the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
High resolution digital outcrop model of faults and fractures in caprock shales, Konusdalen West, central Spitsbergen
Thomas Birchall
Gareth Lord
Simon Oldfield
Lise Nakken
Kei Ogata
Kim Senger
Abstract. Structure-from-motion (SfM) photogrammetry has become an important tool for quantitative characterisation of outcrops. Digital outcrop models (DOMs) allow for the mapping of stratigraphy and discontinuous structures like folds, faults and fractures from cm to km scale and provide solutions that are difficult to constrain through subsurface data alone. With pristine, treeless exposures, the outcropping strata in Svalbard, Arctic Norway, hold exceptional potential for analogue studies and are ideally suited for the acquisition of high-resolution DOMs. We here present the acquisition, processing and integration of the Konusdalen West digital model data set, comprising both DOM and derived digital terrain model (DTM) data. Dronebased image acquisition took place over two weeks in July and August 2020. Fifteen differential GNSS control points were used to georeference and quality assure the model, five of which functioning as reference checkpoints. SfM processing of 5512 acquired images resulted in high-confidence, cm-scale resolution point clouds, textured mesh (DOM), tiled model, orthomosaics, and a DTM. The confidence-filtered dense cloud features a median inter-point distance of 1.57 cm and has an average point density of 3824.9 points m-2. For the five checkpoints, the dense cloud features root mean square errors of 2.0 cm in X, 1.3 cm in Y, 5.2 cm in Z, and 5.7 cm in XYZ. Drops in point confidence and point density are mainly found in areas with reduced image densities, and on the backside of boulders. Increased confidences and densities are present along the western flank of the Konusdalen West outcrop, where a fault-fracture network in mudstone-dominated stata is best exposed and photographed most extensively.
The Konusdalen West DOM and DTM cover a 0.12 km2 area and span a 170 m elevation difference. The mean of the altitude of the checkpoints versus elevation of the dense cloud-derived DTM differed by less than a cm. The dense cloud-derived DTM closely matches an existing lower-resolution reference DTM of the area. The DOM covers the upper two-third of the mudstonedominated Late Jurassic-Early Cretaceous Agardhfjellet Formation. The Agardhfjellet Formation and its time-equivalents are regional cap rocks for CO2 sequestration and petroleum accumulations both on the offshore Barents Shelf and onshore Svalbard. Faults, formation members and established marker beds can be traced in the high-resolution model and have been used for the stratigraphic integration. Additional structural measurements and observations were taken in June 2021 to place the data in the geological context. Top and side-view orthomosaics feature maximum resolutions of 8 mm per pixel, enabling the mapping of fractures and other sub-cm features. The Konusdalen West digital model data set, together with the extensive drill cores through the same section near Longyearbyen, forms an ideal starting point for the generation of high-resolution, outcrop-truthed geomodels that are suitable for numerical modelling of fluid flow and appraisal of the regionally important caprock. Data described in this manuscript can be accessed at Norstore under https://doi.org/10.11582/2022.00027 (Betlem, 2022b).
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Peter Betlem et al.
Status: open (extended)
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RC1: 'Comment on essd-2022-143', David Tanner, 08 Sep 2022
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This is a good, well-written paper. Importantly:
- The article is itself appropriate to support the publication of the data set.
- The data set is truly significant – unique, useful, and complete.
- The data set is of a high quality.
I make the following minor points:
- The title is a bit misleading. It’s not just a DOM of faults and fractures, but also stratigraphic units. Please adjust accordingly.
- It is tantalising to see a small portion of the orthomosaic and the fractures within. There should be a figure of just faults/and fractures, without stratigraphic units.
- Some handmade strike and dip of fractures are given in Table C1. Why not produce a stereonet figure, with measurements made in the field and measurements made in the DOM?
- Line 72-75. Please revise the last line of this paragraph, as it is it doesn’t make sense. Make it in to two sentences.
David Tanner
Citation: https://doi.org/10.5194/essd-2022-143-RC1 -
AC1: 'Reply on RC1', Peter Betlem, 23 Sep 2022
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Dear Dr. David Tanner,
Thank you for taking the time to review our submission and provide us with constructive feedback.
Please see below how we have implemented and addressed your suggestions.
Specified line numbers refer to the original document.Best regards,
Peter Betlem on behalf of all authors.
1. The title is a bit misleading. It’s not just a DOM of faults and fractures, but also stratigraphic units. Please adjust accordingly.
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Agreed. We have changed the title to High resolution digital outcrop model of the faults, fractures, and stratigraphy of the Agardhfjellet Formation caprock shales at Konusdalen West, central Spitsbergen.
2. It is tantalising to see a small portion of the orthomosaic and the fractures within. There should be a figure of just faults/and fractures, without stratigraphic units.
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We have revised Figure 7B to include a larger section of the DOM, and have removed stratigraphic unit boundaries and other markers from the subfigure. Note that digital fracture sets have only been mapped locally for data-usage illustration purposes, and not across the entire model. Nonetheless, Figure 7C has been revised to include a "raw" section (now Figure 7D), with the locally annotated fractures shown in the new Figure 7E. Please find the revised Figure 7 and caption attached.
3. Some handmade strike and dip of fractures are given in Table C1. Why not produce a stereonet figure, with measurements made in the field and measurements made in the DOM?
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We agree with this very good point and have included measurements in stereonets (faults as planes, fractures measured across sub-vertical sections as poles) and a rose diagram (for fracture orientations measured on flat surfaces and across sub-vertical sections) in Figure 7C and 7F. The following section has been added below line 174 in the original manuscript to clarify the methodology:
Fracture sets were analysed and classified through use of the NetworkGT software (Nyberg et al., 2018). Dip angles were calculated from the interpolated planes through line traces with at least 3 points, i.e., 3D data. The same methodology was applied to along-fault groupings of dGNSS field measurements (Table C1) to obtain interpolated fault planes and structural information.
Table C1 only includes strike and dip measurements for faults. We have updated the table caption to better clarify this. The new table caption reads:
Table C1: Structural measurements and ground truthing of selected faults, acquired summer 2021. Easting and Northing given in the WGS 84/UTM zone 33N (EPSG:32633) projection. The calculated orthogonal heights use the Earth Gravitational Model 2008 (Pavlis et al., 2008). Interpolation point set indicates point groupings used for the calculation of GNSS-based fault measurements.
Finally, Betlem (2022c) will be updated with the relevant scripts and Jupyter Lab workflow to document the structural analysis in detail.
4. Line 72-75. Please revise the last line of this paragraph, as it is it doesn’t make sense. Make it in to two sentences.
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We have revised 72-75 to the following:
Geological constraints and inputs are needed to ascertain what may happen following injection. The Konusdalen West outcrop, covering the lower part of the caprock, is ideally suited for this and forms an important analog to assess the impact of faults and fractures on fluid flow in mudstone-dominated sequences (e.g., Ogata et al., 2012).
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CC1: 'Comment on essd-2022-143', Niklas W. Schaaf, 10 May 2023
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The article provides a thorough description of the acquisition, processing, and relevant metrics of a high-quality digital outcrop model. It places the dataset in the geological context on a regional scale, illustrates its suitability to assess the caprock integrity of the Agardhfjellet Fm. in the context of local CO2 sequestration, and suggests further use cases.
Adding to the previous comment minor suggestions are:
- In addition to the detailed Data availability section, it would be nice to see how the data set can be accessed in the introduction, for example, right after stating the issue of data availability for DOMs in lines 48-53.
- Future studies could implement a cross validation approach with respect to the ground control and control points to further increase the accuracy.
Niklas Schaaf
Citation: https://doi.org/10.5194/essd-2022-143-CC1
Peter Betlem et al.
Data sets
Svalbox-DOM_2020-0039 Betlem, Peter https://doi.org/10.11582/2022.00027
Peter Betlem et al.
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