sensors Article Towards Urban Archaeo-Geophysics in Peru. The Case Study of Plaza de Armas in Cusco Nicola Masini 1, Giovanni Leucci 2,* , David Vera 3 , Maria Sileo 1 , Antonio Pecci 1,4, Sayri Garcia 3, Ronald López 3, Henry Holguín 3 and Rosa Lasaponara 5 1 Istituto di Scienze del Patrimonio Culturale–CNR, Seat of Potenza, C.da S. Loya, 85050 Tito Scalo (PZ), Italy; nicola.masini@cnr.it (N.M.); maria.sileo@cnr.it (M.S.); antonio.pecci@unibas.it (A.P.) 2 Istituto di Scienze del Patrimonio Culturale–CNR, Seat of Lecce, Prov.le Lecce-Monteroni, 73100 Lecce, Italy 3 Universidad Nacional de San Antonio Abad del Cusco, Av. de La Cultura 733, Cusco – Peru, Cusco 08000, Peru; david.vera@unsaac.edu.pe (D.V.); sayri.garcia@unsaac.edu.pe (S.G.); comandernod@gmail.com (R.L.); henryholguing@gmail.com (H.H.) 4 Dipartimento di Scienze Umane, Università degli Studi della Basilicata, Via Nazario Sauro, 85, 85100 Potenza, Italy 5 Istituto di Metodologie di Analisi Ambientale–CNR, C.da S. Loya, 85050 Tito Scalo (PZ), Italy; rosa.lasaponara@cnr.it * Correspondence: giovanni.leucci@cnr.it; Tel.: +39-08-3242-2213  Received: 23 March 2020; Accepted: 14 May 2020; Published: 19 May 2020  Abstract: One of the most complex challenges of heritage sciences is the identification and protection of buried archaeological heritage in urban areas and the need to manage, maintain and inspect underground services. Archaeology and geophysics, used in an integrated way, provide an important contribution to open new perspectives in understanding both the history of cities and in helping the decision makers in planning and governing the urban development and management. The problems of identification and interpretation of geophysical features in urban subsoil make it necessary to develop ad hoc procedures to be implemented and validated in significant case studies. This paper deals with the results of an interdisciplinary project in Cusco (Peru), the capital of Inca Empire, where the georadar method was applied for the first time in the main square. The georadar method was successfully employed based on knowledge of the historical evolution of Cusco and the availability of archaeological records provided by some excavations nearby the study area. Starting from a model for the electromagnetic wave reflection from archaeological structures and pipes, georadar results were interpreted by means of comparative morphological analysis of high amplitude values observed from time slices with reflectors visualized in the radargrams. Keywords: urban archaeology; urban geophysics; ground-penetrating radar; Cusco; Inca Empire 1. Introduction In the last decades of the 20th century, urban archaeology has developed in Europe and in North America in response to the rapid urban development and its impact on archaeological documentation. It also arises from the need to overcome chronological and disciplinary barriers in the field of archaeology which limits its effectiveness in answering the questions that the long history of an ancient city typically poses. Thus, urban archaeology represents a cognitive tool for a comprehensive and diachronic analysis of social, cultural, political events that occurred in ancient cities over time. This is true in the case of archaeological ruins, and especially in the case of underground structures that often re-emerge fortuitously and traumatically, following excavations made for the construction of buildings or infrastructures. Therefore, in line with the compromise needed between the planning of city development and the preservation of cultural heritage (with particular reference to the conservation Sensors 2020, 20, 2869; doi:10.3390/s20102869 www.mdpi.com/journal/sensors Sensors 2020, 20, 2869 2 of 16 of buried and of unearthed vestiges), urban archaeology can provide methodological approach and a comprehensive interpretative analysis based on operational tools as geophysical methods. In particular, the understanding of the interface between urban modern settlements and ancient remains in the subsoil constitutes one of the most important challenges not only in archaeology but also in civil engineering and urban planning. To this aim, urban geophysics has a great growing potentiality as an operational tool useful for (i) improving planning and design of urban and infrastructure development and (ii) preserving cultural resources buried into the subsoil of historic cities, as in the case of Cusco—the object of analysis in this paper. 2. Urban Archaeo-Geophysics: Overview and Future Challenges Nowadays, geophysical methods are successfully applied for addressing hydrogeological, civil engineering, environmental, and archaeological issues. In the latter case, these methods are generally more used in suburbs than in urban areas due to the complexity of urban subsoil, especially in cities with a long continuity of life. The presence of modern underground services affects and disturbs natural and artificial stratifications heavily, thus making the interpretation of geophysical investigations very complex and difficult. To this regard, “the London case” (1995) was emblematic. The ground-penetrating radar (GPR) investigations performed by [1] in the subsoil of central London completely failed in the light of the data produced by the subsequent excavations. These unsuccessful results suggested a new approach in the GPR data interpretation based on the identification of a sort of ‘GPR fingerprint’ for a variety of archaeological deposits and features [1]. Nevertheless, “the London case” adversely impacted the development of geophysics for urban archaeology together with two additional reasons: (1) the limits of the GPR data processing tools at that time (before 2000s); (2) and, above all, the low demand of geophysical investigations in urban area. More recently, as in the cases [2–8], successful investigations were conducted. In detail, the subsoil of the ancient city of Potsdam, using the electric, magnetic, electromagnetic methods and GPR methods, which allowed the detection of (i) two ancient ditches around the original town, (ii) the foundations of the city castle, and (iii) the main church was studied by [2]. The subsoil of Kaifeng (Henan, China) nearby a gate of the city walls using GPR and electrical resistivity tomography (ERT) was studied by [3]; thus discovering, after excavation, archaeological remains at different depths dated to different historical periods. The potential of multifrequency electromagnetic induction in archaeology in two heritage sites Han Hangu Pass and Xishan Yang was evaluated by [4], both in urban areas south of the Yellow River. Integrated archaeological records, with historical and cartographic sources, field surveys, remote sensing, geo-magnetometry and geophysical techniques were used by [5]. The aim was to detecting buried remains and reconstructing the landscape of ancient Ravenna, when (5th century A.D.) it was the capital city of the Western Roman Empire. The feasibility of GPR methodology in an urban area for the location of buried structures of archaeological interest was shown by [6]. Shallow cavities in the historical center of Matera using GPR and microwave tomography were detected by [7]. 2D-ERT survey for the identification of buried historical structures beneath the Plaza of Santo Domingo in Mexico City was used by [8]. Results from GPR survey carried out in the historical center of Lecce that have proven very important for the knowledge of buried archaeological evidence were show in [9], in particular, regarding the matter of the Messapian necropolis. The subsoil of San Benedetto del Tronto to identify the presence of natural voids and ancient anthropic underground structures using ERT and GPR was successfully investigated [10]. This brief overview highlights both the potentiality and complexity of urban geophysics along with the needs and importance to give a strong impulse to geophysical investigations in urban areas necessary both to (i) support objective decisions in urban planning and (ii) protect archaeological heritage preserved up to now into the subsoil of cities with a long history. This paper deals with the GPR investigation conducted to explore the archaeologically sensitive areas located into the subsoil of Cusco, the ancient capital city of the Incas Empire. After the Spanish Sensors 2020, 20, 2869 3 of 16 Sensors 2020, 20, x FOR PEER REVIEW 3 of 17 conquest, Cusco was partially rebuilt on the foundations and structures of the pre-existing Inca bcuonilqduinegsst, aCnudsmcoo nwuams epnatrst.ially rebuilt on the foundations and structures of the pre-existing Inca buildTihnegsi navneds tmigoantiuomnsenbtass. ed on the GPR prospection were conducted in the context of a bilateral projecTthbee tinwveeesntiUgantiivoenrss ibdaasdedN oanci othnea lGdPeRS apnroAsnpteocntiioonA wbaedred ceolnCduuscctoed(U inN SthAeA cCon) taenxdt othf eaN bailtaiotenraall Rperosejeacrtc hbeCtwoueennci Ul onfivItearlsyid(CadN NR)a.cTiohnearle dsee aSracnh Aprnotjoenctiow Aabsaadim deedl Caut sacsose (sUsiNngSAthAeCG) PaRndc athpea bNilaittyiofnoarl tRheesdeeatreccht iConouonf chiild odfe Intasltyr u(CctNurRe)s. oTfhaer crheaseeaorlochgi pcarloijnectet rwesatsi naitmheedC uast caossuersbsainnga rtehae. GTPhRes ecappraebliimlitiny afroyr itnhvee sdtiegteactitoionns woef rehicdadrreine dsotruutcttourperso douf cearacnhaaercohloageioc-agle oinptheyressict alinm athpet oCsuuspcpoo rutrtbhaenm aarneaag. eTmheenset apnrdelimmaiinnatreyn ainnvceesotfiguantidoenrsg rwoeurned csaerrrvieidce os,uitn toth perroedspueccet aonf tahrechaarecho-ageeoolopghiycsailcvael smtigapes t.o support the manaTgheemideenat aonfdth me apirnotjeencatnwcae sobf ournndaefrtgerroaunfodr tsueirtvoiucessd, iisnc tohvee rreysopfecInt ocaf -tehrea awrcahllaestorluocgtiucarel svdesutirginegs. the exTchaev aidtieoan omf athdee pforrojseucbt sweravsi cbeormna ainftteern aa nfocer.tuAitloounsg dthisecorovaedrys loefa dInincag-etoraP wlaazlal dsteruAcrtmuraess idnu2r0in14g (tFhieg uexreca1v),atthioense mexadcaev faotrio snusbusenrevaircteh medawinatellnsaonfcteh.i Acklnoensgs trhaen groeafdrosm lea5d0itnog9 t0o cPmla.zTah deey Aarremcaosm inp o2s0e1d4 o(Ff ifignuerley 1d),r ethsseesde egxrcaanvitaetibolnosc kusn(etayrpthiceadl owfaIlnlsc aofa sthhilcakrnmesass orannryg)e, farroemla 5id0 itno 9b0o tchmr.e Tghuelayr aarned coirmrepgousleadr coofu firnseelsyw dirtehssaendi gnrtaernnitael bcloorcekos f(tryupbibcalel omf aInsocan rays,halanrd maaresocnormy)p, oasreed laoidf uinn bevoethn rsetgounleasr saentdi nirmreogrutlaarr. Icnousprsitees owf itthhe abnu iinldteinrngafle acoturree osfo rfutbhbelwe mallassaotntrriyb, uatnadb laerteo ctohme Ipnocsaedp eorfi oudn,ehvoewn esvtoern,etsh eseetx icna vmaotirotnars. aInls ospbitreo uogf hthtet obuliigldhitnpgr fee-aIntucraesc eorfa tmheic wfraallgsm atetnritbsu[t1a1b].leF tion tahlley I,nthcae peexrciaovda, thioonwpevuetra, ltshoe ienxceavvidateinocnes uanlsdoe rbgrroouugnhdt stoer vliigchets p(priep-Ienscoaf cthereaamqiuce dfruacgtmaenndtss e[w11e]r.a Fgien),awllyit, hthmeo esxtcoafvtahteiomn apbuotv ealtshoe ihne aedviodfetnhcee wunaldlser(gFrigouurned1 s)earnvdicoesth (eprispienst roufs tihvee ianqunaedtuurcet, carnods ssienwgethraegwe)a, lwls.ith most of them above the head of the walls (Figure 1) and others intrusive in nature, crossing the walls. Figure 1. Archaeological find in Calle Mantas, one of the entrance ways to Plaza de Armas (courtesy of AFiNgDurIeN 1A. APrecrhcyaeHoulorgtaicdaol fSianndt iilnlá Cn)a.lle Mantas, one of the entrance ways to Plaza de Armas (courtesy / of ANDINA/Percy Hurtado Santillán). In the context of those found in the excavation at Calle Mantas, which can reasonably be repreIsne ntthaeti vceonotfetxhte ooft htherosaer efaosuunndd einr itnhvee setxigcaavtiaotnio, nth aete lCaablolera Mtioanntaansd, wthheiicnht ecrapnr erteaatisoonnaobf lGy PbRe oreuptpreustesnatraetivvee royf ctohme optlhexerd aureeatso uthnedenre eindvteostdigisactrioimn,i nthaete eslaigbnoaralst,ioi.ne .a, nredfl tehceto irnstelirnpkreedtattioonm oafs oGnPrRy soturutpctuutrse asroef vaercrhya ceoomlopgliecxal dinutee rteos tthfreo nmeeredfl teoc tdoirsscrreimfeirnaabtlee tsoigpniapless, io.er.s, trreufcletuctroesrsl ilninkkededt otow mataesroannrdy ssetrwuecrtaugreesn oetfw aorcrhka. eological interest from reflectors referable to pipes or structures linked to water and sCeownesridagerei nngettwhaotrkth. e GPR is the only geophysical method applicable in Cusco, the interpretation was mCaodnesiduesriningg( tih) acto tmhep GarPaRti vise tohbes oenrvlya tgieoonpohfyGsicPaRl mtimethe-osdli caeppamlicpalbitleu dine Cmuaspcos, wthieth inrtaedrparrgetraatmiosn, awpapsr omparidaete ulysignego -(rie)f ecroemncpeadraotnivdei goibtaslecrvarattoiognra opfh yGaPlRon tgimwei-tshli(ciie) tahmepinlitteugdrea timonaposf GwPitRh oruatdpaurtgsrwamiths, aanpcpilrloapryridataetlays egtesom-raedfeeruenpcoefda onnu drbigaintaml caaprtwoigtrhaupnhdy earlgornogu nwditshe r(viii)c eths ea nindteingfroartmioant ioofn GpProRv oiduetdpubtys twheithh isatnocriilclaalryd adtaatsaosuertcse smaandde aurcph aoefo along iucrablarnec omrdaps (Fwigituhr eu2n)d. erground services and information provided by the historical data sources and archaeological records (Figure 2). Sensors 2020, 20, 2869 4 of 16 Sensors 2020, 20, x FOR PEER REVIEW 4 of 17 Fiigurree 22.. Meetthodollogiiccaall aapprroaacch.. 33.. MMeetthhooddoollooggiiccaall AApppprrooaacchh aanndd SSttuuddyy AArreeaa 3.1. Methodological Approach 3.1. Methodological Approach The methodological approach (Figure 2) was based on the integration of a heterogeneous dataset incluTdhineg mientfhoordmoalotigoincapl raopvpirdoeadchfr (oFmiguhries t2o)r wicaasl bsoasuerdce osn, uthreb ainntemgraaptsiown iotfh au hnedteerroggreonuenoduss edravtiacseest, iUnAclVudbiansge dinsfuorrvmeaytsi,oann dprroevsuidltesdf rformomG hPiRstboarisceadl asnoaulrycseiss,. urban maps with underground services, UAVT bhaesehdis stourrivceayl ss,o aunrdce rse,siunlctlsu fdrionmg tGhPeRn abrarsaetdiv aenoanlyessis(t. he so-called Cronicas), published sources, histoTrihcael hicisotnoorigcraalp shoyuracneds, cianrctlougdrianpgh tyhaen ndarertahtnivoeh iosntoersi c(athl eso suor-cceasllaeldlo Cwreodnitchaes)r,e pcounbslitsrhuectdio snouorfctehse, hhiissttoorriiccaall idceovneolgorpamphenyt aonfdC cuasrctoogartaapnhyu rabnadn estchanleo,haisstsohroicwaln soinurFciegsu arello3w. ed the reconstruction of the historical development of Cusco at an urban scale, as shown in Figure 3. Sensors 2020, 20, 2869 5 of 16 Sensors 2020, 20, x FOR PEER REVIEW 5 of 17 FFiigguurree 33.. ((aa)) CCuussccoo ccaappititaal looff IInnccaa EEmmppiriree; ;((bb)) hhiissttoorricicaal lddeevveeloloppmmeennt toof fCCuussccoo aanndd PPlalazzaa ddee AArrmmaass; ; ((cc)) ddeettaaiill ooff PPllaazzaa ddee AArrmmaass wwiitthh iinnddiiccaattiioonn ooff ssoommee mmoorrpphhoollooggiiccaall cchhaannggeess ooff tthhee mmaaiinn ssqquuaarree ffrroomm tthhee CCoolloonniiaall ttoo RReeppuubblliiccaann eerraa.. In particular, the narrative sources [12–14] allowed us to define the main phase of the Cusco In particular, the narrative sources [12–14] allowed us to define the main phase of the Cusco landscape evolution from the last decades of Inca period (1500–1542) to the first centuries of the landscape evolution from the last decades of Inca period (1500–1542) to the first centuries of the Viceroyalty of Peru (1542–first half of the 18th century). Provided the most reliable information on Viceroyalty of Peru (1542–first half of the 18th century). Provided the most reliable information on the Inca Empire and, of course (being recorded coeval to the events), the first decades of Spanish the Inca Empire and, of course (being recorded coeval to the events), the first decades of Spanish rule rule (Figure 3) [12] This allowed us to understand the urban landscape and the architecture during (Figure 3) [12] This allowed us to understand the urban landscape and the architecture during the the transition between Inca and Spanish rule, and, therefore, to capture continuity and discontinuity. transition between Inca and Spanish rule, and, therefore, to capture continuity and discontinuity. In In particular, palaces, churches and monasteries were largely constructed on the foundations of the Inca particular, palaces, churches and monasteries were largely constructed on the foundations of the Inca structures, for example, the Tempe of Sun (also known in Quechua as “Qoricancha”) was demolished structures, for example, the Tempe of Sun (also known in Quechua as “Qoricancha”) was demolished by the Spanish colonists to build the Convent of Santo Domingo, and the Cathedral in the northern by the Spanish colonists to build the Convent of Santo Domingo, and the Cathedral in the northern area of the main square was built following the destruction of “Kiswarkancha”, incorporating the Inca area of the main square was built following the destruction of “Kiswarkancha”, incorporating the stonework into the structure of the new architecture. Inca stonework into the structure of the new architecture. The comparative analysis of the available historical maps of Cusco made it possible to identify The comparative analysis of the available historical maps of Cusco made it possible to identify changes in the building fabric, including some in the main square, and suggested areas to be investigated changes in the building fabric, including some in the main square, and suggested areas to be by GPR. investigated by GPR. The urban map of Cusco with underground services was very useful in the interpretation of The urban map of Cusco with underground services was very useful in the interpretation of georadar maps. In particular, it helped us to reduce the errors in the interpretation of reflectors, georadar maps. In particular, it helped us to reduce the errors in the interpretation of reflectors, differentiating those caused by wall structures from those related to aqueduct and sewer pipes. differentiating those caused by wall structures from those related to aqueduct and sewer pipes. The urban maps have been integrated by a more detailed orthophoto of Plaza de Armas obtained The urban maps have been integrated by a more detailed orthophoto of Plaza de Armas obtained by processing RGB images taken from drone using structure from motion, as shown in Figure 4. by processing RGB images taken from drone using structure from motion, as shown in Figure 4. Finally, regarding the GPR data analysis, beginning from a model to understand the nature of the electromagnetic (EM) wave reflection from archaeological structures and pipes (as reasonably as they are detected; see Figure 1), the GPR result interpretation is performed by means of comparative morphological analysis of high amplitude values observed from time slices with reflectors observed Sensors 2020, 20, x FOR PEER REVIEW 6 of 17 Sensors 2f0r2o0m, 2 0t,h2e8 r6a9dargrams. From their matching, a number of features are extracted and analyzed by mea6nso f 16 of three-dimensional amplitude visualization. Figure 4. Plaza de Armas and the ground-penetrating radar (GPR) surveyed areas. Figure 4. Plaza de Armas and the ground-penetrating radar (GPR) surveyed areas. Finally, regarding the GPR data analysis, beginning from a model to understand the nature of the electrom agnetic (EM) wave reflection from archaeological structures and pipes (as reasonably as they are detected; see Figure 1), the GPR result interpretation is performed by means of comparative 3.2. Study Area: Cusco and Plaza de Armas Over Time morphological analysis of high amplitude values observed from time slices with reflectors observed from the radTahreg srpamacse. oFcrcoumpietdh ebiyr mthaet cchitiyn gof, aCunsucmo baenrd otfhfee taetrurriteosrya rien ewxthriaccht eitd saitns disa tnhael yrezseudltb oyf mtheea ns of threec-odnitmineunosuiso ntraalnasmforpmliatutidonesv cisoumaplilzetaetdio nov. er time by the human groups that have inhabited it. Through the information collected from the archaeological records, in particular the 16th–17th 3.2. StucdeyntAurryea S:pCaunsischo cahnrdonPilcalzeas adnedA hrimstaosriocvael rstTuidmiees, it is known that the city of Cusco is the result of six or more centuries of urban history that can be summarized into five successive historical phases the: Th(ei) Kspillakcee seotctlceumpeinetd (pbrey-Itnhcea acgiety), (oiif) ICncuas, c(ioii)a cnodlontihael, (tievr) rrietpourybliicnanw, ahnidc h(v)i tcosnittesmispotrhaeryr ceistiueslt. of the conItni nuthoisu slotrnagn spfroorcmesast, iobnusilcdoinmgsp leatnedd ostvreertst imunedbeyrwtehnet hduimffearnengt rocuhapnsgtehs,a tdheastvreucitniohna baitnedd it. Througrhectohnesitnrufoctrimonast itohnat csohlalepcetde dthfer ocumrrtehnet hairscthoraiecoal ocgitiyc.a Ilnr pecaortridcusl,airn, praer-tIinccual asertthleem1e6ntths –(1b7eftohrec etnhetu ry Spanish15cthr coennitculreys)a, nocdcuhpisietodr tihcae lasrteuad ciaelsle,dit Aiscakmnoamwan, tlhataetr tkhneocwitny boyf tChue sncaomies tohf eCruesscuol, tdoivf isdixedo irnmtoo re centurifeosuor fnuerigbhabnohrhisotoodrsy. tChuastccoa dnebveelosupemdm alaornizge tdwion troivfiervse, tshuec Rceios sSivalephhyis taonrdic Railop Thualsleusmtahyeo: w(i)hKichil lke settlemwenerte( plartee-rI cnhcaanangelei)z,e(di ib)yIn Icnaca, (iinii )thceo lPoanchiaalc,u(tievc) argeep,u dbulriicnagn w, ahnicdh (tvh)ec doinvtiseimonp oofr anreyigchibtoiersh.oIondst his long prwoecrees sc,hbanugileddi.n Tghsea onrdgasntrizeaettisonu nofd ethrwe espnatcde,i heorwenevtecrh, aconngteins,uedde sttor ubcet idoinviadnedd irne cfoonusr trpuacrttsff i,o ns including the center where the elite lived and the continuous suburbs of the center. In its that shsauprerdoutnhdeincgusr, rseonmteh sisattoelrliitcea lnceiigtyh.boInrhpoaodrtsi cwuelarer ,spetrtele-dIn. cRaegsaertdtlienmg ethnet sm(abienf osqreuatrhee, a1s5 wthe ckennotwur y), occupiefdrotmh ethaere raepcaolrlte bdyA Gcaarmcialamsoa ,dlea tlear Vkengoaw [1n3b] yanthde anrcahmaeeoolofgCicuaslc eov, iddievnicdee d[11in],t othfeo uInrcna ewigahllbs owrheroeo ds. Cusco denevcoemloppaesdseadl oonng thtwe ofarciavdeerss ,otfh teheR iPolaSzaal pdhey AarnmdasR, itohuTus lclounmfiarmyoinwg hthicaht twhee rleimlaittse rocf hthaen ncuelrirzeendt by Inca in tPhlaezPaa dche aAcurmteacsa cgoei,ndcuidrein, gfowr hthiceh mthoestd pivairst,i owniothf ntheoigshe boof rhthoeo dolsdw Aewrekcahyapnagtae dsq. uTahre.o Orgnalny izthaeti on of the scpaathce,dhraolw anedv etrh,ec ohnoutisneus ebduitlot obne tdhiev iSdaepdhii nRifvoeur rhpavaert as,ltienrcedlu tdhienirg othlde lcaeynotuetr. Twhhee croemthpeareisliotne loifv ed and thecocloonnitailn muoapuss wsuithb ulartbers mofapths epucte anltseor i.n Ienviidtsenscuer sroomune dchinanggse,ss oofm thee smataeinll istqeunareei ginh tbhoer shoouothdesaswt, ere settled. Regarding the main square, as we know from the report by Garcilaso de la Vega [13] and archaeo logical evidence [11], the Inca walls were encompassed on the facades of the Plaza de Armas, thus confirming that the limits of the current Plaza de Armas coincide, for the most part, with those of the old Awkaypata square. Only the cathedral and the houses built on the Saphi River have altered their old layout. The comparison of colonial maps with later maps put also in evidence some changes of the main square in the southeast, as shown in Figure 3. The southeast corner of Plaza de Armas is Sensors 2020, 20, 2869 7 of 16 one of the town areas selected to be the object of GPR-based analysis, whose results will herein be shown and discussed in this paper. 3.3. Research Aims The aims of this study are twofold: one technological and scientific, the second archaeological. Aim 1: Evaluation of the potential of GPR in imaging archaeological features in a complex stratigraphy, as can typically be found in an urban area such as Cusco, distinguishing said features from other potential reflectors such as pipes, infrastructures etc. Hence, the archaeo-geophysics challenge as discussed in Section 2. Aim 2: The current debate in the field of Inca archaeology in the city and region of Cusco regards understanding the consistency of previous phases of the Inca. In other words, to what extent is there continuity and discontinuity between the Inca and pre-Inca phases? Additionally, where can this pre-Inca phase can be found? Thus, GPR prospection was also aimed at identifying further anthropogenic layers, exploiting the available archaeological records unearthed by archaeologists during the excavations in Calle Manta, as shown in Figure 1. 4. GPR-Based Investigations of Cusco Several geophysical methods are applied in urban areas with the aim to find buried archaeological structures [15,16]. These allow one to obtain high-resolution images of the subsurface. In this study, we used the GPR method which is based on the detection of variations in the electromagnetic (EM) properties of the subsoil, and subsequently uses these data to identify archaeological features and distinguish them from subsurface pipes. The GPR prospection was carried out with an IDS Hi Mod system with the 200–600 MHz dual-band antenna. Data were acquired in continuous mode along 0.5-m-spaced survey lines in both x and y directions, using 512 samples per trace, 70 ns two-way time (TWT) for 600 MHz antenna and 130 ns TWT for 200 MHz antenna, and a manual time-varying gain function. The transect spacing was 0.5 m in the x and y directions. Assuming the suggestions in [17], the transect spacing should be 0.25 m with only parallel profiles. In theory, this should preclude the use of time slicing on these data. The effective target area is measured by the radius of the Fresnel zone (Fz) or the footprint area [6,15]. Table 1 shows the Fresnel zones at several depth related to the 200 MHz and 600 MHz antennas. Table 1. Fresnel zone diameter at several depths (mean velocity 0.07 m/ns). Depth (m) 0.5 1.0 1.5 2.0 2.5 3.0 200 MHz Fz (m) 0.62 0.85 1.04 1.20 1.33 1.45 Depth (m) 0.5 1.0 1.5 2.0 2.5 3.0 600 MHz Fz (m) 0.35 0.49 0.59 0.69 0.76 0.84 As the anomalies lie within 2 m of the present-day surface and the profiles were acquired in x and y directions, the resolution of time-slices will be near the optimum. Furthermore pioneering work on GPR time slicing (e.g., [15]) has shown that it is possible to obtain useful pattering from time-slices constructed from relatively widely spaced two-dimensional profiles, despite reduced resolution capacity. This is also because for the surveyed area, the expected dimensions of archaeological features range from 0.8 to more than 2 m. Recently, the distance between the GPR transect useful for obtaining excellent results has been discussed with several application examples in [16]. The data were subsequently processed using standard two-dimensional processing techniques by means of the GPR-Slice Version 7.0 software [18]. The processing flow-chart consists of the following steps: (i) Frequency filtering. (ii) Manual gain, to adjust the acquisition gain function and enhance the visibility of deeper anomalies. (iii) Customized background removal to attenuate the horizontal Sensors 2020, 20, 2869 8 of 16 banding in the deeper part of the sections (ringing), performed by subtracting in different time ranges a ‘local’ average noise trace estimated from suitably selected time–distance windows with low signal content (this local subtraction procedure was necessary to avoid artefacts created by the classic subtraction of a ‘global’ average trace estimated from the entire section, due to the presence of zones with a very strong signal). (iv) Estimation of the 2D electromagnetic wave velocity. EM wave velocity was determined from the reflection profiles acquired in continuous mode, using the characteristic hyperbolic shape of reflection from a point source [19]. This is a very common method of velocity estimation and it is based on the phenomenon that a small object reflects EM waves in almost every direction. (v) 2D Kirchhoff migration. (vi) Depth axis conversion using a constant average velocity value of 0.07 m/ns. The migrated data were subsequently merged together into three-dimensional volumes and visualized in various ways in order to enhance the spatial correlations of anomalies of interest. In order to understand the nature of the EM wave reflection from an archaeological structure and a pipe, a 2D model was produced using the reflex software [20]. This program allows the user to generate a synthetic model of what might be expected using known properties of the ground and the geometry of underground features [20]. A model of a stratified subsoil with the presence of pipes and walls was assumed, as shown in Figure 5. Two homogeneous layers with a dielectric constant of 16 and 9 were modelled for the surface terrain and more compact layer, respectively. In the terrain, two pipes with different dielectric constant were inserted. The first one simulated a water-filled pipe (εr = 50), and the second a metal cable-filled pipe (εr = 2). A wall feature, with a dielectric constant Soefn5so,rws 2a0s20p, 2la0,c xe dFOaRt PthEEeRc oRnEVtaIEctWb etween the two modelled layers. 9 of 17 Figure 5. Forward model: two homogeneous layers with dielectric constants of 16 and 9 were modelled Ffoigr uthre t5e.r rFaoinrwanadrdm moroedceol:m tpwaoc thlaoymero,grenspeoecutsiv lealy.erIns twheithte rdriaeilne,cttwrico pcoipnesstawnittsh odfi ff1e6r eanntdd i9e lewcterriec mcoondstealnletdw feore tihnes eterrteradinw aitnhdε mr =or5e0 caonmdpεarc=t 2la.yAerw, raelslpf eacttuivre,lyw. iItnh tahde iteelrercatirnic, ctwonos ptainpteos fw5i,twh adsifpfelarceendt datietlheectcroicn ctaocntsbtaentwt weeenret hinestewrtoedm woditehll εerd =l a5y0e arns.d εr = 2. A wall feature, with a dielectric constant of 5, was placed at the contact between the two modelled layers. The synthetic reflections (Figure 6) demonstrated that the pipes generate reflections when energy is intTerhsee cstyedntihnettihce rseafmleectpioipnes s(pFaigcue,rea s6w) adseemxponecstteradt.eOd ththeartr ethfle cptiiopnese vgeenntserwateer eregfelnecetriaotnesd wathtehne ecnonertgacyt ibs eintwteeresenctseodil i1n athned s2amane dpifproe mspathce, saism wualast exdpwecatleld. . WOthheenr raefdleacrteionne regvyenistsr wefleercet gedenferroamteda abtu trhied coi nttearcfta bcetwheerne stohiel 1E Mandw 2a vaendve flroocmity thdee csirmeausleast,etdh ewpaolll.a Writhyeonf rtahdearre eflneecrtegdy wis arvefelewctielldb ferothme asa bmueriaesd tihnetedrifraeccet whaevreeg tehnee EraMte wd afrvoem vethloectitrya ndsemcriettainsegs,a tnhte npnoala[r1i6ty,2 1o]f. tThhe irseifsletchtedn owrmavael wcaislle bine tmhoe sstagmroeu ansd thcoe nddiriteicotn ws,aavned gtehneererafotered, fmromst rthefle etcrtainonsms aitrteinrgec aonrdteendnas [n1o6r,2m1a].l lTyhpiso liasr tihzed nsoirnmeawl acvaeses. iUns muaolslyt ,garsoruandda rcoenderitgiyonms,o avneds dtheerpeefrorine,t omtohset grreofluenctdio, nmso airsetu rreecorertdeendti oasn ninocrrmeaslleys paonldarEizMedw sainve wvealvoecist.y Udseucarlelya,s eas. raIfdard ernaestrigcyi nmcorevaesse dienevpelro icnitoy tohcec ugrosuant da, bmoouinsdtuarrey ,refoternetxioanm ipnlcer,ewashees nanwda EvMes wenatveer avepliopceitysp daeccer,eaasresfl.e Icft iao ndrwasiltlicb eingcerenaesrea tiend vtehlaotciitsyv oiscicbulersi natt raa cbeosuansdarrye,v feorrs eedxapmolpalrei,t ywshienne waves[ 1e6n,t2e1r] .a pipe space, a reflection will be generated that is visible in traces as a reversed polarity sine wave [16,21]. Figure 6. Forward model: radar section with the shown polarity of the electromagnetic (EM) wave reflected from pipe 1, pipe 2 and the wall. The investigated areas were some sectors of Plaza de Armas (named sectors 1 and 2, respectively) and two arcades on the south eastern side of Plaza de Armas (named sector A and sector B) (Figure 7). In Plaza de Armas, the two sectors are located at the southeast and the northwest of the octagonal fountain, respectively. The two areas, respectively 10 × 23 m and 15 × 25 m, were investigated in both orthogonal directions, with profiles spacing of 0.5 m. At the southern side of Plaza de Armas, two rectangular sectors were investigated. In this area, some pipes are expected to Sensors 2020, 20, x FOR PEER REVIEW 9 of 17 Figure 5. Forward model: two homogeneous layers with dielectric constants of 16 and 9 were modelled for the terrain and more compact layer, respectively. In the terrain, two pipes with different dielectric constant were inserted with εr = 50 and εr = 2. A wall feature, with a dielectric constant of 5, was placed at the contact between the two modelled layers. The synthetic reflections (Figure 6) demonstrated that the pipes generate reflections when energy is intersected in the same pipe space, as was expected. Other reflection events were generated at the contact between soil 1 and 2 and from the simulated wall. When radar energy is reflected from a buried interface where the EM wave velocity decreases, the polarity of the reflected wave will be the same as the direct wave generated from the transmitting antenna [16,21]. This is the normal case in most ground conditions, and therefore, most reflections are recorded as normally polarized sine waves. Usually, as radar energy moves deeper into the ground, moisture retention increases and EM wave velocity decreases. If a drastic increase in velocity occurs at a boundary, for example, when Sewnsaovrse2s0 2e0n, t2e0r, 2a8 6p9ipe space, a reflection will be generated that is visible in traces as a reversed po9loafr1it6y sine wave [16,21]. FFigiguurere6 .6.F Foorwrwaardrdm mooddele:l:r araddaarrs esecctitoionnw witihtht hthees hshoowwnnp poolalarirtiytyo of ft htheee leelcetcrtormomagagnneteitcic( E(EMM) )w wavavee rerflefelcetcetdedf rformomp ippiepe1 ,1p, ippiepe2 2a nadndth tehew walal.ll. TThheei nvinevsteisgtaigteadteadr eaasrewase rewseorme essoemcteo rsseocftoPrlsa zaofd ePAlarzma ads e(n aAmrmedass ec(tnoarms 1eda nsde2ct,orress p1e ctaivnedl y2) , arnedsptwecotiavreclades on the south eastern side of Plaza de Armas (named sector A and se Sensors 2020, y20), axn FdO Rtw PEoE aRr cRaEdVeIEsW on the south eastern side of Plaza de Armas (named sectcotorrB A) ( aFnigdu sreec7to).r InB)P (lFaizgaudree 7A).r Imn aPsl,atzhae dtew Aorsmecatso, rtshea rtewloo csaetcetodras tatrhee loscoautethde aats t a 10 o then sdouththeenaosrt tahnwde tshteo nf othrtehowcetastg oofn ta f l17 he fooucntatgaionn,arle sfpoeucntitvaeinly, . rTehspeetwctiovaerlyea. sT, rhees ptewcoti vaerlyea1s0, ×re2s3pemctaivnedly1 51×0 2×5 2m3, wme raenidn v1e5stigated in both orbteh dogetoencateldd airse schtioownsn, fwrom som × 25 m, were investigated in both ortihthogpornoafi el l e m dsaps irsepca of ticoinn Cgu s, o s wfco i0 .. 5Inm re. gAatrd to th profilest hsepas th coiu e tahrecrandes, se ng of 0s.5id me .o ctfoPr lA was 4 × 45 m At tahzea sdoeuAthremrna s, twhoile se side of rePclt catnogr uBl awr ases c2to ×r s3w5 emre, winhviecsht iwgaetre inve aza de Armas, two rectangular seedc.toInrst shtiigsaatreed only were ina,vseosmtige alpoinpge sthaere loengitud ated. In thisx apreecat,e sd intaol directi ns with profile spacings of 0.5 m. omeb pe idpeetse catreed eaxspeshctoewd nto from some maps of Cusco. In regard to the arcades, sector A was 4 × 45 m while sector B was 2 × 35 m, w hich were investigated only along the longitudina l directions with profile spacings of 0.5 m. 5.5R. Reesusultlstsa nanddD Disicsucussisoionn 5.51..1P. Plalzaazad edeA ArmrmasasS Secetcotror1 1 FiFgiugurere7 7s hsohwows st htehet rtarcaecer erfleeflcetciotinosnsa nadndil liullsutsrtartaetsest htehed idffifefreernecneceb ebtewtweeenenr erfleeflcetciotinosns( n(onromrmalal poploalrairtyit)yg) egneenraetreadtefdro mfrotmhe pthreo bpabrolebaabrclhe aaeroclohgaiecoallofgeiactaul refsea(tyuerlleosw (ybeolxlolwab eblloexd 1la)baenldledth e1)p raonbda btlhee pippreob(raebdlec ipricplee l(arbedel lceidrc2le) .Tlahbeelclehda n2g).eTohfe pcohlaanrigtey otof paorleavreitrys etdo pa orleavreitryserde flpeocltaiorintya tretfhleisctfieoant uarte tihsis cofenafitrumrea itsio cnonthfiartmthatiisofne athtuart ethisisl ifkeealtyurae pisi pliek.eTlyh ae preipflee.c tTiohne rfreoflmecttihoen sflriogmht ltyheu nsldiguhlatltyin ugn(dyuellalotiwng d(aysehleldowco ndtaisnhueodu scloinneti)neuxohuibsi tlsinneo)r mexahl ipboitlsa rnitoy.rmThails preofllaercittiyo.n Tcohuisl drebfelercetliaotned ctooualdp rboeb arbelleataendc ieton ta livpirnogbasbulref aacnec.ient living surface. Figure 7. The 600 MHz processed radar section. Figure 7. The 600 MHz processed radar section. A way of obtaining visually useful maps for understanding the distribution plan of reflection amplitAu dwesayw oitfh oinbtsapinecinifigc vtiimsuealilnyt eursveaflusli smtahpesc froear tuionndoerfshtaonridzionngt athl tei mdiestsrliibceust.ioTnh peslaena roef mreafplesctoinon wahmicphlitthuederesfl wecitthioinn sapmecpilfiitcu tdimeseh inavteervbaelesn isp trhoeje ccrteeadtiaotna osfp heocrifiizeodnttaiml teim(oer sdliecpesth. T),hwesiteh aares meleacptse don tiwmheiicnht etrhvea rle[f2l2e]c.tiIonna agmrapplihtuicdmese hthaovde bdeeevne lporpoejdecbteyd[ 2a3t ]a, tseprmeceifdie“do tviemrlea y(oar ndaelpysthis)”, ,wthitehs atr osenlgecetsetd antidmwe einatkeersvtarle [fl2e2c].t oInrs aa tgrthapehdiecp mthetohfoeda cdhevsleilcoepaerde bayss [ig23n]e, dtesrpmeecdifi “cocvoelorlrasy. Tanhaislytseicsh”n, itqhue estarlolonwgesst foarntdh ewleinakkaesgte roefflsetcrtuocrtsu arte sthbeu drieepdthat odfi ffeaecrhen stlidceep atrhes .asTshigisnreedp srpeseecniftisc acnoliomrsp. rTohviesm teecnhtniniqiume aagllionwg s for the linkage of structures buried at different depths. This represents an improvement in imaging because subtle features that are indistinguishable on radargrams can be seen and interpreted more easily. In the present work the time-slice technique has been used to display the amplitude variations within consecutive time windows of the width Δt = 5 ns, which corresponds to a soil thickness of about 0.15 m if an EM wave velocity of 0.07 m/ns is used. The time slices show the normalized amplitude using a range defined by blue as zero and red as 1. The time slices for the sector 1 are showed in Figure 8. Sensors 2020, 20, 2869 10 of 16 because subtle features that are indistinguishable on radargrams can be seen and interpreted more easily. In the present work the time-slice technique has been used to display the amplitude variations within consecutive time windows of the width ∆t = 5 ns, which corresponds to a soil thickness of about 0.15 m if an EM wave velocity of 0.07 m/ns is used. STenhseorst i2m020e, s20li, cxe FsOsRh PoEwER tRhEeVnIEoWr malized amplitude using a range defined by blue as zero a1n1d of r1e7d as 1. The Steinmsoers s2l0i2c0e, s20f, ox rFOthR ePEsEeRc tRoErV1IEWar e showed in Figure 8. 11 of 17 Figure 8. Plaza de Armas: sector 1: time slices in which some alignments are highlighted. FiguFrigeu8r.e P8l. aPzlazda edAe Armrmasa:s:s seeccttor 1: ttiimee slsilciecse sini nwhwichhi cshomsoem aleiganlmigennmtse anrtes haigrehlhigightheldig. hted. In the slices ranging from 0.2 to 1.1 m in depth, relatively high-amplitude alignments are clearly vIinsitbhIlnee tashsle itc hselesi carenaso nrmagniangliigensgf re fovrmoidme0n 0.c2.2etd too i n1. .t1h me r iaind dadreegpprtahtmh, r, e(rFleailtgaiuvtirevely e7 lh)y.i gIhnhi -tgahhme- atpimmlitepu dlsiletiu caedlsieg innam lFiigegnutmsre ae 8rne rt caslneagarirenlygc learly visibvflriesoimabsl e0t .ha5se ttoah ne0 o.a6mn moam liinae lsdieeespv etivhdi,e dtnhencee cweddei ainnk ttohhbeel irrqaauddeaa arrglgirgranammm (eF(nFitg iu(glrauebr 7eel)l.7e I)dn. 2Ithn) est ehtiememtesi mstloiec seshslo iicwne Fsai gipnurroFebi g8a ubrlareen pg8iipnrega. n ging fTrohme d 0ee from 0.5 to.5 per slices (0.5–1.1 m) show other high amplitude alignment 0 t.o6 0m.6 min idne dpetpht,ht,h theew weeaakk oobblliiqquuee aalilgignnmmenetn (tla(blaeblleedll e2d) s s (eleambelled 3). Moreover, the highest amplitudes were rendered into an2 )issese tmo sshtoows hao pwroabapbrloe bpaipbele. pipe. The Tdheee pdeeerpselirc selsice osurface [21,24,25]. Three- dimensional am(0 s. 5(0–.51–.11.m1 m) s) hshoowwo otthheerr hhiigh amplitude alignments (labelled 3) plitude isosurface renderinggh daimspplaliytsu dame palliitgundmese onft seq(luaable vllaeld . 3). Moreover, the highest amplitudes were rendered into an isosurfaceu e [i2n1 ,t2h4e,2 G5]P. RT shtruedey- dvMiomlouermnesoeiv.o enSrah,la tahdmeinphgli igtiuhs deues stiusaoamlsluypr lfiuatucseed dre esntowd eeirlrlieunmgre idnnidaspteelr aetydhse iasnemt opsualintrufaidsceoesss ,uo frg fieavqciuneag[l 2 vt1ha,2elu 4ea,2 pi5np] e.tahTreah nGrecPeeR -do sfitm urdeeanyl s ional ampvlaiortcluuhdmaeeoi.sl oSgshuicaradflia nscgter uriecstn uudrseusr.ai nlIlngy dtuhisisspe dlcaa ystoes, a itmlhluep mltihitnrueadsthee osltodhf ecesaqel uibsaurlarvtfiaaoclneu sei,s i ngai tvvhienergyG PdthReeli sctaauptedp yetaavsrokan luicnem oeor.df Serrhe aatodl ing is usuaaolrblcythaauineso eulodsgetifcouall i lrsleutsrmuulctitsnu. arFetiseg. utIhnre tsh9e isss hcuoarwsfeas,c ttehhsee, tgthhirvreeisenh-gdolitdmh cenaalsiipborpnaaetilao ranam nispc leait uovdefrery e isadoleslauicrracftaheca ete aousklsoi nigngi cotahrldese 5trr 4tu%oc tures. In thotihbsrtcaeaisnhs eou,ldsteh. feult hreresushltos.l dFicgaulrieb r9a tsihoonwiss athvee trhyredee-ldiicmateenstaiosnkailn aomrpdleitrutdoe oibsotasuinrfaucsee fuuslinrge stuhlet s5. 4F%ig ure 9 showthsrethsheotlhdr. ee-dimensional amplitude isosurface using the 54% threshold. Figure 9. Plaza de Armas: sector 1: three-dimensional amplitude. Figure 9. Plaza de Armas: sector 1: three-dimensional amplitude. Figure 9. Plaza de Armas: sector 1: three-dimensional amplitude. In sector 1, the 200 MHz antenna results do not show results other than those obtained with the 600 MInH sze catnotre 1n,n tha.e 200 MHz antenna results do not show results other than those obtained with the 600 MHz antenna. Sensors 2020, 20, 2869 11 of 16 In sector 1, the 200 MHz antenna results do not show results other than those obtained with the S6e0n0 SsorMs 2H02z0,a 2n0,t ex nFnOa. ensors 2020, 20, x FRO PRE PEERE RRE RVEIEVWIE W 12 1o2f o1f7 17 55..22.. PPlalazzaa ddee AArrmmaass SSeeccttoorr 2 5.2. Plaza de Armas Sect o2r 2 FFiigure Fguigruer 1 100 sshhoowwss tthhee ttrraaccee rreefflleeccttiioonnss aannd the corresponding time slices. In this case, it was also possible to iell u1s0t rsahtoewthse thdei terraecnec reebfleetcwtieoenns adn dth teh ceo crorerrsepsopnodnidnign tgi mtime sel iscleicse. sI.n I nth tihs case, it was also possible to illustrate the di ff reflections (normal polarity) generated frios mcatshee, ipt rwobaas balleso arpcohsaseioblloe gtoic ialllufsetartautree tsh(er d ff edi efrfeence be borexnecsel b twe een re abetwlleedenw r fleefcltions (normal polarity) generated from the probable arch ) anecdtitohnesp (nroobrmaballe ppoilpaeri(trye)d gebnoexralatebdel flreodmP )t.hTe hperohbiagbhle amarcaheaoeloogloigcaicl afle afetuatruerse (sr e(rde db obxoexse lsa blaeblleeldle dw )w a)n adn dth teh pe rporboabbaleb lpe ippiep (er e(rde db obxo lxa blaeblleeldle dP )P. )T. he high ampplliittuu The high amplitd dee aannoommaallyy llaabbeelllleedd mm iiss dduuee ttoo tthhee pprreesseennccee ooff aa ppiit in the surface. The presence of pipes is confirmeuddeb yanaommaaplyo lfaubnedlleerdg mro uisn ddusee rtvoi cthese ipnrCesuesnccoe. of a pt iitn i nth teh seu srufarcfaec. eT. hTeh pe rperseesnecnec oef o pf ippiepse iss is cocnofnirfmirmede dby b ay ma mapa pof o uf nudnedregrrgoruonudn dse srevrivceicse isn i Cn uCsucsoc. o. FFiigFguiugrruee r1e10 01. .0P.Pl aPlzalaazz adaed d eAe ArAmrrmamsa:a ss:e: cssteeoccrtto o2rr: 2(2a:: )( a6(a)0 )600 6M00 0MHMzH aHzn zatenantnentnaen;n apn;r apo;rcpoescroseescdsees rdsae rdaadr aasdre castericostneioc antni oadnn tdiam ntidem steli imscleie c(es0 l.(i50c–.e5– 0(.060. 5.m6– 0 md.6e dpmethpd)te;h p()b;t h)(b )2;)0 (20b0 )M02 MH00zHM aznH atenzntaennnatn;e apn;rn poarc;oepscrseoescdsee srdsa edrdaadrr aasrde csatericostneioc antin oadnn tdaim ntidem tseilm isclei cs(e1li .(c31e–.31(1–.6.13 .m–61 m.d6e dmpethpd)te.h p)t.h ). FFiiFgguiugrrueer 1e11 11 s1shh soohwwosws ttshh teeh tethh trrheereee--ded-iidmmimeennessniioosninoaanll aaalmm amppllpiittluuitddueed iiess ooisssouusrruffaarcfcaeec uue ssuiinnsiggn ttghh teeh 55e00 5%%0 %tthh trrheerssehhsoohllddol..d . Figure 11. Plaza de Armas: sector 2: three-dimensional amplitude: (a) 600 MHz a ntenna; F(bigF)ui2gr0ue0 r1eM1 1.H 1P.zl aPazlnaazt deane d nAea r.Amrams:a sse: csteocrto 2r: 2th: rtheere-dei-mdiemnesniosnioanl aalm apmliptulidtued: e(a: )( a6)0 600 M0 MHzH azn atenntennan; a(b; ()b 2)0 200 M0 MHzH z anatenn 5.3. Plaznatedn a. enAa.r mas Sector A 5.35.. 3P.F lPaizglaauz draee d Ae1 r2Amrsamhso aSwse Scsteoctrht oeAr tAra ce reflections and time slice (0.3–0.5 m depth) for 600 MHz antenna. In thFisigcuarsee ,1i2t wshaos also po Figure 12 shwosw tsh teh t srsaicbele rteofluecntdioenrlsi naendin ttihmees shlaiclleo (w0.s3u–0b.s5u mrfa dceepotfht)h feopr robable archaeological e trace reflections and time slice (0.3–0.5 m depth) fo6r0 600 M0 MHzH azn atnentennan. aI.n I n tfheiaths t uicsa re cs se,a intd the pipes. ase, iwt awsa asl saols op opsosisbslieb lteo t ou nudnedrelirnlien ien i nth teh seh sahllaolwlo wsu sbusbusrufarcfaec oe fo tfh teh pe rporboabbalbe laer achrcaheaoeloogloigcaicl al feafetuatruerse asn adn dth teh pe ippiepse. s. Sensors 2020, 20, x FOR PEER REVIEW 13 of 17 SeSnesnosrosr2s0 22002,02,0 2,02,8 x6 9FOR PEER REVIEW 1213o fo1f 617 Figure 12. The 600 MHz processed radar section and time slice. Figure 12. The 600 MHz processed radar section and time slice. Figure 13. show the Fthigrueree- d1i2m. Tehnes 6io0n0 aMl Hamz prloitcuedssee dis orasduarrf aseccet iuosni nangd t htiem 5e0 s%lic teh. reshold. Figure 13 show the three-dimensional amplitude isosurface using the 50% threshold. Figure 13. show the three-dimensional amplitude isosurface using the 50% threshold. Figure 13. Plaza de Armas: sector A: three-dimensional amplitude for 600 MHz antenna. Figure 13. Plaza de Armas: sector A: three-dimensional amplitude for 600 MHz antenna. In sector A the 200 MHz antenna results do not show results other than those obtained with the 600InM sHecztoarnF tAiegn tunhrae .1230.0 P MlazHa zd ea nAtremnansa: srecstuolrt sA d: toh rneoe-td sihmoewns rioensuall tasm optlhiteurd teh faonr 6th00o sMe Hozb taanitneendn aw. ith the 600 MHz antenna. 5.4. PlIanza sedcetAor mAa tshSee c2t0o0r MB Hz antenna results do not show results other than those obtained with the 5.4.6 P00la zMa Hdez A arnmteans nSae.c tor B Sector B is adjacent to an area used for archaeological excavations which unearthed walls 50 to 90 cSmecttohric Bk iosf atdyjpaicceanl tI ntoc aanas ahrleaar musaesdo nforry ,acrocmhapeosloedgicoaflfi enxeclayvdatrieosnses dwghriacnhi tuenbelaorctkhsedla widailnlsr 5e0g utola r 90c oc 5m.4ur .t Pshl esi aczka o de , witfh t Ayprmica an ina s tle ISneccato ar rnal csBoh relaor fmruabsobnlerym, acsoomnpryo,saendd onf ofitnlealiyd dinrersesgeudl agrracnoiutres belso. cks laid in regular coursesF,iS gweucitrohers a B1n4 isna ntaeddrjna1ac5le snchoto rtewo oatfnh r euartbrebaalc eue msreeadfls eofocntrri oyan,r cashnaadne donlotoitgm liaceiadsl lieincxe craefogvruat6lia0or0n csMo wuHrhsziecsah.n udn2e0a0rtMheHdz waanltles n5n0a t,o re9s0pF ceigmcuti rvtheesilc y1k.4 To ahfn etdyrp a1id5ca asrlhg Iornawcma t sahssehh tlorawarc mea traelsefoalnescrttytih,o crneosem acnpodons tteiinmdu oeo fus flsincree fllfyoe rcd t6roe0rs0ss weMdhH giczrha ancnoitdue l2bd0lo0b ceMkrseH llaazti edadn itnteo nretnhgaru,e lear redspciffoeeucrtriesvneestl,yb w.u Tiitlhhde ia nrnag dinpatrhegarrsnaeamsl sac onshrdeo hwouf amrtu albenbalsfetr etmhqarueseeon nctoraynti,to iannu.doT nuhose tr elsafhlieadcl litonow rrsee wgstuhliiascrh5 c 0co–ou6ur0lsdec smb.e dreeleapte,da tsoso thciraetee d diwffeitrhentwFt bioguupilridepsien s1g4( pi nahndadisc ea1st5 ea sdnhdao shwpu tmihneaF ntir fgaruceerqe ursef1nl4etacatniodn1.s 5Ta)hn.edT s hthiemalilneo tswelriemcsete fidosi r5a 06te–060r0e M flcemHc tzdo earenipsd,a 2ar0sos0uo nMcidaHt1ez.d5 a0 wndtieethne npa, , twroerl epastipeedcstt oi(vinaedllyiac.y aTetheredw raahsdi cpah ringinr Facimlguusd rseehsso t1w4 oantl oldec a1as5lt)r .te hTflrheeece t iconortnesrt(miwneu1doaiuanstd er ewrfel2fel)cetwcotorhsri c wihs haairrcoehu ccnoodnu s1ldi.d5 b0ee rde redeelptaot, erbdeel tawot etadhllr se.e toF aid nliaaffyleleyrr,e ntwht ehbiudchiele dipninecgrlu opdnheeass iteswsa oaron luodcn hadul m1re.9afl–ne2 cf.tr1oermqsu (dewne1eta pat,inoadnn .wd T2ihs)e rw sehlaaitclelhod watroes tcth oiesn o5s0ild–ee6rs0et cdlma tyo de breewp wh, aicslhlsso.i cnFicainltueaddlle yws, iath thleot cwdaeoler peiflepree cost no(iren ndisai mcaaretoedudwn ad3s, p1w. i9hn–i 2Fc.hi1g iumsrae ldss eo1e4cp oa, nnasdni d1de5 ri)se. dTrhetloea itbneedtea rtwmo aethldle.ia Fotielnd raelslfylte ,lctahtyoeerr ria swd aahrroigcurhna dmin 1sc.ls5uh0do dewes erape ,pl oreecalatalet ded rerfeltefloc etaco tlroa rnysearcm owendhsi idwche3r i,e ndwclthuoidcbhees i mstw aaonls holoo cleaosln r(sneidfalmerceetodr ‘stmo ( w’bie1n aFn iwgdu awrlle2. 1)F 5win).hailclyh, atrhee croandsairdgerraemd st os hboe w arlelps.e Fatinedal ly, refltehcet odrse ecpoenrs iodneer eids taor obuen md a1n.9h–o2le.1s (mn admeedp ,‘ man’ din i sF irgeulartee 1d5 t)o. the oldest layer which includes a local reflector named w3, which is also considered to be a wall. Finally, the radargrams show repeated reflectors considered to be manholes (named ‘m’ in Figure 15). Sensors 2020, 20, x FOR PEER REVIEW 14 of 17 Seenssoorrss 2020,, 20,, x28 F6O9R PEER REVIEW 143 off 176 Figure 14. The 600 MHz processed radar section and time slice. Figure 14. The 600 MHz processed radar section and time slice. Figure 14. The 600 MHz processed radar section and time slice. FFiigguurree 1155.. TThhee 220000 MMHHzz pprroocceesssseedd rraaddaarr sseeccttiioonn aanndd ttiimmee sslliiccee aatt 11..22 ttoo 22..00 mm eexxhhiibbiitt aa nnuummbbeerr ooff rreefflleeccttoorrss aanndd hhiigghh aammpplliittuuddee vvaalluueess ccoonnssiiddeerreedd ttoo bbee ttoo wwaallllss,, sshhaallllooww ppiippeess aanndd mmaannhhoolleess.. Figure 15. The 200 MHz processed radar section and time slice at 1.2 to 2.0 m exhibit a number of 6. Cornefclleuctsoirons 6. Conclusiosn asn d high amplitude values considered to be to walls, shallow pipes and manholes. For the first time, geophysical prospection has been performed in Cusco, which within its territory, 6co. CnsoeFnrovcrle ustshtiheo enfgisr rseta tteimsten, ugmeobperhoyfsiacraclh pitreoctsupreacltimono nhuams ebnetsena npdearfrochrmaeeodlo ignic aClursucion,s wdahtiicnhg twoitthheinI nictsa t(ehrarvitionrgy,b ceoennstehrevecsa pthitea gl roefaItnecsat nEummpbireer; osfe earFcihgiutercetu1)r,aal nmdoCnuolmoneniatls and archaeological r For the first time, geophysical prospection has been performeerdas ionf CSouustcho,A wmheircihc uain w. Ts hdeat ithinc i a ng iste tsotu thde Inca (h s territyorsyh,o cwons atvhienggr been the capital of Inca Emp servese tahtep gorteeantteiastl nouf tmhbeeGr PoRf amrceht ihreo;d seine Fimigaugrien 1g), an itectural monumceonm dp Colonial eras of So ts alenxds atratigraphy uputtoh 2Ammedreiceap., Tchhaer caacsteer siztueddyb yshaonwcise ntht ew garllesa, ta pquoteednutciatsl ,osfe twheer GpPipRe lmineetshaondd ino tihmeragstirnugc tcuo rmchpaleeoxl osgtriactailg rruapinhsy d uapti ntog 2to m th de eIenpca, c(hhaarvaicntge rbizeeend tbhye acnapciietanlt owf aInllcsa, aEqmupire; see Figure 1), and Colonriaesl elirnakse odf tSoouunthd Aermgreoruicnad. Tsehrev iccaesse. sTthuedycr suhcoiawl sis tshuee ghraesabt epeontetnhteiainl toefr tphr eedtautciotsn, sewer pipelines and other structures linked e GPR moef tthhoedse ind iivmerasgeinfega ctuormesp.leOxn stthraetibgarsaipshoyf duipr etcot udnadtaerogbrsoeurvned services. The crucial issue has been the interpr to 2 m deep, chadrfrom some archaeological excavations, a modeetal toiof nth oef atnhtehsreo dpiovgeersnei cfesatrtuatriegsr.a Opnh ythoef bCaussisc oofh dasirebcete nd acattear iozbesde rbvye dan fcrioemnt swoamlles , aarqcuhaedeoulcotgs,i csaewer pipelines and other structures linked to underground seravsiscuesm. Tedhei ncrourcdiaelr istosusei mhausl abteeetnh tehwavle erxecflavecattiioonnsf,r oam mwodaelll soaf ntdhep aipneths.roTphoigsehnaics satlrlaotwigerdapuhsyto ocfo Cnudsuccot ahmaso brpeehno laosgsiucaml eadna ilny soisrdoefrg et eo i nsitmeruplraettea ttihoen wofa tvhee sree fdleivcteirosne fferoatmu rwesa. lOlsn a tnhde pbiapseiss .o Tf hdisir heacts dalaltoa observed from some archaeologoicradar features which have been compared with high amplitude vawlueeds ufrso tmo choonrdizuocnt ata ml toimrpehoslliocgeiscaaln a anl ex d tahly csaivations, rees- doifm geeonrsa ad maro fdeel of the a ional aamtuprleitsu wdheincthh hroapvoeg beeneinc csotrmatpigarraepdh wy itohf Cusco h visualization facilitating thei rhiignhte arpmrp alsi tbe etatuiod een v aaslsuuems ferdo min h oorrder to simulate the wave n. The center of Pizlaoznatadl etiAmrem slaisc,esth aenmd tahinrere seq- fdleicmtieonns ifornoaml awalls and pipes. Thi uare of Cuscom, apnlidtutdhee vsoisuutahleizaastti so hna fsa caillliotawtiendg us to conduct a morphological analysis of georadar features which have been comparede wrnitahr chaigdhe aomf t thheir interpretation. The center of Plaza de Armas, the main square of Cusco, and the southeastern arcpal eitusadme evaslquueasre have been inve de of the sam fero smqu haorrei zhoanvtea lb tei smtieg astleicde.s Tanhde itnhtreerep-dreimtation of GPR results sh en investigated. The inteenrpsiroentaatli oanm opfl iGtuPdRe rveis ow the presence of walls, pipelines and ma suulatlsi zsahtoiown tfhacei lpitraetsienngc teh oeifr w inatlelrsp, rpeitpaetliionne.s T nhhoe lceesnitnerth oef sPqlauzaare daen Adramloans,g ththee maarcades. In particular, along the arcades of the square, GPR shows atnhdre me adniheorleens tinla tyheer ss,qaumaroen agndw halio inn gs qtuhea raer ocaf dCeuss. cIon, panardt itchuela sro, uatlohneags ttheren a arcracaddese ooff tthhee ssqaumaere s, qGuPaRre s hhaovwes ffb tehern in ch, two ar ee dvifefsetriegnatt elady. eTrhs,e a imntoenrpgr wethatiicohn e orfe lGatPeRd rtoestuwltos sahnothwr otphoe gperneiscenlacyee orsf wasso , two are allsc,i aptiepdelwinieths apnodte mntaianlhsotlreusc tinu rtehse. sTqhueasree paontde natlioanl gst trhuec taurrceasdaerse. Iinn particular, along the arcades of the square, GPR shows three different layers, among which, two are SeSnesnosrosr2s 022002,02, 02,02, 8x6 F9OR PEER REVIEW 1415o fo1f 617 reraelsaotneadb tloe trweloat aionnthwroitphowgeanlilcs ulanyeearrst hasesdocbiyataerdc hwaietohl opgoitsetnstiinal CstarlulectMuraenst. aT(hFeisgeu preo1te6n),tiaanl dstrfiutcwtuerlels wairthe ianr crheaaesoolnoagbiclea lrerelactoirodns ,winitchl uwdainllgs purnee-aInrtchaecde rbaym airccfhraaegomloegnitsst.sT inh iCs iamllep oMrtaannttar (eFsiuglut rseu g1g6)e,s atsntdh efit pwreesleln cweiothf tharrceheaaenotlhorgoipcaolg erneiccolradyse, rsin: cplrued-Iinncga ,pInrec-aInacnad cceorloanmiaicl , tfhraugsmcoernrtosb. oTrahtiisn gimthpeohrtyapnot threessuislt thsuatgsguegstgse tshtse tphraetsethnecem oaf itnhrseqeu aanrethisrotphoegreensiucl tlaoyfemrsu: pltripe-lIenpcah,a Isnecsao afnhdu cmoalonnfirael,q tuheunst actoiorrnob(Aoruactcinag &t-hCea bhaylpleortoh2e0si1s8 ;tChaiet zsaudgegLesetosn t1h5a5t 4;thVee gma a1i6n0 9s)quare is the result of multiple phases of human frequentation (Aucca &-Caballero 2018; Cieza de Leon 1554; Vega 1609) FFigiguurere1 61.6.C Coommppaarirsiosonnb betewtweeenent htheeG GPPRRa nandde xecxacvavataitoionnr erseusultlst.s. InInt htheef ufututurer,ea, ag egoepophhyysisciacla-li-nintetgegraratetedda apppproroaachch[ 2[266] ]i ninclculuddininggg geoeoraraddaarrw witihthl olowweerrf rferqequuenencycy aanntetnennnaaa annddE ERRTTw wililllb beea dadoopptetdedt otoe xexpplolorerei ning greraetaetrerd depepththt htheeu urbrbanans osoilili nino ordrderert otod deteetcetcta annciceinent t uurbrbanant rtarnasnfsofromrmataiotinonab aobvoevteh ethceh acnhnanelnizealitzioantioonf tohfe tShael pShaylpahnyd aTnudll uTmulalyuomRaiyvoe rRsiwvehresr ewChuersec oCwusacso fowuansd feodunindtehde ipnr teh-Ien pcraee-Irna.ca era. AAuuththororC Conotnrtirbiubtuiotinosn:sC: Conocnecpeptutualailziaztaitoionn: :N N.M.M. .a annddD D.V.V.;.;m metehthooddoolologgyy: :N N.M.M. .a annddG G.L.L; ;s osoftfwtwaarere: : GG.L.L.,., DD.V.V.,., MM.S. and A.P..;S.w arnitd H H.H.H.;.v; avlaildidataitoionn: :D D.V.V., S.G ing—original draft pre.p, Sar.G . a. nadndR R.L..L(.R( RonoanladldL óLpóepze)z;)i;n ivnevsetsigtiagtaitoinon: :a lall;l;d dataatac ucuration: M.S., D.V. and ation: N.M., G.L. and M.S.; supervision: R.L. (Rosa Lraatsiaopno: nMa.rSa.), ;Dp.rVo.j eacntd adAm.Pi.n; iwstrriattiinogn—: Dor.iVg.i;nfauln ddrianfgt apcrqepuaisriatitoionn: : DN.V.M. a.,n Gd.NL.. Man.dA Mll a.Su.;t hsourpsehrvaivseiorne:a dR.aLn.(d Raogsrae eLdastaoptohneaprau)b; lpisrhoejedct veardsmioiniosftrtahteiomn:a nDu.Vsc.r; ifputn. ding acquisition: D.V. and N.M. All authors have read and agreed to the published Fvuenrdsiionng :ofT thhee mpaapneurscsrhipotw. s the results of the research project “Prospección geofísica para determinar las pFousinbdleisnegs: tr“uTchteu rpaaspseort esrhraodwass tehnel arepslualztas doef athrme arsesdeealrcchu spcoroujeticlti z“aPnrdoospuenccriaódna rgdeoefpíseicnae tpraacriaó ndeteterrremsitnrea”r olafs Upnoivsiersidad Nacional de San Antonio CONCblYesT EesCtr-UucNtuSrAasA sCo-tFerOraNdDasE CenY Tla. p Alabzaa dded ealrmCuasco funded by UNSAAC Framewo The research ws adselc oc-ufsucnod uetdilibzyanCdNo Runw riathdafur ndde sp reknaegreeme of JotrianctióLna ntetrbreetween boratsotrrye”o fof PUren-hiviseprsaindiacdA Nrcahcaieoonlaolg dicea lSSacni eAncneto(nLiaoP AASb)a.d del Cusco funded by UNSAAC Framework agreement between CONCYTEC- UNSAAC - FONDECYT. The research was co-funded by CNR with funds of Joint Laboratory of APcrken-owledgments: We acknowledge the archaeologist Angel Sanchez for his support during the geophhyisspicaanl ipc rAosrpchecateioolnosg.ical Science (LaPAS). CAonckflnicotws olefdIgnmteerenstts:: TWhee aacuktnhoowrsleddegcela trheen aorcchoanefloilcotgoisfti nAtnergeeslt .SaTnhcehefuzn fdore rhsish asudpnpoorrto ldeuirnintgh ethdee sgiegonpohfytshiceal stpurdoys;piencttihoencs.o llection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Sensors 2020, 20, 2869 15 of 16 References 1. McCann, W.A. GPR and archaeology in central London. Archaeol. Prospect. 1995, 2, 155–166. 2. 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