Case report

Ground penetrating radar investigations for the restoration of historicbuildings: the case study of the Collemaggio Basilica (L’Aquila, Italy)

Danilo Ranalli, Marco Scozzafava, Marco Tallini *

Dipartimento di Ingegneria delle Strutture, delle Acque e del Terreno, Università dell’Aquila, Monteluco di Roio, 67040 L’Aquila, Italy

Received 11 February 2003; accepted 1 May 2003

Abstract

Ground penetrating radar (GPR) surveys were applied in the preliminary stage of a project of structural monitoring and restoration of thefacade of the Collemaggio Basilica, a medieval church located in L’Aquila (central Italy). GPR surveys were very useful in evaluating the stateof conservation of the facade and in identifying the thickness of its walls, the forms and deterioration of its masonry with its ashlar facing andrubble core, and the forms and locations of its middle cornice supports. GPR was demonstrated to be an ideal non-destructive method toinvestigate ancient structures of high cultural and historical value.© 2004 Elsevier SAS. All rights reserved.

Keywords: Ground penetrating radar; Non-destructive method; Italy; Historical building; Medieval masonry technique

1. Research aims

Non-destructive survey methods are being increasinglyused in different applications, especially in the conservationof cultural and historical heritage. The main feature of thesemethods is their capability of investigating a site or a struc-ture non-invasively, i.e., without digging, boring or alteringits original composition or shape.

Detailed knowledge of the internal masonry structure ofhistorical monuments is key to their restoration. In general,such structure is composed of different types of stones,bricks, with wooden or iron elements inserted into walls andcavities as ties, etc. [1]. Wall thickness and type of founda-tions are also useful information for the planning of struc-tural conservation efforts. Moreover, the recognition of de-tachments and cracks is crucial to verify the stability ofbuildings.

As is obvious, preference should be given to non-destructive techniques while destructive ones (boring anddigging) should be minimised, especially when the buildingsinvolved are highly deteriorated or ancient.

Among non-destructive techniques, GPR is the one whichprovides the most interesting results. The complete architec-tural framework of a building, obtained with GPR or other

non-destructive approaches, makes it possible to plan activi-ties of structural monitoring, conservation, restoration andstabilisation.

2. Experimental

2.1. Introduction

Non-destructive GPR surveys were conducted as part of aproject of conservation of the Collemaggio Basilica, an im-portant medieval church located in L’Aquila. The surveyswere expected to make available a large set of data on thefacade of the basilica, with a view to planning the restorationof its masonry and mitigating its vulnerability to seismicevents. The project required the collection of data on wallthickness, internal masonry structure and location of detach-ments or cracks.

2.2. Historical and architectural outlineof the Collemaggio Basilica

The Collemaggio Basilica (Fig. 1), located in L’Aquilaand built in local and composite Gothic style [2], is the mostfamous medieval church of Abruzzi (central Italy). Its con-struction began in 1287 under Peter of Morrone, the futurePope Celestino V, who was crowned there in 1294.

* Corresponding author.E-mail address: tallini@ing.univaq.it (M. Tallini).

Journal of Cultural Heritage 5 (2004) 91–99

www.elsevier.com/locate/culher

© 2004 Elsevier SAS. All rights reserved.doi:10.1016/j.culher.2003.05.001

During the 14th and 15th centuries, the Collemaggio Ba-silica underwent several changes, both for improving itsappearance and for repairing the damage caused by earth-quakes (the main ones occurred in 1315, 1349, 1461,1703 and 1915).

The wide facade of the basilica is characterised by amagnificent geometrical arrangement of coursed ashlars ofwhite and pink local limestone and marly limestone (Fig. 1).The facade wall consists of inner and outer faces of stonewith coarse-grained rubble core, as confirmed by historicalphotos of the facade damaged by the 1915 earthquake. Themain portal in Gothic style, which was built in the 15thcentury, is decorated with niches, which previously accom-modated statues.

The wonderful Gothic rose window of the basilica con-sists of a double row of small spiral-shaped columns and oflight-weight trefoil arches, while its cornice is decorated withflowers and leaves.

2.3. The GPR method

GPR is a non-destructive method that uses electromag-netic waves to investigate the underground or the internalstructures of natural or human-made objects [3], especiallymetal objects and structures, as well as caves and voids. Its

wave frequency ranges from 10 to 2000 MHz. The GPRsystem consists of a data acquisition unit and of two (trans-mitter and receiver) antennas. The transmitter sends an elec-tromagnetic wave pulse which is reflected back to the re-ceiver by an anomaly, if there is a difference in the dielectricconstant between the anomaly and the surrounding environ-ment. The maximum depth of wave propagation depends onthe wave frequency and on the dielectric constant of theinvestigated medium. The higher the frequency, the smallerthe depth and the better the spatial resolution of the signal.The lower the wave frequency, the higher the depth and thesmaller the spatial resolution. For example, a 100 MHzantenna can investigate a medium down to a depth of about15–20 m with a resolution of about 10–20 cm; a 600 MHzantenna (used in this study) can reach about 4–5 m with aresolution of about 2–5 cm, while a 1600 MHz antenna (theother one used in this study) can reach about 1 m with aresolution of about 1 cm.

The output of the GPR survey is a radar section of theinvestigated medium, where a point-shaped anomaly is out-lined by a hyperbolic trace. The X axis corresponds to thedirection of scanning, the Y axis is the depth. The datacoming from the reflected echoes, saved in the data acquisi-tion unit, are processed by filtering algorithms. The interpre-tation of the radar section permits the recognition of radar

Fig. 1. Collemaggio facade during GPR data acquisition: note its magnificent light red and white ashlar facing (local limestone and marly limestone).

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anomalies and their spatial location. These anomalies mayreflect actual discontinuities, objects or voids.

Generally, the sectors in which GPR is successfully ap-plied are Civil Engineering [4,5] and Engineering Geology[6,7], Geology [8] and Geoarchaeology [9–11]. In the pastyears, GPR has also been applied to the characterisation ofbuildings of high cultural and historical value for conserva-tion and restoration projects [12].

2.4. Experimental data

The goal of this study was to obtain the largest possibleamount of data on the masonry of the basilica facade.

The 600 MHz antenna was used to identify wall thickness,while the 1600 MHz one was used to detect the internalfeatures of its masonry and the possible detachment of itsashlar facing from the rubble core. Data were collectedthrough a grid of radar sections of the facade for both anten-nas. Scan spacing was equal to about 40 cm. The mesh andlocation of the grid of radar sections were chosen on the basisof the geometrical arrangement of the white and pink ashlars(Figs. 1 and 2). Radar sections were located along the verticalor horizontal (course) alignment of the ashlars with a mesh ofthree (rarely four) courses. The radar scanning lines wereplaced in the middle of each course. So, the regular geometri-cal arrangement of the ashlars facilitated the scanning of thefacade without using a complex coordinate system.

The GPR scanned the entire facade (Fig. 2), namely thelower band below its middle cornice, the part around its threeportals and its two lateral rose windows. The survey was alsoextended to two square areas located on the right and leftsides of the central rose window above the middle cornice.

2.4.1. Technical details of GPR data acquisitionFor investigating the facade, use was made of a bistatic

GPR with 600 and 1600 MHz antennas and special softwarefor data collection, processing and interpretation [13]. Thewave velocity was set to 10 cm/ns. The signal acquisitiontime was set to 128 ns for the 600 MHz antenna and to 40 nsfor the 1600 MHz one, corresponding to maximum depths ofabout 6 m (600 MHz) and 2 m (1600 MHz). The collecteddata were processed with the GP0 software [14]. Two filterswere applied to all the radar sections. The first filter (“soilsample”) removed the effect of distortion due to the air–ground interface between the GPR antenna and ground. Asthis phenomenon causes a down-shifting of the radar sec-tions, the filter was necessary to shift the sections upwards.The second filter (“pass band”) removed background noise inthe vertical and horizontal directions.

3. Results

3.1. GPR surveys with medium-frequency antenna(600 MHz)

The thickness of the facade wall was measured with600 MHz radar sections. Total thickness was well recognisedin almost all the facade. The radar signals proved to be lessclear in zones where the inner face of the wall has architec-tural or decorative elements (e.g., columns and arches alongthe nave and the two side aisles) (Fig. 3).

The right side of the facade wall proved to be about 20 cmthinner than the left side, except in its lower band, which wasabout 10 cm thinner (Fig. 4).

This finding was justified by the presence of the tower(Figs. 1 and 4) on the right side of the facade, making it less

Fig. 2. Map of GPR survey grid (1600 and 600 MHz radar sections).

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susceptible to earthquakes. The thicker left side of the facadewall demonstrated that the tower was surely not built later.

3.2. GPR surveys with high-frequency antenna(1600 MHz)

The 1600 MHz GPR survey identified the structural fea-tures of the facade masonry and the detachments of the ashlarfacing from its rubble core. Also the middle cornice-

supporting structures of the facade were identified. The1600 MHz radar sections also showed that the masonrystructure of the investigated portion of the facade consisted ofinner and outer faces of stone with rubble core.

The interpretation of the 1600 MHz radar sections wasmore difficult because the masonry structure is heteroge-neous. In the radar sections, there were many radar echoeswhich might interfere with and hide anomaly signals (detach-ments, voids, degraded mortar).

Fig. 3. Radar sections (600 MHz) showing wall thickness: the strong horizontal anomaly (arrow) marks the masonry–air interface. (a) Wall thickness: 120 cm;(b) wall thickness: 110 cm; (c) wall thickness: 90 cm.

Fig. 4. 3D image of the basilica facade; thickness values are in cm. The left side and bottom part of the facade wall have the highest values.

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Anyway, the interpretation of the radar sections made itpossible to make the following interesting observations.

Generally, the radar sections showed the thickness of theashlars (20–30 cm) and of the internal masonry structure

(Fig. 5). The scans were carried out in the zones of the facadewhere some ashlars had been previously removed. It was thuspossible to calibrate and interpret the radar sections withdirect observations (Fig. 6).

Fig. 5. One thousand and six hundred mega hertz radar section showing anomalies of the ashlar facing thickness (1), middle cornice-supporting through stones(2), joints between the ashlars covering the middle cornice-supporting through stones (3).

Fig. 6. Zones where ashlars were removed for a check and their location on the facade: (a) thickness of one of the ashlars (about 25 cm); (b) fractures of anoriginal medieval ashlar.

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In some cases, the radar sections showed anomalies insidethe ashlar facing, probably corresponding to cracks (Fig. 7),voids or degraded mortar.

Just below the middle cornice, the radar sections exhibitedevenly shaped anomalies. They were also present in a hori-zontal direction with a constant spacing of about 2 m

(Figs. 5 and 7). These anomalies were supposed to corre-spond to elongated through-stones, placed between theashlar facing and the internal masonry in order to support themiddle cornice. The ashlars covering these supporting stonesare about 15 cm thick and thinner than the others (20–30 cmthick). The voids or degraded mortar around these supporting

Fig. 7. Radar sections (1600 MHz) with clear and evenly spaced anomalies, probably linked to the presence of the middle cornice-supporting through stones: 1,supporting through stone; 2, lateral rose window; 3, main portal; 4, middle cornice.

Fig. 8. (a) Section of the facade masonry: 1, lower facade; 2, upper facade; 3, supporting through stone; 4, rubble core; 5, inner face of the facade wall; 6, middlecornice. (b) Scheme of the facade masonry: 1, ashlar facing ; 2, ashlar; 3, supporting through stones; 4, rubble core; 5, inner face of the facade wall. (c) 3D imagesof the middle cornice (6) and of its supporting through stones (3).

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stones made it easier to recognise their shape and location inthe radar sections. Fig. 8 is a reconstruction of the supposedmiddle cornice-supporting stones and the masonry.

Other 1600 MHz radar anomalies were interpreted asdetachments of the ashlar facing from the rubble core. Thesedetachments are localised or distributed in patches of a fewmeters (above all in zones Ib and Ic–Fig. 2, the first letter

refers to the grid column and the second one to the grid row).An example of the 1600 MHz radar section of this area isreported in Fig. 9: the arrows mark a strong radar anomalylocated at depth of approximately 20 cm. This anomaly wasinterpreted as a zone of detachment of the ashlar facing fromthe rubble core, characterised by voids or degraded mortar,although it might also have been interpreted as a zone of

Fig. 9. One thousand and six hundred mega hertz radar section highlighting strong anomalies, probably related to widespread detachments of the ashlar facingfrom the rubble core or to zones of degraded mortar. Fig. 11 shows the trace of this radar section.

Fig. 10. Diagram of the facade of the basilica showing traces corresponding to shallow or deep radar anomalies (1600 MHz): (1) local anomalies between 10 and30 cm of depth; (2) diffuse anomalies between 10 and 30 cm of depth; (3) local anomalies between 30 and 50 cm of depth; (4) diffuse anomalies between 30 and50 cm of depth; (5) zones for which no GPR data are available; (6) zones for which incomplete GPR data are available.

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hidden cracks in the ashlar facing (Fig. 6b). The interpreta-tion of 1600 MHz radar sections was supported by directobservations of fractured ashlars (Fig. 6b).

A complete analysis of the location of 1600 MHz radaranomalies is reported in Fig. 10. Some of the anomalies,presenting a regular pattern, were interpreted as supportingthrough-stones located just below the middle cornice(Fig. 7); while many others are distributed with an irregularpattern all over the facade and interpreted as detachments,cracks in the ashlar facing or in the interface with the rubblecore. This interpretation was validated by checks that weremade in zones where the ashlars had been removed (Fig. 6).

Among the most interesting zones of the facade, Fig. 11shows a zone with high density of local and diffuse1600 MHz radar anomalies. A diffuse and clear anomaly wasdetected here also by thermographic investigations (D. Pa-oletti, G. Schirripa Spagnolo, personal communication). Thiszone is located between the main portal and the right lateralrose window. In this zone, the clear radar anomalies wereinterpreted as detachments. This hypothesis was corrobo-rated by a number of conservation works which were carried

out in the 19th and 20th centuries on the ashlar facing of thiszone (P. Dalla Nave, personal communication).

4. Conclusions

GPR, a non-destructive method, is particularly effective ininvestigating structures or buildings with high historical–cultural value. The numerous data collected on the internalstructure of the Collemaggio facade masonry may be usefulfor restoration and conservation purposes.

As regards the restoration of the basilica facade, the dete-rioration of the ashlar facing and its detachment from therubble core might lead to local or diffuse zones of ashlarfacing instability. The high-frequency GPR easily detectedthe more degraded zones. Moreover, the medium-frequencyGPR antenna, with a very small error margin, revealed for thefirst time a fairly variable wall thickness. This is a significantfinding to be taken into consideration in seismic modelling ofstructures. Hence, both medium- and high-frequency GPR

Fig. 11. (a) One thousand and six hundred mega hertz sections: zone with clear anomalies, which may reflect detachments of the ashlar facing from the rubblecore. (b) Scheme of the same zone showing traces corresponding to shallow or deep anomalies: (1) local anomalies between 10 and 30 cm of depth; (2) diffuseanomalies between 10 and 30 cm of depth; (3) local anomalies between 30 and 50 cm of depth; (4) diffuse anomalies between 30 and 50 cm of depth. The radarsection A–B is showed in Fig. 9.

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surveys can help shed more light on the Collemaggio Ba-silica facade in view of more effective restoration or conser-vation projects.

Acknowledgements

The authors thank M. Pezzuti (B.A.P.—Abruzzo) for thepermission to publish GPR data on the Collemaggio Basilicafacade, B.M. Colasacco (P.S.A.D.—Abruzzo) for the per-mission to publish the relevant photos (Fig. 6) and G.C.Beolchini (D.I.S.A.T.—L’Aquila University) and P. DallaNave for useful suggestions.

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