Abstract
Structure-from-motion (SfM) photogrammetry enables the cost-effective digital characterisation of seismic- to sub-decimetre-scale geoscientific samples. The technique is commonly used for the characterisation of outcrops, fracture mapping, and increasingly so for the quantification of deformation during geotechnical stress tests. We here apply SfM photogrammetry using off-the-shelf components and software, to generate 25 digital drill core models of drill cores. The selected samples originate from the Longyearbyen CO2 Lab project’s borehole DH4, covering the lowermost cap rock and uppermost reservoir sequences proposed for CO2 sequestration onshore Svalbard. We have come up with a procedure that enables the determination of bulk volumes and densities with precisions and accuracies similar to those of such conventional methods as the immersion in fluid method. We use 3D printed replicas to qualitatively assure the volumes, and show that, with a mean deviation (based on eight samples) of 0.059% compared to proven geotechnical methods, the photogrammetric output is found to be equivalent. We furthermore splice together broken and fragmented core pieces to reconstruct larger core intervals. We unwrap these to generate and characterise 2D orthographic projections of the core edge using analytical workflows developed for the structure-sedimentological characterisation of virtual outcrop models. Drill core orthoprojections can be treated as directly correlatable to optical borehole-wall imagery data, enabling a direct and cost-effective elucidation of in situ drill core orientation and depth, as long as any form of borehole imagery is available. Digital drill core models are thus complementary to existing physical and photographic sample archives, and we foresee that the presented workflow can be adopted for the digitisation and digital storage of other types of geological samples, including degradable and dangerous ice and sediment cores and samples.
Highlights
Structure-from-motion (SfM) photogrammetry has led to the rapid evolution of cost-effective digital characterisation of geological, palaeontological and archaeological sites [1,2]
Digitisations with sub-millimetre resolutions have for a long time only been achievable through complex workflows and expensive setups [28], but can increasingly be afforded through consumer-grade software and hardware solutions. Through this contribution we aim to introduce an efficient and high resolution SfM photogrammetry workflow as a tool for the full digitisation of centimetre-scale geological samples, by focusing on the generation of digital core models (DCMs)
root mean square errors (RMSEs) increase, volume overestimates (e.g., DH4-568a, +40%) and underestimates (e.g., DH4-568b, −25%) are associated with fewer Ground Control Points (GCPs) and non-object-surrounding configurations, which is likely a result of insufficient coordinate truthing around the object
Summary
Structure-from-motion (SfM) photogrammetry has led to the rapid evolution of cost-effective digital characterisation of geological, palaeontological and archaeological sites [1,2]. This method has significant potential for the geoscientific community. Through SfM, digital characterisation is readily extended to include sub-decimetre-scale geoscientific samples, and allows for the quantification of data and the subsequent integration of big-data workflows. Digital reconstructions can be used to visualise reservoir analogues (e.g., [3]), are accurate enough for large-scale bulk volumetric calculations (e.g., [4]), and can be used for the characterisation, digital storage, and the reproduction (i.e., 3D printing) of samples (e.g., [5]). Industrial applications of SfM are widespread in the mining industry (e.g., [6]), agriculture and forestry (e.g., [7]), snow science (e.g., [8]), geotechnical monitoring (e.g., [9]), and even in healthcare (e.g., [10])
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