SOURCES OF UNCERTAINTY IN REMOTE STRATIGRAPHIC OBSERVATIONS

Small UAVs (drones), are increasingly useful for field data acquisition in the geosciences. Drone images and videos can be processed via digital photogrammetry to produce a 3D digital outcrop model (DOM). DOMs provide opportunities to collect data from outcrops that are inaccessible. Our study focuses on understanding limitations of virtual digital measurements and interpretations used to create stratigraphic columns by comparing 2D and 3D results to traditional stratigraphic descriptions and measurements observed in the field.

In this study, a drone collected photos and videos of a well-exposed section of the Palm Spring Formation in the Mecca Hills, California, which is divided into lower and upper units by an angular unconformity and a change in overall texture. In the field, 75m of section was measured in 10-cm increments, and markers were placed on the beds throughout the section for use in correlation. Due to the change in bedding above the unconformity, only three meters of the upper Palm Spring Fm. were accessible on the cliff face. Pix4Dmapper was used to produce a point cloud and calculate terrain 2D lengths and projected 3D lengths from a DOM. Additionally, a stratigraphic section from a simple photomosaic of the UAV images was produced.

Initial results between the four stratigraphic logs show inconsistencies in bed thicknesses and lithology. Overall, the discrepancies suggest that distortion within a digitally-produced stratigraphic log is not systematic, and simple scaling will not produce an accurate representation of the section. DOM-based measurements provide more accurate bed thickness measurements than photomosaic measurements, however, all digital sections misrepresent lithology, even when major lithofacies are known based on prior fieldwork. We will present sensitivity of this error to bed orientation, number of images collected and utilized in the point cloud, flatness of rock face, shadow, and processing template. While DOMs make fieldwork more accessible to geologists, understanding sources of error are critical for assessing the uncertainty associated with observations and judging potential correlations with other field results.