Measuring and describing outcrops that you can't actually touch

Outcrops are valuable sources of geologic information, but exposures at mines and quarries or along streams are commonly inaccessible or short-lived. Even when these types of exposures are accessible, expense, safety, and logistical limitations inhibit detailed technical description and surveying, reducing the value of information collected at these sites. Remote-imaging and remote-measurement technologies overcome these obstacles to supplement geological and geotechnical mapping. In many cases, the methods provide critical information where traditional methods are impractical or even impossible. Three remote-sensing technologies, reflectorless total station, terrestrial laser scanning, and close-range photogrammetry, are used to obtain three-dimensional (3-D) georeferenced measurements of geologic features in outcrops and exposures. Reflectorless total station surveying involves in-field collection of surveyed points in combination with detailed descriptions of remotely observed lithologies and sedimentary or structural features along a vertical profile. Imagery from terrestrial laser scanners produces georeferenced point clouds over which conventional color photography can be draped. Processing of close-range photogrammetry produces both a 3-D stereomodel and a georeferenced, orthorectified photograph. Important attributes for mapping lithologic changes and performing slope stability analyses can be derived from measurements and calculations tied to real-world coordinates. Dip and strike inclination can be calculated from the digital imagery for joints, fractures, and bedding surfaces. Additional measurements, such as thickness, area, and ground elevations, can be determined from digital models and combined with other descriptive information, such as materials and sedimentary structures, to improve stratigraphic correlations in 3-D geologic mapping. These techniques also allow inference of material properties where physical samples cannot be obtained for examination. Image processing techniques, customarily used for aerial and satellite imagery, can enhance outcrop photography for interpretation and analysis. In addition, statistical processing can augment conventional, digital, and visible-light photography by enhancing differences in lithology, contacts, moisture, and sedimentary structure. Remote sensing of outcrops, including nonvisible infrared wavelengths, can be used to reliably characterize changes in texture and moisture, which allow accurate and detailed mapping of the distribution of sedimentary structures to help set sampling or testing priorities and provide more complete and precise descriptions of the exposed materials.