Magnetic and radar surveys at Locri Epizephyrii: A comparison between expectations from geophysical prospecting and actual archaeological findings

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29 July 2021

Original Citation:

Magnetic and radar surveys at Locri Epizephyrii: A comparison between expectations from geophysical prospecting

and actual archaeological findings

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DOI:10.1016/j.culher.2019.06.012

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison

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Original

article

Magnetic

and

radar

surveys

at

Locri

Epizephyrii:

A

comparison

between

expectations

from

geophysical

prospecting

and

actual

archaeological

ﬁndings

Chiara

Colombero

a,∗

_,

_Diego

_Elia

b

_,

_Valeria

_Meirano

b

_,

_Luigi

_Sambuelli

a

a_Politecnico_di_Torino,_Department_of_Environment,_Land_and_{Infrastructure}_Engineering,_Torino,_Italy b_Università_degli_Studi_di_Torino,_Department_of_Historical_Studies,_Torino,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received7April2019 Accepted26June2019 Availableonlinexxx Keywords: Ground-penetratingradar Magneticsurvey LocriEpizephyrii Geophysicalprospecting Archaeologicalexcavations

a

b

s

t

r

a

c

t

Inrecentyears,geophysicalmethodshavebeenincreasinglyappliedasapreliminarymappingtoolto guidearchaeologicalexcavations.Despitethereportedgrowingcasehistoriesofgeophysical prospec-tionsforarchaeologicalpurposes,directcomparisonbetweenexpectationsarisingfromthegeophysical resultsandactualfindingsisnotalwayssystematicallyperformed.Acriticalcomparisonbetween pre-excavationgeophysical-guidedhypothesesandpost-excavationarcheologicalevidencesisproposedin thiswork.AtestsitewithinthearchaeologicalareaofLocriEpizephyrii(Calabria,southernItaly)was chosenforthispurpose.Anunsurveyedrectangulararea(31×26m)wasinvestigatedwithhigh-density ground-penetratingradar(GPR)andmagneticprofiles.Severalanomalousalignments,bothcompatible andobliquewithrespecttotheorientationoftheGreek-Romancityplan,werepreliminarlyobservedin thegeophysicalresults.Theuseoftwodifferenttechniquesallowedforcomparisonofanomalousareas, enhancingthelikelihoodoffindingfeaturesofsignificance.Twoarchaeologicalsoundingswerelater carriedoutintheareasshowingthemostpeculiargeophysicalanomalies.Severalstructuresrevealing thesameorientationoftheancientcityplanandbelongingtoatleasttwodifferentbuildingphaseswere unearthedfromadepthof15–25cmbelowthegroundsurface.Asystematiccomparisonbetween geo-physicalandarcheologicalresultswasthencarriedout.Ingeneral,wallsshowingdifferentconstruction typologies(opustestaceum,opusincertumoropuscaementicium)werefoundtogeneratesimilarradar anomalies.Artifactsmadewithmaterialssimilartothebackgroundsandyalluvialdepositswerenot identifiedbybothgeophysicaltechniques,asinthecaseofanunearthedchannelbank,madeoflocal sandstoneblocks.GPRwasgloballyobservedtodetectburiedstructuresbetterthanthemagneticmethod, probablyduetobackgroundgeologicalvariableslinkedtothepresenceofFe-richmineralswithinthe backgroundsediments,generatingnoisierandscatteredmagneticgradientmaps.

1. Introduction

Geophysical methods have been applied to archaeological prospection since the half of the past century [1,2]. With advancesininstrumentationandprocessingtechniques, geophys-icalprospectingisnowadaysa largelyappliednon-invasivetool toinvestigatearchaeologicalstructures.Geophysicalmethodscan beproﬁtablyappliedasapreliminarymappingtooltoguidethe archaeologicalexcavationsinunknownareas.Evenifadense spa-tialsamplingisneededtoobtainhigh-resolutionresults,theycan cover wide areaswithreasonable execution timesand costs.A

∗ Correspondingauthor.

E-mailaddress:chiara.colombero@polito.it(C.Colombero).

reviewoftheavailablemethodscanbefoundinLinford[3]and Gaffney[4].Awidevarietyofsuccessfulapplicationsarereportedin Piroetal.[5].Betweentheavailablemethods,ground-penetrating radar(GPR)canbeusedforthehigh-resolutionimagingof near-surface targets [6–9]. With advances in software and imaging techniques,GPR datainterpretationfor archaeologicalpurposes isevolvingfromtheanalysisofsingle2-Dproﬁlestothe recon-structionof 3-D volumes,betterenabling thespatial tracingof thedesired targets[10–12]. High-densitymagnetic prospecting can be complementary used. Since the magnetic equipment is a portable instrument carried bythe user,themethod hasthe advantage of providing a large amount of data in a fast way and on anyterrain conﬁguration [13–15].A proper contrastin electromagnetic and magnetic properties between the artifacts andthesurroundinggeologicalconditionsishoweverneededto

https://doi.org/10.1016/j.culher.2019.06.012

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison Fig.1. Geographicallocation(a)andaerialviewofthearchaeologicalareaofLocriEpizephyrii(b).Theareainvestigatedwithbothgeophysicalsurveysandarchaeological excavationsishighlightedwiththeredrectangle.Geometricdetailsofthestudiedarea,withlocationofsomegeophysicalproﬁles(blackandbluedashedlines)forfurther analysisofdataandresults(c).

recoverageophysicalsignalindicating,abovethenoiselevel,the presenceofburiedstructures.Acriticalcomparisonbetweenthe pre-excavationresultsofGPRandmagneticprospectionsandthe correspondingarcheologicalﬁndingsispresentedinthisstudy.An exempliﬁcativegeophysicalcampaignconductedinthe archaeo-logicalareaofLocriEpizephyriiischosenforthispurpose.

1.1. Archaeologicalaspects

ThearchaeologicalsiteofLocriEpizephyriiislocatedinCalabria (southernItaly),ontheshoreoftheIonicSea,afew kilometers southofthemoderncityofLocri(Fig.1).Itsmillenarianhistory[16] beginsbetweentheendofthe8th_and_the_beginning_of_the_7th_c._BC

[17]withthearrivalofagroupofsettlersfromGreekmotherland. Fromthatmomentonwards,thecitydevelopsinthecourseofthe ages,fromGreekcolonytoRomanmunicipium,tilltheunavoidable declineandabandonmentbythelastinhabitantsseekingshelterin thenearbyhills,wheretheyfoundedthenewcityofGerace.

TheUniversityofTurinhasbeenconductingregular excava-tionsatthesitesince1969.Theexplorationsmainlyfocusedon thecoastalplainoftheancientcity,bothintraandextramoenia, andunearthedpartsoftheGreekfortiﬁcationsystem,ofthe pot-ters’quarterandoftheresidentialarea,aswellasontwosacred buildings[18–21].Anewresearchprogramstartedin2002[22–26] (seealso[27–29])focusingonthecentral,ﬂatareaoftheancient polis(6in Fig.2a),ontheseaward side oftheso calledCasino Macrì,aruralbuildingdatingtothe18th–19thc.AD,erectedon theruinsofaRomanimperialthermalcomplex(2inFig.2a).The aimwastoacquirenewdataaboutthetopographyanddiachronic developmentofthisarea,previouslyneglectedbysystematic inves-tigation,aswellasamorein-depthknowledgeabouttheRoman

andpost-Romanphases.Acomplexstratigraphicsequencewasin factbroughttolight,datingfromthefoundationoftheGreekcity tothelateantiqueperiod.Theexcavationsconfirmedthecrucial importanceofthissectorduringtheentirelifespanofthesite, regardingthewaterregimentation,thetopographyandthe evolu-tionoftheurbanfabric.Fromtheorigins,thecentralareaofthe colonywascharacterizedbythepresenceoftheMilligri(Fig.2a),a seasonalstreamdescendingfromagorgeinthehighlandbehindthe site,whichwasresponsibleforthedestructionofthemostancient structuresandoccupationlevelsreachedduringtheexcavations, belongingtothe7th c.BC.Toregulatewaterflowsandprevent thesedevastatingfloods,theLocriansbuiltamonumental chan-nelabout24mwide(inredinFig.2b),runningfromthehillto thesea,dividingtheplainareaofthecityintwoparts[23,24,26]. Thismomentousstructuredatingtothe6thc.BC,whichis unpar-alleledamongWesternGreekwaterregimentationdevicessofar, influencedthereorganizationoftheurbanspacewhichledtothe developmentofaperstrigasplan.Thisgridplan(Figs.2aand3b) ischaracterizedbylong,narrowinsulaedelimitedbylargeroad axesapproximatelyparalleltotheshore(plateiai;12mlarge)and orthogonalnarrowcross-streets(stenopoi;circa4m).

Thecentralareaofthecityunderwentaradicaltransformation andspacere-shapingduringtheClassicalage,intheﬁrstdecades ofthe4thc.BC[23,26]:thechannelwasnarrowedtolessthan7m (ingreeninFig.2b)andﬂankedbytworoads,stenopoiS7andS8; insulaI7wascreated,revealingacomplexstratigraphylike steno-posS6andtheadjacentinsulaI6,whichhadbeenoccupiedsincethe Archaicperiod.Aritualarea(BinFig.2b;[23])wasbuiltininsulaI8 onthenorthbankoftheArchaicchannel,countingacourtyard,a corridor,severalrooms,wellsandnumerousdevicesrelatedtothe ritualuseofwater.Cultpracticesareattestedbyfurnishingsand

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison Fig.2.LocriEpizephyrii:a:blackboldline:perimeteroftheancientcitywalls;inblue:ancientstreamsandchannelsﬂowingfromNWtoSE;1.NationalArchaeological Museum;2.CasinoMacrìmodernbuildingandRomanbaths;3.Aphroditegate;4.ancienttheatre;5.Petraramonumentalarea;6.areaoftherecentexcavationsbythe UniversityofTurin;b:zoomontheareaoftherecentexcavationsbytheUniversityofTurin:CasinoMacrìmodernbuildingandRomanbaths(A);Greekritualarea(B);large late-antiquebuilding(C);I5-I8:ancientinsulae;S5-S8:ancientstenopoi(narrowstreets);inred:banksoftheArchaicchannel;ingreen:banksoftheClassicalchannel;in grey:areaofthegeophysicalprospections(bothmodiﬁedafterElia[26]).

votiveofferingsrelatedtopre-weddingceremoniesandablutions. Thesacredareawasinusetillthelate3rdc.BC,whenthe build-ingwassuddenlyabandonedandtheritualdevicesintentionally obliterated.

Attheendofthe3rdc.BCthebedofthenarrowwatercourse, withitsthickalluvialsediments,wastransformedintoasmallroad [24,26].Thisradicalchangeaffectingthecentralareaofthecityis partofthegeneraldismantlingand/ormodiﬁcationoftheprevious waterregimentationsystem,probablyduetodefensivereasons,as itisattestedinvarioussectorsofthecity,liketheAphroditegate nearthesea(3inFig.2a).Theareacontinuestobeoccupieduntilthe middleofthe1st_c._BC._Afterwards,_the_meager_evidence_retrieved

andthelackofstructuresindicatethatthestreetandthehousesin thecentralsectorwereabandoned.

Arenewalstarted inthe2nd c.AD withthe erectionofthe nearbybathsnamedaftertheCasinoMacrìandtheconstruction offurtherbuildings,someofthemprobablyrelatedtothethermal complex,likethepillar-buildingfacingstenoposS7(DinFig.3a; [24,25]).Thissector,togetherwiththePetraraarealocatedwest(5 inFig.2a;[28]pp.563–567;[25]),becomesnowthemonumental poleoftheRomanimperialcity,aperiodinLocrianarchaeology whoseknowledgeisstillfarfromsatisfactory.

Afurthermonumentalphaseisdocumentedduringthe3rdand late4thc.AD[25]whenevidenceofrestructuringandremodelingis knownatseveralareasofthecity,likePetraraandtheGreektheatre (4inFig.2a),whichisnowtransformedinordertoserveasan amphitheater.Inthecentralsectorofthecity,inthesouthernpart oftheexploredarea,abuildingiserectedinthisperiod(inyellow

inFig.3a),latersubstitutedbyanewconstruction,unearthedforat least22m(CinFig.3a),probablycorrespondingtoapublicbuilding facinganopenspaceonthewestside.Thesetwobuildingsoverlay insulaeI6andI7andstenoposS6,andaretheﬁrstsignsinthearea oftheobliterationanddismantlingofthepreviousurbanplanper strigas,whichhadbeenuninterruptedlyrespectedsincetheArchaic period[25].

Thefinaloccupationphase,betweentheendofthe5thandthe 7th–8thc.AD,ischaracterizedbyalternatingsmallresidentialunits andburialsaccompaniedbyraregravegoods.Thebuildingsand ashortportionofarudimentaryroadshowacompletely diver-gentorientationincomparisontothepreviouscityplan.Moreover, large-scaleinterventionscutearlierlevelsandlargepitsaredugall overthearea,filledwithceramicsanddebris(Fig.3a).Thearea previouslycorrespondingtothemonumentalcenteroftheRoman municipiumisnowtransformedinanirregularsettlement,asit hap-penstoothersectorsoftheancientcitylikeTribona-Paleapoliand QuoteSanFrancesco(Fig.3b).Thesiteisdefinitivelyabandonedby the8thc.AD.

1.2. Geologicalsetting

Intheearlystageofthedesignofageophysicalprospection,a properanalysisofthetargetandsurroundingenvironment materi-alsisawell-establishedrequirementforasuccessfulinvestigation [30].TheareaofLocriEpizephyriilaysonalluvialdepositswith dif-ferentgrainsize(fromclaystogravels)oftheﬁumare(streams)of GeraceandPortigliola,ﬂowingtothenorthandsouthofthearea

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison Fig.3.LocriEpizephyrii:a:detailedmapofthearearecentlyexcavatedbytheUniversityofTurin:largelate-antiquebuilding(C);structurepertainingtoapillarbuilding(D); I6-I7:ancientinsulae;S6-S7:ancientstenopoi(narrowstreets);b:plainareaandperimeteroftheancientcitywalls(partim)withlocationoftheareashownin(a)highlighted withtheredrectangle(bothmodiﬁedafterEliaetal.[25]).

respectively.Thesourceareaofthesesmallriversislocatedinthe Aspromontereliefs.Particularly,plutonicrocks,especiallyPermian granites,andaheterogeneoussedimentarysequence (conglomer-ates,sandstones,pelitesandclays)startingfromtheOligoceneto present,areerodedandtransporteddownstreamalongtheriver flows.Especiallyduetotheplutonicrocks,significantcontentin magnetiteandFe-richmineralswithinthesedimentsofthe archae-ologicalarea,potentiallyaffectingthemagneticsurvey,couldnot beaprioriexcluded.Inaddition,thesedimentaryrockscan poten-tiallyrepresentasignificantsourceofclaysinthecoastalsediments. ThisaspectcanpotentiallylimitthepenetrationofEMsignalsfor GPRprospecting.Despitethisconsideration,thegrainsizeofthe sedimentsintheareaisprevalentlysandyandthewaterlevelin theareaisdeeperthan7mfromthegroundfloor.Theseconditions wereconsideredpromisingforGPRsurveying,whilethepresence ofmagneticmineralscouldnotbeeasilyassessed.Nevertheless, alsoamagneticsurveywasattempted.

2. Researchaim

Evenifgeophysicalprospectingfor archeologicalpurposesis welldocumentedin literature,witha widevariety ofcase his-tories,few studies comparing the geophysicalresults withthe actualarchaeologicalﬁndingsortrialexcavationsarepresentin literature[31–34].LinfordandDavid[35]triedtocarryouta semi-quantitativecomparison betweenextensivegeophysicalsurveys (mainlymagneticacquisitions)andlaterarcheologicalexcavations.

Theavailabilityof simultaneousgeophysicalmeasurementsand excavationsprovidedtheopportunitytoattemptanobjective com-parisononﬁve testsites. Nevertheless,theauthorshighlighted manylimitingfactors(e.g.fewcasestudies,variablegeology, over-burden,topographyandsurfaceconditions,recentland-useand seasonality)and problemsindataconsistency(e.g.spatial sam-plingandgeoreferencingofprospectionsandexcavations,variable acquisitionparameters,subjective evaluationsondata interpre-tation)preventingageneralquantiﬁcationoftheeffectivenessof pre-excavationgeophysicalprospections.

Inthiswork,wefocusonthedirectcomparisonbetween geo-physicalresultsandarchaeologicalexcavationsinordertoprovide furtherinsight andcriticalanalysisontherelationshipbetween geophysicalanomaliesand buried structures.The archeological areaofLocriEpizephyriiischosenastestsitetoconductthisanalysis, sinceapartofthecentralsectorofthearearecentlyinvestigatedby theUniversityofTurinseemedtobeparticularlypromisingfor geo-physicalprospections(Figs.1and2b).LocatedbetweentheCasino Macrìbaths(AinFig.2a)andthelongbuildingunearthedinthe southernpartoftheexcavation(CinFig.2a),itisincludedinthe regularplanofGreekoriginandalsoshowedcluesoflaterevidence, belongingtotheimperialandlate-antiqueperiods.

3. Materialandmethods

Arectangularareaof806m2_(ABCD,_Fig.₁_b_and_c)_was_selected

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison referencecoordinatesystem(xandydirectionsinFig.1c).They

directionoftherectangleisorientedatanangleofN30◦E, coinci-dentwiththeconventionalorientationadoptedinarchaeological literature(e.g.[25]).Asmallrectangularsectorincludedinthe sur-veyarea(EFGH)wasnotinvestigatedduetoongoingarchaeological excavationsonsite.Thesurveyareaincludesthewidthofinsula I7andstenopoiS6andS7,anditsnorthsideapproximately cor-respondstotheexpectedcontinuationofthesouthbankofthe channelofClassicalage(ingreeninFig.2b).Thenorthwestcorner

issituatednearafragmentofanopusincertumwallerectedonthe southlimitofthesouthbankofthechannel.

3.1. Geophysicalprospecting 3.1.1. GPRsurvey

TheGPRsurveywasperformedwitha500-MHzGSSIantenna connectedtoaIDSK2unit.Atotalof103proﬁleswereacquired alongthey-direction(Fig.1c)oftheinvestigatedrectangle,with

Fig.4.Soundings(FandN)withdetailoftheunearthedarchaeologicalstructures(orthophotobyN.Masturzo,UniversitàdegliStudidiTorino,DepartmentofHistorical Studies).Theelementsarelabelledwithdifferentnumberedcodes(US=stratigraphicunit).Thelocationofthesoundingswithrespecttothegeologicalprospectionsisshown inFig.8.

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison aproﬁlespacing(x)of0.3m. Theaveragetraceintervalalong

eachproﬁlewas0.03m,foranaveragenumberofabout800traces alongeachproﬁle.Traceswereacquiredatasamplingfrequency of10.24GHz,foratotalrecordingtimeof50ns.

3.1.2. Magneticsurvey

Onthesame surveyarea, amagnetic surveywasperformed withaGSM-19GFOverhausermagnetometerinwalking-gradient mode.TheTotalMagneticField(TMF)andtheTMFverticalgradient wereacquiredwithloweranduppersensorslocatedrespectively at0.2mand0.8mabovethegroundsurface.Atotalof124 mag-neticproﬁleswereacquiredalongthey-direction(Fig.1c)ofthe investigatedrectangle,withaspacing(x)of0.25m.The acquisi-tionratewassetto2readings/s,foranaveragedistanceof0.125m betweensubsequentmeasurementsalongeachproﬁle.

3.2. Archaeologicalexcavation

Awidesounding(FinFig.4),measuring11.50×9m,wasopened duringthe2017and2018campaignsintheareawhereboth geo-physicalprospectionshadshowntwosetsoforthogonalanomalies. Aftertheremovalofmodernlayersbymechanicalmeans,afew Romanstructuresbearingmortaremergedatadepthof15–25cm. TheyshowthesameorientationoftheGreek-Romancityplanand belongtoatleasttwodifferentbuildingphasesofamonumental construction,recognizedontechnicalandstratigraphicbases.They correspondtotheouterwallsofthenorthwestcornerandtoafew innerwalls.

The walls belonging to the ﬁrst phase (US 1020/1021, US 1023/1024-1059-1025inFig.4)measureabout60cminwidthand showopustestaceumontheouterside,whiletheinnersideisinopus incertum.US1021andUS1024-1059-1025areperfectlyaligned

Fig.5.GPRprocessingsequenceontheexemplificativeGPRprofile44(bluedashedlineinFig.1c).Processedradargramsafterstarttimemoving(a),dewow(b),divergence compensation(c),band-passfiltering(50–550Hz)(d),backgroundremoval(e).

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison andbelongtotheoriginalnorthfac¸adeofthebuilding.Theextant

partsreach30cmcircainheight,whilethefoundationshavebeen onlypartiallyinvestigated.

Twowallsinopuscaementicium,measuring50–55cminwidth (US1133and1022),belongtothenorthenlargementofthe build-ing.Theyriseatahigherlevelincomparisontothepreviousones andjustpreservetheirfoundations.US1022,moreover,isdirectly builtontheblocksofUSSPA1019,themonumentalsouthbankof thechanneldatingtotheClassicalperiod,anditkeepsthesame orientation.

Furtherstructures(US1154,1155and1247)wereunearthedin theeasternpartoftheareadelimitedbyUS1022and1025.They areparallelandshownorth-southorientation.

Asmallersounding(NinFig.6),measuring3.50×2.50m,was dug6.80msouth of F. It partly corresponds to a trench exca-vatedduringtheNinetiesandlaterﬁlled,wherethegeophysical prospectingshowedaT-shapedanomaly.Theexcavationbrought tolightanopusincertumnorth-southstructure(US1162)perfectly alignedtothewestouterwallunearthedinsoundingF(US1020, 1133).Aperpendicularshortstructuremadeinthesametechnique (US1164)belongstothesamephase,whileaslightlydivergent rubblewall(US1165)issurelylater.

4. Results

4.1. GPRresults

Raw radargrams were processed with the same processing sequence,involving:

• starttimemovingincorrespondenceofthemainbang(i.e.ﬁrst reﬂectedGPRpulse)toobtainacorrecttraveltime;

• dewow (high-pass ﬁltering to remove electronic noise low-frequencytrend);

• divergencecompensation(torecovertheamplitudeofthe deep-estreﬂections);

• band-passﬁlterintherange50-550Hz(toattenuatenoise out-sidethefrequencybandofinterest);

• backgroundremoval(subtractingtheaveragetracetoattenuate thehorizontalclutteralongtheproﬁles);

• ﬁttingofthediffractionhyperbolapresentintheradargramsto estimatethevelocityofpropagationintheground;

• migrationofthehyperbolawiththeaverageestimatedvelocity (0.14m/ns).

Fig.6. GPRtime-slicesrelatedtodifferentinvestigationdepths:0.62m(a);0.82m(b);1m(c);(d),(e),(f)arethesameof(a),(b)and(c)respectively,withindicationofthe mainreﬂections(thedashedlinescorrespondtothemostuncertainpatterns).Ingrey:smallareaexploredbyarchaeologists,inaccessibleforgeophysicalprospections.

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison AnexempliﬁcativeGPR sectionacquiredinthecenterofthe

investigatedarea(profile44,Fig.1c)isshownthroughthe process-ingsequence(stepsi–v)inFig.5.Evenifsubtlelateralandvertical variationsarealreadydepictedintheprocessedradargrams,GPR sectionswereassembledintheir3-Dspatialconfiguration(x,y, time/depth)inordertomapandfollowthespatialcontinuityof GPRanomaliesindirectcomparisonwithmagneticand excava-tionresults.AvolumeofGPRdataundertheinvestigatedareawas obtained.Time-slices(Fig.6)wereextractedfromthisvolume,with averticalintegrationof1ns(half-lengthoftheantennaperiod),to investigatethedistributionoftheamplitudeofreflection(AOR)in mapsparalleltothegroundsurface,correspondingtoprogressively increasingtwo-way-times(twt),andthustoprogressively increas-ingdepthsofinvestigation.Givenaconstantvelocityofpropagation (

v

)oftheEMpulseintheinvestigatesubsoil,isindeed straightfor-wardtoestimatethedepthzrelatedtoeachtime-slice,following:

z=1₂twt (1)

Threeofthemoresigniﬁcanttime-slicesobtainedfromthe pro-cessingofGPRdataareshowninFig.6.Theslicesarereferredto increasinginvestigationdepths.Particularly,consideringan aver-ageconstantvelocityof0.14m/nsintheshallowsubsurface,the depthofeachtimeslicewascomputed,accordingtoEquation1,in 0.62m(Fig.6a),0.82m(Fig.6b)and1m(Fig.6c)fromtheground surface.

Theresultsshowastratiﬁedconﬁgurationinwhichrecent ele-mentsmaybepresentintheshallowsubsurface(e.g.alignmentsof AORmaximaatx<10m,highlightedwithdashedlinesinFig.6d) anddisappearatdepth.

Peculiarobliquealignmentsarefoundinthe0.82-mtimeslice (dashedlinesinFig.6e).Thesepatternsarenotaswelldeﬁnedin theshalloweranddeepersection(Fig.6dandFig.6f).

Besidetheobliqueorientations,sharporthogonalreﬂectionsare presentinallthetime-slices,fromthex-distanceof15mtowards ESE.Thesehigh-amplitudepatternsshowgoodspatialcontinuity andseemtodelineateconcentricrectangularstructures.The dis-continuityintheAORmaysuggesttointerprettheseanomaliesas buriedwallsummits,layingatvariabledepths.

4.2. Magneticresults

A total number of 26,600 magnetic measurements were acquiredonthesurveyarea.Afteroutlierrejection,only26,150 high-qualitymeasureswereconsideredforfurtherprocessing.The gradientmapwasobtainedbymeansoftriangularinterpolation overax-ygridof0.3×0.15m.

Duetothepresenceoffewlocalizedgradientmeasurements withhigh absolutevalues (>1500nT/m), possibly hidinglower amplitudegradientfluctuationsoverthesurveyarea,absolute val-ueshigherthan150nT/mwerefilteredout.Thisoperationreduced thedatasetto25,344measures(gradientmapofFig.7aandb). Afurtherfilter,excludingdata withabsolutevalue higherthan 30nT/mwasfinallyappliedtoadditionallyenhancesmaller mag-neticanomalies(gradientmapofFig.7candd),reducingthedata setto19,682measurements.Thankstothedenseinitialsampling, afterthefilterapplicationanaverageof26readings/m2_was_still

obtained.

Thegradientsmapsobtainedfrommagneticdataprocessingare reportedinFig.7.Theﬁlteredmapwithabsolutevalues<150nT/m (Fig.7a)isdominatedbyafewhigh-gradientanomalies. Partic-ularly,thetwomainanomalies(locatedaroundx=14m,y=20m andx=18m,y=3m)maybeattributedtoshallowbodieswithhigh magnetizationanddecimetricdimensions.Severalanomaliescan beapproximatelyinterpretedasmagneticdipolesalignedtothe presentmagneticNpole(blackarrowsinFig.7c).Theseare prob-ablyduetomaterials,whichhavebeenmagnetizedbytheEarth’s

Fig.7.Magneticgradientmaps:measurementswithabsolutevalues<150nT/m(a);measurementswithabsolutevalues<30nT/m(b);(c)samemapas(a)withindication ofthemainanomaliesattributabletodipoles.BlackandbluearrowsmarktheorientationsalignedandnotalignedwiththepresentNmagneticpolerespectively;(d)same mapas(b)withindicationoftheanomaliespossiblyattributabletoburiedstructures(thedottedlinescorrespondtothemostuncertainpatterns).

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Please cite this article in press as: C. Colombero, et al., Magnetic and radar surveys at Locri Epizephyrii: A comparison magneticﬁeld,withoutresidualmagnetization.Nevertheless,some

anomaliesshowadipolarconfigurationwithorientationdiverging fromthepresentmagneticfield(bluearrowsinFig.7c).These orien-tationssuggestthepresenceofbodieswithresidualmagnetization, notgeneratedfromtheinteractionwiththepresentEarth’s mag-neticfield.Thefilteredmapwithabsolutevalues<30nT/m(Fig.7b)

showsdiffusemagneticnoise,whichcanbeduetothepresence ofnaturalmaterialwithlow-intensitymagnetizationbutstrong enoughtogeneratethenoisyconﬁgurationofthemap. Neverthe-less,somegeometricpatternscanberecognized(yellowlinesin Fig.7d),globallyinagreementwiththehigh-amplitudealignments detectedwithintheGPRtime-slices(Fig.6).

Fig.8.Comparisonbetweengeophysicalresultsandarchaeologicalsoundings.ZoomonthedeeperGPRtimeslice(Fig.6candf)(a)comparedtoarchaeologicalstructures(b). Zoomonthemagneticgradientmap(measurementswithabsolutevalues<30nT/m,Fig.7candd)(c)comparedtoarcheologicalstructures(d).Ineachsection,theperimeter ofthesoundingsisdelimitedbyblackboldlines.Greenline:geophysicalanomalycorrespondingtoaburiedstructure.Redline:geophysicalanomalynotinterpretedasa buriedstructure(butcorrespondingto).Greyline:geophysicalanomalynotcorrespondingtoaburiedstructure.Violetlineandshadedarea:buriedstructurenothighlighted asgeophysicalanomaly.

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5. Discussion

Theresultsofthearcheologicalexcavationhighlighteda sig-niﬁcantcoincidence betweengeophysicalanomaliesand buried structures.AdirectcomparisonbetweenGPR,magneticresultsand archeologicalsoundingsinshowninFig.8.Geophysical anoma-liesinterpretedaspossibleburiedstructuresbeforetheexcavation (rightcolumnsofFigs.6and7),andeffectivelycorrespondingto ﬁndingswiththesamelocationandorientation,arehighlighted withgreen dashedlines. Theseinclude theperpendicularouter wallsinthefragmentsclosesttotheintersection(US1022,1133 and1020inSoundingF,Fig.4)forbothsurveys,whilethesouth continuationofthestructure(US1162inSoundingN,Fig.4)is depictedonlyinGPRtimeslices.Noclearmagneticanomaliesare foundintheareaofSoundingN(Fig.8c).

Incontrasttosuccessfullocations,archeologicalﬁndingsnota prioriidentiﬁedbyanygeophysicalanomalyarehighlightedwith thevioletlinesandshadedareasinFig.8.

Thehighestnumberofthesemismatchesisfoundinthe mag-neticresults.Nevertheless,somediscrepanciesarepresentalsoin theGPRtimeslice.Inparticular,thealignmentofwidesandstone blocks(withsizeofapproximately0.6_×0.6_×1.10m)formingthe south bank of the channel dating to the Classical period (US SPA1019,Fig.4)doesnotshowadifferentgeophysicalresponse withrespecttothesurroundingmaterial.Thiscanbejustifiedbythe weakcontrastbetweenthepoorly-coherentsandstoneblocksand theloosesandydepositsofthebackground.Bycontrast,thewalls andstructuresmadebystifferpebblesandclaybricks,cemented bymortar,offerasharpercontrastwiththesurroundingmaterial. Otherminorstructuresmadeofthesamematerials(US1247,1154, 1155,1165,Fig.4),withorwithoutmortar,arehowevernot iden-tifiedinthegeophysicalresults.Thearcheologicalexcavationsalso broughttolightfragmentsofstructureswhosesizewasprobably toosmalltosharplyappearinacleargeophysicalanomaly.These elementsarehighlightedinredinFig.8.Theeffectoftheirpresence remainedembeddedintheanomaliescausedbybiggerstructures (US1021inSoundingF,US1164inSoundingN,Fig.4).Asan exam-ple,themainanomalyofUS1020masksthepresenceofUS1021 intersection.EvenifitspresenceisquiteclearintheGPRslicesafter comparisonwiththeexcavationresults(seeforexampleFig.6band c,wherealocalmaximumintheamplitudeofreflectionisfound attheintersectionlocation),itwasnot aprioriinterpretedasa reliableevidenceofaburiedobject.

Finally, there are geophysical anomalies that were initially interpretedasburied structuresbuthad novalidationafterthe excavation.Thesefeaturesarehighlightedwithgreydashedlines inFig.8.Inparticular,inthemagneticresultstwoanomalous align-mentswereoriginallyidentiﬁedaspossiblestructures,parallelto they-directionoftheinvestigatedrectangleandtoasetofwalls. TheirlocationisnotmatchingwiththeparallelstructuresUS1133, 1020and1023.Themagneticanomaliesseemconverselyto high-lightalateralcontrastbetweenstructuresofdifferentheight.

NostructuresclearlyrelatedtotheobliqueGPRanomaly inter-secting the SW corner of Sounding N were found during the excavation.Thispeculiar alignment of maximain amplitude of reﬂection can be related to deeper structures and is however locatedatthelimitoftheexcavatedarea.

Therelativedepthofthedifferentstructureshighlightedinthe GPRtimesliceswasgenerallyconﬁrmedduringthe archaeologi-calexcavation.Progressivelygoingdeeper,theimprintofshallow laterstructureswascoherentlyfoundtodisappearinthe geophys-icalresults.Somediscrepanciesinquantitativedepthestimation betweentimeslicesandﬁeldconditionswerehoweverobserved. Thisdifferenceisprobablyduetotheheterogeneoussite veloci-ties,slightlyvaryingbothlaterallyandvertically,withrespectto theuniformvelocityadoptedfortime-to-depthconversion.

No clear relationship between the construction typologies recognizedin thesoundings(opus testaceum,incertumand cae-menticium)and theintensityof thegeophysicalanomalies was established.

6. Conclusions

A direct comparison between geophysical prospections and archeologicalexcavationswascarriedoutinthiswork,withtheaim ofverifyingthecorrespondencebetweenexpectationsandactual findings and analyzing the possible influence of materials and constructiontypologiesonradarandmagneticdata.Apromising researchsectorwasidentifiedasatestsitewithinthe archaeolog-icalareaofLocriEpizephyrii(southernItaly).Arectangularsurface (width=31m,length=26m)wascoveredwithhigh-densityGPR andmagneticprofiles.Theuseoftwodifferentgeophysical tech-niquesallowedforcomparisonofanomalousareas,enhancingthe likelihoodof findingfeatures ofsignificance. Severalanomalous alignments,bothcompatibleandobliquewithrespecttothe orien-tationoftheGreek-Romancityplan,werepreliminarlyobserved inGPRtimeslicesandmagneticgradientmaps.These geophys-icalresultsguided thelocation oftwoarchaeological soundings insectorsoftheinvestigatedrectangleshowingthemost pecu-liargeophysicalanomalies.Severalstructuresshowingthesame orientationoftheancientcityplanandbelongingtoatleasttwo dif-ferentbuildingphaseswerefoundfromadepthof15–25cm.Walls belongingtothefirstphaseshowedopustestaceumandopus incer-tumontheouterandinnersiderespectively.Aperfectalignment ofthewallfragmentswasobservedbetweenthetwosoundings, suggesting acontinuous structure. Twowallsin opus caementi-ciumbelongingtothesecondphasecorrespondtoanenlargement ofthebuilding.Allthesewallstructures,independentlyfromthe constructiontypology,werecorrectlyidentifiedby GPR anoma-lies,whilethemagneticgradientmaponlypartiallylocatedthese structureswithinthelargerarchaeologicalsounding.Furtherminor structures,unearthedin theeastern partofthemainsounding, despitethesimilarconstructiontypology,showednoevidenceon thegeophysicalresults.Thiscanbeprobablyinterpretedasdue tothesmalldimensionsanddifferentorientationsofthesetargets inasmallarea,generatingthereforealessclearcontrastwiththe surroundingmaterialsifcomparedwiththeotherdetectedwalls.

ThemonumentalsouthbankoftheClassicalchannel,formed byanalignmentofbigsandstoneblocksrunningparalleltoone ofthewallelongations,wasdisclosedinthesamesounding.Also inthiscase,theweakcontrastbetweenthepoorly-coherent sand-stoneblocksandtheloosesandydepositsofthebackgrounddid notgenerateGPRormagneticsignalswhichcouldbeclassiﬁedas anomalies.

The GPR resultswere globally foundtobetter highlightthe buriedstructureswithrespecttothenoisymagneticdata, prob-ablyduetobackgroundgeologicalvariableslinkedtothepresence ofFe-richmineralswithinthealluvialsedimentsofthesite.

Acknowledgments

TheauthorsaresincerelygratefultoDiegoFrancoforhishelp ingeophysicaldataacquisition.

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