Using the micro - resistivity method to detect hispanic ancient floors at Nombre de Dios, Panamá

Alexis Mojica¹, Louis Pastor², Richard Vanhoeserlande³, María Salamanca Heyman⁴

¹ Laboratorio de Ingeniería Aplicada, Centro Experimental de Ingeniería, Universidad Tecnológica de Panamá E-mail: alexis.mojica@utp.ac.pa.

² Laboratoire Sisyphe - Unité Mixte de Recherche 7619, Université Pierre et Marie Curie, France, Cooperation Géophysique pour l'Amérique Central. E-mail louis.pastor@upmc.fr

³ Service Proffesionnalisation, UFRde Physique, Université Paris Diderot - Paris 7, France, Cooperation Géophysique pour l'Amérique Centrale. E-mail: vanhoe@paris7.jussieu.fr

⁴ Department of Anthropology, The College of William and Mary, USA. E-mail: mafesalam@hotmail.com

Record

Manuscript received: 15/05/2010 Accepted for publication: 03/11/2010

ABSTRACT

Keywords: Electric Prospection, apparent resistivity, apparent resistivity mapping, floor of pebbles, Nombre de Dios, 3D modeling.

RESUMEN

Este artículo describe la aplicación del método de prospección eléctrica para la detección de un piso de tipo hispánico asociado con el lugar denominado "Nombre de Dios". Este sitio representa una importante y antigua colonia española fundada en la época de la conquista europea en el continente americano alrededor del año 1510. La prospección eléctrica desplegadaen elsitio estuvo conformadaporun mapeo de resistividad aparente en un área pequeña(10 x10) mobtenidaa través de un arreglo de electrodos polo - polo y se desarrolló durante la época de verano. Los resultados obtenidos en el proceso de interpolación para la obtención del mapa de resistividad del subsuelo mostraron un grupo de anomalías geoeléctricas con valores altos, las cuales fueron asociadas con pisos de guijarros construidos durante este período. Las áreas con bajos valores de resistividad aparente se asociaron a los materiales sedimentarios que caracterizan la región. Más tarde, los trabajos de excavación llevados a cabo en una de las zonas con anomalías eléctricas fuertes concordaron con los resultados de esta prospección geofísica, un sondeo de 0,5 x 0,5 x 0,2 m reveló la parte superior de un nivel de guijarros, y esta estructura colonial descasaba sobre una capa húmeda de material sedimentario superficial.

Por debajo de los pisos hispánicos, la humedad resultó ser más elevada. Posterior a las excavaciones, y con el objetivo de generar un mapa sintético de resistividad eléctrica con las mismas características observadas en el mapa de valores de resistividad medida, se desarrolló un modelo geoeléctrico 3D incorporando las principales características de piso de piedra y de los materiales del entorno detectados durante el excavación.

Palabras clave: Prospección Eléctrica, resistividad aparente, mapeo aparente resistividad, guijarros,Nombre de Dios, modelo 3D.

Introduction

The city of Nombre de Dios was one of the first European settlements founded in the Atlantic coast of the Isthmus of Panama and in American mainland and for this reason, it is considered as one of the oldest Hispanic sites. This city was founded in the year of 1510 by Diego de Nicuesa and its importance resided on its belonging to the port of the Atlantic that connected Spain with many colonies ofSouth America. After this port was abandoned in 1597 because of the attack of the English pirates, Nombre de Dios was forgotten for four centuries, fact that helped it to preserve part of its archaeological remains (Salamanca, 2007). After these events, the city is occupied and nowadays it is consolidated as one of the main section called Santa Isabel's district, province of Colón (North of Panama).

With the passing of time the population went growing and some places lie below certain structures for what it was necessary to carry out an archeological evaluation of the place; and based on this, a multi-disciplinary team accomplished certain scientific activities that represent the electric prospection. This arises from the fact that the study of the physical properties of the subsoil, through measurements realized in the surface of it can give referent lights to the presence and or absence of archeological buried features. These archeological elements are characterized for presenting physical properties (electric, magnetic, thermic or electromagnetic) different to the way that surrounds them, which make them potentially detectable. These techniques of exploration of the subsoil have been successfully applied in some archeological sites of colonial type in the American continent. (Pastor etal, 2001; Lascano et al, 2003; Mojica etal, 2004; Caballero etal, 2004; Chávezetal, 2005; Mojica, 2007a, 2007b; Mojica and Garcés, 2008; Mojica etal, 2009). These techniques, which are categorized as non-destructive, play a very important role in the process ofthe extended description ofan archeological site.

A geophysical prospection based in the electric method was developed in the southwest part of the actual settlement of Nombre de Dios, with the principal objective to detect pavements or floors characteristic of the period of the Spanish conquest.

Nombre de Dios is located to 23 km east of Portobelo, at the Atlantic coast ofthe Isthmus ofPanama (Figure 1). The place, repose on a sedimentary formation of the Secondary Period in which limestones and tuffs become present from the Ocu Formation. Few kilometers from the site, it is encountered the Rio Hato Formation of the Quaternary, which is characterized by the existence of conglomerates, sandstones, lutites, tuffs and non-consolidated sandstones.

Related to the historical context, mentioning that Christopher Columbus in his fourth voyage was searching a new route toward the Asiatic continent, and he went over the Caribbean coast of the Isthmus of Panama and found (in 1503) in its central zone, the first Spanish village on the continent called Santa María de Belén, which time later was going to be destroyed by the natives of the region. After that, numerous expeditions were carried out in the all region with the objective to find deposits of precious metals. At a later time The Spain in the desire to colonize mainland decides to divide the explored zone into two governments: Nueva Andalucía and Castilla de Oro. The government of Castilla de Oro was ranged from Cabo Gracias a Dios (located in the actual Nicaragua) to the boundary with the government of Nueva Andalucía which is located in the middle of Uraba's gulf (actual Colombia). In the year of 1510, it was founded in Castilla de Oro's government, a port in the Caribbean coast of Panama which was named Nombre de Dios; later on this port played a very important role in the commerce between Spain and the originating wealth of Panama City, as a result ofthe conquest process ofSouth America.

Geophysical test principles and field procedures

The use of the techniques of geophysical prospection in archaeological explorations is a topic that has been strongly compiled in a utter literature and as example of it we can find the works of Scollar et al. (1990), Dabas etal. (1998), Milsom (2003), Campana and Piro (2009) among others. One of the most important geophysical techniques used in archaeology is the electric prospection (Tabbagh, 1992; Dabas et al. 1998); which is used to measure differences in the electric resistivity of the subsoil. This physical parameter of the subsoil constitutes the base of this type of explorations, and the same one is affected by the differences in the porosity of the grounds, its content ofwater, its chemical nature and its thermodynamic properties (Nover, 2005). The device ofelectric prospection used in this studyis composed ofa source ofAC power, which is connected to two metallic electrodes (A and B) inserted in the ground. When a certain current intensity i (in mA) flows through the subsoil another couple of electrodes (M and N) register the difference of electricpotential V(in mV) generated in the subsoil. Thanks to the data of V, i and a parameter k (in meter) that depends of the geometry of the four electrodes aligned in the surface it is possible to determine the distribution of the electric apparent resistivity (pap) in the subsoil through the following equation:

For effects ofmapping this physical parameter, it is necessaryto maintain fixed the distance among these electrodes, displacing them all equallythrough the separate profiles. The exploration depth doesn't vary and it is determined through the separation among the electrodes.

With the idea ofobtaining an image at level ofpixels on the variations of the electric resistivity of the ground for a constant depth, it was used a pole-pole electrodeconfiguration,whichis composedbyamovilpartthatcarrieswooden electrodes withthe coupleofAandMelectrodes separatedinadistanceof0.50 m; the othercouple ofelectrodes (Band N) is locatedfarfrom the prospecting area (see Figure 2).

The nature of the superficial ground (in this case sedimentary) the low level of contamination and the superficial archaeological evidence (ceramic remains and metallic materials characteristic of the period) constituted the necessary conditions for the development of an electrical prospection. However, elements likethedensityofthe floraandsome topographicalaspects were the factors that influenced in the election ofthe area whose dimension of its surface was of(10x10)m (see Figure 3). The measurements of the difference ofelectric potential were carried out throughout profiles of10 m oflongitude separated to a distance of 0.50 m among them, being obtained a total of441 registrations. There were used in this study, electrodes ofstainless steel of30 cm oflongitude by5 cm diameter. The used resistivimeterconsisted ofasource of AC powerof50 V ofoutput and a maximum currentelectricityof5 mA.

Geophysical results, interpretation and archaeological excavation

The result ofthe interpolation on a 0.25 m regular grid obtained in this study is shown in Figure 4(a). The apparent electrical resistivity values range is extended between 90 and 360 ohm.m. This result shows two strong anomalies (A and B) in dark tonality with values of apparent resistivity greater than 180 ohm.m, which can be associated to cultural material. It also shows up a conducting area with anomalies in light gray tonality, with values of apparent electrical resistivity below 180 ohm.m which can be associated to the sedimentary material characteristic of the place. The dotted lines in the map correspond to the possible limits ofthese strong anomalies.

Based on this result, an archaeological excavation of(0.5 x 0.5 x 0.2) m of dimension that was developed in the central coordinates x = 9 m andjy = 4m (Figure 4 (b)). The result obtained in the excavation revealed the existence ofa superficial wet layer of 0.20 m of thickness constituted by sedimentary material (organic material) characteristic ofthe drystation. Then it was detected a floor of pebbles very similar to the ones found in the hispanic site of Old Panama, located in the pacific coast of the isthmus of Panama. The thickness average of this layer oscillates among the 0.10 m. The two layers lie on the same sedimentarymaterial, but with astrong moisture level.

A Posteriori 3D forward modelling

With the objective of deepening in the results obtained in the electric and archaeological prospections, it was intended the reconstruction of a synthetic mapofapparentresistivitybasedon thedimensionsoftheprospectedarea,the electrode configuration used (pole-pole in this case,) the dimensions and geoelectric characteristics ofthe structures contained in the subsoil (superficial layer with organic material, floor of pebbles and sedimentary ground, characteristic ofthe site). The response to the 3D model ofelectricalresistivity appliedinthisworkwascalculatedthroughanumericalapproximationoffinite difference.Thismethodisusedtosolvethedistributionoftheelectricpotential V due to a punctual source of electric flow in the surface of a half-space that possessesathree-dimensionalarbitrarydistributionofelectricalresistivity(Dey and Morrison, 1979). These potential and electric current are related to the electrical resistivityofthe structures bythe equation:

Thanks to the transformationofthe integralvolume ofthe leftmemberof the equation (3) to an integral surface through Green's theorem; a separated equation is obtained for a single node inside the mesh. This equation possesses boundfunctions to:(i) the geometryofthemeshandthe value ofthe electrical resistivity(bydefault) ofeachrectangularprism, and(ii) the unknown value of the electricpotential(V).Iftheytake into account the differentialequationsof all the nodes ofthe analyzed mesh, it is obtained a matrix representation in the way:

In this equation C represents a matrix related to the geometry and the distribution of the electrical resistivity in the mesh, which corresponds to well-known terms, V is another matrix that contains the unknown values ofthe electric potential in all the nodes, and I represents a matrix related with the well-known terms of intensity of electric current. The program used in this study was Res3Dmod (version 2.0, developed by M.H. Loke) and it is based on the works ofDeyand Morrison (1979). In the 3D electrical resistivitymodel proposed in this study, there were established 4 levels composed byrectangular prisms whose characteristic are shown in Table 1. The values of electrical resistivity assigned to the rectangular prisms of the proposed model were chosen according to the Standard Guide of Using the Direct Current Resistivity Method for Subsurface Investigation (Designation D 6431 - 99).

Figure 5(a) shows the three-dimensional representation ofthe group of structures and Figure 5 (b) a plan viewofthe same structures extendedamong 0.20 and0.30 mofdepth.The applicationoftheprogramgeneratedasynthetic dataset of apparent resistivity to different depths with 5% of noise (given in terms of the separation of the voltage of electrodes and electricity in the pole-pole setting); from the generated data, there were extracted those correspondingto the firstlevelofdepthfor all the profiles, thatis to sayforthe distance AM = 0.5 m and after that, the next was to map this information using the same procedure than the one carried out with the data ofapparent electrical resistivitymeasure.The result ofthis process is shown in Figure 6 whichis very similarto the oneobtainedin Figure 4(a),whichmeansthatthedistributionof the rectangular prisms of Figure 5, constitutes a good model for the representation andstudy of the electric anomalies associated to colonial floors.

Discussion

The results of the electric prospection developed in the site of Nombre de Dios, have revealed the existence of strong electric anomalies of the subsoil with values above 180 ohm.m and that they are associated to a floors of pebbles system, which were very common in the colonial period. The range of apparent resistivity values obtained for the superficial layer is associated to the sedimentary material of the Secondary Period that is also composed of organic material. In this same aspect, the synthetic map generated by the three-dimensional modeling through the discretization of the subsoil for finite difference, possesses similar characteristic to the map of apparent electrical resistivity measued of Figure 4 (a), with a first layer of rectangular homogeneous prisms of 0.20 m of thickness and 300 ohm.m of resistivity; representing to the superficial sedimentary layer. The second layer (located between 0.20 and 0.30 m of depth) is more complex and it reveals a group of prisms with 3 different values of electrical resistivity: the rectangular prisms of 2500 ohm.m that represent the system of floors of boulders, the prisms of 300 ohm.m associated to sedimentary floor withorganic components, and those of 90 ohm.m with the same characteristics that the floor type described previously, but with certain level of moisture. Below these two levels, it shows up a group of homogeneous prisms that are extended until 1 m of depth and of 90 ohm.m of electrical resistivity.

Even the prospected area doesn't possess a significant extension, the results obtained in it demonstrates in first place, a good lateral resolution referring to electric mapping with electric anomalies that define the limits of floors of pebbles characteristic of the foundation period of the old city. Thanks to this geophysical intervention it became evident the existence of archaeological features of Hispanic type, being demonstrated in this way, the historical importance of the site. In this same aspect the three-dimensional model of rectangular prisms proposed in this work, constitutes a good model for the representation of old floors and the surrounding subsoil. Based on the previously exposed thing, it becomes necessary a more extensive exploration to detect other archaeological features of this site, so that the life forms of the first European in the Isthmus of Panama can be understood.

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Acknowledgment

The authors want to extend their gratefulness to the Universidad Tecnológica de Panamá and College of William and Mary for the offered support. To theEmbassyofFranceinPanamaandtheirCulturalService, andto CCCAC. To Mrs. Angelica Deglass, president of the quinquennium committeeofthe cityofNombre de Dios, andto Mr. Guido Olmos forallthe facilities and permissions he offeredto the development ofthis experience. To theauthoritiesofNombredeDiosfortheirsupport,inthisinterestingproject.