Workflow for capturing information and characterizing difficult-to-access geological outcrops using unmanned aerial vehicle-based digital photogrammetric data

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Abstract

This paper aims to present a methodological approach for capturing information and characterizing difficult-to-access geological outcrops using unmanned aerial vehicle-based digital photogrammetric data, which has been growing in importance as a three-dimensional modeling method along with the use of 3D geomodelling, geological, stratigraphic and structural software packages, and specialized programmed algorithms in complex geological cases. In this way, it is possible to document rock outcrops, geological structures, stratification or foliation plans, geometry of outcropping lithologies, underground and surface mining works, karst systems, etc. The data obtained will then serve as a basis for the geomodelling of the geological structure of mineral deposits and oil and gas. Traditionally, the photogrammetry technique in Geosciences has been limited to simplifying and improving the work of surface mapping, topography, cartography, interferometry patterns, surface geomorphology and spectral analysis of high-resolution satellite images. However, currently, the evaluation of the discontinuities of a rock massif can be carried out, the structural domains with high precision, in a short time and in a complete way remotely, taking the information gathered in outcrops to other scenarios so that the work be interactive.

Introduction

With the technological advances achieved by the different companies dedicated to the manufacture and marketing of unmanned aerial vehicles (UAVs) it has been achieved that this type of technology is available to multiple users according to their needs, with a modern technology that years ago It was restricted for military use only. UAVs are equipment designed to operate without crew and with the ability to fly for a certain time and at a distance from who operates without major difficulties. There is a wide variety of models, which include fixed-wing UAVs and quadcopters of different sizes and specifications according to their application. UAVs designed for agriculture, for example, are very robust and load-bearing, unlike those designed with a more commercial or fun purpose which are very compact and versatile, yet they share skills with which they are capable to maintain a stable and reasonably safe flight. In the last two decades, new computational tools have allowed great advances in different fields of science. Currently, through the implementation of digital photogrammetry, it has been possible to obtain high-resolution digital elevation models. The development of open source triangulation algorithms, such as Structure From Motion, together with the increasing capacity of processors and the use of UAVs, have allowed the development of data extraction software from images, whose uses They have diversified in fields such as medicine, architecture, arts and sciences on Earth, among others [1]. However, in Geosciences, the use has been limited mainly to topographic, geomorphological, and slope stability studies [2]. Geological mapping is an activity that can be particularly complicated in mountainous areas [3], [4], where in many cases it is difficult to access rock outcrops. UAVs have a great advantage over conventional field work as they can acquire data faster and more economically [5]. According to [6], two aspects must be considered before carrying out an aerial acquisition using UAVs: the choice of the acquisition system and the data processing strategy, which are fundamental for the superposition of images and distance. Commercial UAVs [7] usually acquire Red-Green-Blue, RGB [8] and near-infrared, NIR images [9]. A digital elevation model could be considered as one of the most important spatial datasets in a Geographic Information System (GIS) [10]. In literature [10], [11], [12] there are three commonly used terms related to elevation models, digital elevation model (DEM), digital terrain model (DTM), and digital surface model (DSM); and the difference between the three terms is not clearly and universally agreed, but some common terms may apply. A DEM is a regular gridded matrix representation of the continuous variation of relief over space. A DEM is hypothetically free of trees, buildings, or other nonground objects, providing the basis for modeling and analysis of spatial-topographic information. DEM analysis includes four steps: (1) Acquisition of data, (2) Data modeling, (3) Data management, and (4) Application development [13]. A DSM is an elevation model that includes the tops of everything, including buildings, vegetation, and the ground where there is nothing else on top of it [12]. It includes digital representations of topographic surfaces, digital models of gradient, aspect, land surface curvature, and other surface attributes [10]. A DTM is a more generic term referring to a DEM with one or more types of terrain information, such as terrain morphological features, drainage patterns, and soil properties. A DTM model, which includes only terrain information related to height, might be considered as a DEM [10]. DTM generation using traditional land surveying methods are costly and complex to obtain [14]. Using a photogrammetric approach, DSMs, DTMs and orthophotos can be generated automatically [15], [16]. [17] explain that these models obtained can be used by geologists to enrich and extend different points of view to verify the quality and validity of the conventional geological mapping acquisition. However, typical workflows to obtain geological mapping products use to couple low-resolution sattellyte imagery and time consuming high quality field measurements, which requires precise knowledge of the earth's surface. Geological research with UAVs is carried out worldwide, and the application of this technology is becoming increasingly popular. Regional studies, such as landslide monitoring [18], terrain analysis [19], deformation analysis [20], geological structure mapping on a small scale [21], monitoring of geothermal environments [22] can be achieved through UAVs. In Colombia, the use of UAVs has been limited in detailed geological studies. Colombia has a complex topography. It is common to find outcrops with difficult or even dangerous access. These difficulties include cliffs, high slopes, river banks, high stream rivers, and heavily dense forests. As a consequence, observation work is limited to the most accessible outcrops, which, in general, have limited exposure, have the intervention of civil works on the road, loss of contrasts due to alteration and smoke contamination, in general, modifications due to anthropic intervention. As part of a novel technology and in which, to this date, there are few geological works. The use of UAVs provides enormous technological advantages in carrying out extensive field work in areas difficult access areas as a complement to geological and geomorphological cartography. At the moment, Industrial Information Integration Engineering (IIIE) is an emerging subject that has attracted much attention by the Information and Communication Technologies (ICT), which provide methods for solving complex problems based on collecting, processing and reporting different types of data [23], [24], [25]. According to Xu [24], the discipline structure of IIIE has five layers. The acquisition, integration, processing and interpretation of data from rocky outcrops difficult to access by UAV can be located in the second layer of this methodological structure of the IIIE whose foundation is interdisciplinary engineering, since it involves theoretical concepts and aerospace engineering techniques for the acquisition of information, computer science for data processing and earth sciences and geographic information systems for the integration and interpretation of information. The present work illustrates some relevant aspects to the use of UAVs and their applications for geological purposes, such as the use of 3D models obtained through digital photogrammetry, to simplify and optimize geological survey of rock massifs, and present complex outcrops with high degree of detail. On the other hand, this would facilitate 3D visualization, reducing information bias due to the classic 2D representation.

Section snippets

Flights and system configuration

UAVs represent an advanced technology that in recent years has begun to be used to map surface structures [1, 26], becoming almost independent from the ground control station and capturing images at predetermined positions [27]. UAVs are equipped with navigation technology, which is now used in geosciences as photogrammetric equipment to document geological outcrops and other surface objects [1, 5, 14, [28], [29], [30], [31]].

Data acquisition

Depending on fieldwork, the acquisition system is chosen, depending on the type of activity to be performed, since they have different yields in terms of payload, flight time, and stability for data acquisition. Depending on photogrammetric purposes, fixed-wing UAVs are indicated in large areas (1.5 km radius), and multi-rotor UAVs are suitable for areas of lesser extent (400 × 400 m2) or where a vertical flight is necessary [43]. Data acquisition is key because it depends not only on the

Data processing and integration

The data processing strategy must consider flight planning, the definition of the survey area, number of lines, and relative flight altitude. Candidate outcrops selection was made to carry out its photogrammetric survey according to the following parameters: location and remoteness of the data processing center or urban area; geological unit to raise; evaluation of transport and access roads; selection of candidates according to geological criteria and difficult access according to previous

Integrated workflow for building digital outcrop models

The creation of digital outcrop models (DOMs) using UAVs is a cost effective solution to introduce the advantage of the field observations into more accurate subsurface models [51]. In the first instance, a preliminary fieldwork was carried out in order to evaluate areas of interest for gathering information from difficult-to-access geological outcrops (Fig. 8). Once the main geographical features, climatic conditions, outcropping lithologies were recognized to carry out the UAV flights, three

Future work

The applications of the outcropping models obtained from photogrammetry of images with drones have shown exponential growth in the last decade [86], [87], with which even standard datasets have been established for the test and reproduction of new methodologies [88]. In addition, future analysis would include object classification such as the use of convolutional neural networks or standard classifiers techniques such as K nearest neighbors or support vector machines, to extract different

Conclusion

In this paper, we introduce a workflow for capturing information and characterizing difficult-to-access geological outcrops using UAV-based digital photogrammetric data. The benefits that would be obtained with the use of UAVs are translated in the amount of useful information that could be contributed to the characterization of outcrops of rock of difficult access, whose data obtained will then serve as a basis for modeling in geosciences. The present work is fundamental since it will allow

Credit author statement

The corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors.

All authors carried out investigation, conceptualization, methodology and writing, supervision, reviewing and editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the Industrial University of Santander for financially support the development of fieldwork for capturing images of outcrops difficult to access with the use of UAVs and photogrammetry data processing. The manuscript benefited from the constructive comments of reviewers. We are most grateful to these people and institutions for their support.

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