Capineri, Lorenzo

Falorni, Pierluigi

Frosinini, C.

Macchioni, N.

Mannucci, M.

Olmi, R.

Palanti, S.

Penoni, S.

Pieri, S.

Priori, S.

Riminesi, C.

Santacesaria, A.

Todaro, C.

English

Florence Research

778 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012Microwave Reflectometry for the Diagnostic of Cultural HeritageAssetsL. Capineri1, P. Falorni1, C. Frosinini2, M. Mannucci3, N. Macchioni4, R. Olmi7,S. Palanti4, S. Penoni5, S. Pieri7, S. Priori7, C. Riminesi6, A. Santacesaria2, and C. Todaro51Dipartimento Elettronica e delle Telecomunicazioni, Universita` di Firenze, Firenze 50139, Italy2Opificio delle Pietre Dure, Via degli Alfani 78, Firenze 50100, Italy3LegnoDOC, Via Pier della Francesca, b39, Prato 59100, Italy4CNR, IVALSA, Via Madonna del Piano 10, Sesto Fiorentino 50019, Italy5Faberestauro, piazza Ginori 13, Sesto Fiorentino 50019, Italy6ELab Scientific, 17 Fra’ D. Bartolomeo Street, Firenze 50132, Italy7CNR, IFAC, Via Madonna del Piano 10, Sesto Fiorentino 50019, ItalyAbstract— A non destructive diagnostic method for cultural heritage assets based on mi-crowave reflectometry has been studied and developed. This new method is implemented with aridged antenna operating in the range 1.5–3GHz for the detection of reflected wave amplitude andphase variation due to the dielectric properties of the materials under investigation. Particularinterest has been devoted to wood structures attacked by fungi and insects and to mural paintingswith delaminations due to subsurface humidity. A simplified model of the antenna coupled to thematerial has been implemented in terms of a cascade of transmission lines in order to investigatethe response of the system in presence of dielectric discontinuities. A series of laboratory modelshave been developed to assess the capability of the method to display in real time images of acalculated parameter which includes both amplitude and phase variation respect to a referenceresponse taken from a homogeneous portion of the investigated item. A novel compact easy touse in-situ optical system mounted on the antenna has been developed to correlate the positionsof the antenna on the surface with the optical digital picture, avoiding cumbersome wheel opticalencoding which need flat surfaces and raster lines for guiding the manual scan.1. INTRODUCTIONMicrowaves can penetrate up to several centimetres in dry wood [1] or plaster and brick or stonewalls [2]. In the operating frequency range of the instrument (1.5–3GHz) there is also a good sen-sitivity to the moisture content, that can be a source of degradation both in wooden structures andin mural paintings, favouring the development of biotic decay in the former and the developmentof detachment of the painted layer in the latter [3, 4]. Preliminary experience with the holographicradar have pointed out that using single emitting frequency and two antenna polarizations it ispossible to get high resolution images of impedance discontinuities at shallow depth by using thephase variation information. Moreover the investigation of cultural heritage assets like paintings onwood or frescos often implies to work on thickness of the order of a few wavelengths. Then the useof high frequency pulsed radar becomes difficult due to the overlapping of the finite pulse lengthreflected from the item interfaces [5–7]. This research work is aimed to a new microwave reflec-tometry system for the investigation for several materials like stone, plaster and wood. The depthanalysis is performed thanks to a modulated continuous wave operating in the 1.5–3GHz frequencyrange. The electromagnetic wave penetration in a medium depends on the dielectric characteristicsof the material: moisture, presence of cavities, and material inhomogeneities can be detected ex-ploiting the dielectric “contrast” among different regions. The specifications of the system shouldtake into account the application requirements in not easy areas, such as restoration sites, includingportability (weight and reduced size) and autonomy (battery operation). These features should beadded to the responsiveness, reliability and, possibly, the low cost. The implementation of themicrowave reflectometer prototype consisted in:(1) The development of a generator/receiver that complies with electromagnetic compatibilityissues. The prototype described in this paper has been based on a commercial Vectorial NetworkAnalyzer.(2) An optical compact low cost system for tracking the scan head with accuracy better than1 cm that takes into account different environmental illumination conditions.(3) The development of models to study the interaction between the electromagnetic signal andthe different media — e.g., wood, stone, plaster.Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 779(4) The development of software for signal analysis and imaging of the reflectometric response.The results of the prototype applied to several case studies will be presented for two areas ofinterest in the cultural heritage: diagnostics of frescoes and mural paintings, diagnostics of woodenstructures and artefacts. In both cases the objective is the detection of material discontinuities(voids, delaminations, knots) and inhomogeneities (in particular, moisture), by analysing imagesobtained by scanning the head of the microwave reflectometer on the object under investigation.With regard to the study of wall paintings, the analysis should also give information about thetexture of the support layer.Concerning wooden artefacts, two types of structures are of interest: structural elements (beams)and panel paintings. For both structures, laboratory models have been developed: the presenceof fungal decay and pockets of moisture have been simulated in wooden beams, as well as paintedwoods boards attacked by xylophageous insects.These activities should hopefully lead to the creation of a database of “signatures” that willlater be used to interpret the results of on site diagnostic case studies that will be discussed in thepresentation.2. THE TRANSMISSION LINE MODELThe interpretation of the reflection measurements requires the development of a model of theinteraction between the antenna and the material under investigation. A full-wave model canbe implemented using well known numerical approaches, e.g., the finite element method, but thesolution is quite time consuming and not very flexible.An analytical approach is very desirable, as it allows studying the dependence of the responseon the geometry and on the dielectric properties. A zero-order approximation is therefore assumed,consisting in representing the material under investigation as a layered medium excited by a planewave. Such a system is described by an equivalent circuit consisting in a series of transverseelectromagnetic (TEM) lines with characteristic impedances depending on the dielectric constant ofeach layer. As an example, Figure 1 represents a wooden structure with a cavity inside, representedby the line section identified by the characteristic impedance Z2.The reflection coefficient Γ at the wood surface is defined in the following equation:Γ = (f, d1, d2) =Z0 − Zi1(f, d1, d2)Z0 + Zi1(f, d1, d2)(1)where Zi1 is the impedance at the air-material interface, defined by:Zi1(f, d1, d2) = Z1Zi2(f, d2) + iZ1 tan(k1d1)Z1 + iZi2(f, d2) tan(k1d1)(2)di is the thickness of the layer i, f is the operating frequency and ki the wavenumber in the i-thmaterial.We can observe that the reflection coefficient can be measured at different discrete frequenciesin order to get the information about impedance variation (amplitude and phase) into the material.3. EXPERIMENTAL APPARATUSThe experiments have been done by using a set-up consisting of a portable reflectometer modelAnritsu S331E using a double-ridged antenna as a probe head, shown in Figure 2. In our experi-ments the antenna has been manually scanned over the surface of the investigated item without airgap. This is not a limitation for the investigation of paintings or frescos because a thin (few mmthickness) protective layer can be used to couple the antenna to the surface.Figure 1: The TEM model of a wooden structure under investigation.780 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012(a) (b)Figure 2: Measurement setup on (a) a wood beam and (b) a particular of the double-ridged antenna.Figure 3: Wood beam utilized for measurement, internal cavity in the centre of the beam and image obtainedthrough elaboration of the signals obtained with the double-ridged antenna on wood surface with opticalpositioning system.4. IMAGING OF THE PARAMETER KεFor each antenna position the reflection parameter is acquired together with the antenna position.A parameter called Kε has been defined (see Equation (3)) to obtain a single display representationof the amplitude and phase variation of the reflection coefficient respect to a homogeneous portionof the material assumed as reference response (Γref).Kε =|Γref ||Γ| cos [arg (Γ)− arg (Γrif)] (3)where Γref = Z0−Z1Z0+Z1 is the reference reflection coefficient.A preliminary experiment has been carried out to test the feasibility of the method on a woodbeam where in the central position a cavity was excavated. Different wood blocks, artificiallyattacked by insect, positioned inside the cavity simulated different grade of biotic decay. (seeFigure 3(a)).On the investigated surface the wood beam shows some knots and vein patterns. On theFigure 3(b) the image ofKε in false color (yellow and red) where all above features can be describedaccurately. The position of the ridged antenna has been acquired with an optical system withposition accuracy better than 1 cm that is a fraction of the wavelength.5. CONCLUSIONThe developed microwave reflectometry have been tested on laboratory models and has demon-strated sufficient sensitivity to detect dielectric variations inside the material. The imaging of acalculated parameter which includes both amplitude and phase variation is investigated with theperspective of facilitating the interpretation of acquired images by a final user when operatingin-situ or in a restoration laboratory.ACKNOWLEDGMENTThe author wish to acknowledge the financial support to Rimidia Project of the Tuscany Region(rimidia.det.unifi.it).Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 781REFERENCES1. Bucur, V., “Techniques for high resolution imaging of wood structure: A review,” Meas. Sci.Technol., Vol. 14, 91–98, 2003.2. Capineri, L., P. Falorni, G. Borgioli, T. Bechtel, S. Ivashov, A. Zhuravlev, I. Vasiliev, M. Par-adiso, and G. Cartocci, “Application of the holographic radar RASCAN to cultural heritageinspection,” Archaeological Prospection, Archaeol. Prospect., Vol. 16, 1–13, 2009.3. Olmi, R., S. Priori, D. Capitani, N. Proietti, L. Capineri, P. Falorni, R. Negrotti, and C. Rim-inesi, “Innovative techniques for sub-surface investigations,” Materials Evaluation, MaterialsEvaluation, Vol. 69, No. 1, 89–96, 2011.4. Bison, P., G. Cadelano, L. Capineri, D. Capitani, U. Casellato, P. Faroldi, E. Grinzato, N. Lud-wig, R. Olmi, S. Priori, N. Proietti, E. Rosina, R. Ruggeri, A. Sansonetti, L. Soroldoni, andM. Valentini, “Limits and advantages of different methods and techniques for moisture contentevaluation in masonry,” Materials Evaluation, Vol. 69, No. 1, 111–116, 2011.5. Razevig, V. V., S. I. Ivashov, I. A. Vasiliev, A. V. Zhuravlev, T. Bechtel, and L. Capineri,“Advantages and restrictions of holographic subsurface radars experimental evaluation,” XIIIInternational Conference on Ground Penetrating Radar, 657–662, Lecce, Jun. 21–25, 2010.6. Capineri, L., P. Falorni, M. Inagaki, T. D. Bechtel, V. V. Razevig, and C. G. Windsor, “Valida-tion of a theoretical model for quantitative interpretation of holographic radar response from aninclined plane reflector,” XIII International Conference on Ground Penetrating Radar, Lecce,657–662, Jun. 21–25, 2010.7. Ivashov, S. I., V. V. Razevig, I. A. Vasiliev, A. V. Zhuravlev, T. Bechtel, L. Capineri, P. Falorni,and T. Lu, “Holographic subsurface radar as a device for NDT of construction materials andstructures,” International Symposium on Nondestructive Testing of Materials and Structures,28, Istanbul, Turkey, May 15–18, 2011.

Microwave Reflectometry for the Diagnostic of Cultural Heritage Assets

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Microwave Reflectometry for the Diagnostic of Cultural Heritage Assets

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