1. Introduction
Moisture detection is one of the most relevant issues within Hydrogeophysics. Ground Penetrating Radar (GPR), in particular, can accurately assess the level of moisture in the subsoil. This type of geophysical non-destructive technique (NDT) can be used to accurately determine the moisture content below the earth’s surface [
1]. This electromagnetic (EM) technique can be used to estimate material parameters and to particularly obtain valuable information about the presence of moisture in an investigated medium. Moreover, this novel approach toward data analysis uses the acquired amplitude attribute information of the GPR signal with a standard, common-offset, and ground-coupled antenna configuration [
2,
3]. Under these conditions, where the antenna offsets are small, the direct GPR signal consists of a complex combination of the air and ground waves and carries information about the physical properties of the surrounding material. This is because the amplitude, shape, and duration of the signal change acts as a function of the EM properties of the soil [
4,
5].
The main results are as follows: (i) a systematic change in the amplitude and duration of early ground-coupled GPR signals is induced by spatial variations in the shallow soil’s dielectric properties which, in this case, depend only on the distribution of water content in the subsurface medium; (ii) this new approach towards GPR data analysis is highly suitable for creating detailed maps of variations in the electric permittivity of the shallow subsurface (e.g., water content); (iii) the evaluation of the shallow soil’s dielectric properties using this novel technique, instead of more traditionally configured radar systems, could represent a practical way to rapidly characterize materials at high spatial resolutions [
1,
2,
3,
4,
5].
Several controlled experiments, numerical simulations, and “real-life” applications have been conducted to study the effects of EM parameter variations on antenna-material coupling. However, this approach toward evaluating variations in the shallow dielectric permittivity and conductivity (i.e., the presence of moisture) in a soil is more reliable and consistent than other methods [
6,
7,
8,
9,
10,
11,
12,
13,
14,
15].
Given the recent advancements in commercial geophysical systems, researchers are now exploring new spatial and temporal monitoring techniques which are capable of providing valuable information regarding the soil water content at the intermediate scales. The estimation of water content in this scale range is important in many hydrological processes, such as water infiltration, percolation, runoff, and evapotranspiration, that affect the investigated medium [
16,
17,
18].
GPR has gained popularity in engineering and Cultural Heritage Preservation sectors, in particular, thanks to its applicability regarding water content estimation, damage evaluation, long-term masonry changes, restoration management, and location of cracks [
3,
4].
Herein, the acquired data is part of a broader study to plan a proper and more elaborate restoration of a precious building in the city-center of Rome. The aim of this study is to assess the capacity of surface GPR techniques to non-invasively monitor the internal moisture dynamics in a cultural heritage site. In particular, the novelty of this paper is the possibility to apply the real-time characteristics of the GPR methodology by acquiring the data and, at the same time, communicate the damages to the restorers for their immediate operations. Here, we would like to propose a different use of the GPR in a very specific situation within a limited time of intervention for both the GPR acquisition and the architectonic restoration. The novelty is exactly this “immediate” use of the GPR results for the restorations within the same morning of the operations.
2. Materials and Methods
The area under investigation was the Turkish Room at the Villa Medici situated on Pincio Hill in Rome, Italy. This hill does not belong to the seven major Roman hills because it is outside the pomoerium, which is the sacred wall of antique Rome. Beautiful ornamental facades, elaborate entrance gates, gardens replete with fantastic water displays, and antique statues once formed the stage for the grand theatrical entertainments of the Villa. The Villa Medici, founded by Ferdinando I de′ Medici—the Grand Duke of Tuscany—and presently a property of the French State, has housed the French Academy in Rome since 1803 [
19].
The Turkish Room, one the most picturesque areas of the building, which is situated in a tower, is an unusual boudoir with neo-Moorish decorations and has frescos overlooking Rome. It was designed in 1833 by Horace Vernet, then the Director of the French Academy, who had been inspired by a sojourn in Algiers at the time of the French conquest. Between 1961 and 1967, Balthus, then the head of the Academy, carried out a vast restoration campaign of the palace and its gardens and provided them with modern equipment. Recently, this room has been under heavy restoration due to damage, related to the presence of humidity, in the walls containing the detached portions of the frescos and decorations (
Figure 1) [
20].
The involved GPR instrument was the bistatic FINDAR (Sensors & Software, Inc. Mississauga, ON, Canada) system equipped with a 500 MHz antenna. Two one-way grids of 5 × 5 m (one X-oriented, one Y-oriented), with a distance of 0.1 m among profiles, were acquired on the terrace above the Turkish Room′s ceiling (
Figure 2). No other processing was required beyond the standard one (Automatic Gain Control—a method of overcoming unwanted clutter echoes, DeWOW—removing the low-frequency component, the so-called “wow,” of every trace, and the Average Envelope Amplitude of the signal) [
21]. This is due to the aim of this paper: real-time acquisition, results, and operations. The signal penetration velocity was calculated using the hyperbola calibration and verified using the actual measurements of the exposed masonry, which was 0.12 m/ns.
3. Results and Discussion
Thanks to its high sensitivity to moisture, GPR investigation was necessary to localize the possible presence of moisture inside the ceiling of the room and to understand the level of structural damage entailing the presence of humidity. This was necessary to plan a proper and focused restoration plan of the Turkish Room vault.
Figure 3 shows a radargram collected almost in the middle of the vault below with a direction facing North–South. It is possible to clearly see the inner structure of the ceiling. The thickness and the geometry of the vault are particularly detectable due to the contrast between the architectonic structure and the air. Moreover, the composition of both the terrace pavement and the ceiling filling is visible—first, an almost-homogenous layer of bricks is present above with a thickness of about 0.5 m, and different grouting materials below. In the middle of the radargram, there is an evident hyperbolic event at a depth of about 0.20 m. This is due to the presence of an electrical cable, as the following depth-slices analysis will illustrate. It is worth noting that, because of its geometry, the vault thickness varies from a maximum of about 1.1 m to a minimum of about 0.70 m.
The depth-slices highlight the different kinds of targets (
Figure 4). Starting from the near-surface one, two elongated anomalies are visible. These two evidences are possibly due to the presence of electrical cables at a depth of about 0.20 m inside the brick layer. The longest one is partially detected by the previous radargram, as shown in
Figure 3.
In
Figure 5, the results clearly highlight the presence of a strong anomaly related to the high presence of moisture in a specific part of the internal vault below. The intensity and definition of the anomaly allows us to understand the level of damage and to plan a focused restoration without entirely dismantling the ceiling of this precious room. The potential of such an NDT investigation, using the GPR, allows us to detect the presence and the dimensions of a moist layer inside this architectonical structure and (more importantly) to locate it in depth. In particular, there are two damaged areas—one larger than the other one—in a specific area of the vault (the southern side) at a depth of about 0.85 m. Due to the lateral position and the depth of both the anomalies, it is possible to identify the humidity present at few centimeters behind the frescos of the vault below. In
Figure 5, to the right, their position is indicated; the restoration started exactly from that portion of the room’s ceiling.
Finally,
Figure 6 confirms the moisture presence, thanks to both the “time-stretched signal” and “ringing” effects. When GPR data are acquired using a fixed-offset, ground-coupled GPR antenna configuration, the dielectric permittivity affects both the amplitude and duration of the GPR signal; therefore, higher amplitudes and shorter wavelets are associated with lower permittivity [
5,
7,
8]. The ringing anomalies in the radargrams with very low EM signal amplitudes—due to high attenuation, poor antenna coupling, and temporal stretching—can be analyzed in order to estimate the physical properties of near-surface materials in archaeological as well as cultural heritage sites [
4,
8,
22]. Note that the signal amplitude is affected by variations in the EM parameters and particularly by permittivity changes. Hence, it is also possible to determine the medium’s volumetric water content using an appropriate petrophysical relationship [
16,
18,
23].
4. Conclusions
The results highlighted in this paper demonstrate the ability of an NDT, such as the GPR, to locate and estimate the extent of water ingression in order to develop restoration plans at the same time of the GPR investigations before permanent damage occurs. Rapidly and non-invasively, the GPR acquisition allows for the restoration managers to immediately develop strategies for the preservation of such a precious cultural heritage site. The restorers, right after the presentation of the GPR results, found the damaged areas behind the frescos and began to re-establish the original condition of the vault by removing the excess moisture in the area as displayed by the GPR investigations.
Author Contributions
Conceptualization, P.M.B. and C.F.; Data curation, P.M.B. and C.F.; Formal analysis, P.M.B. and C.F.; Funding acquisition, C.F.; Investigation, P.M.B. and C.F.; Methodology, P.M.B. and C.F.; Project administration, P.M.B. and C.F.; Resources, P.M.B. and C.F.; Supervision, P.M.B. and C.F.; Validation, P.M.B. and C.F.; Visualization, P.M.B. and C.F.; Writing—original draft, P.M.B. and C.F.; Writing—review and editing, P.M.B. and C.F.
Funding
This research received no external funding.
Acknowledgments
The authors would like to thank Valeria Di Tullio and Emiliano Ricchi for their logistics support during the measurements.
Conflicts of Interest
The authors declare no conflict of interest.
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