGeoscience and Remote Sensin