Engineering Geological Mapping for the Preservation of Ancient Underground Quarries via a VR Application

Abstract

Underground monument preservation is tightly linked to geological risk. The geological risk management of underground structures typically relies on a preliminary site investigation phase. Engineering geological mapping—as a key site investigation element—is largely based on manual in situ work, often in harsh and dangerous environments. However, although new technologies can, in many cases, decrease the on-field time as well as eliminate inaccessibility issues, the example presented in this study demonstrates a special challenge that had to be addressed. The ancient underground marble quarries of Paros Island in Greece constitute a gallery complex of a total length of 7 km and only two portals, resulting in total darkness throughout almost the full length of the unsurveyed galleries. As such, the entire survey and engineering geological mapping solely relied on a virtual reality application that was developed based on a digital replica of the quarries using laser scanning. The study identifies several critical locations with potentially unstable geologic structures and computes their geometrical properties. Further numerical analyses based on data extracted directly from the digital replica of the rock mass led to the definition of appropriate risk mitigation measures along the underground marble quarries.

1. Introduction

Engineering geological mapping constitutes a key element of the preliminary site investigation phase in geological risk management projects. Underground structures such as quarries are typical engineering works with high geological and geotechnical risk dependence for their success and sustainability. To study the stability and ensure the safety of the people interacting with the quarry, a detailed characterization of the rocky formation in terms of engineering geological properties is an absolute necessity for the subsequent steps.

This process not only assesses the feasibility of proposed interventions but also informs the selection of construction methods that will maintain stability within the natural environment [1]. Engineering geological research and mapping focus on understanding the interactions between the geological environment, engineering conditions, and geodynamic processes to predict potential changes [2]. Each site represents a dynamic geological system shaped by rocks, soils, water, and geomorphological features. Engineering geological maps simplify these complex systems, offering critical models for evaluating geological conditions relevant to project planning and risk assessment.

The role of engineering geologists in the field is primarily to define through observations the potential failure mechanism of the rock. This would in turn orient the in situ mapping towards the appropriate parameters for collection that are required by the correspondent analysis [3]. This detailed information is crucial for informed decision-making in engineering design and risk management [4]. However, traditional mapping techniques often fall short of capturing all these complexities, highlighting the need for new methods that better represent these factors. This calls for innovative approaches that can transform not only the data collection process but also the visualization and analysis of geological environments, making the information more accessible to professionals with various backgrounds.

Accurate mapping is also crucial for archeological research and the effective planning of protective measures and presentation strategies for a complex monument of human activity over time. Detailed and precise records provide a comprehensive understanding of the monument’s structure, historical context, and condition, which is essential for developing informed conservation approaches. Such documentation allows researchers to trace changes over time, assess risks, and propose interventions that respect the monument’s integrity. Furthermore, it aids in the careful curation and public display of the monument, ensuring its cultural and historical significance is both preserved and properly communicated to future generations.

In this work, the case study concerns ancient underground marble quarries located in Marathi, on the island of Paros, Greece. Constituting one of the world’s finest quality marbles, Parian marble exhibits a distinctive combination of characteristics that have been highly valued throughout history (Figure 1). Structurally, Parian marble is composed primarily of fine-grained calcite, with an exceptionally uniform white color and significant translucence, which is due to the purity of calcite and minimal presence of impurities such as iron oxides [5]. This quality made it ideal for use in some of the most famous sculptures of antiquity, such as the Venus de Milo and Hermes of Praxiteles, where translucency conveyed a life-like quality and depth [6].

Beyond its visual properties, Parian marble is highly valued for its superior geotechnical characteristics. Its low porosity, high density, and compact microstructure confer exceptional resistance to physical wear, abrasion, and environmental weathering, thereby ensuring its durability under demanding conditions [7]. The marble’s high degree of isotropy and homogeneity ensures that mechanical stresses are distributed uniformly throughout the material, enhancing its stability and workability, which is crucial for both sculptural and engineering applications [8] (Figure 2). These qualities are particularly advantageous in the subterranean environment of the Parian quarries, where the marble’s interlocking calcite crystal structure provides significant resilience to flexural stresses, while its low porosity limits water infiltration, thereby reducing the risks associated with freeze–thaw cycles. As a result, Parian marble’s resilience and adaptability have facilitated its continued use and high esteem in both ancient and contemporary projects, underscoring its historical significance and enduring legacy.

Recent studies on the ancient marble quarries at Marathi [9,10] have highlighted the significant impact of the tectonic setting and structural characteristics on both the extraction process and the associated geohazard risks (Figure 2). The presence of complex joint systems and potential structural failures necessitates comprehensive analysis to ensure safe excavation practices. Researchers have examined the stability of these rock masses, addressing the geological and mechanical challenges posed by the multiple joint families present in the marble, which directly influence the stability and safe accessibility of the quarries.

Those characteristics dictate a direct analysis against structurally governed failure mechanisms. The collection of field measurements such as (a) joint orientation, (b) discontinuity spacing, (c) joint roughness, and (d) joint aperture are typical input measurements for this type of structural analysis. However, even though such data collection campaigns usually involve straightforward and standardized procedures, in the case of Paros’ underground marble quarries, the situation was rather challenging.

The ancient underground marble quarries of Paros, particularly renowned for their high-quality “lychnite” marble, have a history stretching back to the Neolithic period. The term “lychnite” comes most probably from the use of marble oil lamps (“lychnoi”) to illuminate the tunnels, a detail recorded by Pliny the Elder [11]. Initially exploited through surface mining, extraction shifted underground as quarrying reached deeper levels, driven by the increasing costs of removing overburden. These underground quarries formed intricate networks of galleries and chambers, with two main extraction methods, the chamber-and-pillar technique on gentle slopes and stepwise excavation in steeper areas.

Notably, the first systematic study of the quarries was conducted by Andreas Kordellas in 1884, followed by the extensive research of Professor Manolis Korres in the late 20th century (Figure 3), who documented the condition of the quarries after modern exploitation and provided crucial insights into ancient extraction methods, tool marks, and the estimated volumes of marble extracted [11].

The quarries were fully exploited from the fifth century BC onwards, when Paros was considered the wealthiest of the Cycladic islands with its marble cherished for classical sculpture. Though they continued to operate into the Roman era, activity dwindled with the rise of Christianity and the decline of large-scale marble production [12]. After a long period of inactivity, renewed interest emerged in the 19th century, when various European companies, including a Belgian and later a Greek enterprise, attempted to revive marble extraction.

Despite renewed interest during that period, efforts were ultimately unsuccessful, and all extraction ceased by 1900. Since then, the galleries have been abandoned, with minimal maintenance carried out. The latter, together with the geometry of the galleries, have resulted in the formation of a harsh environment for any activity (Figure 4). Specifically, the total length of the galleries is completely dark, with zero natural or artificial lighting and a ceiling height even below 1.5 m in parts.

In addition, deep holes, accumulated debris, and the short width of the passages in places disfavors even further the navigation and accessibility to “mappable” rock structures. Considering the above challenges and the total length of the galleries’ complex, the engineering geological mapping of that environment would include exposure to major risks for the researchers, with extensive exposure time.

Today, various reality capture devices such as laser scanners allow for the reliable generation of digital replicas of the surrounding environment. There have been solutions for multiple applications, including both indoor and outdoor settings with a variety of ranges. Building on these advancements, VR (virtual reality) applications have seen a tremendous rise in many industries, where the user can be virtually “transferred” to another place, navigate, observe, and even interact within it. For instance, ref. [13] used VR environments in the context of teaching rock engineering, ref. [14] utilized virtual reality to map fractures within a challenging natural environment combining LiDAR scans and high-resolution imagery, and ref. [15] reconstructed and studied the interior of a karst cave based on laser scanning and photogrammetry.

This idea was decided to be applied to the engineering geological mapping case of the underground quarries for its various advantages that include the following:

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Laser scanning is fully operational in total darkness;

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Underground exposure time would decrease by approximately 10% of the conventionally required time;

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Access to even inaccessible areas;

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Detailed inspection and mapping from the comfort of the office.

Considering the challenges posed by the Paros underground marble quarries, the key priorities for engineering geological mapping are centered on safety, efficiency, comprehensive data acquisition, and advanced analytical capabilities. The foremost priority is ensuring the safety of researchers. By incorporating VR technology based on laser scanning, the hazards of physically navigating the dangerous quarry environment are eliminated, creating a safe, controlled workspace without compromising the quality of the data collected.

A key objective in this approach is the thorough acquisition of essential geological data, even in a virtual environment. The VR application enables the collection of critical information for structural analysis while maintaining a high level of accuracy, comparable to traditional on-site methods. Central to this approach are both accuracy and efficiency, with the digital replica generated through laser scanning, capturing the quarry’s geometry and geological features in precise detail, ensuring reliable measurements for further analysis. The VR platform enhances these capabilities by allowing users to conduct detailed virtual inspections through realistic visualizations, while efficient data management supports the easy recording, organization, and export of information for integration with geotechnical models. VR technology further improves efficiency by significantly reducing the time needed for data collection, providing seamless navigation through the virtual quarry and granting access to areas that would otherwise be challenging or unsafe to explore. This combination results in a streamlined workflow and faster, more comprehensive data acquisition across the entire complex.

Beyond gathering data, this approach also integrates engineering geological analysis within the VR environment itself. Researchers can not only collect information but also conduct real-time stability assessments and hazard evaluations. Planned tools within the system will enable the analysis of rock mass behavior, simulate potential failure mechanisms, and evaluate the impact of proposed interventions. These capabilities aim to enhance decision-making efficiency while reducing the need for reliance on external software.

An additional advantage of VR technology is its ability to share findings with a broader audience. The platform offers an immersive and engaging method for presenting complex geological data, making it accessible not only to researchers but also to the general public. Its interactive features enhance communication by allowing users to virtually explore the quarry, observe geological features, and intuitively grasp the research outcomes. This versatility makes the VR platform an invaluable tool for both scientific research and educational outreach.

2. Materials and Methods

2.1. UAV Photogrammetry Portal Survey

The 3D reconstruction of the underground quarries began with a UAV-based photogrammetry survey focused on the portal areas. The surveyed site includes two significant ancient quarries, the Nymphs’ quarry and the Pan’s quarry. This approach involved two main phases: (1) a total survey covering the entire area, which included both quarries (Figure 5), and (2) two additional detailed surveys (Figure 6 and Figure 7) concentrating on the portal areas of the Nymphs’ quarry, utilizing both nadir and oblique flight configurations. This application serves two primary purposes, which involve (a) assessing the stability of the portal slopes and (b) providing georeferencing for the entire survey area. The former is, however, beyond the scope of this study.

The total survey (

Table 1. Details of the photogrammetric survey program.

Survey Name	Paros Ancient Quarries Total Survey
Date of Capture	9 October 2022
Coverage Area	0.347 km2
Number of Photos	876
Reprojection Error	2.32 cm
Total Number of Points in Point Cloud	278,416,392 [points]
Resolution	2.23 cm
Coordinate System	GGRS87

) was designed to capture the surface terrain of the entire quarry complex, focusing on both surface features and mainly the portals of the quarries. A DJI Phantom 4 RTK drone, equipped with a 20-megapixel camera, was used to capture high-resolution imagery with a ground sampling distance (GSD) of 2.23 cm at an altitude of 100 m. A single control point, corresponding to the location of the RTK equipment, was used to ensure precise georeferencing. The RTK system provided real-time, high-precision georeferencing, ensuring spatial accuracy within less than 1 cm. Covering 0.347 square kilometers, the total survey documented the surface terrain and key features of the Pan’s quarry entrance. The survey resulted in 876 images, which were processed using the structure-from-motion (SfM) photogrammetric technique as described in [16]. This process generated a dense 3D point cloud with nearly 280 million points.

The data processing was carried out using the Agisoft Metashape v2.1.2 software suite. The workflow began with image alignment, where key points were detected across overlapping images. Based on these key points, an initial sparse point cloud was generated. Multi-view stereo (MVS) photogrammetry was then applied to reconstruct a dense point cloud from the aligned images. Finally, a detailed mesh model was created from the dense point cloud, resulting in an accurate and comprehensive representation of the surface terrain. The entire dataset was subsequently georeferenced to the Hellenic Geodetic Reference System of 1987 (EGSA87), allowing for an integration into a broader geospatial framework and facilitating further analysis and regional applications.

At the Nymphs’ quarry, two UAV surveys were also conducted to capture detailed structural information around the portal entrances, using a combination of nadir and oblique flight paths. The nadir mission (

Table 2. Details of the nadir photogrammetric survey program at Nymphs’ quarry.

Survey Name	Nymphs’ Quarry Nadir Survey
Date of Capture	10 October 2022
Coverage Area	0.025 km2
Number of Photos	241
Reprojection Error	0.03 cm
Total Number of Points in Point Cloud	137,950,849 [points]
Resolution	1.47 cm
Coordinate System	GGRS87

) was set at an altitude of 35 m, focusing specifically on the quarry portal. This mission achieved a GSD of 8.8 mm, enabling the capture of finer details necessary for precise analysis. A total of 241 images were collected with 85% overlap to ensure accuracy in the 3D reconstruction.

In addition to the nadir mission, an oblique flight path (

Table 3. Details of the oblique photogrammetric survey program at Nymphs’ quarry.

Survey Name	Nymphs’ Quarry Oblique Survey
Date of Capture	10 October 2022
Coverage Area	0.0023 km2
Number of Photos	69
Reprojection Error	0.008 cm
Total Number of Points in Point Cloud	43,733,990 [points]
Resolution	1.16 cm
Coordinate System	GGRS87

) was employed to capture the steep slopes and rock faces surrounding the portal. A total of 69 images was taken from different angles to document the complex topography of the near-vertical surfaces, with a GSD of 5.8 mm for even greater detail in the analysis. The collected data were also processed using Agisoft Metashape to generate a high-resolution 3D model of the portal, which will serve as a foundation for geotechnical evaluations, focusing on slope stability analysis.

2.2. Laser Scanning Underground Survey

The second phase of the works included the main laser scanning survey of the underground spaces. The Leica BLK 360 laser scanner system was used for the underground survey. This system allows sequential scans with sufficient overlap to be registered with each other directly after acquisition using the tablet Cyclone app provided by Leica. The scanning projects were initiated from the portals—scanned from their on-ground part as well—to enable absolute georeferencing by registering the underground model to the RTK-based georeferenced photogrammetry model of the portals.

The survey of the Nymphs’ quarry consisted of 103 scan positions (Figure 8 and

Table 4. Laser scan exports from the Nymphs’ quarry survey, showing total point cloud points and surface density (r = 0.5) for the north, middle, and south galleries.

Nymphs’ Quarry Survey
Total Number of Points in Point Cloud Surface Density (pts/m2)	North Nymphs’ gallery
	573,250,629
	155,982
Total Number of Points in Point Cloud Surface Density (pts/m2)	Middle Nymphs’ gallery
	259,385,758
	180,020
Total Number of Points in Point Cloud Surface Density (pts/m2)	South Nymphs’ gallery
	403,625,373
	146,540

) with an average advance step of 5 m, resulting in a final point cloud of 1,236,261,760 points and a density of 160,847 points/m² surface. The relative mean registration error among the individually registered point clouds was calculated at 4 cm, while the registration error between the laser scanning underground and the photogrammetry on-ground point clouds was 3 cm.

The survey of the Panos quarry consisted of 40 scan positions (Figure 9 and

Table 5. Laser scan exports from the Pan’s quarry survey, showing the total number of points in the point cloud and the surface density (r = 0.5).

Pan’s Quarry Survey
Total Number of Points in Point Cloud Surface Density (pts/m2)	1,096,742,193
	164,406

) with an average advance step of 5 m, resulting in a final point cloud of almost 1.1 billion points and a density of 164,406 points/m² surface. The relative mean registration error among the individually registered point clouds was calculated at 5 cm, while the registration error between the laser scanning underground and the photogrammetry on-ground point clouds was 3 cm.

2.3. Virtual Tour Application Development

After the completion of the data acquisition and preprocessing stages described in Section 2.1 and Section 2.2, the high-resolution 3D data were used for the development of a desktop application that would allow for navigation into the underground quarries and the high-resolution engineering geological assessment through VR (virtual reality).

The prerequisite to the development of the VR application is the generation of a 3D digital replica of the underground marble quarries, including the on-ground portal areas. The 3D point clouds were uniformly resampled at 1 cm point spacing and used as the basis for the generation of the 3D mesh model. The surface reconstruction was conducted using the Poisson method [17] at a resolution of the 12th level of the point cloud’s computed octree. This process led to mesh models with a total of almost 195 million vertices and nearly 390 million triangles, reconstructing a surface area of 12,642 m². However, to optimize rendering times, parts of the model—not of engineering geological interest for inspection—were smoothed out using the open-source MeshLab software package. Such parts include the floor surface across the entire model (~25–30% of the total surface) and other caving structures distant to the walking path that were important to create a realistic depth perception but a high resolution was not required for inspection. The optimized mesh consisted of nearly 190 triangles in an OBJ format.

The application was developed within the open access Unity 2021.3.11 game engine for the Windows operating system (Figure 10).

2.4. Engineering Geological Mapping Using Virtual Tour Application

This chapter explores how the VR system enabled the production of engineering geological exports from these virtual tours and the subsequent implications for geotechnical analysis.

2.4.1. Critical Structure Recognition and Geotechnical Classifications

Through the VR application, critical geological structures in the Nymphs’ and Pan’s quarries were recognized and classified remotely with precision. This high-resolution georeferenced digital environment facilitated the identification of unstable rock masses, overhanging sections, and areas prone to structural failures. This approach provided a detailed understanding of site-specific hazards, as the structures could be examined from multiple angles and under different conditions—such as varying perspectives, simulated lighting, and elevations.

These features were particularly valuable in overcoming challenges posed by darkness, low ceiling heights, and debris accumulation, which often obscure critical details during traditional field surveys. Adjustments in virtual lighting helped reveal features such as joint fractures and overhangs that might otherwise be difficult to detect in these difficult conditions. The ability to navigate and inspect inaccessible areas, such as narrow tunnels or high-hazard zones, was one of the primary benefits of this technology.

Additionally, the application enabled a detailed division of the study area into engineering geological units (EGUs), essential for assessing the mechanical behavior of different rock masses. The digital environment offered a clear visualization of rock mass structures and geological features (e.g., faults, karstification zones), enabling the classification of rock units based on the digital visual recognition of these features. This was key to identifying potential instability zones.

Furthermore, the platform enhanced geotechnical classifications by applying systems such as the Geological Strength Index (GSI) [18] and the rock mass rating (RMR) [19].

The GSI classification was conducted through detailed inspection of surface roughness, the structure of the rock mass, and the interlocking of intact blocks within the 3D environment. This approach provided a more accurate and immersive evaluation, closely mimicking in situ fieldwork but with greater flexibility and precision.

As for the RMR classification, five of the six parameters were measured within the VR environment. These included the rock quality designation (RQD) using scanline techniques [20], the spacing of discontinuities, the condition of the discontinuities based on surface roughness, groundwater conditions indicated by the presence of small dripstone formations on the gallery roofs, and signs of extensive dissolution, as well as the orientation of the discontinuities in relation to gallery geometry. The uniaxial compressive strength (UCS) of the intact rock, which could not be directly obtained from the VR system, was determined through laboratory tests on samples collected during the laser scanning underground survey. Joint shear strength [21] tests were also performed on selected samples to further refine the analysis of discontinuity conditions.

2.4.2. Discontinuity Measurements and Hazard Identification

Following the critical structure recognition and geotechnical classifications, another key advantage of the integrated system was the ability to extract precise digital measurements of tectonic discontinuities. Key parameters, such as orientation, spacing, and the persistence of joints, were captured efficiently within the 3D replicas, offering valuable insights for further analysis [22]. This capability also supported detailed structural analysis, allowing for a close examination of joint patterns and the identification of critical planes of weakness, particularly in high-hazard sections of the quarries, where slippage along discontinuities posed a significant threat.

In addition to structural evaluations, the methodology allowed for volumetric assessments of rock blocks, which were essential for determining the geometric characteristics of unstable rock masses. The position, volume, shape, and dimensions of hazardous blocks were accurately identified and measured based on the high-resolution raw 3D data. This included the volumetric analysis of fallen blocks on the quarry faces near tunnel portals and within the tunnels. These volumetric measurements played a crucial role in assessing the stability risks of each block and informing geotechnical interventions.

The system was also instrumental in identifying high-hazard zones, including areas vulnerable to structural failures, potential roof collapses due to overstressed rock masses, or pillar instability. Hazardous zones with overhanging rock masses or visibly deteriorating sections were identified for immediate attention, helping prioritize geotechnical efforts and the implementation of stabilization measures.

3. Results

3.1. Interactive VR Application

The methodology applied for the engineering geological mapping of the ancient underground marble quarries in Paros Island resulted primarily in an essential virtual tool that was made available to both the research team and stakeholders. The developed VR application included the wider area of the two main gallery complexes (Nymphs and Pan). The extent of the virtual environment within which the user could navigate is illustrated in Figure 11 and Figure 12 for both galleries. The importance of combining the on-ground and underground conditions in a united environment also assisted in the communication of the quarries’ complexity to audiences that never had the chance to visit the galleries.

The result includes a high-resolution georeferenced digital environment—as depicted in the snapshots in Figure 13, Figure 14 and Figure 15—where all the required geometric rock mass properties could be extracted from within the comfort of the office without the in situ risks and in a user-friendly interface. This turned out to promote interoperability within the research team, since it was possible to make interpretations collaboratively in a conference room, simplifying data transfer protocols and optimizing the deliverance timeframes.

Apart from the commodity provided to the research team by the utilization of the VR application for mapping purposes, the results also included the demonstration of the quarries’ structure to non-technical users by predefined virtual tours along critical sections. This practice has proved essential to the communication of the results to the local community, as well as to interested contractors for the application of the proposed works.

Furthermore, the combined 3D models enabled the creation of detailed planimetric representations for each quarry, aiding in the visualization of critical areas and the interpretation of the spatial relationships within the underground structures, as illustrated in Figure 16.

3.2. Assessment of Engineering Geological Condition

The next section presents the results from the engineering geological assessments carried out at the Nymphs’ and Pan’s quarries, using the data and analyses derived from the VR application and subsequent numerical modeling.

Starting with the classification of the formations, the detailed VR-based analysis and digital mapping of the quarries led to the identification of three main engineering geological units (EGUs).

The first unit (EGU 1) encompasses thick-bedded, compact marble, predominantly white to light gray in color with moderate fracturing. This rock mass is largely intact, and the discontinuity surfaces exhibit good-to-moderate quality. Key structural elements include persistent bedding planes, which form the roof slabs of the quarries, alongside sparse vertical discontinuities. This unit is widespread throughout the Nymphs’ and Pan quarries and is generally stable, with its mechanical behavior mainly influenced by the orientation of discontinuities relative to the underground openings.

The second unit (EGU 2) is concentrated around the portal areas, where the marble exhibits a more fragmented structure. The discontinuities in this zone are of moderate quality, with smoother, weathered surfaces showing signs of degradation. The structural behavior of this unit is compromised by the presence of multiple intersecting discontinuities, which create potentially unstable blocks.

The third unit (EGU 3) is associated with zones of significant karstification, where dissolution has created voids that severely weaken the rock mass. These voids, ranging from 10 to 40 cm, are concentrated along major tectonic lines on the walls and roof. These features reduce the overall integrity of the rock mass, increasing the risk of instability, particularly in areas with poor discontinuity quality.

For each of the identified EGUs, a geotechnical classification was carried out using both GSI and RMR systems. The detailed results of the GSI and RMR classifications for each EGU are presented in

Table 6. Table of proposed parameters per engineering geological unit.

Engineering Geological Unit	Unit Weight (kN/m3)	RMR	Hoek–Brown Parameters				Mohr–Coulomb Parameters
			σci (MPa)	GSI	Ei (GPa)	mi	φ (°)	c (MPa)
EGU1	26–27	72	100	65–70	15	9	34	6
EGU2	26–27	47	100	45–50	15	9	28	4.5
EGU3	26–27	44	100	35–40	10	6	21	3.0

.

In both the Nymphs’ and Pan quarries, the use of VR-based models allowed for a detailed extraction of tectonic discontinuity measurements, providing essential data on the structural integrity of the rock mass. These measurements included the orientation, spacing, and persistence of joints, all critical factors in assessing the stability of the tunnels. Particularly in the Nymphs’ quarry, the persistence and orientation of joints in the roof sections of the northern tunnels contributed significantly to the identification of slippage planes and zones at high hazard of instability. The joints displayed considerable persistence, forming slippage planes that directly impacted the roof’s stability, leading to increased risk of block detachment.

The structural analysis of discontinuities further highlighted the critical zones of weakness within the rock mass. By closely inspecting the texture of the fractures within the digital environment, it became apparent that many of these fractures exhibited rough surface textures, which increased the potential for slippage. This analysis was particularly valuable in identifying critical planes of weakness in the high-hazard zones, where block detachment was most pronounced. The detailed inspection of the structural characteristics allowed for a comprehensive understanding of how these fractures would behave under stress, particularly in areas where structural support was limited.

Kinematic analysis of the tunnels revealed high potential of planar and wedge failure hazards in EGUs 2 and 3 due to the intersection of several discontinuity sets at unfavorable angles, particularly near the intersections of galleries. Here, the intersection of multiple joint sets and the geometry of the excavation resulted in an increased likelihood of gravitational failures, further emphasized by the overhanging and suspended blocks. The VR system was crucial in identifying at first these high-hazard zones throughout the quarries, particularly areas susceptible to structural failure (Figure 17, Figure 18, Figure 19, Figure 20, Figure 21, Figure 22, Figure 23 and Figure 24). For instance, sections with large overhanging rock masses or visibly deteriorated rock were clearly highlighted for further analysis (Figure 22 and Figure 23). Additionally, the volumetric assessments performed within the VR environment offered precise measurements of the size and geometry of both fallen and suspended blocks.

After the assessments conducted within the VR system, data from the 3D models were exported for numerical stability analysis using Rocscience software, including Unwedge and RS2. A detailed wedge stability analysis was performed using Unwedge to identify unstable rock blocks formed by the intersection of discontinuities within the tunnels (Figure 24 and

Table 7. Wedge Information table.

a/a	Wedge Type	Factor of Safety	Wedge Volume (m³)	Apex Height (m)	Color
1	Lower Right Wedge	Stable	0.008	0.19	Red
2	Lower Right Wedge	4.543	0.093	0.29	Green
4	Roof Wedge	1.638	14.208	3.09	Dark Green
5	Lower Left Wedge	20.759	4.837	2.07	Brown
7	Upper Left Wedge	2.132	0.001	0.07	Purple

). Different tunnel cross-sections were assessed under two scenarios, the current conditions and potential seismic loading with a horizontal acceleration of 0.04 g. For each scenario, support measures, such as rock bolting, were applied to ensure stability, aiming for a factor of safety (F.S.) greater than 1.5. The Barton and Bandis failure criterion [23] was used to evaluate the shear strength of the discontinuities, considering both mechanical and geometric characteristics (JRC, JCS, φ), with safety factors calculated according to Eurocode 7 (EC-7) standards.

Once the wedge stability analysis was completed, a 2D finite element modeling (FEM) analysis was conducted using RS2 v11 software from Rocscience Inc. to further assess the stability of critical sections in the Nymphs’ and Pan’s quarries, particularly in areas showing potential roof collapse or pillar failure (Figure 25, Figure 26 and Figure 27). Four cycles of FEM analysis were carried out on sections exhibiting significant instability. The model geometry was based on the detailed geometry and discontinuities identified through the VR models. The generalized Hoek–Brown failure criterion [24] was applied to simulate the behavior of the rock mass under stress, incorporating parameters such as the GSI. This allowed for a realistic understanding of how the marble responded to various loading conditions, especially in areas with pre-existing fractures. The incorporation of mapped discontinuities enabled a detailed analysis of potential slippage or failure along these planes, highlighting areas where additional support was necessary.

Insights gained from both analyses led to the proposal of several protective measures tailored to the specific conditions of the site.

Inside the galleries, the measures that were advised include the removal of hazardous overhanging blocks, anchoring of unstable masses, and wire mesh installation at critical points on the roof. For the stabilization of pillars and surrounding debris, the options considered were the stabilization of the pillars with a support framework to prevent collapse and the removal of sterile materials.

In high-hazard areas with overhanging marble blocks and cracks (Figure 28), the installation of geomechanical monitoring systems, such as crack meters, were recommended to continuously monitor structural movements.

These interventions were communicated and visualized through the VR environment, providing stakeholders with a clear understanding of the proposed solutions and confidence in their effectiveness for mitigating the identified hazards and ensuring long-term stability.

4. Discussion

The integration of virtual reality (VR) into the engineering geological mapping of the Nymphs’ and Pan quarries on Paros Island has significantly enhanced how we approach such complex environments, offering clear advantages over traditional methods. This approach allowed us to overcome the challenges posed by the difficult and hazardous underground conditions while also improving the quality and efficiency of data acquisition and analysis.

The VR application facilitated the safe and efficient gathering of critical engineering geological data. By virtually “entering” the quarries through high-resolution 3D models, we were able to conduct detailed inspections and engineering geological assessments without the need for prolonged physical presence in the tunnels, mitigating the risks associated with unstable rock masses, darkness, low ceilings, and narrow passages. The platform allowed us to capture comprehensive geological data—such as joint orientations, spacing, and persistence—with a level of accuracy that rivaled traditional fieldwork, all from the safety of an office setting. This substantially reduced on-site time, improving the overall safety of the research team.

One of the most significant advantages of the VR system was its capacity to streamline the entire data acquisition process, resulting in a faster workflow. By using laser scanning to create detailed 3D models and a VR platform to conduct virtual geological mapping, we achieved a higher level of data accuracy and detail. The integration of engineering geological analysis directly into the VR environment further eliminated the need for manual data transfers between systems, allowing for real-time decision-making and immediate hazard identification, all within the virtual environment.

The application’s role in advancing geotechnical classifications also cannot be overstated. Although we had to face the limitation of not having color information in the data due to total darkness of the underground environment and the inability to photographically enhance the scans, we were able to apply detailed classifications, such as the Geological Strength Index (GSI) and rock mass rating (RMR), directly to the digital models, improving the precision of our assessments. The digital measurements and structural analyses, supported by kinematic evaluations, underscored the thoroughness of the VR approach, as it provided a level of access and precision not achievable through traditional methods.

In addition to these technical advantages, the VR application offered a way of communicating complex geological information to stakeholders. The interactive nature of the virtual tours allowed non-technical users to engage with the quarries’ structural issues in an intuitive and accessible way, facilitating a deeper understanding of the site’s geotechnical risks and proposed interventions. By allowing users to explore the quarries virtually, stakeholders, including contractors and local authorities, could better grasp the critical zones that required attention. This enhanced communication supported the decision-making process and fostered a more collaborative approach to hazard mitigation.

Looking towards the future, the potential of VR applications in engineering geological mapping is vast. The current system demonstrated how VR can serve as an indispensable tool for structural analysis, but there is room for further development. Future iterations of the application could integrate advanced numerical modeling directly within the VR environment, enabling users to not only explore the site but also run simulations and analyze the results in real-time. For instance, combining stability analyses with the virtual platform would allow for dynamic evaluations of how different interventions (e.g., rock bolting, mesh reinforcement) would perform under various loading conditions. This would further streamline the workflow by integrating all necessary analysis tools into a single, user-friendly interface, enhancing both the depth of analysis and the efficiency of implementation.

In conclusion, the VR-based approach has radically improved the quality of engineering geological mapping in the challenging environment of the ancient Paros quarries. By providing a safer, more efficient, and highly accurate method of data acquisition and analysis, this technology has set a new standard for site investigations in hazardous or inaccessible environments. The ability to communicate complex geological information to a broad audience, coupled with the potential for the future integration of advanced analytical tools, positions VR as a crucial tool for the future of geotechnical engineering and geological risk management.

Finally, accurate three-dimensional digital mapping and geotechnical characterization is vital for archeological research and the strategic planning of protective measures and presentation approaches for a complex monument of human activity over time. This detailed mapping enables researchers to monitor changes, evaluate risks, and propose interventions that honor the monument’s integrity. Additionally, it supports thoughtful curation and public engagement, ensuring the monument’s cultural and historical value is preserved and effectively conveyed to future generations.

5. Conclusions

The integration of virtual reality (VR) technology into engineering geological mapping has revolutionized the way complex underground environments, such as the ancient marble quarries on Paros Island, are studied. Traditional methods of geological risk management relied heavily on in situ fieldwork, which posed safety risks and was limited by either inaccessibility or lack of light. The VR-based approach, utilizing laser scanning to create a detailed digital replica of the quarry, has overcome these limitations. It has allowed researchers to gather comprehensive geological data, perform structural analysis, and identify hazards in a safe, controlled environment without compromising the accuracy or detail of the data.

This innovative method also streamlined the entire data acquisition and analysis process, significantly reducing the time and effort required for on-site surveys. The VR platform enabled real-time virtual inspections and assessments, improving workflow efficiency and data accuracy. By integrating engineering geological analysis directly into the virtual environment, researchers were able to make immediate decisions and conduct hazard assessments without needing to manually transfer data between systems.

The engineering geological assessment of the Nymphs’ and Pan quarries, conducted using VR-based analysis and numerical modeling, provided a comprehensive understanding of the site’s stability. The ability to accurately measure and analyze critical geological structures within the virtual model further enhanced the precision of classifications like the Geological Strength Index (GSI) and rock mass rating (RMR), offering a more thorough understanding of the site’s geotechnical conditions. The identification of three main engineering geological units (EGUs) revealed varying levels of rock mass integrity, from the stable, thick-bedded marble in EGU 1 to the karstified and structurally compromised rock of EGU 3. Kinematic analyses and volumetric assessments highlighted critical zones of weakness, particularly in areas near the portals and tunnel intersections, where wedge failures and block detachment posed significant risks. Numerical modeling further confirmed the need for stabilization measures, such as rock bolting, mesh reinforcement, and pillar support, to enhance safety.

Beyond its technical advantages, the VR system has proved invaluable for communication and collaboration with stakeholders. By providing an immersive, interactive way to explore the quarries, the platform made complex geological information accessible to non-experts, improving the decision-making process for the long-term stability of the quarries. This engagement, combined with the potential to integrate advanced modeling tools into future VR applications, positions this technology as a critical asset for geotechnical engineering and geological risk management in the future, especially for hazardous or inaccessible sites.