Quantitative structural analysis of fractures using digital outcrop models

Different remote sensing technologies, including photogrammetry and LIDAR (light detection and ranging), allow collecting three-dimensional (3D) data sets that can be used to create 3D digital representations of outcrop surfaces, called digital outcrop models (DOM). The main advantages of photogrammetry over LIDAR are represented by the very simple and lightweight field equipment (a digital camera), and by the arbitrary spatial resolution, that can be increased simply getting closer to the outcrop or by using a different lens. The quality of photogrammetric data sets obtained with structure from motion (SFM) techniques has shown a tremendous improvement over the past few years, and this is becoming one of the more effective ways to collect DOM data sets. The Vajont Gorge (Belluno Dolomites, Italy) provides spectacular outcrops of jurassic limestones (Vajont Limestone Formation) in which mesozoic faults and fracture corridors are continuously exposed. Some of these faults acted as conduits for fluids, resulting in structurally controlled dolomitization. A 3D DOM study, based on a photogrammetric SFM data set, was carried out, aimed at enabling interdisciplinary characterization and reconstruction of coupled brittle deformation and fluid flow processes. For this study we used a DOM (730 m × 360 m × 270 m) consisting of continuous triangulated surfaces representing the outcrop, textured with high-resolution images. Interpretation and modeling performed on this data set include (1) georeferencing of structural measurements and sampling stations; (2) tracing of stratigraphic boundaries, structural surfaces, and dolomitization fronts (ground-truthed); (3) correlation and extrapolation of realistic 3D surfaces from these traces; and (4) development of a 3D geological model at the scale of the Vajont Gorge, including stratigraphy, faults, dolomitization fronts, and volumetric meshes suitable for the statistical analysis of structural, diagenetic, and geochemical parameters. The DOM study highlighted the close relationship between faults and dolostone geobodies, demonstrating that dolomitization was guided by fluid infiltration along Mesozoic normal faults. In order to explore the uncertainty associated with the 3D model of irregularly shaped dolostone bodies, three different 3D dolostone geobody realizations have been modeled, providing a minimum, intermediate, and maximum estimate of the dolostone/limestone volumetric facies ratio, while honoring the field constraints.

Ground-based Lidar (light detection and ranging) can instantly record the 3-dimensional digital outcrop characterization with a high density and high positional accuracy. It is the new method to capture the geological information from outcrop. This paper presents the new methodology for digital outcrop modeling and geology information extraction based on ground-based Lidar. A complete workflow from data acquisition and processing, 3D digital outcrop model building to geology information extraction is discussed. Based on the digital outcrop model, a method of automatic extracting the dip and strike information of structural surface is researched. And a method of automatic lithology classification is studied too. The 3D digital outcrop model has been applied to geological description and analysis in Xinjiang, China.

Summary In recent years, the evolution of Unmanned Aerial Vehicles (UAV), together with advances in photogrammetry and Structure from Motion techniques (SfM) have enhanced the role of Digital Outcrop Models (DOM) in fracture studies, due also to the possibility to obtain accurate 3D measurements from large and inaccessible areas. In this work, we applied the Digital Outcrop Model techniques, developed by UAV Photogrammetry, to the study of fracturation of a strongly prograding carbonate buildup in the Dolomites. About 250 fault and fractures where measured from the Digital Model, and more than 400 fracture trends were collected in the field , both on the platform-top surface and in the overlying Heiligkreutz formation. Opening mode joints and neptunian dykes, orthogonal to the direction of progradation (i.e. margin parallel) were detected at seismic scale and in the field, and interpreted as early-stage fractures. Moreover, the carbonate body records two main tectonic events, which contribute to form the present-day fracture network: the Jurassic extension, forming N-S trending joints and normal faults, and the Alpine compressive phase. Syndepositional fractures are preferentially reactivated in strike-slip, showing that their orientation with respect to the stress field can influence the subsequent evolution of the fracture pattern.

Digitate shallow-water deltas are commonly found in modern lakes and bays, as well as within cratonic petroliferous basins. They develop one or multiple sinuous finger-like sands (i.e., bar fingers), including high-RSI (sinuosity ratio of distributary channel and bar finger ≥1) and low-RSI (RSI < 1) types. Bar fingers consist of four types of subenvironments, that is, distributary channels, point bars, mouth bars, and levees. However, the internal architecture within the above subenvironments is still unclear. This paper documents the internal architecture of a digitate delta based on the integration of shallow-core and ground-penetrating radar data from the Ganjiang Delta, China, coupled with Delft3D simulations. Our results show that multiple convex-up muddy-silty accretion beds are developed in mouth bars, which top lap the side of the distributary channels or point bars and down lap the bottom of the mouth bar. The accretion beds have low dip angles (<2°), which is slightly higher for the upper accretion beds. Point bars, unique to the high-RSI bar finger, develop multiple inclined silty drapes, which top lap the top of the point bar. The cohesive levee and backwater effect impede the migration of the distributary channel, resulting in silty drapes with high-dip angles (can be >10°) compared with those in the supplying river. This dip angle exhibits a negative relationship with downstream distance and a positive exponential relationship with lateral migration distance. Silty drapes become dense along the migration direction of the distributary channel. The levee develops multiple horizontal muddy accretion beds. The high-RSI bar finger develops a large number (>3) of accretion beds in mouth bars with high dip angles, and a large number of accretion beds in thick levees, compared with the low-RSI bar finger. The results of this paper provide insights into the prediction and development of cratonic digitate shallow-water delta reservoirs.

Historical shear deformation of rock fractures derived from digital outcrop models and its implications on the development of fracture systems

The IGCP 714 project “3GEO – Geoclimbing & Geotrekking in Geoparks” is financed by the International Geoscience Programme (IGCP) and supported by the International Union of Geological Sciences (IUGS). In this paper, we report on the results of the first phase of the project focused on the criteria to be adopted to select geodiversity sites equipped for climbing or trekking. The selection of geoclimbing sites and geotrekking routes is aimed at combining multimedia tools and outdoor activities for Geosciences promotion and conservation in UNESCO Global Geoparks, aspiring geoparks or geoparks project, and also in protected areas featuring geoheritage sites. Indeed, both outdoor activities and multimedia tools favour the pursuing of many of the United Nations Sustainable Development Goals (e.g., 3, 4, 8, 11). An international consortium of geoscientists from 12 different countries selected, through the proposed procedure, 22 geoclimbing sites, and then they also detected 30 geotrekking routes mirroring Earth geodiversity. At some test sites (geoclimbing and geotrekking) multimedia tools and digital outcrop models have been developed through different methodological approaches (e.g., Structure from Motion and Multi-View Stereo photogrammetry), to open the way to the second part of the project still in progress. These sites and the relative virtual models are herein also shown. The final aim of the IGCP 714 project is indeed to create an open data repository (digital outcrop models, videos, virtual tours, photos, scientific information, and interpretations) to upload data of the selected sites to mirror Earth geodiversity for different users including tourists and school groups.

The dual laterolog response of fractured-vuggy reservoirs based on conductivity tensor and Maxwell–Garnett mixing rule

Research subject. The hill morphology formed in the study area, which is part of the Semilir Formation, is suspected to be a product of Opak fault reactivation. Further investigation is needed to substantiate this claim.Aim. To investigate the characteristics of the Opak Fault in the Semilir Formation. The study area is a red zone, which is an area damaged by the 2006 Opak Fault earthquake.Materials and Methods. This study was interpreted through integrated data that combined the Digital Elevation Model (DEM) from DEMNAS (Indonesian National Digital Elevation Model), the Digital Outcrop Model (DOM), and extensive field observations. The methodology employed involves investigating DOM interpretation combined with field surveys and morphotectonic methods.Results. We have identified the study area as divided into two segments: the northern and southern segments. The characterization of these segments is based on the differences in the alignment of landforms associated with the general direction of the hills and analysis from rosette diagrams. In the northern region, elongated hills are oriented in the NE-SW direction, whereas in the south, they exhibit an N-S block lineation. Field observations reveal the presence of a high-angle fault at observation point A (Fig. 6a), indicating reactivation, and a reverse fault at observation point D1 (Fig. 6b). The rosette diagram results indicate a compression direction of NW-SE in the northern segment and N-S in the southern segment.Conclusions. Integrating UAV photogrammetry and Digital Elevation Model (DEM) data has significantly enhanced our understanding of the geological structures within the Semilir Formation in the Opak Fault Zone. The analysis identifies the study area as a reactivation zone of the Opak fault, indicated by the presence of high-angle reverse faults. This reactivation zone and the landscape morphology consisting of hills are interpreted as a zone of destruction resulting from Opak fault movement, previously identified as a sinistral strike-slip fault.

Increasing political and social awareness of the importance of protecting the geological heritage is compelling geoscientists to consider new methods for reconciling conservation and exploration of their research sites. Terrestrial Light Detection And Range (LiDAR) imaging is an accurate method of collecting 3D spatial data that has so far been under-utilized in the geological sciences. This aim of this paper is to assess the value of integrated LiDAR and photogrammetric imaging as a tool for synchronizing scientific exploration with conservation of geological heritage sites. Fumanya (Catalonia) is one of the most important Cretaceous tracksites in Europe, but the nature of exposure of the track-bearing surface has hindered quantitative documentation of the ichnites. Using integrated Light Detection And Range (LiDAR) imaging and photogrammetry it has been possible to construct high-resolution Digital Outcrop Models (DOM) of the tracksites. Photo-textured DOMs are a powerful visualization tool and function as fully 3D interactive databases that preserve information about the site that would otherwise be lost to erosion. LiDAR-derived DOMs have the potential to contribute profoundly to future geoconservation projects, particularly as a tool for documenting and monitoring heritage sites and promoting education and tourism. LiDAR scanning also provides sufficient resolution to perform robust quantitative analysis of dinosaur tracks.

To enhance the automation and spatial accuracy of geological outcrop information extraction, this study proposes a three-dimensional (3D) modeling method oriented toward multi-class semantic boundary representation. High-resolution outcrop images at the centimeter scale were acquired using UAV-based oblique photogrammetry. A modified U-Net model, integrating a self-attention mechanism and a boundary-weighted IoU loss, was developed to achieve fine-grained semantic segmentation of four explicit classes: microbial reef, grain beach dolomite, micritic dolomite, and background. To address the occlusion and label conflict issues inherent in multi-view imaging, a spatially-constrained projection strategy based on Z-buffer depth testing and weighted majority voting was proposed, accurately mapping 2D semantic labels onto 3D photorealistic model surfaces. Following high-precision 3D reconstruction using ContextCapture, a multi-scale semantic visualization platform was ultimately constructed based on Cesium. Experimental results demonstrate that the proposed method increases the boundary-aware semantic segmentation performance (Boundary F1-Score) to 78.64% and limits the mean projection error to within 3.2cm, significantly outperforming existing mainstream approaches. This research provides an effective pathway for the automated recognition of complex geological structures and the development of digital twin models.

From conventional outcrop datasets and digital outcrop models to flow simulation in the Pont de Montanyana point-bar deposits (Ypresian, Southern Pyrenees)

Fracture intensity and associated variability: A new methodology for 3D digital outcrop model analysis of carbonate reservoirs

Fracture/fault slip data has long been used to calculate the paleo-strain/stress. Recent development of techniques for automatic extraction of fracture surfaces from digital outcrop models and estimation of historical shear deformation on rock fractures provide an efficient way of quantitatively acquiring large amount of high quality fracture/fault slip data from outcrops. This study shows how to interpret these data. The strain tensors of the slip data are grouped into populations corresponding to far-field strain events and local strain events using a clustering analysis technique. The results from outcrops in the eastern Tian Shan area give a clear picture of the paleo-strain variation over space and time, and also throw light on the relationship between paleo-strain, fracture development and different rock formations in a thrusting environment.

Permeability in crystalline rocks, considered for use as geothermal reservoirs or deep geological repositories, is controlled by networks of well-connected fractures. Fracture connectivity depends on fracture orientation and density, which are influenced by tectonic and non-tectonic stresses, pre-existing foliations, and fracture zones. This study investigates fracture variability in the Devonian Wilsons Promontory granitic batholith of southeast Australia, which intruded Devonian metasediments that are unconformably overlain by Cretaceous rift-related sedimentary rocks.Outcrop analogues allow 2D and 3D observation of fracture networks at scales from centimeters to hundreds of meters, complementing sub-meter-scale borehole data and regional lineament mapping. Additionally, digital outcrop models from uncrewed aerial vehicle (UAV) surveys enable fracture characterization in outcrops that are difficult to physically access, such as the granites in the study area. In this study, over 2,500 fractures were mapped and characterized from a UAV-derived point cloud. Most fractures strike NNW-SSE to N-S; they are are interpreted as extensional and to have formed coevally with NNW-SSE striking joints in outcropping Cretaceous rocks during regional uplift under NNW-SSE horizontal compression. Domains characterized by distinct fracture patterns are separated by meter-scale fracture zones, suggesting structural segmentation within the granite.Future work will investigate the geometry and origin of these domain-bounding fracture zones and their links to mechanical heterogeneities in the granite. These insights will inform discrete fracture network (DFN) and hydrological models of granite reservoirs and repositories for spent nuclear waste. They will also support comparisons of brittle deformation in granitic versus siliciclastic rocks under shared tectonic regimes, relevant to energy projects involving multi-level, multi-lithology reservoirs.

Building a training image with Digital Outcrop Models