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Article

Reconstruction of a 3D Bedrock Model in an Urban Area Using Well Stratigraphy and Geophysical Data: A Case Study of the City of Palermo

by
Alessandro Canzoneri
,
Raffaele Martorana
*,
Mauro Agate
,
Maurizio Gasparo Morticelli
,
Patrizia Capizzi
,
Alessandra Carollo
and
Attilio Sulli
Department of Earth and Marine Sciences (DiSTeM), University of Palermo, 90123 Palermo, Italy
*
Author to whom correspondence should be addressed.
Geosciences 2025, 15(5), 174; https://doi.org/10.3390/geosciences15050174
Submission received: 8 April 2025 / Revised: 6 May 2025 / Accepted: 7 May 2025 / Published: 14 May 2025
(This article belongs to the Section Geophysics)
Browse Figures
Versions Notes

Abstract

:
A multidisciplinary approach was employed to construct a three-dimensional model of the bedrock top surface within the Palermo Plain, Sicily, Italy. This urban area is characterized by a dense and extensive built environment that largely obscures its geological features, thereby emphasizing the value of geophysical methods for enhancing subsurface understanding. In this sector, Numidian Flysch deposits constitute the geological bedrock of the plain. The morphology of the top surface of this unit was reconstructed by integrating borehole stratigraphic data with both passive and active seismic surveys. Ambient noise recordings were analyzed using the Horizontal-to-Vertical Spectral Ratio (HVSR) method to obtain spectral curves. These were then inverted into seismostratigraphic models using shear wave velocity profiles derived by Multichannel Analysis of Surface Waves (MASW) and lithostratigraphic information from borehole logs. Finally, the depth of the top of the Numidian Flysch, determined from both the borehole data and the inverted seismic models, was interpolated to generate a comprehensive 3D model of the bedrock top surface.

1. Introduction

This study presents a multidisciplinary approach aimed at reconstructing a 3D geological model of the top of the bedrock within the Palermo Plain, located in the northwestern sector of Sicily, Italy. This buried surface corresponds to the top of the Oligo-Miocene Numidian Flysch deposits and, overall, does not outcrop in the investigated area. The study realized a reconstruction of the upper surface of this lithological unit by integrating data acquired by active and seismic surveys and stratigraphic logs analysis.
The adopted workflow underscores the value of an interdisciplinary approach in enhancing the understanding of subsurface features in urban environments. In such contexts, integrating existing geological data with geophysical surveys has become increasingly crucial for several reasons. Firstly, conducting new direct investigation is often highly invasive, posing risks of interference with underground infrastructure and urban utilities [1,2,3]. Furthermore, direct methods, such as core drillings or trenching, are subject to complex regulations and protocols, require detailed logistical planning, and entail high costs, unlike many geophysical surveys [1,2,3,4,5,6,7,8]. In addition, direct investigations usually yield point-specific data, often limited to shallow depths, whereas non-invasive methods can provide spatially extensive information at greater depths [9,10,11,12]. Considering these factors, the integrated use of geological and geophysical data proves to be a powerful tool for urban subsoil characterization.
Several previous studies have highlighted the significance of defining the geological setting of urban areas as a key factor in risk prevention and mitigation [13,14,15,16,17,18]. In this regard, the Palermo Plain serves as a valuable case study for demonstrating the importance of integrating diverse datasets to delineate subsurface geometries and assess geological hazards, especially in areas where natural features are largely obscured by urban development. The city of Palermo, the fifth most populous in Italy, has undergone continuous urban expansion since the 8th century BC, evolving through successive dominations. Initially confined within its historic walls, the city has progressively extended outward, transforming the landscape and concealing many geological and geomorphological features [18,19,20]. These transformations include alterations to natural watercourses such as the Oreto, Kemonia, and Papireto rivers, which have been redirected, partially enclosed, or drained over time [17,21]. Post-World War II developments further modified the coastal morphology due to the accumulation of anthropic fills in areas near the sea [17,19,20,22]. Notably, it was only in 1962 that Palermo adopted a formal master plan, leading to decades of unregulated expansion during the 20th century. As a result, various sectors of the city now face hazardous conditions caused by uncontrolled urbanization, which has obscured the geomorphological and geological features of the plain [18,23].
The most relevant geological hazards in the Palermo Plain include seismic [24,25,26,27] and hydrogeological hazards [18,28,29,30]. Understanding the depth and the extent of the bedrock is fundamental for assessing these hazards. In the studied area, the Numidian Flysch is typically considered the bedrock due to its higher shear wave velocity and superior mechanical and physical properties compared with the overlying units [24,31,32,33]. These clayey and silty deposits also act as hydraulic boundaries due to their markedly lower permeability relative to the overlying formations, like calcarenites, alluvial deposits, and eluvial–colluvial sediments [34,35,36].
Given these considerations, the present study offers a valuable case study for demonstrating how a 3D subsoil model, based on diverse and integrated datasets, can effectively support urban geoscience. The multidisciplinary approach employed, consistent with methodologies adopted in previous research on coastal plains [37], emphasizes the potential of combining geological and geophysical data to more accurately reconstruct the geometry and properties of bedrock surfaces. In particular, HVSR (Horizontal-to-Vertical Spectral Ratio) surveys, effectively constrained by MASW (Multichannel Analysis of Surface Waves) data and supported by borehole lithostratigraphy, were integrated to delineate the upper level of the Numidian Flysch.

2. Geological and Geophysical Setting

2.1. Geological Features of the Palermo Plain

The Palermo Plain is located in Sicily’s northwestern sector (Figure 1A) and has altitude values ranging between 0 m and 150 m a.s.l. The study sector is situated between the Tyrrhenian Sea and a belt of Meso-Cenozoic carbonate reliefs, which surrounds the plain towards the west and south (Figure 1B).
The area under investigation corresponds to the plain section shown in Figure 1C, that is enclosed between the coastline and the areas surrounding the Oreto River’s path specifically. Within the urban center, the northern limit of the examined area is marked by the Harbor and Boccadifalco Airport areas, while the southern limit is identified in the Acqua dei Corsari and Ciaculli sectors. The densely urban net of the Municipality of Palermo hides the deposits that characterize the plain, as visible in the geological map of Figure 1C.
The area is characterized by Triassic–Holocene deposits as shown in the simplified stratigraphic column of Figure 1D [38]. It lies along the northern margin of western Sicily, where a Neogene, SE-verging orogenic wedge (Sicilian fold and thrust belt) develops [39]; the reliefs surrounding the Palermo Plain represent the topographical expression of this wedge. They are made up by Upper Triassic–Oligocene, mainly carbonate successions, covered by Upper Oligocene–Lower Miocene terrigenous deposits (Numidian Flysch) [40,41]. These rocks that create the relief surrounding the study area have been interpreted and recognized by some boreholes at depths over a hundred meters [17] and represent the substrate on which the plain of Palermo developed.
In further detail, this substrate comprises Meso-Cenozoic shallow- and deep-water silico-carbonate successions (belonging to the Panormide and Imerese domain, respectively) that outcrop along the main relief enclosed in the study area, like the Caputo, Cozzo Meccini, Orecchiutta, and Grifone mountains. Above this carbonate substrate, the Upper Oligocene–Lower Miocene deposits of the Numidian Flysch (FN) lie; they consist of marl and sandy clays accumulated in a foredeep basin, that mainly outcrop between the Monreale and Villagrazia areas and are partly dislocated above the Mesozoic units due to thrust structures. The Panormide and Imerese successions are overall buried beneath the urban center area, although a couple of small outcrops of Numidian Flysch outcrop near the Harbor and Acqua dei Corsari sectors [40,41].
Along the plain, the previously mentioned Mesozoic–Cenozoic substrate is unconformably covered by 1.5–0.8 Ma aged, clayey sediments and mostly carbonates, arenitic, and sandy deposits, that outcrop across a large sector of the plain (Figure 1C,D; [38,42,43]). These deposits are composed of blue clays (BC) (locally known as the “Ficarazzi Clays”) [44] and partly bioclastic, variously cemented, calcareous sands (of the “Palermo Calcarenites” (CC)). These lithologies, forming on the whole a shallowing upward succession, have been assembled in an “unconformity bounded unit” named Marsala synthem. The rocks of the Marsala system form an extended sedimentary prism whose thickness reaches a hundred meters along the coast. Proceeding inward, they reduce in thickness and taper at the foot of the mountains with a thickness close to 0 m. This sedimentary prism is well stratified, with layers dipping towards the sea by 5° on average. This stratigraphic setting results from a marine transgression that affected the area in the Lower Pleistocene [45]. Above these Lower Pleistocene marine rocks, the coastal conglomerates and sands of the “Sintema of Buonfornello–Campofelice” (middle Pleistocene) and of the “Sintema of Barcarello” (upper Pleistocene) may also outcrop. These latter rocks consist of conglomerates, variously cemented sandstones, and colluvial deposits of coastal to continental (foothill) environments [17]. Holocene deposits, accumulated along the coast and within the current hydrographic network, complete the Quaternary sedimentary succession. An almost continuous strip of debris fans and colluvial eluvium deposits outcrops along the morphological slope break connecting the mountains and the plain.
All of the middle Pleistocene–Holocene deposits have been assembled and named as “Upper deposits” (UD; Figure 1D), with topsoil and anthropic fill also included in this unit.
Deposition during the middle Pleistocene to Holocene follows frequent (about 100 kyr) and high amplitude (about 100 m) sea level change, combined with slow and lasting tectonic uplift over time; both controlled the deposition of Sintema of Buonfornello–Campofelice and Sintema of Barcarello.
A series of compressive tectonic events, active since the middle Miocene, is responsible for the current structural setting of the Meso-Cenozoic carbonate substrate beneath the Palermo Plain. In general, two main tectonic events have occurred: event 1 is characterized by the development of the main thrusts responsible for the building of the orogenic wedge; event 2 is marked by the development of high-angle transpressive faults associated with significant vertical displacements. The main tectonic features observable in the study area [40,41], which locally juxtapose the Oligo-Miocene Numidian Flysch deposits with the Meso-Cenozoic carbonate bodies, can be attributed to the latter tectonic event. Immediately to the southwest of the Palermo Plain, two significant left-lateral transpressive tectonic features (the Monreale–Pioppo lineament and the Villa Ciambra–Altofonte fault) [39] have been mapped (Figure 1C), which limit a topographically incised sector in the NE–SW direction, within which part of the drainage system of the Oreto River develops.
Historical and instrumentally recorded seismic activity has affected the Palermo Plain; the most recent events are related to the “southern Tyrrhenian seismogenic zone”, which is characterized by low-medium magnitude and upper crustal depth seismic events [23,24,25,46].

2.2. Velocities of Shear Waves Within the Palermo Plain Deposits

Several studies have been conducted on the geophysical characterization of the Palermo Plain deposits, specifically on the shear wave velocity values.
These works mainly provide shear wave velocity (Vs) parameters obtained by active seismic surveys [47,48] or geotechnical analysis [24,31,32,49] and were often utilized to highlight areas more susceptible to seismic shaking and to analyze the degree of damage that affected buildings in the city of Palermo due to historical earthquakes, mainly in the urban center [24,25,26,32,33,49].
The velocities of shear waves generally described in these works for the primary lithologies present in the subsoil plain are summarized in Table 1. The Upper Triassic–Oligocene carbonate succession (CA) deposits usually reach and exceed more than 800 m/s; a wider velocity range, varying between 450 and 1900 m/s, is commonly attributed to Cenozoic Numidian Flysch (FN). The degree of the alteration of the deposits found in outcrops influenced the low speeds of the wider velocities range; the higher values characterize the deeper and partly tectonized section of these deposits, which can be considered as hard schist. The velocity of the shear waves of the Palermo Calcarenites (CC) is, on average, 550 m/s and can reach values equal to or greater than 800 m/s for some more cohesive and cemented sections of this lithology. The blue clays (BC) usually show mean values of approximately 450 m/s. The Upper Pleistocene–Holocene deposits (UD) present generally lower velocities that vary between 100 and 450 m/s.

3. Materials and Methods

3.1. Boreholes

A comprehensive collection of borehole logs was consulted to characterize the subsoil of the Palermo Plain. This database, known as City–GIS, was developed in a GIS environment and is maintained by the Department of Earth and Marine Sciences of the University of Palermo (DISTEM). It includes over 2500 borehole stratigraphic and geotechnical records [50,51].
The database provides a wide range of information, including lithological descriptions, aquifer depth, and lithotype thickness and depth. The lithological data are categorized into 87 distinct facies. Two examples of the data structure are shown in Figure 2.
A total of 1404 stratigraphic logs from the study area were extracted from the City–GIS database, as shown in Figure 3. Most of the data are concentrated in the historic city center, particularly in the zone between the Harbor and the mouth of the Oreto River. The Settecannoli sector also shows good data coverage, whereas the southwestern part of the study area is relatively underrepresented. The depth of the boreholes ranges from 2 to 170 m.
Based on age, composition, and genesis, the borehole lithologies have been grouped into the five geologic macro-categories shown in Figure 1D. These broader classifications—CA, FN, BC, CC, and UD—simplify the original 87 facies categories to facilitate the construction of a coherent 3D model of the subsurface at the scale of interest. For example, 30 facies corresponding to different types of calcarenites were consolidated under the category “Palermo Calcarenites” (CC). This simplification supports a more uniform and effective 3D modeling process. Numidian Flysch deposits were identified in only 281 boreholes, as shown in Figure 3.

3.2. Environmental Noise Acquisition

Ambient seismic noise, primarily composed of surface waves from anthropogenic and natural origins [52,53,54,55,56,57,58,59,60] provides a continuous stochastic signal suitable for passive seismic analysis. This signal can be recorded using single-station measurements and analyzed via the Horizontal-to-Vertical Spectral Ratio (HVSR) method [52,61,62,63,64].
The HVSR method estimates a site’s resonance frequency by computing the ratio between the average horizontal components (H) and the vertical component (V) of the ambient noise spectrum [52,53,54,55,56,57,58,59,60,65]. The resulting H/V spectral curve may exhibit peaks that correspond to site resonance frequencies, typically caused by low impedance sediment layers overlaying high impedance substrates [66,67,68,69,70]. When appropriately constrained, these curves can be inverted to derive shear wave velocity models of the subsoil, as validated in previous studies [37,71,72,73,74,75,76].
A total of 54 environmental noise surveys, represented by green triangles in Figure 3, were analyzed. These surveys were commissioned by the Municipality of Palermo in 2016 as part of the general urban planning process in collaboration with the University of Palermo.
Data were acquired using a triaxial seismometer produced by MoHo Srl (Bologna, Italy), firmly coupled with the ground to ensure optimal signal transmission. Each recording lasted 30 min and was sampled at 128 Hz.
HVSR analysis was performed using the Grilla 7.0 software, developed by Moho Srl, Bologna, Italy, adopting SESAME (2004) guidelines for data quality control. Transient noise was excluded by subdividing the time series into 60 s windows and rejecting those with spectral standard deviation exceeding 1.5 dB in the 0.5–10 Hz range. H/V peaks were considered stable when the standard deviation at the peak was below 5% and the curve showed >80% coherence [77,78,79].

3.3. Active Seismic Survey

The Multichannel Analysis of Surface Waves (MASW) technique was employed to determine shear wave velocities in the subsurface. This method exploits the dispersive nature of surface waves in layered media, where waves exhibit different phase velocities at varying frequencies due to subsurface heterogeneity [80,81,82,83].
MASW is an active survey method in which surface waves are generated mechanically—typically using a slide hammer—and recorded by geophones aligned on the surface. The signals are sent to a seismograph, and the fundamental mode of Rayleigh wave dispersion is identified by tracking peak spectral amplitudes. Dispersion curves derived from this analysis are then inverted to obtain subsurface Vs (shear wave velocity) profiles [84,85,86,87,88].
Surveys were conducted at multiple sites within the study area and adjacent sectors. Some seismic arrays were acquired under the “MODEnergy” project, as part of a collaboration between DISTEM, CURSA (University Consortium for Socio-economic Research and the Environment), and the TERIN Department (Energy Technologies and Renewable Sources Department) of ENEA (National Agency for New Energy Technologies and Renewable Sources) [89].
The seismic arrays used geophones with a 4.5 Hz proper frequency. Arrays included between 12 and 24 sensors, spaced 2 to 3 m apart. Two external shots were performed at each end of the array with 5 m offset using a 5 kg sledgehammer. The analysis of data was conducted using WinMasw 7.0 software, which was developed by Eliosoft, Florence, Italy.

4. Results and Discussion

4.1. MASW Analysis

The MASW survey data were analyzed with the support of the stratigraphic information from the nearest City-GIS boreholes, which were used to constrain the inversion process.
The acquired datasets allow us to determine the shear wave velocity (Vs) parameters. The superficial cover layers (UD) exhibit Vs values ranging from 180 to 410 m/s. Deposits associated with the calcarenites (CC) show values ranging from 300 to 750 m/s, while those for blue clays (BC) range from 370 to 650 m/s. To assess model reliability, dispersion spectra and comparisons between measured and theoretical curves are presented in Figure 4 and their representative Vs models derived from the MASW analysis are shown in Figure 5. The inversion misfit values, which quantify the uncertainty associated with the shear wave velocity estimates, ranges between 0.19 and 0.29.
The data acquired in this study were integrated with results from previous research. The computed Vs values align with the ranges reported in Table 1, validating the use of these velocities as constraints in the inversion of HVSR curves into 1D Vs models. This process also incorporated geological information derived from borehole data.
Although the Vs ranges attributed to the Numidian Flysch and blue clays appear broad, the actual velocities used in the interpretation are more restricted, as they are locally constrained through lithological data from stratigraphic logs and MASW profiles. This accounts for the significant heterogeneity within these formations, which include quartz arenites, siltstones, compact clays, and sandy interbeds.

4.2. HVSR Qualitative Analysis and Inversion into Subsoil Velocity Models

The HVSR data, processed using Grilla 7.0 software, were pre-processed to remove external disturbances that could compromise the reliability of the H/V curves. This filtering excluded time windows likely influenced by non-stochastic anthropogenic noise, a particularly important step for higher frequency peaks, where such disturbances are common [90,91,92,93,94].
Analysis of the complete set of HVSR curves (Figure 6A) revealed distinct frequency ranges with recognizable peak distributions. These peaks were correlated with the main stratigraphic boundaries in the Palermo Plain subsoil, as described in Figure 1D.
A prominent peak in the 0.8–2.5 Hz frequency band likely corresponds to the interface between the Pleistocene clays (BC) and the underlying Numidian Flysch (FN). The broad shape of this peak is attributed to the transition between Oligocene–Miocene Flysch deposits and the deeper Meso-Cenozoic carbonate basement (CA) [47]. Secondary peaks, generally found between 10 Hz and 30 Hz, are associated with the boundary between Pleistocene–Holocene deposits (UD) and the underlying Palermo Calcarenites (CC). Additionally, a sharp velocity inversion, indicated by a H/V ratio below 1, was observed within the 2.5–3 Hz to 15–20 Hz range, suggesting a transition from calcarenite deposits (CC) to blue clays (BC).
The robustness of these correlations is supported by agreement between HVSR data and previous borehole-based studies, as shown in Figure 6, especially in the Oreto River area. A notable correlation was found between HVSR peak frequencies (0.5–2.5 Hz) and the depth of the Numidian Flysch reconstructed from 16 boreholes (Figure 6B) in previous studies [32]. Kriging interpolation of peak frequencies produced iso-frequency contour maps (Figure 6B), showing that higher frequency values correspond to shallower Flysch depths (e.g., in the hospital area) while lower frequencies match deeper Flysch depths (e.g., in the Guadagna area and north of the Oreto River mouth).
Subsequently, 54 HVSR curves were inverted into Vs–depth models. Given the non-uniqueness of HVSR inversion, which can yield multiple equivalent solutions [95,96,97], stratigraphic and Vs constraints from Table 1 were used to improve the reliability. This approach, validated in previous studies [37,48,72,97,98,99,100,101], yielded reliable Vs–depth models (Figure 7). Layers in these models with similar Vs values were then grouped into the lithological macro categories defined for the Palermo Plain subsoil.

4.3. Reconstruction of the Numidian Flysch Surface Model

All available data on the Numidian Flysch top surface elevation—derived from boreholes and seismostratigraphic modeling—were interpolated to reconstruct its geometry across the study area. The Natural Neighbor interpolation method was chosen due to the non-uniform spatial distribution of the data [102,103] and implemented using Surfer software (Golden Software LLC).
The surface was reconstructed only in regions where sufficient boreholes or HVSR data were available (Figure 8A). The eastern sector was excluded due to a lack of deep boreholes which did not reach the Oligo-Miocene deposits in that area.
A thickness map of Quaternary sediments (Figure 8B) was produced by summing all layers classified as BC, CC, and UD that are lying above the Numidian Flysch top surface. This map indirectly indicated the depth of the Flysch from ground level and revealed thickness from 10 m to 130 m. Maximum values occurred in the Guadagna and Belmonte Chiavelli areas, while minimal thicknesses (10–30 m) are observed along an almost continuous SW–NE corridor including Monreale, Villa Tasca, and the Harbor. Intermediate thicknesses are noted in the central areas (from 40 m to 60 m), and deeper (from 70 m to 110 m) on the right side of the mouth of the Oreto River. A drop trend is visible along a north–south direction, sharply interrupted only in the hospital and Villagrazia areas, where low thicknesses are shown along a narrow southwest–northeast shape. This reconstruction is consistent with models developed by previous studies [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47] (see Figure 6B, [32]).
To reconstruct the Flysch altitude (Figure 8C), thickness values were subtracted from a 2 × 2 m resolution Digital Elevation Model (DEM). While the density of HVSR measurements is relatively limited, the use of a 2 × 2 m resolution DEM improves the accuracy of surface georeferencing and contributes to a more reliable integration and visualization of the geophysical models.
The resulting surface ranges from approximately 290 m a.s.l. in the west to around 90 m below sea level in the east. Slope gradients are steeper in Monreale and Villaciambra, close to the mountains, while gentler slopes dominate the Palermo Plain. Plateaus appear near Villa Tasca (about 80 m a.s.l.) and the area between the historic center and the hospital (from 10 m to 20 m b.s.l.). A third flat area, enclosed between the Oreto River mouth and the Guadagna and Settecannoli areas. In this sector, the depth ranges from −40 to −60 m.
The contour line patterns on Figure 8C follow a general SW–NE direction. Some strong curvatures are located around the flat areas mentioned above. In the hospital, Villagrazia, and Harbour areas, steep and circumscribed elevation increases are observed. Moreover, the Oreto River area presents a small but continuous change in curvature that can be recognized along the river paths. Finally, the contour line patterns became nearly E–W in the area enclosed between the Papireto and Kemonia hidden river paths.
Quantitative validation with previous studies such as Gianmarinaro et al. [32] was not possible due to the unavailability of original datasets; however, future work may incorporate independent data sources (e.g., groundwater level measurements) to improve the robustness of model verification.

4.4. Analysis of the 3D Model of the Numidian Flysch Top Surface

A 3D representation of the altitude model of the Numidian Flysch was reconstructed by applying a vertical exaggeration of 10 to the surface using the above-mentioned Surfer software. This representation, shown in Figure 9A, highlights the features of the models produced and allows for the linking of them with the tectonic structures defined by the previous studies [40,41], as reported in Figure 9B. For the same reasons, two 3D views of the model, realized using the software Move v.2022.1 provided by Engineering and Structural Geology Software, are reported in Figure 10A,B. These views, produced by applying a vertical exaggeration factor of 6, enhance the highest and lowest sectors of the Numidian Flysch top surface and the presence of significant morphologies.
The presence of Pliocene transpressive faults systems [104], previously defined as extensional structures, named the Monreale graben [105], seems to find continuity in the high morphology structures, represented in the models produced and characterized by an SW–NE direction; these are distinctly visible in the Monreale, Villa Tasca, and Papireto sectors and between Villaciambra and Villagrazia.
Among these structures, the Numidian Flysch top surface shows a more depressed shape; this low morphology has most likely created the morphological conditions that subsequently guided the development of the drainage system along a preferential SW–NE direction. Evidence of this assumption is the abrupt change in the morphology of the altitude contour lines in the area of the Oreto River from the Villaciambra and Villagrazia areas to the mouth of the river. Similarly, the high structures in the Villa Tasca area and near the north sector of the historic center look to partially confine the Kemonia and Papireto River paths, respectively.
Within the depressed area, the model describes another Numidian Flysch high structure close to the hospital sector. This circumscribed morphology, clearly visible in Figure 10A,B, can be related to a structural pattern described in the close Cozzo Meccini area already interpreted as a high structural morphological feature of the Meso-Cenozoic basement [39].
In the eastern sector, the model exhibits the maximum depth close to the Oreto River mouth; higher values are instead described in the Harbor area and the Numidian Flysch outcrops in a nearby, unmodeled area, present in the north. A similar pattern is reconstructed in the unmodeled Settecannoli area, where the Numidian Flysch top rises in elevation, as described in previous studies, and outcrops in the Acqua dei Corsari sector. Considering the above-mentioned evidence, the general morphology of the Numidian Flysch surface in the whole studied area should be represented as a synform structure, as described in a previous study reported in Figure 10C; this structure should be confined in the north and south sectors by antiform structures partly constituted by the contribution of wide-angle faults [26,32,47,49].
Finally, in the modeled sector, a general drop trend of the surface according to a NE–SW direction can be recognized; this can be associated with a wider SSW–NNE antiform structure, which influenced the late Quaternary structural evolution of the Palermo Mountains complex and their deposits [106]. The Palermo Plain area analyzed should represent, in this way, a segment of the flank of this main regional structure.

5. Conclusions

Here, the study and analysis realized in the exanimated sector of the Palermo Plain have enabled a more detailed characterization of the subsurface in a densely urbanized area through a multidisciplinary geological and geophysical approach.
Initially, more than 1000 previously collected borehole data were re-evaluated and standardized to create a coherent model of the bedrock surface, represented by Numidian Flysch deposits in the study area. Subsequently, active seismic surveys were performed in several parts of the city to determine Vs parameters. The resulting shear velocities were consistent with values reported in earlier studies for the various lithologies composing the subsurface of the plain.
Furthermore, ambient noise data were analyzed using the HVSR method, generating H/V spectral ratio curves. The inversion of these microtremor data, constrained by boreholes information and Vs velocity profiles, enabled us to estimate the depth of the Numidian Flysch at 54 new locations within the urban area. This step significantly expanded the available dataset and improved the spatial resolution of subsurface information in previously under-sampled areas.
Two comprehensive maps were produced: one representing the thickness of the Quaternary deposits, and another depicting the elevation of the Numidian Flysch. Furthermore, a three-dimensional model of the bedrock surface was developed to illustrate its morphological characteristics in relation to existing stratigraphic and tectonic data from previous studies and geological maps.
The model derived through this integrated approach holds potential for various scientific applications, including stratigraphic, paleoenvironmental, and paleomorphological reconstructions. For example, future survey steps should consider the definition of the bedrock depth in the eastern sector of the Palermo Plain. A new campaign of HVSR and MASW surveys could deep model this area, without deep boreholes, to obtain the a complete model. Additionally, considering that the modeled surface is associated with a shear wave velocity typically exceeding 800 m/s, it can serve as a valuable tool for urban planning and seismic hazard mitigation in the investigated area. Taken together, this study offers a compelling example of the effective application of a multidisciplinary workflow in urban environments, aimed at enhancing our understanding of subsurface conditions. The integration of geophysical datasets has provided a coherent model of the subsurface structure of the Palermo Plain. However, further work is necessary to enhance spatial coverage in areas with limited data availability, particularly in the eastern sector. Given the absence of deep boreholes in this zone, future investigations should prioritize the acquisition of HVSR and MASW data. These methods, which have proven effective in other parts of the plain, could support the extension of the current model and improve the definition of the seismic bedrock surface without requiring invasive interventions.

Author Contributions

Conceptualization and methodology, A.C. (Alessandro Canzoneri), R.M., M.A., M.G.M., P.C., A.C. (Alessandra Carollo) and A.S.; software, A.C. (Alessandro Canzoneri), R.M., M.G.M., P.C. and A.C. (Alessandra Carollo); validation, R.M., M.A., M.G.M. and P.C.; formal analysis and investigation, A.C. (Alessandro Canzoneri), P.C. and A.C. (Alessandra Carollo); resources, R.M., M.A., M.G.M., P.C. and A.S.; data curation, A.C. (Alessandro Canzoneri), R.M. and M.A.; writing—original draft preparation, A.C. (Alessandro Canzoneri); writing—review and editing, R.M., M.A., M.G.M., P.C. and A.S.; visualization, A.C. (Alessandro Canzoneri), R.M., M.A., M.G.M., P.C., A.C. (Alessandra Carollo) and A.S.; supervision, R.M. and M.A.; funding acquisition, R.M. and A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the European Union NextGenerationEU Mission 4 “Education and Research”, Component 2 “From Research to Business”, Investment 3.1 “Fund for the realization of an integrated system of research and innovation infrastructures”, Project IR0000037—GeoSciences IR and by MOD-Energy Project: “Application and Dissemination of Intervention Models in Smart Lighting, Street, and Building Systems for Reducing Energy Consumption in Metropolitan Cities”, Research Agreement ENEA-TERIN/CURSA/DiSTeM, Project Code: E61C22001310001, 2024.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to funder restrictions.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. (A) Representation of the study area within Sicily and (B) in the northwestern sector of the island; (C) geological map of the study area; (D) schematic stratigraphic column of deposits found in the Palermo Plain subsoil, modified from previous study [38].
Figure 1. (A) Representation of the study area within Sicily and (B) in the northwestern sector of the island; (C) geological map of the study area; (D) schematic stratigraphic column of deposits found in the Palermo Plain subsoil, modified from previous study [38].
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Figure 2. Samples borehole charts analyzed. Each chart includes data on latitude, longitude, altitude, water table depth, and lithotype thickness. Lithologies, labeled with acronyms, are associated with broader lithological macro-categories, as reported outside the chart.
Figure 2. Samples borehole charts analyzed. Each chart includes data on latitude, longitude, altitude, water table depth, and lithotype thickness. Lithologies, labeled with acronyms, are associated with broader lithological macro-categories, as reported outside the chart.
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Figure 3. Map showing the distribution of geological and geophysical surveys. The grey lines represent topographic contour lines.
Figure 3. Map showing the distribution of geological and geophysical surveys. The grey lines represent topographic contour lines.
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Figure 4. Examples of the dispersion curves produced from MASW analysis. The corresponding MASW survey locations (labeled) are shown in Figure 3. The color scale in the dispersion spectra indicates the amplitude of ground motion, from blue (lowest amplitude) to red (highest amplitude).
Figure 4. Examples of the dispersion curves produced from MASW analysis. The corresponding MASW survey locations (labeled) are shown in Figure 3. The color scale in the dispersion spectra indicates the amplitude of ground motion, from blue (lowest amplitude) to red (highest amplitude).
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Figure 5. Examples of Vs models derived from MASW. The corresponding MASW survey locations (labeled) are shown in Figure 3. Schematic columns on the right represent lithological macro-categories correlated with seismostratigraphic layers. The green background indicates the parameter search space used for model calibration.
Figure 5. Examples of Vs models derived from MASW. The corresponding MASW survey locations (labeled) are shown in Figure 3. Schematic columns on the right represent lithological macro-categories correlated with seismostratigraphic layers. The green background indicates the parameter search space used for model calibration.
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Figure 6. Comparison of HV curve peaks and stratigraphy of the Palermo Plain: (A) HVSR curve dataset and corresponding peak frequency clusters (red/purple boxes) attributed to different lithological boundaries. Dotted lines indicate peaks over 30 Hz which can be due to transient noise and, consequently, subjected to more accurate revision; (B) analysis of the spatial distribution of the peaks in the 0.8–2.5 Hz range by iso-frequency contour lines (B′), compared with the Numidian Flysch depths (B″) from prior borehole studies [32].
Figure 6. Comparison of HV curve peaks and stratigraphy of the Palermo Plain: (A) HVSR curve dataset and corresponding peak frequency clusters (red/purple boxes) attributed to different lithological boundaries. Dotted lines indicate peaks over 30 Hz which can be due to transient noise and, consequently, subjected to more accurate revision; (B) analysis of the spatial distribution of the peaks in the 0.8–2.5 Hz range by iso-frequency contour lines (B′), compared with the Numidian Flysch depths (B″) from prior borehole studies [32].
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Figure 7. Vs models from HVSR curve inversion. Labels correspond to passive seismic acquisition locations shown in Figure 3. The red line represents the experimental HVSR curves with their standard deviation ranges (grey lines); the blue curve describes the synthetic model computed. Right-side tables represent interpreted Vs models and related lithologies.
Figure 7. Vs models from HVSR curve inversion. Labels correspond to passive seismic acquisition locations shown in Figure 3. The red line represents the experimental HVSR curves with their standard deviation ranges (grey lines); the blue curve describes the synthetic model computed. Right-side tables represent interpreted Vs models and related lithologies.
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Figure 8. (A) Modeled sector of the Numidian Flysch. (B) Thickness map of the Quaternary deposits. (C) Altitude map of Numidian Flysch top surface. Color scales and values are shown in figure legends.
Figure 8. (A) Modeled sector of the Numidian Flysch. (B) Thickness map of the Quaternary deposits. (C) Altitude map of Numidian Flysch top surface. Color scales and values are shown in figure legends.
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Figure 9. (A) Color relief representation of the Numidian Flysch altitude model applying a vertical exaggeration factor of 10. (B) Analysis of whole studied area. The following are represented on the map: the modeled Numidian Flysch top surface elevation, the Numidian Flysch outcrops, and the principal tectonic structures already shown in Figure 1 [40,41]. The axes of the supposed folds and the morphology of hidden river courses, derived from previous studies, are also represented [17,18].
Figure 9. (A) Color relief representation of the Numidian Flysch altitude model applying a vertical exaggeration factor of 10. (B) Analysis of whole studied area. The following are represented on the map: the modeled Numidian Flysch top surface elevation, the Numidian Flysch outcrops, and the principal tectonic structures already shown in Figure 1 [40,41]. The axes of the supposed folds and the morphology of hidden river courses, derived from previous studies, are also represented [17,18].
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Figure 10. Three-dimensional views of the Numidian Flysch altitude model with an exaggeration factor of 6 in (A,B); (C) Modified section derived from a previous studies [26] crossing the area studied (C) and corresponding to the S–S′ trace shown in Figure 9B.
Figure 10. Three-dimensional views of the Numidian Flysch altitude model with an exaggeration factor of 6 in (A,B); (C) Modified section derived from a previous studies [26] crossing the area studied (C) and corresponding to the S–S′ trace shown in Figure 9B.
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Table 1. The velocity range of the main lithologies of the Palermo Plain subsoil. The values are described in previous works and obtained through geophysical surveys and geotechnical tests.
Table 1. The velocity range of the main lithologies of the Palermo Plain subsoil. The values are described in previous works and obtained through geophysical surveys and geotechnical tests.
LithologiesLower Vs Value
(m/s)		Upper Vs Value (m/s)
Upper Triassic–Oligocene Carbonate (CA)7001000
Numidian Flysch (FN)4501900
Blue Clays (BC)1001250
Palermo Calcarenites (CC)1001850
Middle Pleistocene—Holocene “Upper deposits” (UD)60450
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MDPI and ACS Style

Canzoneri, A.; Martorana, R.; Agate, M.; Gasparo Morticelli, M.; Capizzi, P.; Carollo, A.; Sulli, A. Reconstruction of a 3D Bedrock Model in an Urban Area Using Well Stratigraphy and Geophysical Data: A Case Study of the City of Palermo. Geosciences 2025, 15, 174. https://doi.org/10.3390/geosciences15050174

AMA Style

Canzoneri A, Martorana R, Agate M, Gasparo Morticelli M, Capizzi P, Carollo A, Sulli A. Reconstruction of a 3D Bedrock Model in an Urban Area Using Well Stratigraphy and Geophysical Data: A Case Study of the City of Palermo. Geosciences. 2025; 15(5):174. https://doi.org/10.3390/geosciences15050174

Chicago/Turabian Style

Canzoneri, Alessandro, Raffaele Martorana, Mauro Agate, Maurizio Gasparo Morticelli, Patrizia Capizzi, Alessandra Carollo, and Attilio Sulli. 2025. "Reconstruction of a 3D Bedrock Model in an Urban Area Using Well Stratigraphy and Geophysical Data: A Case Study of the City of Palermo" Geosciences 15, no. 5: 174. https://doi.org/10.3390/geosciences15050174

APA Style

Canzoneri, A., Martorana, R., Agate, M., Gasparo Morticelli, M., Capizzi, P., Carollo, A., & Sulli, A. (2025). Reconstruction of a 3D Bedrock Model in an Urban Area Using Well Stratigraphy and Geophysical Data: A Case Study of the City of Palermo. Geosciences, 15(5), 174. https://doi.org/10.3390/geosciences15050174

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