Studying the Construction of Floor Mosaics in the Roman Villa of Pisões (Portugal) Using Noninvasive Methods: High-Resolution 3D GPR and Photogrammetry
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remote sensing Article Studying the Construction of Floor Mosaics in the Roman Villa of Pisões (Portugal) Using Noninvasive Methods: High-Resolution 3D GPR and Photogrammetry Bento Caldeira 1,* , Rui Jorge Oliveira 2 , Teresa Teixidó 3 , José Fernando Borges 1 , Renato Henriques 4 , André Carneiro 5 and José Antonio Peña 3 1 Institute of Earth Sciences/Physics Department, University of Évora, Rua Romão Ramalho, 59, 7002-554 Évora, Portugal 2 Institute of Earth Sciences, University of Évora, Rua Romão Ramalho, 59, 7002-554 Évora, Portugal 3 Andalusian Institute of Geophysics, University of Granada, Campus Universitario de Cartuja, 18071 Granada, Spain 4 Institute of Earth Sciences, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal 5 Centre for Art History and Artistic Research/History and Archaeology Department, University of Évora, Largo dos Colegiais, 2, 7000-803 Évora, Portugal * Correspondence: [email protected] Received: 29 June 2019; Accepted: 9 August 2019; Published: 12 August 2019 Abstract: Over the past decade, high-resolution noninvasive sensors have been widely used in explorations of the first few meters underground at archaeological sites. However, remote sensing actions aimed at the study of structural elements that require a very high resolution are rare. In this study, layer characterization of the floor mosaic substrate of the Pisões Roman archaeological site was carried out. This work was performed with two noninvasive techniques: 3D ground penetrating radar (3D GPR) operating with a 1.6 GHz central frequency antenna, which is a very high-resolution geophysical method, and photogrammetry with imagery obtained by an unmanned aerial vehicle (UAV), which is a very high-resolution optical method. The first method allows penetration up to 30–40 cm depth and 3D models can be obtained, and with the second method, very high detail surface images and digital surface models can be obtained. In this study, we analyze a combination of data from both sensors to study a portion of the floor mosaic of the Pisões Roman Villa (Beja, Portugal) to obtain evidence of the inner structure. In this context, we have detected the main structural levels of the Roman mosaic and some internal characteristics, such as etched guides, internal cracking, and detection of higher humidity areas. The methodology that we introduce in this work can be referenced for the documentation of ancient pavements and may be used prior to carrying out preservation activities. Additionally, we intend to show that a Roman mosaic, understood as an archaeological structure, does not consist of only beautiful superficial drawings defined by the tesserae, but these mosaics are much more complex elements that must be considered in their entirety for preservation. Keywords: subsurface detection; GPR and photogrammetry UAV; Roman mosaic structure; noninvasive archaeological inspection 1. Introduction The Roman Villa of Pisões (Figure1) is located 10 km south of Beja (Portugal). This villa was discovered accidentally in 1967 and is considered by the academic and scientific community to be a heritage site of the greatest cultural relevance and was classified as a property of public interest in Remote Sens. 2019, 11, 1882; doi:10.3390/rs11161882 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, x FOR PEER REVIEW 2 of 21 heritage site of the greatest cultural relevance and was classified as a property of public interest in Remote Sens. 2019, 11, 1882 2 of 20 1970. The excavations made during the 1970s revealed a pars urbana (residential house) of a singular monument of the Roman province of Lusitania composed of more than 40 divisions, distributed around1970. The a small excavations peristyle made in the during tradition the 1970s of the revealed Italian aatrium, pars urbana which (residentialis gathered house) around of the a singular four- colonnadedmonument ofspace the Roman(Figure province 1). The ofsouth Lusitania facing, composed large porticoes of more with than 40columns divisions, open distributed to face a aroundlarge watera small mirror peristyle with ina thelength tradition of 40 m, of theone Italian of the atrium,largest whichof this istype gathered in private around residences the four-colonnaded throughout thespace Iberian (Figure Peninsula1). The south [1]. The facing, architectural large porticoes design with and columns type openof structures to face a large [2], waterrefinement mirror of with the a decoration,length of 40 especially m, one of the largestgreat quality of this typeof the in floor private covering residences materials, throughout mosaics the Iberianand marbles Peninsula [3,4], [1 ]. bringThe architecturalto mind a luxurious design andresidence type of with structures proven [ 2occupation], refinement between of the decoration,the 1st and 6th especially centuries the A.D. great [1].quality In addition of the floor to the covering residence, materials, the thermal mosaics baths and are marbles considered [3,4], bringto be tothe mind most a r luxuriouselevant examples residence ofwith private proven baths occupation [5], which between are remarkably the 1st and well 6th-preserved centuries in A.D. some [1 compartments,]. In addition towith the an residence, entire heatingthe thermal system baths that allowed are considered the circulation to be the of mosthot air relevant between examples double walls of private and under baths the [5 pavements], which are ofremarkably the rooms. well-preservedThis complex system in some of compartments,brick arched roof, with known an entire as hypocaust, heating system is exceptionally that allowed well the preservedcirculation [6]. of hot air between double walls and under the pavements of the rooms. This complex system of brick arched roof, known as hypocaust, is exceptionally well preserved [6]. FigureFigure 1. 1. LocalizationLocalization of of the the Roman Roman Villa Villa of of Pisões Pisões in in Portugal Portugal map, map, and and the the plan plan view view of of the the pars pars urbanaurbana with with the the (A (A) )residential residential area area and and (B (B) )thermal thermal baths. baths. 1 1.. North North entrance entrance corridor, corridor, where where the the studiedstudied mosaics mosaics are are located; located; 2 2.. columned columned atrium; atrium; 3 3.. reception reception room room (tablinium (tablinium);); 4 4.. d diningining room room (triclinium(triclinium);); 5. 5. bedrooms; bedrooms; 6.6. larder larder and and kitchen; kitchen; 7.7. former former atrium atrium with with square square basin basin for for rainwater rainwater (implivium(implivium);); 8. 8. south south porticoe; porticoe; 9. 9. swimmin swimmingg pool pool (natatio (natatio);); and and 10 10.. m mausoleums.ausoleums. In In the the baths: baths: a. a. DressingDressing room room ( (apodyterium);apodyterium); b.b. furnacefurnace (praefurnium); (praefurnium); c. c hot. hot water water room room (caldarium); (caldarium d.); warm d. warm room room(tepidarium); (tepidarium e. cooling); e. cooling room room (frigidarium); (frigidarium f. swimming); f. swimming pool pool (natatio); (natatio and); g.and latrines. g. latrines. Despite the relevance attributed to this villa, the pars rustica and pars fructuaria remain Despite the relevance attributed to this villa, the pars rustica and pars fructuaria remain underground and undiscovered. The knowledge about the excavated areas remains poor due underground and undiscovered. The knowledge about the excavated areas remains poor due the the scarcity of documentation resulting from the excavations made at that time, a lack of stratigraphic scarcity of documentation resulting from the excavations made at that time, a lack of stratigraphic sequences, and the absence of other studies performed thereafter. sequences, and the absence of other studies performed thereafter. In 2017, the University of Évora assumed the tutelage of the villa with the commitment to promote In 2017, the University of Évora assumed the tutelage of the villa with the commitment to multidisciplinary scientific research to study, preserve, value, and disseminate that heritage [7]. Thus, promote multidisciplinary scientific research to study, preserve, value, and disseminate that heritage the investigation returned to Pisões, and the ongoing work aims to conduct a multidisciplinary research [7]. Thus, the investigation returned to Pisões, and the ongoing work aims to conduct a approach based on two essential pillars: a) Analysis and interpretation of structures and artifacts multidisciplinary research approach based on two essential pillars: a) Analysis and interpretation of already known and b) research of underground structures. Due to the fact that the preservation aspect structures and artifacts already known and b) research of underground structures. Due to the fact guides the dynamics of this plan, all activities are defined to develop, refine, and apply research Remote Sens. 2019, 11, 1882 3 of 20 methodologies that minimize impacts, rigorously reproduce the geometric and functional memory of the structures, and ensure the scrupulous preservation of all data collected throughout the whole site. Archaeological geophysics are noninvasive survey techniques that are sometimes used for the analysis of architectural elements and artifacts of the Roman villae [8,9], but in the Roman villae located in Lusitania, these methodologies have rarely been used. The study of Roman material culture found in archaeological excavations still mostly relies on typological and comparative approaches.