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3DelectricalprospectioninthearchaeologicalsiteofElPahñú,HidalgoState,CentralMexico

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DenisseArgote-Espino

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AndresAndrade

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GerardoCifuentes-Nava

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3D electrical prospection in the archaeological site of El Pahñú, Hidalgo State,Central Mexico

Denisse Argote-Espino a,*, Andrés Tejero-Andrade c, Gerardo Cifuentes-Nava a, Lizbeth Iriarte a,Sabrina Farías b, René E. Chávez a, Fernando López b

a Instituto de Geofísica, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, México D.F., C.P. 04510, Mexicob Escuela Nacional de Antropología e Historia, Periférico Sur esq. calle Zapote, Col. Isidro Fabela, México D.F., Mexicoc Facultad de Ingeniería, Universidad Nacional Autónoma de México, Circuito Escolar, Ciudad Universitaria, Coyoacán, México D.F., C.P. 04510, Mexico

a r t i c l e i n f o

Article history:Received 3 June 2012Received in revised form9 August 2012Accepted 22 August 2012

Keywords:3D ERTGeophysical surveyEl Pahñú archaeological siteCentral Mexico

a b s t r a c t

The implementation of a 3D Electrical Resistivity Tomography (ERT-3D) survey was carried out in ElPahñú archaeological site, Hidalgo State, Central Mexico. A combination of a new ERT arrays allowedstudying the subsoil beneath the Main Pyramid built near the edge of a plateau, along with anotherimportant structure (the Tecpan), which was a smaller structure that lodged the governmental council inpre-Hispanic times. The recorded information was acquired through the combination of several elec-trodic designs: L-Corner (LC), Equatorial (Eq), and Minimum Coupling (MC). For the Main Pyramid, theelectrodes were set up around the perimeter of the structure, since they were not permitted to beinserted over the edi!ce, thus preventing damages to the architectonic elements. The second structureallowed inserting electrodes on selected spots within the architectonic space. The combination of thedifferent arrays made possible the acquisition of 1204 apparent resistivities beneath the Main Pyramidand 2460 resistivity data beneath the Tecpan. The apparent resistivity data were inverted to obtaina three dimensional display of the subsoil electrical resistivity beneath the archaeological structure. Theinterpreted resistivity model under the Main Pyramid displayed a highly resistive structure towards itsnorthern face that could be associated with in!ll. Such material was employed by the ancientconstructors to level the terrain close to the edge of the cliff. Another interesting anomaly was foundtowards the central portion of the structure that could be associated to a foundation offer. The inter-pretation of data beneath the Tecpan identi!ed the structural foundations and other interestinganomalies related to the different occupational times. The investigation supported the archaeologicalinvestigation of the site, suggesting areas of potential geological risk and of archaeological interest. Forexample, the Main Pyramid presents serious stability problems, indicating that the in!ll has weakened,producing cracks threatening long-term pyramid integrity.

! 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The archaeological site of El Pahñú is located within the Mez-quital Valley (Fig.1, inset), in Tecozautla municipality, Hidalgo State.The Mezquital Valley covers more than 7000 m2 of the northernMesoamerican limit, and includes thewestern side of Hidalgo State,the northern portion of the Mexico State and a limited area of thesouthern limits of Queretaro State (López Aguilar and Fournier,2009). The region forms part of the Central Mexico highlands,within the physiographic province of the Central Volcanic Belt(CVB).

El Pahñú forms part of a complex system of pre-Hispanicsettlements called Las Mesas Culture. The principal characteristicof these communities is the construction of their ceremonialcenters near the edge of high plateaus that can reach elevations of200 m above the surrounding valley. The discovery of this siteoccurred in the 1990’s during the prospection labors of the Valle delMezquital Archaeological Project, and was carried out byresearchers and students from the Escuela Nacional de Antropo-logia e Historia (ENAH). The other sites discovered, besides ElPahñú, included: Zethé, Zidadá, Taxangú and El Cerrito. These !vesites create part of the regional development recently named“Xajay”.

Several !eldworks campaigns occurred in different seasons(1995, 2001, 2007-1, 2007-2 and 2011), which have focused on

* Corresponding author. Tel./fax: !52 5556750029.E-mail address: efen!@gmail.com (D. Argote-Espino).

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Journal of Archaeological Science

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0305-4403/$ e see front matter ! 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jas.2012.08.034

Journal of Archaeological Science 40 (2013) 1213e1223

investigating the origin of this cultural system, its transition fromthe Classic Period to the Epiclassic (800e900 AD), the role that theCoyotlatelco groups played and the rising of Tula site, as well as theidenti!cation of the attributes of the Xajay territorial and politicallyautonomous unity (López Aguilar et al., 2006; López Aguilar et al.,2007; López Aguilar and Vilanova de Allende, 2008; LópezAguilar, 2011).

El Pahñú is considered the site with the highest hierarchicalstatus and comprises four ceremonial assemblies. The mainassembly is located at the northern edge of the plateau (Fig. 1A),and consists of several buildings around a main square (some ofthem are still unveiled). At this time, only two have been excavated:the Main Pyramid (Fig. 1B) to the north and the Tecpan (Fig. 1C) tothe eastern side of the square. The position of each buildingcorresponds to the particular way that the Xajay society used toconsecrate ritual spaces.

Each building had two constructive phases. The !rst substruc-tures were related to the foundation of the site, around the year of450 AD, and show a unique architectonic style for the region. Asimple "ight of stairs at the south and a double "ight to the north,a dice style ending, and platforms and sloping surfaces (known astalud-tablero) with proportions dissimilar to the predominantTeotihuacan style of this period. Moreover, their eastern façadepresent a distinctive element, an E glyph, which represents !re andwater in bas-relief. The second constructive phase has been dated

around 621 AD, and implied several desacralization processes ofthe substructures and the placing of offerings that marked thebeginning of the new period. The architecture lost its unique styleand took up the features present in contemporary sites along theregion oftenly found in Central Mexico. The Main Pyramid is thecore of a newer 9m tall pyramid, composed of three stepped bodiesbuilt in talud-tablero style (Fig. 1B).

In 2011, the investigation of Pahñú Special Project (LópezAguilar, 2011) began. Among other concerns, there was the chal-lenge of opening the archaeological site of El Pahñú for publictourism. The objectivewas to show to the visitors a comprehensibleimage of the two constructive phases and the ceremonial use of thissite. An important issue for the planning of the project was the littletime available for the excavation of the structures. One dif!culty theproject had to face was the structural instability of the MainPyramid due to the geological materials (tepetate). These supportthe structure and the man-made !lling over which the pyramid isseated towards the edge of the cliff. Such in!ll was used to level theterrain.

According to Gama-Castro et al. (2007) de!nition, the name“tepetate” refers to indurated horizons, compacted or cemented,that are very common in the pediments of Mexican volcaniclandscapes, with soft thin soil layers covering it or outcropping inthe surface. Tepetates exhibit a matrix composed by sands, silt andminor proportions of clay. The United States Department of

Fig. 1. Topographic model of the archaeological site of El Pahñu is shown (A). The archaeological site of El Pahñú is located at the northern edge of a plateau. The Main Pyramid (B)and the Tecpan (C) are also depicted.

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Agriculture contemplates compacted or cemented soil layers,exclusively by pedological processes, as paralithic materials(similar to rocks), and considers tepetate similar to fragipans andother duripans (SSS-USDA, 1998). But, due to the variability of theirphysical and chemical properties, there is not yet a precise de!ni-tion of tepetates (Velásquez-Rodríguez et al., 2001). Therefore, theterm “tepetate” remains applied without an exact Englishequivalent.

Tepetate under soils can generate landslides and erosionfavoring lateral run off due to their low porosity and high imper-meability. These type of materials compromised the stability of thebuilding presenting the risk of collapse, especially in the northernhalf of the site nearest to the edge of the plateau. Furthermore, inthe 1940’s, the pyramid suffered two lootings. The !rst one brokethe structure, leaving a fragile substructure façade in its NW corner.The second looting occurred directly in the core of the pyramid,destroying part of the inner central portion of the pyramid.

Looking for a faster and effective way to investigate the site inlight of the time and stability constraints, a geophysical survey wascarried out between the months of August and October, 2011. A 3DElectrical Resistivity Tomography (ERT-3D) in its galvanicmodewasapplied within the site of El Pahñú. The main goals of the surveywere to: 1) identify the precise position, volume andmorphology ofthe man-made prehispanic in!ll beneath the Main Pyramid todetermine geological risks that could compromise the stability ofthe structure, and 2) to determine the presence of anomalies thatcould correlate to elements of archaeological interest of previousconstruction phases in the “Tecpan”.

2. 3D Electrical Resistivity Tomography (ERT-3D)

The Electrical Resistivity Tomography (ERT) is a geophysicalmethod that allows the investigation of the horizontal and verticalvariations of the electric resistivity of the subsurface materials. Thesubsoil is often studied as two-dimensional cross-sectionsmeasured along lines in a predetermined direction. We refer to thismode of acquisition as ERT-2D. Speci!cally, the measurements areconducted with an electric current injected into the ground at a setof electrodes while a receiver device observes the response in theelectric !eld. A perturbation in the !eld will be due to the electricalresistivity of the underground features. The ERT-2D geophysicalcharacterization allows detecting various objects like voids, faultsand fractures (Arango-Galván et al., 2011), contaminated ground-water (Rucker et al., 2009a), and bedrock topography and archae-ological targets (Cardarelli et al., 2008).

In the last three decades ERT-3D techniques have been devel-oped, like the Roll-Along method to acquire data in three dimen-sions (Dahlin and Bernstone, 1997). Such methodology consists onsetting parallel lines that cover the study area. Observations aremade in the ‘x’ and ‘y’ directions, which commonly employ thePoleePole array. A large amount of data can be obtained by usingthis technique, however data processing is cumbersome andsometimes data inversion takes too much time. Rucker et al.(2009b) presents a list of large 3D resistivity studies over the past20 years that outlines many strategies that have been taken toacquire these data.

Loke and Barker (1996) designed a processing approach todecrease the number of data maintaining resolution and quality ofmodeled data. This technique was named ‘cross-diagonal survey’,where the potential (V) observations are made in the lines ofelectrodes lying along 0, 45 and 90" to the current electrode. Theamount of data is reduced to the half, while keeping signi!cantquality and resolution. Aizebeokhai et al. (2009) recommenda maximum separation between pro!les of 4a (with a being theelectrode separation) in designing the survey grids. Such rule

grants good quality and resolution of the 3D resistive image of thesubsoil. These techniques are suitable to open areas in the search ofkarstic zones (Dahlin et al., 2002; Ogilvy et al., 2002).

The above strategies for electrode placement assume noadministrative or physical restrictions. The geophysical explorationfaces then a great challenge when surveying sensitive areas that donot allow positioning of geophysical pro!les in equidistant parallelmeshes, perhaps due to obstacles like houses, buildings orarchaeological and historical monuments (may damage the struc-tures). Having this in mind, a new strategy was developed to surveyEl Pahñu site, since theMain Pyramid in particular suffers of seriouscracks and !ssures in its structure that jeopardize its stability. Thenew methodology avoided inserting the electrodes directly withinthese archaeological structures.

3. Geoelectric survey

A geophysical survey was designed to study the two mainimportant structures within the archaeological site of El Pahñu: theMain Pyramid, and the Tecpan. A Syscal Pro Switch equipment,manufactured by IRIS Instruments (France) with 10m long electrodeconnection cables, was employed. The design of the SyscalPro48instrument allows a maximum of 48 electrodes used simulta-neously. The electrodes (3/400 copper bars hydrated with a CuSO4solution) were set in the ground a day before the data acquisitionstarted in order to provide more stable readings. Numericalmodeling of the acquired apparent resistivity data was performedwith EarthImager 3D software (Copyright 1999 Advanced Geo-sciences, Inc), through a numerical inversion process that trans-forms apparent resistivities to estimate the real resistivities ata given depth. In addition, all data were topographically corrected.

The geophysical survey designs included multi-electrode arraysof different types: L-Corner (LC) (Tejero-Andrade et al., submittedfor publication), Equatorial (Eq), and Minimum Coupling (MC).These special electrode arrays were employed to study thesubsurface of both archaeological structures, i.e. the Main Pyramidand the Tecpan.

3.1. The Main Pyramid

The ERT-3D survey of the Main Pyramid required the insertionof a set of 44 electrodes in the ground with a 3 m equidistantspacing between electrodes, forming a square of 33# 33m2 aroundthe studied buildings (Fig. 2). Circles indicate the position of eachelectrode and numbers indicate how the electrodes were set. Thesolid black square indicates the location of the acquisition console(SYSCAL-Pro, between electrodes #24 and #25). The new meth-odology to acquire the subsoil apparent resistivity data were ob-tained by using different arrays that are explained below.

3.1.1. L-corner (LC) arrayThis ERT array is discussed in detail by Chavez et al. (2011) and

Tejero-Andrade et al. (submitted for publication). This set is basedin a traditional WennereSchlumberger (WS) array and is the mostcomplicated of the three proposed arrays. This technique is done inthree steps. In the !rst step the L set is carried out (Fig. 3A) byshifting simultaneously, one electrode at a time, the current elec-trodes, A and B (in black), and the potential electrodes, M and N (ingray). The arrows de!ne the direction of movement. In thiscon!guration, theWS arraymoves along the square, starting with Ain the electrode #5 and ending when A reaches electrode #44. Inthe next level of observation, data is taken with a WS array openedby two positions. By this mean, the current electrodes start atpositions #5 and #10, whereas potential electrodes are now atpoints #7 and #8; the process is repeated until electrode A reaches

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again position #44. This procedure can be repeated n times in orderto have different levels of observation. This process is very similarto an ERT-2D if the electrodes were stretched out to form a straightline.

For the second step, the Corner array is surveyed. The currentelectrodes (black dots) are !xed in two opposite corners of thesquare (Fig. 3B), as well as the potential electrodes (gray dots).These move one position in the direction of the arrows until theyreach the end of the line. For starting electrodes #5 and #6 end atelectrode positions #13 and #14, respectively, while starting elec-trodes #25 and #24 end at electrode #17 and #16 position. In thenext level of observation, the current electrodes move back to theirposition in the opposite corners of the square (#5 and #25), but thepotential electrodes are now at #7 and #23. The array is moved oneposition in the direction indicated by the arrows until electrodepositions in #14 and #16 are reached. All the procedures are iter-atively repeated until just one observation can be done, that iswhen current electrodes are in position #5 and #25 and potentialelectrodes are in position #14 and #16.

The last step for the corner array (Fig. 3C) shows the sameposition of the current electrodes as above. The difference now isthat the potential electrodes (gray dots) move together in a clock-wise direction. For the !rst level of observation, the current elec-trodes (black points) are in #5 and #25 and potential electrodes(gray dots) are in #24 and #23. The entire array is moved indirection of the arrow until electrodes positions #16 and #15 arereached. The next level takes place by moving the current elec-trodes again to #5 and #25, the potential electrodes are now in #23and #22 and the array is moved until electrodes #16 and #15 arereached. The procedure is repeated until just one observation canbe completed, when the current electrodes are in #5 and #25 andthe potential electrodes are in #16 and #15.

The corner array is repeated for each corner of the square whichencloses the structure. The calculated position at depth of the

Fig. 3. The ‘L’ (A) and ‘Corner’ (B and C) arrays designed to survey the subsoil beneath the Main Pyramid. The arrows indicate the direction of measurements that the SySCAL-Promust follow. Coordinates of each electrode are loaded into the SySCAL-Pro system and position of current (black) and potential (gray) electrodes are given. Then, automatically, theERT device will survey the area.

Fig. 2. The ERT-3D array employed in the Main Pyramid conformed by 40 electrodes(numbered open circles) surrounding the structure. Numbers indicate the positionread by the SySCAL-Pro.

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apparent resistivities obtained through the observation sequencesare displayed by the commercial software Electre-Pro (Copyright2010 IRIS Instruments). This program estimates the depth of theapparent resistivity following Edwards (1977) rule, where an‘average depth’ is computed, according to the array employed(Tejero-Andrade et al., submitted for publication). Coordinates ofeach electrode as well as the observation sequences are introducedinto the program. The software automatically displays the positionof each apparent resistivity point at depth to be later measured inthe !eld.

The estimated 3D distribution of resistivities at depth (emptycircles) is depicted in Fig. 4 for the Main Pyramid. The position ofeach electrode is shown as small black dots forming a square. Theestimated depth of investigation was 8.25 m. The L array is shownin (D) and the Corner setting is displayed in (E). Combining botharrays, the total number of observation points was 640. Observingthe differences in both diagrams, the L array mostly covers an areaclose to the edges of the square, whereas the corner array producesa better coverage towards the deep central portion of the square. Ineither case, no data are found near the surface of the cube. It means,that if the combined dataset of D ! E data was inverted, theresistivity cubewill lack of resolution near the surface. Such processwas applied for the rest of the arrays employed.

3.1.2. Equatorial (Eq) arrayThis method employs two parallel ERT lines in the square

(Fig. 5A), where the current electrodes (black dots) are kept !xed atthe beginning of each line (current electrodes #25 and #35, forexample). The potential electrodes (gray dots) will thenmove in thedirection of the arrows (electrodes #24 and #36) until the totalnumber of electrodes in each line is completed (end electrodes #5and #15). The current electrodes move one electrode up (to, say,electrodes #24 and #36) and the potential electrodes shift oneplace as the sequence continues. The procedure is also carried outin the SoutheNorth parallel lines of the square, setting the currentand potential electrodes similarly as explained before.

A variation of the Eq array can be performed (Fig. 5B) by !xingagain the current electrodes (black dots) at the opposite corners ofthe square (electrodes #34 and #16). The potential electrodes willbe in positions #14 and #36. Such electrodes will move in thedirection shown by the arrows, while the current electrodes stay inthe same position. The !rst set of observation ends when thepotential electrodes reach positions #6 and #44. The procedurestarts again by moving the current electrodes to position #33 and#17 and returning the potential electrodes to position #36 and #14and moving them in the direction indicated by the arrows. Theacquisition process is completed when the total number of possiblepositions have all been occupied. Note that this methodologypossesses symmetry. The sequence can be continued by shifting theelectrodes to the opposite corner, repeating the process asexplained before.

The resulting subsurface resistivity observations, estimated bythe software Electre-Pro (Copyright 2010 IRIS Instruments), aredisplayed in Fig. 5C and D. Diagram in Fig. 5A corresponds to theresistivity observations for the array displayed in Fig. 5C. Thecentral portion of the square is well covered at depth, twoperpendicular series of observations are estimated. An interestingset of observed resistivity points are provided by the diagram ofFig. 5D corresponding to the array described in Fig. 5B. The set ofobservations are given as a diagonal series of data points, whichcross at the center of the surveyed square. The combination of bothseries of observations provided 912 measured points.

3.2. The Tecpan

The Tecpan is a much smaller edi!ce, with approximatedimensions of 22 # 22 m2. The archaeologists were interested onsurveying a larger area around the edi!ce for a better character-ization of the subsoil beneath the Tecpan, and to de!ne theextension of this pyramid at depth as well as to !nd resistivityanomalies of archaeological interest. Therefore, an area of33 # 33 m2 was covered, which allowed the placement of twodifferent sets of ERT arrays (Fig. 6).

The !rst array employed was the square (Fig. 6A), deploying 44electrodes in the ground. The second ERT set was formed with fourpro!les comprising 12 electrodes each (a total of 48) forming a grid(Fig. 6B), crossing the archaeological structure. In both arrays,electrodes were inserted carefully on speci!c spots de!ned by thearchaeologists. Therefore, the edi!ce did not suffer any majordamage. In both cases, a 3 m separation between electrodes waskept during the whole survey. It is important to note that thesecond array accomplishes amaximum separation of 10m betweenparallel lines of electrodes, a proposal set forth by Aizebeokhai et al.(2009).

The surveying process for the square array was exactly the sameas the procedure followed for the Main Pyramid, employing thearrays explained before. In the second con!guration, we employedthe traditional ERT-2D WennereSchlumberger, Eq and MC arrays.Fig. 7A depicts the 3D view of the estimated locations of apparent

Fig. 4. View of the attribution points for the disposition of the electrodes around theMain Pyramid for the ‘L’ (D) and ‘Corner’ (E) arrays. The solid lines in the surfacerepresent the electrical tomography lines. The points at depth represent the position ofthe resistivities observed in the subsoil. Such positions are calculated with the Electre-Pro software (Copyright 2010 IRIS Instruments).

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resistivities at depth for the WS array (Electre-Pro software,Copyright 2010 IRIS Instruments). These four pro!les depict a serialset of 2D earth resistivity models. Fig. 7B shows the result ofapplying the Eq array. This technique was carried out in parts,starting with the closest parallel lines and proceeding as explainedabove, then taking the !rst line with the third and the second withthe fourth pro!le, and !nally with the !rst and last line. That is thereason that Fig. 7B depicts !ve series of observed resistivity points.

The closest lines will produce shallower data points, and as thedistance between parallel lines increase, deeper estimated loca-tions for the resistivities are calculated. The last array employedwas the MC described below.

3.2.1. Minimum Coupling (MC) arrayTwo parallel lines are also needed to carry out the apparent

resistivity observations. The current electrodes are set at the

Fig. 5. Electrode observations carried out for the Eq parallel (A) and Diagonal (B) arrays. Arrows depict the direction of electrode shifting. Position of the electrodes around the MainPyramid for the Equatorial (Eq) parallel (C) and diagonal (D) arrays.

Fig. 6. Location of the electrodes around the Tecpan is depicted. 44 electrodes surround the structure (A) similarly to the methodology explained for Fig. 2. It was possible to haveelectrodes within the structure at selected positions (B) to carry on a traditional ERT-3D survey.

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corners of each ERT pro!le (Fig. 8), as done in the previous array. Inthis setting (Fig. 8A), the current electrodes are !xed (A, B), whereasthe potential (M, N) electrodes move along the ERT pro!lefollowing the direction of the arrow. Such process continues until

the last electrode is reached. Then, the current electrodes move oneelectrode space up and the process start again, moving the poten-tial electrodes once more. The sequence is complete when the lastelectrode is reached. Then, the same procedure follows by shiftingthe potential electrodes to the contiguous line (Fig. 8B) and theprocess is repeated in a similar way. Fig. 8C shows the apparentresistivity distribution at depth obtained with this process. Thesequence is done between closest lines and then for the fartherlines. In total, six patterns of resistivity positions are estimated bythe Electre-Pro software (Copyright 2010 IRIS Instruments).

The !nal observed resistivities at depth are obtained bycombining all the arrays discussed previously. Fig. 9A depicts thetotal resistivity points measured at depth for theMain Pyramid, andFig. 9B shows the location at depth of the total observation pointsfor the Tecpan. These diagrams display the total points (1552 pointsfor the Main Pyramid and 2358 points for the Tecpan) employed tocompute the 3D resistivity models.

4. Results

Data measured in the SySCAL-Pro console were downloadedinto a PC and analyzed for noisy measurements. Input !les werethen prepared the output !le to be inverse modeled with thecommercial software EarthImager 3D, which was originally basedon the smooth model inversion algorithm of Constable et al. (1987)and DeGroot-Hedlin and Constable (1990). The interpretation ofthe resistivity data was displayed in a 3D working cube, wherecolors represent the variations of the real resistivity at depth. Theconstructive materials used by the ancient dwellers of El Pahñúwere recollected from the surrounding areas. Therefore, the resis-tivity variations de!ned by the models will be of the same order ofthe geological material found in the site, and the geometry of theinverted models will play an important role in the detection of thepatterns of archaeological interest. The discussion will focus on theresults obtained for the Main Pyramid and the edi!ce known asTecpan.

Fig. 7. WennereSchlumberger (A) and Eq parallel (B) arrays are depicted. Sucha distribution of resistivities at depth was obtained with the second array (Fig. 7B).

Fig. 8. Electrode shifting method is shown (A and B) to estimate the resistivity distribution at depth (C) for the Minimum Coupling (MC) array.

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4.1. The Main Pyramid

The resulting model converged to an RMS of less than 10%. InFig.10Athe results for the3D inversionaredepicted for the resistivitiesdistribution under the Main Pyramid. The working cube hasa 33# 33m2 dimensionwith an estimated depth of 10m. The brokenline de!nes the footprint of the Main Pyramid. The image is viewedfrom the top with SWeNE geographical orientation. The computedmean resistivity value is around1645Ohm-mand canbe associated tothegeological horizon (tepetate). Tepetate is an indurated soil horizon,hardened by compaction or cementation, principally composed ofmaterials of volcanic origin of the Quaternary period. Due to itsphysical properties (high densities ranging between 1.7 and 1.9 g/cm3,a low porosity of 13e24%, poor fertility, water holding capacity andhydraulic conductivity), tepetate layers block water in!ltration,favoring lateral run off of soils deposited beneath it (Gama-Castroet al., 2007). This is an issue for the archaeologist, since it compro-mises the stability of the main structure and its future conservation.

The high resistive elements (>3000 Ohm-m) correspond toarti!cial !lling materials deposited by the ancient builders forterrain leveling and burying of foundations. These high resistivities,with respect to the tepetate, are due to a less compact and moreporous material. Fig. 10B shows the morphology, volume andposition of the arti!cial !lling (A). A resistivity interval of3500 Ohm-m up to 6000 Ohm-m is displayed (broken square in thecolor scale), showing the in!ll beneath the Main Pyramid. Sucha feature possesses an irregular shape and covers most of thenorthern half of the base of the pyramid, with an approximate areaof 15 m wide (Y axis) and 25 m long (X axis). Arrows infer thelandslide surfaces of the !lling material beneath the tepetate, justbeneath the foundations of the northern wall of the Main Pyramid,which indicates a potential damage of the structure.

Fig. 10A also shows several smaller high resistivity bodies (B, C,D, E, F and G) that are near the surface. Fig. 11C shows much clearlyanomaly B. This is an ellipsoidal body near the center of the cube, in

Fig. 9. Disposition of the total number of apparent resistivities to be measured atdepth combining the methods presented, for the Main Pyramid (A) and the Tecpan (B).

Fig. 10. 3-D display of the real resistivities distribution under the Main Pyramid (A). Dotted line depicts the limits of the Main Pyramid and the black dots mark the position of theelectrodes. Letters (AeF) indicate important detected features. A resistivity interval (marked in the color scale) is obtained; main resistivity anomalies can be clearly observed. Viewfrom the northern face of the cube. The landslide surface of the arti!cial !lling (A) is marked with discontinuous lines (B and C). (For interpretation of the references to color in this!gure legend, the reader is referred to the web version of this article.)

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the SE quadrant, with a maximum NeS diameter of 9 m, an EeWdiameter of 4 m, a depth to the top of 2 m, and an estimatedlength of 4m, approximately. This body could represent a natural orarti!cial void carved inside the tepetate and re-!lled afterwards.The content of this !lling cannot be determined by resistivity alone.Element D, in the center of the west wall, corresponds to an exca-vated offering box. Due to their similar characteristics, elements C,E, F and G could also represent ceremonial deposits, past lootings ornon reported exploration labors.

Fig. 11A displays the topographic model of the Main Pyramidfrom above. The inverted resistivity model is superimposed on it. Itis possible to observe the position of the unconsolidated in!ll (A)with respect to the main edi!ce. An image of the present-daycondition of this portion of the pyramid is shown in Fig. 11B(right) taken 10 m away from the northern facade. The !ssures andcracks (shown in the photograph) detected in the structure areclear evidence of the risk of collapse of this archaeological feature. Acouple of wooden poles were set to support part of the structure

preventing that section from falling down. The in!ll should behardened by pumping into the subsurface special materials thathelp to make the subsoil more stable and capable to support theMain Pyramid. Anomaly (B) could be associated with a buriedoffering, found towards the SE section of the Main Pyramid.Archaeologists believe that such an interesting resistivity featurecould be appealing for excavation. However, they should have to digmore than 8 m to reach the ground level, destroying part of thepyramid. Then, they should dig other 2 m to reach the top of theresistive structure. Archaeologists think that such a procedurecannot be done at this moment, due to the conditions of the MainPyramid. In the future, if the technology is available, a directionalwell can be designed to reach such an anomaly to con!rm its origin.

4.2. Tecpan

The distribution of true resistivities at depth for the Tecpan isshown in Fig.12. The limits of the Tecpan are represented by dashed

Fig. 11. The topographic model and the corresponding resistivity 3D model are superimposed on a 90" view. The in!ll material (A) affects the NW portion of the structure. Aninteresting anomaly (B) can be associated to a foundation offer. Image to the right depict the subsidence effects on the main building, !ssures and cracks are deteriorating thestability of the edi!ce.

Fig. 12. A 3-D distribution for the inverted real resistivities of the Tecpan is depicted in two different views. Three important anomalies can be observed. A low resistive anomaly (A)is found within the limits of the Tecpan. Anomaly B depicts an interesting morphology, suggesting the entrance of a cave. Anomaly C could be associated to a more consolidated tuff.Finally, dotted lines de!ne a pre-Hispanic "oor, located few centimeters deep from the surface.

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lines. The central resistivity value is 1000 Ohm-m, approximately,that can be associated to a healthy tuff (tepetate), suggesting thatlesser !lling material was employed in this site for the constructionphases of this building. Probably, because the terrain in this part ofthe area is almost "at. Thus, terrain leveling for the foundations ofa smaller building was less demanding. Nevertheless, there aresome anomalies that seem to be important from the archaeologicalpoint of view.

The image depicts a low resistivity anomaly A (100 Ohm-m,observed in Figs. 12 and 13A), surrounded by a high resistivityanomalies (w1000 Ohm-m), of rectangular shape with dimensionsof 20# 15m2. Such an effect might represent the inner open area ofa room while the high resistivity bodies correspond to the wallfoundations closing the room. Since this building had a political-administrative function, the size suits an adequate space forpeople gathering. Feature B shows a high resistivity anomaly(w10,000 Ohm-m) suggesting the entrance of a cavity of approxi-mately 4 m wide and 6 m high. Tunnel entrances can be observedon the very step walls of the tuffs walls of the cliffs around thearchaeological site. Thus, it is dif!cult to assure if such a featurepossesses a natural origin or it is a man-made structure. A highresistivity expression is de!ned by C. This feature might correspondto the presence of highly consolidated geological horizon. However,archaeologists believe that might be part of the in!ll materialdeployed by pre-Hispanic dwellers to level the terrain. The dottedline shown in Fig. 12A and B suggest the presence of compactedmaterials that could be associated to a "oor or a platform, overwhich the Tecpan was built. It is few centimeters deep and looksuncontinuous around the pyramid. Previous archaeological exca-vations done on the site had found a hardened soil layer coveredwith calcareous materials (stucco) over which the structures wereerected. This might correspond to a stucco "oor of the !rstconstruction moment of the archaeological site.

The average resistivity media has been removed in Fig. 13, andonly selected resistivity intervals corresponding to a very low and

high resistivity values are left. The resistivity intervals of 96 Ohm-me260 Ohm-m, and 3000 Ohm-me10,000 Ohm-m are displayedin this image. The !rst interval presented corresponds tomaterials lesser consolidated, where feature A matches the limitsof the Tecpan. This could be an evidence of an inner room,beneath this structure. On the other hand, anomaly C depictsa high resistive bulk of more than 6000 Ohm-m that can beassociated to a well consolidated tuff. Anomaly C re"ects a highresistive material, which can be interpreted as a cavity entrance.The morphology of such resistive material suggests a tunnelpartially collapsed. This is due to the lack of continuity towardsthe eastern section of the Tecpan (observe Fig. 12A and B). Sucha structure deepens up to 7 m deep.

Finally, Fig. 13 depicts the resistivity cube overlapped with thetopographic model of the Tecpan. Resistive feature A !ts well overthe western portion of the Tecpan. The interpreted geometry ofsuch low resistive material seems to de!ne the foundations of the!rst constructive phase of this pyramid. Anomaly B is of greatextension and it is very likely that this material was employed tolevel the terrain at this point, towards the SE of the structure. Theinteresting resistivity element depicted by letter C shows anevidence of the existence of a tunnel in the direction to the Tecpan.It is interesting to note, that the resistive structure apparently endsat thewestern stairway of the pyramid to an approximate 2m deep.

5. Conclusions

The new ERT-3Dmethodology applied to survey two discoveredarchaeological structures solved properly the questions involved inthis investigation with a few losses of shallow information. Theresults provided useful data about the distribution of archaeolog-ical and geological elements, where interesting resistive featureswere detected. In the Main Pyramid, the presence of an arti!cial!lling deposited over the tepetate horizon was detected, warningabout the landslide planes and the lateral run off of soils

Fig. 13. A 90" view is presented for the Tecpan showing two resistivity intervals. Anomaly A possesses and interesting geometry suggesting a small gathering room or part of thefoundations of the !rst constructive phase. Anomaly B might correspond to a Tuff more consolidated, and !nally C suggest a cavity or tunnel probably partially collapsed. Anomaly Cpossesses continuity towards the main stairway of the Tecpan.

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underneath the northern portion of the pyramid. There were alsodetected high resistivity anomalies of lower dimensions that can berelated to the position of arti!cial voids due to ritual activities,lootings or archaeological excavations.

\In the Tecpan, archaeological elements related to the !rstconstructive phase of the site were identi!ed. Findings in thesouthern entrance mound show that a geophysical survey is moresuccessful when applied to non altered structures. The presence ofan elongatedhigh resistive feature suggests thepresence of a tunnel,in the direction of the western stairway of this edi!ce. This featuremight be partially collapsed. A low resistive anomaly detectedbeneath the Tecpan suggests the presence of a buried room or thefoundations of the !rst constructive phase of this edi!ce.

The methodology applied in this research establishes new toolsfor shallow underground investigation, especially when facing thedif!culty of having buildings that cannot be altered by any means.The method presented here is much lesser invasive and opens thepossibility to study both pre-Hispanic pyramids and Colonialedi!ces. Even if resolution at very shallow depths is not very good,the resistivity distribution computed at depth provides of veryuseful information to characterize the subsoil beneath affectedstructures.

Acknowledgments

Wewould like to thank Dale Rucker, Jorge Gama-Castro and theanonymous reviewers for their assertive comments on how toclarify and improve the article.

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