Application of laser scanning for rapid geologic documentation of trench exposures

Trenching of a known or inferred fault is a traditional technique to verify the presence of the fault and potentially determine relative palaeoseismic slip rates and magnitudes. These procedures have been employed and improved over the past 50 years and have been particularly applied by engineering geologists concerned with local and regional tectonics that affect large structures such as dams and power plants as well as local habitable structures. Traditionally, however, trench excavation and documentation (logging) is time-consuming and laborious, requiring experienced geologists to identify plausible trench locations and to manually document the exposures in the field. Differences of interpretation are common; hence, many jurisdictions require an independent reviewer to field check trench exposures and to assess the reasonableness of the geologic interpretations. In this paper, we describe a less costly, novel technique to enhance the quality and rapidity of geologic trench documentation. Specifically, we use terrestrial Light Detection and Ranging (LiDAR) scans, coupled with camera imagery, to collect three-dimensional point clouds. These data are then converted into trench-wall images that are readily accessible for analysis even after trench closure. The procedure also provides grain-size (texture) data that are useful for identification and correlation of discrete, trench-exposed stratigraphic markers that are offset or that overlie an imaged fault without being broken. Then, depending on sediment dating, a fault can be deemed “active” or “hazardous” depending on the local classification, and this information can thus be used by planners and others involved in hazard and risk reduction.