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Sources of Uncertainty in Remote Stratigraphic Observations
Marlow, JoAnna Guadalupe
Marlow, JoAnna Guadalupe
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2021
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Geology
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http://dx.doi.org/10.34944/dspace/6868
Abstract
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 “return” to an outcrop after fieldwork is complete, collect data from outcrops that are inaccessible, or may even provide opportunities to radically increase data volume of geometric characterizations of geological structures. Our study focuses on understanding the limitations of digital measurements and interpretations used to create stratigraphic columns by comparing 2D and 3D results to traditional stratigraphic descriptions and measurements from fieldwork. 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, 100 meters of section were measured in 10-cm increments. Markers were placed on the beds throughout the section and surveyed by GPS; these markers were captured by subsequent high-resolution aerial imagery. A low-resolution DOM, a high-resolution DOM, and a high-resolution with video DOM were created in Pix4DMapper via Structure from Motion (SfM) photogrammetry. The resulting dense point clouds and 3D textured meshes were used to measure projected 3D lengths for each DOM and to create stratigraphic columns from each DOM. Additionally, a stratigraphic section from a simple photomosaic of the UAV photos was created.
The comparisons between the five methods yielded inconsistent bed thickness measurements and lithologic facies. Overall, the discrepancies suggest that differences between a digitally produced stratigraphic log and a stratigraphic log produced using traditional field techniques are not systematic nor due to distortion of digital models, and simple scaling will not produce a completely accurate representation of the section. DOM-based measurements provide more accurate strike and dip measurements of stratigraphic layers, leading to more accurate bed thickness measurements than 2-dimensional photomosaic measurements or field measurements. However, all of the digital stratigraphic sections misrepresent lithology due to image distortion and smearing at the grain scale when producing the digital model, so that clear identification of lithology is difficult even when major lithofacies are known based on prior fieldwork. The sensitivity of errors in bed thickness is due to the number and the types of images and video data collected and utilized in the point cloud as well as the processing template chosen. While DOMs can provide access across large outcrops and potentially generate large data sets, understanding sources of error is critical to assess uncertainty in these results and thus the potential for misinterpretation. This assessment requires initial fieldwork with traditional methods to calibrate the DOM’s analysis followed by fieldwork to validate DOM’s results. Thus, if used in conjunction with fieldwork, the digital techniques may be able to substantially improve data collection, serve as a long-term record for continued research, and provide a critical platform for integrating new data sets and research collaborations.
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