In this paper we propose an integrated approach to
diagnostic prospecting applied to the cathedral of Matera, in
Southern Italy. In particular, we have performed both an ultrasonic
tomography and a high frequency GPR prospecting
on some pillars of the Church to investigate about possible
structural yielding and a GPR prospecting at lower frequencies
on the floor, where also a linear inversion algorithm has
been applied to the data

GUIDA, Antonella Grazia

MASINI, Nicola

PAGLIUCA, ANTONELLO

R. PERSICO

Advances in Geosciences

In this paper we propose an integrated approach to diagnostic prospecting applied to the cathedral of Matera, in Southern Italy. In particular, we have performed both an ultrasonic tomography and a high frequency GPR prospecting on some pillars of the Church to investigate about possible structural yielding and a GPR prospecting at lower frequencies on the floor, where also a linear inversion algorithm has been applied to the data

N. Masini

R. Persico

A. Guida

A. Pagliuca

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Adv. Geosci., 19, 17–22, 2008www.adv-geosci.net/19/17/2008/© Author(s) 2008. This work is distributed underthe Creative Commons Attribution 3.0 License.Advances inGeosciencesA multifrequency and multisensor approach for the study and therestoration of monuments: the case of the Cathedral of MateraN. Masini1, R. Persico1, A. Guida2, and A. Pagliuca31CNR-IBAM (Istituto per i Beni Archeologici e Monumentali), C.da S. Loja 85050 Tito Scalo (PZ), Italy2DAPIT, University of Basilicata, Potenza, Italy3Polytechnic of Bari, Bari, ItalyReceived: 4 July 2008 – Revised: 14 October 2008 – Accepted: 3 November 2008 – Published: 14 November 2008Abstract. In this paper we propose an integrated approach todiagnostic prospecting applied to the cathedral of Matera, inSouthern Italy. In particular, we have performed both an ul-trasonic tomography and a high frequency GPR prospectingon some pillars of the Church to investigate about possiblestructural yielding and a GPR prospecting at lower frequen-cies on the floor, where also a linear inversion algorithm hasbeen applied to the data.1 IntroductionThe preservation of artistic and architectural heritage repre-sents a bench mark of the cultural development of a soci-ety. To this end it is necessary to preserve both the materialcomponents and the knowledge of those values (historical,artistic, symbolical etc.) which are intangible aspects of thecultural heritage.These two scopes can be reached by means of a multidisci-plinary and interdisciplinary approach where non-destructivetesting (NDT) provide a significant source of information onmaterials, state of conservation and aspects related to build-ing history.In particular, some case histories (Binda et al., 2003;Masini et al., 2007) show how complex it is to design therepairing interventions on historic stone masonry buildings.Most of them are characterized by load bearing structureswhich are highly inhomogeneous and this makes the struc-tural modelling very difficult. For this reason, any repair-ing work needs the knowledge of the technique of con-struction and the characterization of the state of conserva-tion of the manufactured article, with particular referenceto voids, cracks and decay. To do this, in the past the onlyCorrespondence to:N. Masini(n.masini@ibam.cnr.it)way was based on destructive survey through coring or localdemolition.Fortunately, nowadays an increased awareness of the cul-tural value and the brittleness of artefacts and monumentsto be restored often prevent from further destructive investi-gations. This has favoured the use of non-destructive tests(NDT) as ground penetrating radar (GPR), sonic and ultra-sonic tests.As is well known, the techniques are based on differentprinciples because the probing signals have different physicalorigin: stress waves for sonic and ultrasonic tests and Elec-tromagnetic (EM) waves in the case of radar. The theoreticalcapabilities and limitations of the techniques are strictly re-lated to the frequency band of the signals.The frequency is related both to the resolution and to therate of energy absorption, thus limiting the size of the build-ing element that can be tested and characterized. Therefore,the attenuation of the signal and the resolution requirementshave both to be considered in order to choose the optimalfrequency band.Moreover, with regard to the stress waves, the band can bemeaningfully influenced by the characteristics of the surfacematerials, and the presence of a thick plaster can filter out thehighest frequencies of the radiated band. This reduces theresolution and the capability to detect the inner morphologyof masonry structures in detail with ultrasonic waves. In suchsituation the radar tests show some advantages.In this paper we present the results of integrated NDT(GPR-ultrasonic) performed in the Cathedral of Matera, inorder to answer to needs of the conservation and questions ofthe archaeological research.The paper is organized as follows: in Sect. 2 the case his-tory of the cathedral of Matera is described. In Sect. 3 theinvestigation and the processing are summarized and, finally,in Sect. 4 conclusions follow.Published by Copernicus Publications on behalf of the European Geosciences Union.18 N. Masini et al.: A multisensor approach for the study of monumentsFig. 1. The cathedral of Matera.2 The case history of the Cathedral of MateraThe Cathedral of Matera (Fig. 1) is one of the most remark-able witnesses of the so-called “Romanico Pugliese”, that isa kind of Romanesque style spread in the Apulia region andin other neighbouring areas. It is located on a panoramic spurfacing the famous “Sassi” of Matera, included in the WorldHeritage.The church has a Latin Cross plan, composed of three lon-gitudinal aisles cut transversally by a transept. It was builtbetween 1230 and 1270, on two pre-existent crypts and thefoundations of a church dedicated to St. EustachioMost of the external structures preserves its original Ro-manesque characteristics, but the inner of the church appearsinstead modified by successive changes, as the central nave,mostly ascribable to the 17th century. However, some moreancient details dating back to the first half of the 13th cen-tury still remain also inside the church, as for instance a richvariety of capitals of different shapes (leafs, crowns, anthro-pomorphic and zoomorphic motifs).Currently the Cathedral is under restoration. The worksare prevailingly aimed at the structural rehabilitation of pil-lars, columns, walls and roofing.In particular, it is worth to quote some vertical and sub-vertical cracks which affect two polystyle pillars of the tri-umphal arch of the choir. They could be the effect of a deflec-tion phenomenon to which Superintendance of ArchitecturalHeritage of Basilicata tried to find a structural solution inthe restorations carried out in 1989. Each polystyle pillars iscomposed of a square element, with three semicolumns andfour pilasters, on its sides. They are constructed in masonry,in its turn composed by calcarenite ashlars and a central nu-cleus filled with incoherent material of calcarenite nature anddark soil, as shown by some core tests made during the 1989restorations.Another instability phenomenon affects one of the mono-lithic columns. It is composed by dense limestone and isaffected by some cracks on the basement, as well.In order to investigate both the state of conservation ofbuilding elements and the possible presence of previousremains, the Superintendance of Architectural Heritage ofBasilicata has commissioned a geophysical prospecting cam-paign to the Institute for Cultural Heritage and Monumentsof the National Research Council (IBAM-CNR). It has beenthought of to perform a GPR prospecting on the floor, inthe central nave and the accessible areas of the lateral naves,which are partially filled with scaffoldings.In fact, the flat surface of the floor and the presum-ably quite dry underground (together with the kind of targetlooked for) made a GPR prospecting the most promising onthe soil.For the study of the two pillars GPR investigations havebeen performed, whereas for the column GPR and ultrasonictests have been compared in order to have more data for thediagnosis.3 Investigations carried out: data acquisition, process-ing and resultsThe GPR prospecting has been performed making use ofthe collaboration and the instruments of the Centro di Com-petenza Innova of the Campania Region. The GPR wasa SIR3000 equipped with four linearly polarized anten-nas, with nominal central frequency of 150, 400, 600 MHzwith regard to the prospecting on the floor (however, themost clear results have been obtained from the antennas at400 MHz) and 1000 MHz for the prospecting on the pillars.The ultrasonic tests have been performed by using a pulsegenerator MAE A5000U, equipped with a contact probe withfrequency of 55 KHz.A multifrequency approach by using only the GPR hasbeen adopted for the investigations performed on the floor,whereas GPR data acquired with the antenna at 1000 MHzhave been exploited for the analysis of some masonry pillars.Finally for the analysis of the monolithic column, the GPRhigh frequency scans have been compared with ultrasonictests, as will be shown.Also a more refined diagnostic technique has been ex-ploited to process the data. In particular, some data havebeen elaborated by means of a linear inverse scattering al-gorithm based on a 2-D scalar model in frequency domain.A technical exposition of the technique would require somespace and so it is avoided here. However, this algorithm hasbeen exploited for years, and the interested reader can finddetails in (Persico, 2006; Persico, 2007; Persico and Sol-Adv. Geosci., 19, 17–22, 2008 www.adv-geosci.net/19/17/2008/N. Masini et al.: A multisensor approach for the study of monuments 19Fig. 2. (A) Radar scans near the altar, focalised by means of a linear inverse scattering algorithm;(B) Raw data relative to the images ofFig. 2A.dovieri, 2008) and references therein. Here, we just limit tosay that this technique allows to get a reliable focalisation ofthe targets together with a stable result theoretically account-ing for the losses in the soil. The most meaningful resulthave been obtained near the altar (just up the upper limit ofthe image on the left hand side), and are shown in Fig. 2A.In order to deliver the insight specifically provided by theprocessing, let us specify that the zone near the altar relativeto the scans of Fig. 2A was filled with a metallic scaffold-ing for the restoration. We think that the mutual distancesand the periodicity of the “point-like” scattering targets putinto evidence by Fig. 2A are due to the metallic scaffolding,or at least the scaffolding affects meaningfully the results.For the inversion of Fig. 2A, a frequency step of 20 MHz hasbeen exploited and the data have been decimated so to ex-ploit a spatial step of 4 cm. The relative permittivity of thesoil has been evaluated equal to 5, based on the availablea-priori information, and a small conductivity of 0.005 S/mhas been assumed. The exploited band for the inversion was200–600 MHz, based on the spectra of the data. Let us out-line that the raw data (shown in Fig. 2B don’t allow to cluesuch a feature, that is therefore made much more clear by thefocussing procedure.The most interesting results achieved from the subsoil in-vestigation are well resumed by the slice in Fig. 3. The hori-zontal slice has been obtained from B scans with the antennasat 400 MHz. The scans were parallel to the central nave withregards to the lower part of the figure, but they were orthogo-nal to the central nave with regard to the upper part of Fig. 3.The scans have been taken in continuous mode and at thedistance between any scan line and the next one was 50 cm.The time-depth conversion have been performed accountingfor a relative permittivity of the soil equal to 5, retrieved asa-priori information from the fact that we have been said thatthat the soil underlying the church was essentially constitutedFig. 3. Horizontal GPR slice at about the depth of 0.8 m. The letters“a” and “b” denote the presence of anomalies which could be relatedto tombs., whereas the letter “c” indicates anomalies which could beconnected to buried masonry structures.by calcarenite (there was no possibility to perform excava-tion tests). The features put into evidence from Fig. 3 areconfirmed also by several further horizontal slices not shownhere for sake of brevity. Near the second column on the righthand side of the central nave, at the depth of 60 up to 120 cma strong anomaly is evident (see “a” in Fig. 3). It may cor-respond to a void related to a tomb. This anomaly is partic-ularly important, because the basis of the column near it isfractured, and this might be connected with the subsidencecorresponding to the anomaly. Another anomaly on the leftnave (“b”, Fig. 3) may be another tomb.Anomalies of possible archaeological interest are betweenthe first and the second span, from the altar, at the depth of80 up to 150 cm. In particular, the slice (“c” Fig. 3) putsinto evidence anomalies oriented according a “π” shape. Ifwww.adv-geosci.net/19/17/2008/ Adv. Geosci., 19, 17–22, 200820 N. Masini et al.: A multisensor approach for the study of monumentsFig. 4. Right-hand pillar: the radargrams in X and Y direction and their interpretation.it were related to masonry structures it would suggest and ssquare apse. The existence of a previous church, known bythe documents, makes this anomaly worth to be consideredfor future archaeological trials.Let us now pass to the GPR scans performed on buildingelements (pillars and columns). As said, these have been ob-tained with the 1000 MHz antenna, whose band (evaluatedfrom the data) was about extended from 800 to 1600 MHz.Figure 4, show the radargrams acquired along the X and Ydirections on the right pillar of the triumphal arch (the scalesare in metres). Under the radargrams, we have find usefulto show also the arithmetic average of all the radar traces.The interpretation has been made easier by the informationderived by some core test performed in 1989. This allows toknow that the inner structure is like that shown in Fig. 5.The scans 1–49, in X direction, put in evidence six verticalreflections (B1, B2, B3, C1, C2, C3 in Fig. 4): some relatedto interfaces between different materials, other interpretableas material discontinuities (e.g. ashlars joints or cracks).In particular, both C3 and C2 could be interpreted as lackof connection between ashlars respectively of semicolumnand pilaster, and pilaster and square pillar. Whereas C1 andB2 may be related to interfaces between calcarenite blocksand incoherent soil, and B3 suggests the presence of an ash-lars joint. Finally it is worth to mention a single and clearhyperbola near the left semicolumn. It may be caused by asAdv. Geosci., 19, 17–22, 2008 www.adv-geosci.net/19/17/2008/N. Masini et al.: A multisensor approach for the study of monuments 21Fig. 5. Right-hand pillar: inner structure of the pillar.Fig. 6. Radar scan on the monolithic column. Right-hand panel: raw data (the time is in seconds). Left-hand panel: the data after interfacecancellation and time-depth conversion (the depth is in meters).shapeless stone material (rubble) as a fracture. This would bealso coherent with some cracks clearly visible on the surfaceof the semicolumn.The scans 50–89, in Y direction, show an area character-ized by some hyperbolas (A) and six vertical reflections (B1,B2, B3, C1, C2, C3). The hyperbolas are of difficult inter-pretation. They may be related as to shapeless materials asmetallic bars with strengthening function inserted during therestoration occurred in 1989.As regards the reflections, it is reasonable to attribute themeither to interfaces between different materials (B1), or tojoints between calcarenite ashlars (see B2 and B3 in Figs. 4and 5).The GPR prospecting carried out on the left-hand pillarallowed (as in the case of the right-hand pillar) to character-ize its inner structure and to detect joints and cracks, as well.Further details are not shown here for sake of brevity.Finally, the scans performed on the monolithic column(the third on the right side of the central nave), put in evi-dence at heights between 50 and 100 cm from the roof, sig-nificant reflections, which may be connected as to fracturesas to lithological interfaces (Fig. 6). Let us remind that thebase of the column is affected by cracks.Therefore, in case of fracture we would be in presence ofa serious structural instability. In order to investigate thisfundamental question, ultrasonic tests have been performedat the heights of 65, 100 and 145 cm. The frequency usedwww.adv-geosci.net/19/17/2008/ Adv. Geosci., 19, 17–22, 200822 N. Masini et al.: A multisensor approach for the study of monumentsFig. 7. Scheme of the measurement points for the ultrasonic tomog-raphy of a section of the column.for the longitudinal velocity measurements was 55 KHz, intransmission mode. For each velocity measure, oppositepoints on the column have been taken, according the schemeshown in the Fig. 7. By means of an home made software,the measured velocities have been processed in order to ob-tain tomographies (Fig. 8).As shown in Fig. 8, the tomographies do not put in evi-dence either significant variations in velocity or alignmentsof points with low value velocity which might be relatedto cracks. Therefore, it is more reasonable to interpret theanomalies observed in the radargrams as lithological inter-faces.4 ConclusionsA multifrequency and multisensor approach proved to be re-liable for the inspection of the subsoil and load bearing struc-tures of a monument, in order to support intervention strate-gies of rehabilitation and to provide useful information forfuture archaeological investigations.In particular, GPR prospecting with medium frequency an-tenna allowed to detect buries structures likely to be of ar-chaeological interest.Moreover, the identification of a void near a column af-fected by cracks on the base make necessary to proceed inthe investigations, in order evaluate its role in the instabilityFig. 8. Ultrasonic tomography performed on a section of the columnat height of 65 cm.phenomenon. With regard to this point, let us remind thatthe integration of ultrasonic tests and GPR prospecting withhigh frequency antenna seem to exclude a structural yield-ing. This leans towards a foundation yielding as cause of thecracks on the base of the column.Finally, the high frequency GPR scans provided with aresolution sufficient to characterize building elements and todetect discontinuities and cracks.Edited by: F. SoldovieriReviewed by: two anonymous refereesReferencesBinda, L., Saisia, A., Tiraboschi, C., Valle, S., Colla, C., and Forde,M.: Application of sonic and radar tests on the piers and wallsof the Cathedral of Noto, Constr. Build Mater, 17(8), 613–627,2003.Masini, N., Nuzzo, L., Rizzo, E.: GPR investigations for thestudy and the restoration of the Rose Window of Troia Cathe-dral (Southern Italy), Near Surf. Geophys., 287–300, 2007.Persico, R.: On the role of measurement Configuration in Contact-less GPR data Processing by Means of Linear Inverse Scattering,IEEE Trans, On Antennas and Prop AP, 54(7), 2062–2071, July2006.Persico, R.: “Paralipomena” on uniqueness in inverse scatteringfrom a finite number of data, Ann. Geophys., 50, 165–175, 2007,http://www.ann-geophys.net/50/165/2007/.Persico, R. and Soldovieri, F.: Effects of the Background Removalin Linear Inverse Scattering, IEEE Trans. Geosci. Remote Sens.,46(4), 1104–1114, 2008.Adv. Geosci., 19, 17–22, 2008 www.adv-geosci.net/19/17/2008/

A multifrequency and multisensor approach for the study and the restoration of monuments: the case of the Cathedral of Matera

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A multifrequency and multisensor approach for the study and the restoration of monuments: the case of the Cathedral of Matera

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