Capturing Sedimentary Outcrops in 3D; Putting the Geologists' Needs into Practice.

Abstract

Outcrops of unconsolidated sediments provide very valuable information about Dutch geology, but are vulnerable to erosion and destruction by man. By digitally preserving them in 3D, these outcrops can be visualized, analyzed and revisited in the office. An effective way to digitally capture outcrops is to create a Digital Outcrop Model (DOM) by using Structure from Motion, a low-cost, user-friendly photogrammetric technique. The intended use of a DOM will determine the minimum needed requirements in terms of detail, precision, and accuracy. In its turn, the requirements will determine the methods that are used during data collection. This study will answer the question of how sedimentary outcrops can effectively be captured as DOMs, to be applied in geology as usable 3D models. To define the usability of a DOM, a needs assessment was performed among 15 specialized earth scientists. They were asked about their current fieldwork practices, the potential use of DOMs in their work, and the minimum needed requirements. The information was used to create three DOMs on different scale levels of an active clay and sand quarry, in Brunssum, the Netherlands. The data acquisition was performed using a high-end digital camera and a UAV (drone) to capture data from inaccessible and invisible areas. The study shows that DOMs are seen as powerful communication tools by a large majority of interviewed geologists, for example, to discuss with colleagues, to present results, or to introduce a new audience to geology. The most striking result is that the need for less-detailed overview models exceeds the need for the high-resolution models of outcrop sections. Overview models of large outcrops offer the ability to create virtual viewpoints, swap between scales, and collect data from areas that are physically unreachable, which will all help to see the larger geological perspective of an area. Even though the high-resolution detail models can provide geologists with information on sediment grain sizes and colors, it is generally believed that geological interpretations should not be made solely based on information derived from DOMs. However, if the models are accurately georeferenced and combined with other geo-data in a GIS, it will be a very valuable new addition to geological research. Finally, regardless of the use of the model in the short term, creating DOMs offers the opportunity to secure and archive sedimentary outcrops for feature research, even after their destruction.