Bollettino di Geofisica Teorica ed Applicata Vol. 58, n. 4, pp. 431-444; December 2017

DOI 10.4430/bgta0200

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GPR investigation to map the subsoil of the St. John Lateran Basilica (Rome, Italy)

S. Piro1, i. HayneS2, P. Liverani3 and D. Zamuner1

1 Institute for Technologies Applied to Cultural Heritage ITABC, CNR, Roma, Italy2 School of History, Classics and Archaeology, Newcastle University, UK3 Dipartimento di Storia, Archeologia, Geografia, Arte e Spettacolo SAGAS, Università di Firenze, Italy

(Received: March 21, 2017; accepted: July 5, 2017)

ABSTRACT TheSt. JohnLateranBasilica is thePope’sCathedral and thefirst public buildingconstructedforChristianworship.Thecomplexhasbeenthefocusofsundryexcavationssincethe1730s.Thesehaverevealedtracesoftheearliestphasesofthebuilding,alongwithpartsoftheCastra NovaoftheImperialHorseguard,abathcomplexandpalatialhousing.Interpretationoftheseexcavationsis,however,difficult;andmostofthemareeitherundocumentedoronlypartially recorded.Thegeophysicalprospection isgenerallyconsideredastheattempttolocatestructuresofarchaeologicalinterestburiedinthenaturalsubsoil,butinmanycases,whenappliedinurbancentres,thisattemptcouldfailduetotheeffectanddisturbancescausedbyrecentman-madestructuresinthesubsoil,coveringanysignal related topossiblearchaeologicalstructures. In thepresent paper the groundpenetrating radar (GPR) surveys carried out in the urbanarchaeologicalsiteofSt.JohnLateranBasilica,inRome,characterisedbydifferenttargets and environmental conditions, are presented and discussed. This site ischaracterizedbyartificialmediumasroadpavement,outsidethebasilica,andancientbuildings,belowthecurrentbasilica.ThepaperillustratesthegroundpenetratingradarGPRsurveysandtheobtainedresults.

Key words:St.JohnLateranBasilica,groundpenetratingradar,urbangeophysics,barracksoftheimperialhorseguards.

© 2017 – OGS

1. Introduction

1.1. The St. John Lateran Basilica complexThearchaeologicalareaoftheLateraninRomeliesimmediatelywithintheAurelianWallsnear

thegateoftheVia Tusculana,undertheCathedralofSt.Johnandtheneighbouringbuildings.Itisanareaofgreathistoricalimportance.ThedomusofthefirsttwocenturiesA.D.weresupersededbytheCastra Nova Equitum SingulariumofSeptimiusSeverusandlaterbytheConstantinianBasilicaandtheLateranbishopric.

Anexceptionalbuildinginitsownright,theConstantinianBasilicaofS.GiovanniinLateranoholds the titleofcaput et mater of thechurchesofCatholicChristendom.Thebasilica is thePope’sownchurch,apioneeringstructureandasiteofremarkablearchaeologicalimportance.

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Extensive excavations beneath the complex have revealed not only the remains of the firstpowerfullyinfluentialbasilicaandbaptistery,butalsostructuresfromstillearlierperiods.Chiefamongthesearethebarracksoftheimperialhorseguards,substantialpalatialbuildings,abathhouse, and a streetwith houses.These areas are remarkablywell preserved and a substantialnumberoffrescoesandmosaicsremainin situ.

Extensiveresearchbyprof.PaoloLiverani(FlorenceUniversity)hasenrichedourunderstandingofthecomplex,butamajorcollaborativeprojectisrequiredtointerprettheremainsunearthedbypreviousexcavators.Theearliestrecordedexcavationstookplacein1730,thedeepestlie5.5mbelowgroundlevel.

Thisproject isundertakinganintensivescientificsurveyof theentirestructureto integrateinformation from standing buildings, excavated structures and sub-surface features throughthe collaboration of Newcastle University (UK), FlorenceUniversity (Italy) and Institute forTechnologiesAppliedtoCulturalHeritage(ITABC-CNR,Italy).

1.2. Research designTheLateranprojectaimstoundertakeafullyintegrated3Dsurveyoftheexcavationsunder

SanGiovanniinLaterano.Aparticular concern is tofindanapproach thatwill notonly allowcollaborationbetween

researchersusingarangeofestablishedandinnovativemethods,butalsoamethodthatallowsanintegratedapproachtothethree-levelsofstructuraldatathatformthecomplex(Gaffneyet al.,2008).Therearethestandingfeaturesonthemoderncitysurface,asseeninFig.1.Thesecanonlybefullyunderstoodinrelationtosubsurfacefeaturesandviceversa.

Therearealsotheextensiveandinter-cutstructuresthatformtheopenedareaofexcavations.Finally,therearetheunexcavateddepositsthatlayeitherbeyondtheimmediateconfinesofthesiteorbeneaththeareaopenedtodate.

Theaimofthegroundpenetratingradar(GPR)surveysistoidentifyRomanandhigh-medievalageremainswhichcouldenhanceunderstandingoftheancienttopographyandtheurbanevolutionofthestudyarea.Themaingoalsofthissurveyarethefollowing(Fig.1):

1. to determine the full plan of theSantaCroceOratory, built byPope Ilaro (5th century)anddestroyedbySixtustheFifth;partofthisbuildinghasbeenidentifiedbyOlofBrandtwithintheexcavatedareaadjacenttotheBaptistery;

2. todetermine the fullextentof thepalatialhousing foundbelow thewesternpartof theBasilica;

3. todeterminethelimitsofCastra Nova Equiutum Singularium,thebarracksoftheimperialhorseguardsestablishedbytheemperorSeptimiusSeverus;

4. to locate the remainsof thebuildingsof theLateranPatriarchy.Theseareknown fromrenaissanceplansbutupuntil now it hasnotprovenpossible to locate themall on theground.

Inthepresentpaper,theresultsobtainedtoinvestigatethepoints2,3and4arepresentedanddiscussedthroughthecomparisonwiththehypothesispresentedbythearchaeologists.

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Fig.1-ArchaeologicalmapoftheSt.JohnLateranBasilica(Rome).ThegreencolourindicatetheareasinvestigatedwithGPR;thenumbersarerelatedtotheobjectivesofthesurvey(courtesyofprof.P.Liverani).

2. Archaeological prospection in urban centres

Therearemanyimportantresearchandtechnicalissuesrelatedtotheinvestigationinurbanareatolocatesubsurfacecavitiesand/orarchaeologicalremains,toproducehazardmapping,thatisofthehighestpriority,andarchaeologicalriskmap.Thisisespeciallysoincivilengineering,where it isofkey importance formanagingsafeurbanandcivil construction. Inmanycases,cavities,suchassubsidencefeatures,voidsandcollapsesrepresentdisruptionstothegeometryofanoriginallynear-horizontallayeredsystem.Geophysicaltechniquescanbeusedtoidentifythefeaturegeometriesbycontrastsinthephysicalproperties,butcanbegreatlyimpededbyculturalfeaturesthatinterferewithinstrumentmeasurements(utilities,structures,surficialdebris).

Sitepreparationcanfacilitatethefieldworkandcanimprovetheaccuracyofthegeophysicalsurveysandinterpretations.Themostusefulsitepreparationsareperformanceofatopographicsurveytocreateasitegrid,placementof locationmarkersandremovalofsurficialdebrisandobstructive vegetation. Conditions of the geophysical field project that are subjected to site-specificdesigninclude:selectionofappropriatetechniques,instruments,accessoriesandsettings;performanceofdual-techniquesurveys for redundantsitecoverage;performanceof follow-upsurveysandselectionofinstrumentmeasurementdensity.

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Thecriticalphaseofthegeophysicalsurveyinurbanareaistheinterpretationofthecollecteddataandthecharacterizationofthedegreeofconfidenceintheinterpretations.

Theurbansubsoilconsistsoftenofmanylayersdocumentingthehistoryofaplace,keepingrecordsofalternatingphasesofconstructionanddestruction.Theshallowsubsurfaceofmoderncitiescontains reamsofpipes,cellars,wells,cavities, tunnels,gravesand foundationwallsofformerhouses, churches and town fortifications.Underneath the tarmacof city roads and thepavingstonesoftownsquareslayersofsandandgravelarecriss-crossedwithmodernfibreopticandtelephonecablesandcenturyoldsewerpipesmixedwiththedebrisofbrickbuildings.

Thegeophysicalprospectionofurbancentres,whichhavebeenabandoned in thepastandtodayarelocatedunderbarrenland,posedifferentmethodologicalpossibilitiesandchallengescomparedtosurveysconductedinmoderncitycentres.

While the first sites may successfully be prospected employing aerial photography,magnetometry, GPR, Earth resistance and inductive electromagnetic methods (Campana andPiro,2009),manyofthesemethodsaredifficultorimpossibletouseintheurbancentre.

Themostpromisingnon-destructivegeophysicalprospectionmethodforuseinurbancentresisGPR.GPRmeasurementsarelessaffectedbythepresenceofmetallicstructurescomparedtomagnetometerprospectionandtheyresultinthelargestamountofdataofallcommonlyemployednear-surfacegeophysicalmethods,providingdetailed three-dimensional informationabout thesubsurface (Utsi, 2006;Piscitelliet al., 2007;Trinkset al., 2009;Leucciet al., 2011, 2012a,2012b;Moscatelliet al.,2013;Materaet al.,2016).TheGPRisanelectromagneticimpulsivemethod much suited for shallow depth investigations, as it can supply subsurface profilesgroupedinverticalradarsections.Thetransmitter-receiverantennaispulledalongthesurfaceofasite,signalsaresentwithahighlydirectiveradiationpatternintothegroundandechoesarereturnedfromtargetsinthegroundwithinafewmetres.Theemittedradarsignalisapulseofelectromagneticradiationwithnominalfrequencyvalueintherange15-2500MHz(1MHz=106Hz).Thevelocityofanelectromagneticwaveinairis30cm/ns(1ns=10-9s).Insoilsthevelocityisless,astypicalvaluesareintherange5-15cm/ns(GoodmanandPiro,2013).

Whilegeophysical prospection is generally considered as the attempt to locate structuresofarchaeologicalinterest,inmanycases,whenappliedinurbancentres,thisattemptcouldfailduetotheeffectanddisturbancescausedbyrecentman-madestructuresinthesubsoil,coveringanysignalrelatedtostructuresofarchaeologicalinterest.Modernundergroundstructures(asbarsandslabsofreinforcedconcrete,metallicpipes,cablesandassociatedtrenchesandbuildingdebris)occurringatshallowdepthoftendisplayastrongercontrastinphysicalpropertiesrelativetothesurroundingsubsoilthanlesswellexpressedarchaeologicalstructures,whichoftenareburiedatgreaterdepth.

ChallengesforGPRprospectionincitycentreslieinthelargenumberofobstaclespresentin the urban environments.Traffic islands,metallic gully covers, lamp posts, buildings, treesandparkedvehiclescauseirregularsurveygeometries,holesinthesurveyedareaanddisturbinganomaliesintheGPRmeasurements.

3. Data acquisition

Forthemeasurements,outsideandinsidetheSt.JohnLateranBasilica,aGPRSIR3000(GSSI),equippedwitha400MHz(GSSI)bistaticantennawithconstantoffsetanda70MHz(Subecho

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Radar)monostaticantennawereemployed.Somesignalprocessingandrepresentationtechniqueshavebeenusedfordataelaborationandinterpretation.GPRsurveyswereperformed,employingtheSIR3000(GSSI)tosurveytheselectedareasoutsidetheBasilicainSt.JohnLateransquareandinsidetheBasilica(Fig.2).

Thehorizontalspacingbetweenparallelprofilesat thesitewas0.50m,employingthetwoantennas.Radarreflectionsalongthetranseptswererecordedcontinuously,withdifferentlength,acrossthegroundat40scan/s;horizontalstackingwassetto3scans.

IntheareaoutsidetheBasilica,atotalof777adjacentprofilesacrossthesitewerecollectedalternativelyinforwardandreversedirectionsemployingtheGSSIcartsystemequippedwithodometer.Allradarreflectionswithinthe90ns,for400MHzantenna,and195ns,for70MHzantenna,[two-way-travel(twt)]timewindowwererecordeddigitallyinthefieldas16bitdataand512samplesperradarscan.

IntheareainsidetheBasilicaatotalof192adjacentprofilesacrossthefournaves,thetransept,and theentrancewerecollectedalternatively in forwardand reversedirectionsemploying theGSSIcartsystemequippedwithodometer.Allradarreflectionswithinthe120nsfor400MHzantenna(twt)timewindowwererecordeddigitallyinthefieldas16bitdataand512samplesperradarscan.

Velocities of 0.08m/ns and 0.10m/ns, respectively outside and inside the Basilica, wereestimatedusinghyperbolaefittinginGPR-SLICEv7.0imagingsoftware(Goodman,2016).

4. Data processing and presentation

Reflectionprofileswereanalyzedforpreliminaryidentificationoftheburiedfeaturesandforcalibrationof the instrument.Reflectiondata, collected inprofileswith0.50m spacing,wereprocessedusingstandardtechniques(Neubaueret al.,2002;Leckebusch,2003,2008;Conyers,

Fig.2-St.JohnLateransquare,Rome.LocationoftheareainvestigatedwithGPRsystems.

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2004;Goodman et al., 2004, 2009;Linford, 2004;Goodman andPiro, 2008, 2013; Piro andGoodman,2008;Leucciet al.,2011;PiroandCampana,2012).

Thebasicradargramsignalprocessingstepsincluded:i)postprocessingpulseregaining;ii)DCdriftremoval;iii)traceresamplingalongtheprofile(anaveragedtraceevery4cm);iv)bandpassfiltering;v)migration;andvi)backgroundfilter.

Reflection amplitudemapswere constructedwithin various time (and corrected to depth)windows to show the size, shape, location and depth of subsurface archaeological structures(Neubaueret al.,2002;Conyers,2004;Gaffneyet al.,2004;Linford,2004;GoodmanandPiro,2008,2013;Leckebusch,2008;PiroandGoodman,2008;Goodmanet al.,2009).Theseimageswere created using the spatial averaged squared wave amplitudes of radar reflections in thehorizontalaswellasintheverticaldirection.Thesquaredamplitudeswereaveragedhorizontallyevery0.25malong the reflectionprofiles3ns (for400MHzantenna)and6ns (for70MHzantenna)timewindows(witha10%overlappingofeachslice).Theresampledamplitudesweregriddedusingtheinversedistancealgorithmwithasearchradiusof0.75m.

Asexamples,theunmigratedreflectionprofiles,collectedinthesectorA1oftheinvestigatedarea(Fig.2),showingreflectionsfrombodiesinthegroundatabout10-40ns(twt),for400MHzand20-60ns(twt)for70MHzantenna,arepresentedinFigs.3and4.

All theGPRdatawereprocessed inGPR-SLICEv7.0GroundPenetratingRadar ImagingSoftware(Goodman,2016).

Fig.3-ExamplesofGPRprofilescollectedwith400MHzantenna:a)andc)fieldprofiles,b)andd)unmigratedfilteredprofiles.

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Fig.4-ExamplesofGPRprofilescollectedwith70MHzantenna:e)andg)fieldprofiles,f)andh)unmigratedfilteredprofiles.

5. Results and interpretation of the GPR data

5.1. St. John Lateran square - area A1 and B1 (antenna 400 MHz)TheGPRamplitudemaps,relatedtotheprofilescollectedwith400MHzantennahavebeen

analysedandourattentionhasbeenfocusedtothefollowingtime-windows:19-22,25-28,30-34,41-45,47-50and58-61ns(twt),correspondingtotheaveragedestimateddepthsof0.88,1.10,1.30,1.70,2.00and2.40mrespectively,withoutconsideringthefirstportionofthesubsoilwhichischaracterisedbythepresenceofmanysurfaceutilities.

FewGPRtime-slices,characterisedbyinterestinganomalies,arepresentedanddescribedinthefollowingfigures,firstlyfortheSt.JohnLateransquareandsecondlyfortheareainfrontoftheBasilica.

Fig. 5 shows the anomalies located at the estimated depth of 0.88 m [19-22 ns, (twt)],individuated in the areaA1 and B1.At this depth, the area is characterized bymany strongreflectionsduetothepresenceofutilities(6-7)andofportionofpossiblestructures(1-2-3-4-5).

Thesizeoftheanomalies,indicatedbelow,areapproximate:(1)stronganomalywithlinearorientationanddimensionx:13.0m,y:3.5m;(2)anomalywithsemi-circularorientationwithdiameter:9.0mandsize2.5m;(3)anomalywithaveragedimensionofx:2.0m,y:7.0m;(4)twoparallelanomalieswithaveragedimensionx:2.0m,y:7.0m;(5)anomalieswithdifferentsizeanddimension13.0×17.0m;(6)and(7)anomaliesduetoutilitieswithdifferentsize.

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Fig. 6 shows the anomalies located at the estimated depth of 1.75 m [41-45 ns, (twt)],individuatedintheareaA1andB1.Atthisdepththeanomaliesareconfirmedintermoflocationandsize,butwithreducedintensity.Thesizeoftheanomalies,indicatedbelow,areapproximate:(1)thisanomalyisstillpresentwithdimensionofx:19.50m,y:3.00m;(2)thisanomalyisstillpresentwith an averagedimensionof230m2; (5) the corresponding anomaly is presentwithdimensionof125m2;(8)twoparallelanomalies,withthesamedimensionx:1.70m,y:3.00m;(9)twonewanomalieswithanaveragesurfaceof30m2.

Fig.5-St.JohnLateransquare.AreaA1andB1,GPR400MHz,slicesattheestimateddepthof0.88m.

Fig.6-St.JohnLateransquare.AreaA1andB1GPR400MHz,slicesattheestimateddepthof1.75m.

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Fig. 7 shows the anomalies located at the estimated depth of 1.35 m [30-34 ns, (twt)],individuatedintheareaA2,B2andC2.Atthisdepth,theareaischaracterizedbymanystrongreflectionsduetothepresenceofutilities(1)andofportionofstructures(2-3-4-5-6-7).

Thesizeoftheanomalies,indicatedbelow,areapproximate:(1)theseanomaliesareduetothepresenceofutilities (pipesandgully-holes); (2)and(3)mustbeconsidered together, theyarecharacterisedbystructureslocatedperpendiculareachotherinsideatotalsurfaceof960m2.Incorrespondenceofanomaly(3)asequenceofsquaredanomaliesatadistanceof1.00meachotherarevisible;(4)linearanomalywithlowintensityx:40.40m,y:2.00m;(5)anomalywithasquaredshapewithdimension3.70x4.50m;(6)anomalywithaveragedimension2.30x4.20m;(7)thiszoneischaracterisedbysmallanomalies.

Fig.7-St.JohnLateransquare.AreaA2,B2andC2,GPR400MHz,slicesattheestimateddepthof1.35m.

TheGPRimageischaracterizedbythepresenceofstructureswithdifferentdimensionandgeometricalshapes.Atthisestimateddepth,squaredblocks,amongthemcontiguous,arrangedintheLshapewithanangleof90°andwithanaveragesurfaceof130m2,arevisible.Betweenthese,thesouthernstructurewithrectangularshapeisofparticularshape.Thisischaracterizedalongtheleftsidebyasequenceofsmallsquaredroomswithaveragedimensionof1.2×1.2m.Infrontoftherectangularstructure,twosemi-circularanomalies,withangularopeningof180°anddiameterof10marevisible.

Fig. 8 shows the anomalies located at the estimated depth of 2.00 m [47-50 ns, (twt)],individuatedintheareaA2,B2andC2.Atthisdepth,theareaischaracterizedbymanystrongreflectionsduetothepresenceofutilities(1)andofportionofstructures(2-5-8-9).

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Thesizeoftheanomalies,indicatedbelow,areapproximate:(1)theseanomaliesarerelatedonly to gully-holes; (2) the traces of the corresponding anomalies are still present; (3) threeanomalieswith perpendicular orientation,with an averagedimensionof x: 16.0m, y: 1.7m;therearealso twoanomalieswithdimensionx:2.0m,y:22.7m; (5) and (6) theseareasarecharacterisedbysmallreflections;(7)thiszoneischaracterisedbydiffusedanomalieswhichcanbeduetothepresenceofcollapseofstructuresandchangingoftopographicallayerintheground.Thelightgreyareaischaracterisedbyattenuatedreflectedsignalscorrespondingtoaportionofsubsoilwithhighattenuation.

5.2. St. John Lateran square - area A1 and B1 (antenna 70 MHz)TheGPRamplitudemaps,relatedtotheprofilescollectedwith70MHzantennahavebeen

analysedandourattentionhasbeenfocusedtothefollowingtime-windows:35-42,47-53,59-65,71-77,89-95and112-118ns(twt),correspondingtotheaveragedestimateddepthsof1.7,2.1,2.6,3.0,3.8,and4.7m,respectively.

Fig.9showstheanomalieslocatedattheestimateddepthof6.00m.Atthisdepth,theareaischaracterizedbyanomaliescontainedintwosectors.

The size of the anomalies, indicated below, are approximate: (1) is characterised (portionvisible)bydimensionx:1.4m,y:13.0mandx:1.4m,y:10.0m;(2)ischaracterisedbydimension(portionvisible)x:17.0m,y:3.5m.

Fig.8-St.JohnLateransquare.AreaA2,B2andC2,GPR400MHz,slicesattheestimateddepthof2.00m.

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5.3 Results of the survey inside the BasilicaThesurfaceinsidetheBasilicahasbeensubdividedin8differentsectorsasindicatedinFig.

10.TheGPRsurveyshavebeenmadeduringJanuary2016,employingthesameGPRsystemequippedwith400MHzantenna.

Timeslicedatasetsweregeneratedbyspatiallyaveragingthesquaredwaveamplitudesofradarreflectionsinthehorizontalaswellastheverticaldirection.Thesquaredamplitudeswereaveragedhorizontallyevery0.25malongthereflectionprofiles4ns(for400MHzantenna)timewindows(witha5%overlappingofeachslice).Theresampledamplitudesweregriddedusingtheinversedistancealgorithmwithasearchradiusof0.75m.

TheGPRamplitudemaps,relatedtotheprofilescollectedwith400MHzantennahavebeenanalysedandourattentionhasbeenfocusedtothefollowingtime-windows:3-7,10-14,17-21,24-28, 38-42, 52-56, 59-63, 66-70, 73-77, 80-84, 87-91 and94-97ns (twt), corresponding totheaveragedestimateddepthsof0.4,0.7,1.0,1.4,2.0,2.5,3.0,3.5,4.0,4.5,5.0,and5.5m,respectively.

Fig. 10 shows the anomalies located at the estimated depth of 2.00 m [38-42 ns, (twt)],individuatedintheareainsidetheBasilica.Atthisdepth,theareaischaracterizedbymanystrongreflectionsduetothepresenceofportionofdifferentstructures.

The size of the anomalies, indicated below, are approximate: (a1) linear anomalies, withdimensions 1 x 4m, related to the Borromini’s works; (a2) this area has been characterisedbypreviousexcavationsand the individuatedanomaliesaredue to themodernsupportof the

Fig.9-St.JohnLateransquare.AreaA1andB1,GPR70MHz,slicesattheestimateddepthof6.00m.

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pavementandtothestructuresstillpresentintheground;(a3)twoanomalieswithanaveragedimensionof x: 4m, y: 1.6m; (a4) isolated anomalywith dimensionof 4.3×3.5m and (a5)anomalywithdimensionof4.7x3.4m.

Fig. 11 shows the anomalies located at the estimated depth of 4.20 m [80-84 ns, (twt)],individuated in thearea inside theBasilica.At thisdepth, theareaarecharacterizedbymanystrongreflectionsduetothepresenceofportionofdifferentstructures.

Thesizeoftheanomalies,indicatedbelow,areapproximate:(a2)thisareashowsfewlinearanomaliesrelatedtothestructuresstillpresentintheground;(a3)twoanomalies,atthisdepthwithperpendicularorientationeachotherandwithanaveragedimensionofx:4.3m,y:1.6mand1.7×4.7m;(a5)togetherwiththeindividuatedanomalyitispresentasecondanomalywithdimension1.6×10.8mand(a6)anomalywithdimensionof1.6×3.0m.

6. Conclusion

GPRsurveysattheLateranhaveproducedsignificantandfruitfulresults.Theuseof400and70MHzantennahaveenabledtoreachdepthsofupto3.4and6.7m,respectively,intheexternalareaofBasilica.

Fig.10-St.JohnLateranBasilica.GPR400MHz,slicesattheestimateddepthof2.00m.

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Fig.11-St.JohnLateranBasilica.GPR400MHz,slicesattheestimateddepthof4.20m.

Thelocation,depth,shapeandsizeofburiedbuildingsweredeterminedusingGPR,whichcanbethenintegratedwiththeresultsobtainedwiththeotheremployedmethods.

TakingintoaccountthedifferentaimsoftheinvestigationsandtheobtainedresultsitispossibletoobservethatintheareaA1theanomalies,locatedinthedepthinterval0.80-2.00mwiththe400MHzantenna(Figs.4to7),canbeduetostructuresrelatedtothemedievalage;whiletheanomalylocatedwith70MHzantenna(Fig.9)presentsagoodcorrespondencewiththeexpecteddirectionoftheexternalwalloftheCastra.

IntheeasternpartofthebasilicaareaA2,thelocatedanomalies,characterisedbymanyregularandinterestingshapes,canbeduetotheremainsofthebarracksoftheimperialhorseguards.

The results obtained inside the basilica have helped the archaeologists to recognize thereinforcementsmadewithBorromini’sworks(Fig.10)andfewindicationthroughanomalya3(Fig.10)oftheremainsoftheexternalwallofthefirstCostantineBasilica.

Thepresentprojectisstillinprogressandnewsurveyshavebeenplannedbelowtheactuallevelandintheeasternpartofthebasilica.

Acknowledgements. The authors are very grateful to Daniele Verrecchia for his fruitful collaborationduringthesurvey,theteamofNewcastleUniversityforthetopographicsupportandAuthoritiesofSt.JohnLateranBasilicafortheirhelpduringthesurvey.ThepresentworkhasbeenpartofthelastConferenceofGruppoNazionalediGeofisicadellaTerraSolida,Lecce2016.

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REFERENCES

CampanaS.andPiroS.;2009:Seeing the unseen. Geophysics and landscape archaeology.CRCPress,London,UK,376pp.,doi:10.1002/arp.365.

ConyersL.B.;2004:Ground-penetrating radar for archaeology.AltamiraPress,WalnutCreek,CA,USA,203pp.,doi:10.1002/arp.288.

GaffneyV.,PattersonH.,PiroS.,GoodmanD.andNishimuraY.;2004:Multimethodological approach to study and characterise Forum Novum (Vescovio, Central Italy).ArchaeologicalProspection,11,201-212.

GaffneyV.,PiroS.,HaynesI.,WattersM.,WilkesS.,LobbM.andZamunerD.;2008:Three-Tier visualization of San Giovanni in Laterano, Rome, Italy.In:ExtandedAbstract,12thInt.Conf.Groundpenetratingradar,Birmingham,UK,<www.eurogpr.org>.

GoodmanD.;2016:GPR-Slice 7.0, Manual.<www.gpr-survey.com,January/2016>GoodmanD.andPiroS.;2008:Ground penetrating radar (GPR) surveys at Aiali (Grosseto).In:Seeingtheunseen.

Geophysicsandlandscapearchaeology,CampanaandPiro(eds),CRCPress,Taylor&FrancisGroup,Oxon,UK,pp.297-302.

GoodmanD.andPiroS.;2013:GPR Remote Sensing in Archaeology (Geotechnologies and the environment series).Springer(ed),Berlin,Germany,233pp.,doi:10.1007/978-3-642-31857-3.

GoodmanD.,PiroS.,NishimuraY.,PattersonH.andGaffneyV.;2004:Discovery of a 1st century Roman amphitheater and town by GPR.J.Environ.Eng.Geophys.,9,35-41.

GoodmanD.,PiroS.,NishimuraY.,SchneiderK.,HongoH.,HigashiN.,SteinbergJ.andDamiataB.;2009:GPR Archeometry.In:Groundpenetratingradar:theoryandapplications, H.M.Jol,ElsevierSci.(ed),pp.479-508.

Leckebusch J.; 2003: Ground-penetrating radar a modern three-dimensional prospection method. Archaeolog.Prospect.,10,213-240.

LeckebuschJ.;2008:Semi-automatic feature extraction from GPR data for archaeology.NearSurf.Geophys.,6,75-84.

LeucciG.,MasiniN.,PersicoR.andSoldovieriF.;2011:GPR and sonic tomography for structural restoration: the case of the cathedral of Tricarico.J.Geophys.Eng.,8,S76-S92,doi:10.1088/1742-2132/8/3/S08.

LeucciG.,D’AgostinoD.andCataldoR.;2012a:3D High resolution GPR survey yields insigths into the history of the ancient town of Lecce (south of Italy).Archaeolog.Prospect.,19,157-165,doi:10.1002/arp.1423.

LeucciG.,MasiniN.,PersicoR.,QuartaG.andDolceC.;2012b:A multidisciplinary analysis of the crypt of the Holy Spirit in Monopoli (southern Italy).NearSurf.Geophys.,10,1-8,doi:10.3997/1873-0604.2011032.

LinfordN.;2004:From Hypocaust to Hyperbola: Ground-penetrating Radar surveys over mainly Roman Remains in the UK. Archaeolog.Prospect.,11,237-246.

MateraL.,PersicoR.,GeraldiE.,SileoM.andPiroS.;2016:GPR and IRT tests in two historical buildings in Gravina in Puglia. Geoscientific instrumentation methods and data systems.Geosci.Instrum. Methods DataSystems,5,1-10,doi:10.5194/gi-5-1-2016.

MoscatelliM.,PiscitelliS.,PiroS.,StiglianoF.,GiocoliA.,ZamunerD.andMarconiF.;2013:Integrated geological and geophysical investigations to characterize the antropic layer of the Palatine hill and Roman Forum (Rome, Italy).Bull.EarthquakeEng.,12,1319-1338,doi:10.1007/s10518-013-9460-5.

NeubauerW.,Eder-HinterleitnerA.,SerenS.andMelicharP.;2002:Georadar in the roman civil town Carnuntum, Austria. An approach for archaeological interpretation of GPR data.Archaeolog.Prospect.,9,135-156.

Piro S. and Goodman D.; 2008: Integrated GPR data processing for archaeological surveys in urban area. The case of Forum (Roma, Italy).In:ExtandedAbstract,12thInt.Conf.Groundpenetratingradar,Birmingham,UK,<www.eurogpr.org>.

PiroS.andCampanaS.;2012:GPR investigation in different archaeological sites in Tuscany (Italy). Analysis and comparison of the obtained results.NearSurf.Geophys., 10,47-56,doi:10.3997/1873-0604.2011047.

PiscitelliS.,RizzoE.,CristalloF.,LapennaV.,CroccoL.,PersicoR.andSoldovieriF.;2007:GPR and microwave tomography for detecting shallow cavities in the historical area of Sassi of Matera (southern Italy).NearSurf.Geophys.,5,275-285.

TrinksI.,KarlssonP.,BiwallA.andHinterlaitnerA.;2009.Mapping the urban subsoil using ground penetrating radar - challenges and potentials for archaeological prospection.ArchaeoSci., 33 (supp.),237-240.

Utsi E.; 2006: Improving definition: GPR investigations at Westminster Abbey. In: Proc. 11th Int. Conf. Groundpenetratingradar,DanielsJ.J.andChenC.C.(eds),Columbus,OH,USA,<www.eurogpr.org>.

Corresponding author: Salvatore Piro Consiglio Nazionale delle Ricerche, ITABC P.O. Box 10, 00015 Monterotondo Scalo (Roma), Italy Phone: +39 06 90672375; e-mail: salvatore.piro@itabc.cnr.it