geophysical studyof a pre-hispanic lakeshore settlement, chiconahuapan lake, mexico

Geophysical Studyofa Pre-HispanicLakeshore Settlement,ChiconahuapanLake,Mexico

R.E.CHA¤ VEZ1*, A.TEJERO2,D.L. ARGOTE1ANDM.E.CA¤ MARA3

1 InstitutodeGeofısica, UniversidadNacionalAuto¤ nomadeMe¤ xico, Cd. Universitaria, CircuitoExterior s/n,04510Me¤ xico D. F., Mexico2 Facultad de Ingenierıa, Universidad NacionalAuto¤ noma deMe¤ xico, Circuito Interior s/n, 04510Me¤ xicoD.F., Me¤ xico3 ETSII-UniversidadPolite¤ cnica deMadrid, C/Gutie¤ rrez Abascal, 2. 28006Madrid, Spain

ABSTRACT Resultsofacombinedgeophysicalsurveycarriedoutalong thenorthernshoreof LakeChiconahuapanincentralMex-icoarepresented.Thesite containsaseriesofmoundsfoundbetweenthe lakeshoreand the ceremonialcentrenamedLa Campana-Tepozoco Hill.Those features are the remains of habitation complexes built by the lake residents about1100 years ago. Archaeological excavations performed towards the northeastern margins of the lake uncoveredrectangular clusters of basaltic rocks forming the foundations of ancient dwellings, surrounded by a barrier (‘bordo’),built toprevent floods.Amoundlocatedclose to the ceremonialcentrewasstudiedusing theverticalmagneticgradient(VMG), ground-penetrating radar (GPR) andelectric tomography (ET) methods. Interestingmagnetic anomalieswerefound towardsthe centralportionof themoundandoneofthesewasmodelledwitha three-dimensionalmagneticpoly-gon.TheGPRstudiesunveiledacircular featureinthecentralportionofthemoundthatcouldbeassociatedwithabordo.A rectangular structurewas also interpretedwithin the limits of that structure, which isprobably the location of ancientdwelling foundations.Electric tomographyprofileswere collected in the area, around theVMGanomaly.The resistivitymodel computed shows the presence of a high-resistivity layer, which displays discontinuitieswithin the area definedby the bordo. Presumably, the ancient settlers built their constructions on top of this horizon.The geophysical resultsreveal a rather complex habitat within the mound.The larger size of the bordo (ca. 30m in diameter) compared withothers already studied by archaeologists makes us believe that the purpose of the site was of administrative use orhostedawealthygroupofpeople.Dwelling foundationsare foundwithin the limitsof thebarrier.Copyright# 2009 JohnWiley & Sons,Ltd.

Keywords: LakeChiconahuapan;Pre-Hispanic;bordo;verticalmagneticgradient;ground-penetratingradar;electrictomography

Introduction

This paper presents the results of a geophysical surveycarried out to map buried remains of ancientfoundations on top of a mound located towards thenorthern margins of the Chiconahuapan Lake. Thismound is found in the vicinity of the La Campana-Tepozoco pyramid. This feature is about 1 m above themean lake elevation of 2570m (a.s.l.) today and it is

believed to be part of an elite residential or admin-istrative zone, due to its size and proximity to themonument site of La Campana-Tepozoco pyramid(McClung and Sugiura, 2002), located to the east of thestudied mound.

The area is located in the southeastern portion of theValley of Toluca (Figure 1A), northwest of Santa Cruztown in the state of Mexico (Figure 1B), along thenorthern margin of the Chiconahuapan Lake, approxi-mately 56 km southwest of Mexico City. The LaCampana-Tepozoco site functioned as a religious–civic centre during the Epiclassic Period from aboutAD 700 to AD 900 (Sugiura and Serra, 1983), whichcontrolled the area both politically and economically

Archaeological ProspectionArchaeol. Prospect. 17, 1–13 (2010)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/arp.367

* Correspondence to: R. E. Chavez, Instituto de Geofısica, Universi-dad Nacional Autonoma de Mexico, Cd. Universitaria, CircuitoExterior s/n, 04510 Mexico D. F., Mexico.E-mail: [email protected]

Copyright # 2009 John Wiley & Sons, Ltd. Received 30 June 2009Accepted 28 July 2009

(Sugiura, 2000). The main archaeological feature is the8m tall La Campana-Tepozoco pyramid built on top ofa 3m height man-made platform, visible in the satelliteimage shown in Figure 2. A topographic map has beenalso superimposed on the satellite image, whichdepicts elongated features that appear to be hillocksapproximately 1m height above the ground level (atleast sevenmounds can be observed in the topographicmap of Figure 2). Archaeologists have identified about30 features of this type along the western and northernportion of the Chiconahuapan Lakemargins, although,more than a 100 might have been occupied in the pastat different times during this short period of time(Sugiura and Serra, 1983). Pre-Hispanic construction inthis area was commonly protected from floods duringthe rainy season by small barriers called ‘bordos’, whichsurrounded the habitation complexes (McClung andSugiura, 2002).A comparison of ceramic materials found within

mounds suggests that important social differences

existed between the inhabitants in the vicinity of the LaCampana-Tepozoco pyramid and those elsewherealong the lake shore (Sugiura, 2000). For instance,public buildings as well as dwellings inhabitedperhaps by priests, royalty and high status peoplewere rectangular in shape, larger and built withdressed stone, whereas smaller earthen domesticconstructions of lower status people were foundtowards the lake shore zone (Sugiura and Serra, 1983)McClung and Sugiura (2002) excavated one of these

mounds located south of the studied area, in themargins of the lake, uncovering a structure with acircular wall 0.2m height and diameter of over 10m.Its perimeter was delineated by equidistant spacedwooden posts that supported the walls, which weremade of adobe blocks and wattle and daub or otherperishable materials (McClung and Sugiura, 2002).Rectangular shaped rock clusters of volcanic originformed the foundations of the dwellings. Dwellingfloors appear to have been renovated periodically with

Figure 1. Locationof thestudiedarea (A) taken fromchart E14A48 (INEGI,2006) and the corresponding topographicmodel (B); theChiconahuapanlake canbeseen towardsthe centralportionof themap.Geologicaloutline (C) ofa smallportionof theValleyof Tolucaisalso shown (modified fromArce et al., 2003).

Copyright # 2009 John Wiley & Sons, Ltd. Archaeol. Prospect. 17, 1–13 (2010)

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2 R. E. Chavez et al.

layers of soil and gravel perhaps with the intentionof elevating the living surfaces during continuoussubsidence of the mounds. This population centrepresumably declined as a religious and commercialtrading place around the year AD 1000, perhaps due tolocal climatic changes that affected the water levels ofnearby lakes and rivers (Sugiura, 2000).

The geophysical methods used were vertical mag-netic gradient (VMG), ground-penetrating radar(GPR) and electric tomography (ET). The results ofthese surveys were compared and contrasted toresolve the dwelling foundations, the limits of thebarrier (bordo) and position at depth of archaeologicalfeatures (Cardarelli et al., 2008). Size of dwellings isimportant in order to establish the use of the complexbuilt here. Such a characteristic is an indicator on theuse the ancient settlers gave to this site.

Geophysical studies

The Campana-Tepozoco site is situated inside thegeologically active Upper Lerma Basin filled with

igneous materials formed by a large Plinian event11 600 yr BP, named Upper Toluca Pumice (UTP;Caballero et al., 2001) (Figure 1C). A stratigraphicprofile for the unit was obtained from several shallowboreholes by Caballero et al. (2001) and Lozano-Garciaet al. (2005) on the northern and western margins of thelake. They found that the UTP unit is a volcaniclasticbed found at depths ranging from 3 to 4.5 m, with atotal thickness of more than 1m. Susceptibilitymeasurements made in situ on core samples deter-mined a value of K¼ 0.1T 10�3 SI for the sedimentarycover (mainly volcanic ashes) and K¼ 1T 10�3 SI forthe UTP. Such a geological context and the charac-teristics of the expected archaeological remains led us toemploy the vertical magnetic gradient (VMG), ground-penetrating radar (GPR) and electric tomography (ET)methods. The magnetic method (VMG) was used tomap the distribution of the basaltic clusters composingthe foundation of the dwellings, which are highlymagnetic and located on top of the UTP layer. The GPRsurvey was carried out within the same grid toestimate the geometry of the UTP layer as well as toimage the location of the possible bordos, as the method

Figure 2. Satellite image (obtained from Google Earth, 2007) displays the context of the studied area (rectangle) and the topographic contours(0.25m) of the surrounding area.Dashed circle displays the location of La Campana-Tepozoco pyramid.This figure is available in colour online atwww.interscience.wiley.com/journal/arp


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Pre-Hispanic Lakeshore Settlement, Chiconahuapan Lake, Mexico 3

has good vertical depth control. The first two methodswere carried out over an area of 70T 57m2. Finally, theETmethod was used to provide information regardingthe resistivity distribution at depth of the UTP. The ETstudy comprised six profiles around the centre of thestudied mound.

Vertical magnetic gradient

A high-resolution vertical magnetic gradient (VMG)survey was carried out employing a proton EnvimagScintrexmagnetometer, with a resolution of 0.1 nT. Thesensor configuration had a vertical separation of 1m.Magnetic measurements were taken at intervals of0.5m on south–north oriented lines with a distancebetween transects of 1 m, producing a total of 7980magnetic readings. A VMGmapwas obtained where aseries of small dipole anomalies can be distinguishedtowards the southern and eastern portions of the grid(Figure 3). In particular two VMG anomalies areclearly observed in Figure 3 (A and B), and depictthe presence of foundations, probably similar tothose reported by McClung and Sugiura (2002). Thesewere made by a rectangular cluster of basaltic rocks.The magnetic map in general contains a high degreeof noise perhaps due to shallow heterogeneities inthe subsoil, such as exposed basalt stones or pottery

fragments, which are commonly found in archae-ological sites. In order to isolate the dipolar anomaliesand to reduce the noise level, a spectral analysis (Spectorand Grant, 1970; Chavez et al., 1995) was applied todelimit the wave number interval of interest and toeliminate the noisy portions of data collected withinthe studied area (Figure 4).In this filtering process to eliminate noise the power

spectrum of the VMG field was first calculated, andthe logarithm of the power spectrum <E(k)> wascomputed in terms of the radial wavenumberk¼ (u2þ v2)1/2 (Figure 4). Spector and Grant (1970)stated that a clear separation in the spectrum should beobserved, as they demonstrated theoretically with asynthetic two-dimensional block ensemble. However,it is not always possible to clearly define a ‘break’ in thespectrum (Gupta and Ramani, 1980), which helps us todisregard regional (deep) from residual (shallow)magnetic effects. Residual magnetic signals could berelated to archaeological features, which are ourinterest. Different strategies have been developed(Pilkington and Cowan, 2006) to define such linearfeatures. One of these employs the theoretical relation-ship between the spectrum <E(k)> and the radialwavenumber k in terms of the model or modelsemployed by Spector and Grant (1970), which is dis-cussed in detail by Chavez et al. (1995). In this method

Figure 3. Theverticalmagneticgradient (VGM) fieldobtained (contour linesare1nTm�1).AandBdepict the twomost importantdipolaranomaliesfound in the surveyedarea.


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the slope inferred within the wavenumber interval canbe expressed in terms of a mean depth, wherethe anomalous magnetic bodies lie. A low-wavenum-ber interval (0 < k < 0.05m�1) can be defined, whichcorresponds to ‘deep’ magnetic sources (stripedportion to the left of diagram). A regional magneticexpression is estimated with a mean depth of30� 0.1m (Figure 4) associated with the lava flowsfrom previous volcanic events that formed the UTPlayer. Such eruptions were reported by Siebe et al.(1995). This geological horizon was the product ofvolcanic eruptions of the Nevado de Toluca volcano,located in the southwest portion of the Toluca Valley,near to the studied area, to the west.

An intermediate wavenumber interval (0.05 < k <0.32m�1) associated with residual anomalies can alsobe characterized. Following Spector and Grant (1970),an average depth of 1.4� 0.4m can be estimatedassociated with this intermediate interval (Figure 4).

This value can be associated to the mean depth ofshallow magnetized bodies, which may be of archae-ological interest. Such bodies probably lie on top of theUTP, as McClung and Sugiura (2002) have reportedelsewhere.

A high wavenumber portion can be identified in theinterval (0.32 < k < 0.70m�1), which corresponds tothe noise level in the magnetic data, shown as a stripedsection to the right in Figure 4. Because of the noisecomposition, the adjusted slope to this set of datashould be around zero (Spector and Grant, 1970). Thisportion of the spectrum reflects the errors related toacquisition and processing of the observed magneticdata. Thus, it is possible to remove this region byapplying a suitable filter.

A band-pass filter (Butterworth filter) was appliedto isolate the interval of interest (residual portion),which is probablymeasuring properties of materials ofarchaeological relevance. Later, a horizontal gradient

Figure 4. Spectralanalysisofdata fromFigure3.Shadedzonesdepictunusedportionsofthespectrum.Thecentralpart representstheeffectoftheresidual field employed, where an average depth of1.4� 0.4m is associatedwith shallow structureswithin thewavenumber interval of 0.05< k<0.32m�1.


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filter was applied to the filtered map to delimit theextension of the magnetized bodies (Figure 5), whichenhanced the edges of the magnetic bodies, detectingchanges in the magnetic anomalies in both the x and ydirections (Chavez et al., 1995;Argote andChavez, 2005).The horizontal gradient map of Figure 5 displays

several dipolar anomalies with gradient valuesbetween 2nTm�1 up to 9 nTm�1 aligned in the S–Ndirection in the central portion of the surveyed area.These are related to magnetization contrasts (magneticsusceptibility K), between what are probably archae-ological magnetic features and the volcaniclastic UTPlayer upon which they rest. The magnetic patternsobtained are probably associated with remains of thedwelling foundations (Sugiura, 2000; McClung andSugiura, 2002). Two magnetic anomalies can beobserved (A and B in Figure 5), with anomaly A closethe centre of the mound, and B towards the westernedge of the mound. On the other hand, the magneticpattern observed in feature B is more likely the resultof a more complex structure or series of structures,whose magnetic signatures superimpose. It is there-fore possible that this feature is a collapsed structure,withmaterials forming the structure beingmagnetizedin different directions (Chavez et al., 1995).A visual analysis of the main magnetic anomalies

displayed in Figure 5 (A and B) indicates that thearchaeological structures possess remnant magneticeffects, since the anomalies are not oriented in thedirection of the Earth’s magnetic field at this latitude

(io¼ 478 and do¼ 68, inclination and declination angles,respectively). As it is not obvious to assume or estimatethe body’smagnetic orientation in regard of the Earth’smagnetic field direction (assuming an inductioneffect), a numerical approach can be applied toestimate the magnetic parameters characterizing theobserved magnetic signal. In order to transform amagnetic anomaly possessing a remnant effect toperform a more defined interpretation, a reduction tothe pole, for instance (Chavez et al., 1995, 2001) can beapplied. If an observed magnetic anomaly possesses aremnant effect, a numerical transformation can beapplied to eliminate such a feature. Thus, interpret-ation and modelling processes can be performed,since it can be assumed that the buried body wasmagnetized solely by the present Earth’s magneticfield. Roy and Aina (1986) mention that magnetictransformations are governed by a simple differentialrelation between the observed and the transformedquantities, and obtained an expression in the spacedomain. Correa (1990) and Chavez et al. (1995)represent this general expression in the wavenumberdomain (p,q) in terms of an observed magnetic fieldTðp; qÞ and its transformed field T

0ðp; qÞ as:

T0ðp; qÞ ¼ M

0

0ðp; qÞM0

mðp; qÞM0ðp; qÞMmðp; qÞ

Tðp; qÞ (1)

Where M0(p,q) is a function that depends on thedirection of the geomagnetic field measured in the

Figure 5. FilteredVMGmapisobtainedafterapplyingabandpassfilter (Butterworth type).ObservethatanomaliesAandBcanbeseenmuchmoreclearly.Noisydata hasbeen removed.


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observation region, and Mm(p,q) depends on thedirection of the remnant magnetization field. Thesefunctions are expressed as:

M0ðp; qÞ ¼ ipLþ iqMþ kNMmðp; qÞ ¼ iplþ iqmþ kn

(2)

were (L,M,N) and (l,m,n) are the direction cosines in thedirection of the magnetization fields (geomagnetic andremnant, respectively). The primes in Equation (1)indicate the new transformed field functions.

Although Roy and Aina (1986) do not providean analytical expression for such a mathematicaloperator, following their ideas it is possible to obtainan expression to perform this operation. ForEquation (1), T(p,q) is the observed anomaly producedby a body with induced plus remnant magnetizationsand T’(p,q) is the transformed anomaly produced bythe same body with purely induced magnetization.Since the measured and transformed componentsare the same, the factor M

00=M0 ¼ 1. Therefore,

Equation (1) reduces to:

T0ðp; qÞ ¼ M0ðp; qÞ

Mmðp; qÞTðp; qÞ (3)

whereM0m ¼ M0. In the present investigation, we have

worked with the VMG anomaly, instead of the totalfield magnetic anomaly. However, the transformationfunction does not change and can be expressed as:

VMG0ðp; qÞ ¼ ipLþ iqMþ kN

iplþ iqmþ knVMGðp; qÞ (4)

Note that the magnetic direction (induced andremnant) must be known in order to use Equation (4).The remnant magnetic field direction can therefore beinferred using an indirect method (Zietz and Andrea-sen, 1967). We estimated such parameters followingChavez et al. (1995), who describe in detail such amethodology. We have estimated values of 908 and 08for the inclination and declination angles respectivelyfor anomaly A (Figure 5).

A detailed analysis is carried out for the VMGanomaly A (Figure 6A). Expression (4) can beprogrammed in the wavenumber domain to computethe transformed VMG anomaly A depicted inFigure 6B. This anomaly can be modelled usingthree-dimensional polygonal models (Correa, 1990).Figure 6C depicts the computed VMG field obtainedwith a simple polygon projected to the surface. Finally,Figure 6D displays a three-dimensional view of theposition at depth of the modelled magnetic body. Thissuggests the features beingmapped are 1.4mdeep and0.5m thick with 4m length on each side. The

susceptibility contrast that better fits the data has avalue of K¼ 8.7� 10�3 SI. This value corresponds wellto basalt rocks and the model probably represents thefoundations of a rather larger habitat (McClung andSugiura, 2002).

Ground-penetrating radar

A Geophysical Survey Systems, Inc. (GSSI) SIR-2000was utilized in this investigation with a 200MHzshielded monostatic antenna manufactured also byGSSI. The survey consisted of a total of 56 E–Wtransects, 70m long, with a separation of 1m betweenlines. The data were filtered in the frequency domainusing the software RADAN V.6 (2004). The DN(distance normalization) processing included in theRADAN software enabled the establishment of aconstant horizontal scale with distance betweenmanual marks of 5m.

One of the problems arising in the field, as well as inthe interpretation process was to adequately infer acorrect estimate of the relative dielectric permittivity(RDP). An experiment was carried out at the site inorder to estimate the RDP, using an iron pipe buried1m beneath the ground. Several experimental profileswere collected perpendicular to this object (Conyersand Lucius, 1996) and the hyperbolic reflectionindicated an average RDP of 10 for this area, a valueassociated with unconsolidated soils and sediments(Conyers, 2004). Therefore, the 100 ns window used todisplay the GPR data corresponds to an approximatedepth of 4.7m.

A horizontal amplitude slice from 20 to 40 ns (1–2m)was constructed using software RADAN V.6 (2004)software (Figure 7A). The most interesting amplitudefeature is visible towards the central part of the slice(Figure 7A) where a circular reflective feature 30m indiameter probably represents a preserved bordo. Therim of this structure surrounds an interestingrectangular feature (A), which could be associated tothe remains of the foundations of an importantstructure located along the central part of the rim.McClung and Sugiura (2002) suggested that spaces ofthis size (10� 10m2, approximately) might have beenused as a meeting place. Also, it is possible to seetowards the western portion of the amplitude slice(Figure 7A) another feature (B), which coincides withthe position of the VMG anomaly B (Figure 5).

Reflection profiles depict three layers, the lacustrinesediments at the top, which possesses an averagethickness of 1m. Then, the UTP layer in the middle,where its thickness ranges between 1m and 1.5m andits depth to the top is 1 m, approximately. Finally, the


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homogeneous lacustrine mud is located at the bottomof the reflection profile. This layer attenuates theradar energy at this depth due to its high electricalconductivity.The limits of the bordo can be seen in the GPR profiles

in Figure 7B, and coincide with a topographic low ofthe UTP horizon where the infill material is thicker.White arrows in Figure 7B (line GPR-30) depict thelimits of the squared feature. In reflection profile GPR-18 (Figure 7B); circle A shows an alteration in the UTPlayer that may indicate the remains of a foundation.

Electric tomography

Six ET profiles were surveyed across the mound(Figure 5). Resistivities were measured with STING(AGI) equipment, using a Wenner–Schlumbergerconfiguration for better vertical and horizontal resol-ution (Tejero et al., 2002) with an electrode spacing of

1m. Three profiles were collected in the E–W directionwith a length of 60m (profiles ET-1, ET-2 and ET-3),and the other three profiles in the S–N direction withlengths of 40m (ET-4, ET-5 and ET-6). Real resistivitieswere computed by an inverse process employing thealgorithm RES2Dinv (Loke and Barker, 1996; Tejeroet al., 2002). An iterative process was carried out foreach set of data until changes in the inverted resistivityvalues did not vary by 5% of error. This was achievedin approximately seven iterations for most cases. Inparticular transects ET-1 and ET-4 required five andeight iterations, respectively, to reach the minimumerror specified.Figure 8 shows the inverted profiles in a

three-dimensional view. We have only plotted aresistivity range between 230 ohm-m to 750 ohm-m.Such values correspond to pumice materials compris-ing the UTP layer (Telford et al., 2004). The resistivelayer is approximately 1m thick in the six profiles andit is buried 1m in average beneath the surface. The

Figure 6. StepsfollowedtomodelVMGanomalyA.(B)AccordingtoZietzandAndreasen(1967)theremnanteffectcanberemoved.(C)Therefore,wehave used three-dimensionalmagnetic blocks to compute the synthetic response. (D) Three-dimensionalviewof themagneticmodel computed.


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Figure 7. (A) Three-dimensionalvisualizationofaprocessedGPRslice0.5mthickistakenatadepthof1.5 m.AgeneralviewoftheUTPlayerat thatdepth isobtained.Thedashedlinesshow themain featuresdepictedin thisslice.Thebordo limit isshown, aswellasmagnetic featuresAandB thatmaybeassociatedwithdwellingfoundations. (B)ObservedprofilesGPR30,18and06arepresented.TheUTPlayercanbeseenintheseradargrams.It is also possible tomap the discontinuities in this layer.Circled A correlateswith the position at the surface of VMGanomaly A shown in Figure 5.Arrowsdepict the location of the bordo limits, where the UTP layer showsa discontinuity.


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resistive layer is not continuous within the bordo limits.Evidence of this horizon can be observed to the northin the same Figure 8. A conspicuous curve can bedrawn following these discontinuities in the UTP,which coincides with the bordo limits defined by theGPR (Figure 8). Such low resistivity values (<250 ohm-m) are related to the infill material already observedin the GPR profiles (Figure 7B). This infill couldhave been deposited by the ancient settlers to coverthe extracted materials from the UTP layer and setthe foundations made of clusters of basaltic rocks(McClung and Sugiura, 2002). The bottom layer isassociated with a highly saturated (of water) horizon,about 4m to 5m deep, beneath the UTP horizon. Thatunit probably corresponds to the lacustrine mudreported by Lozano-Garcia et al. (2005). The associatedresistivities ranging between 100 and 200 ohm-m werenot plotted. The ET interpretation helped to map ahigh-resistivity layer associated with the UTP horizon.This layer is not continuous within the ‘bordo area, andportions of this resistive layer are observed towardsthe northern portion of the area.

Integrated Results

A comparison of the results obtained in this study aswell as the topography of the mound itself is shown in

Figure 9. Figure 9A shows the horizontal magneticgradient anomalies (Figure 5) on top of the mound’stopography. A group of weak magnetic (horizontalgradient) anomalies can be observed towards thecentral portion of the mound. It is interesting to notethat anomaly A is found towards the southern portionof the bordo limits, and not coincident with the centralpart of the mound, as expected. Further, a series ofweak magnetic (horizontal gradient) anomalies arefound at the central portion of the mound’s topo-graphy. On the other hand, the VMG (horizontalgradient) anomaly B is found at the outskirts of themound, totally away of the ‘bordo’ limits, towardsthe left of Figure 9A. It seems that remains of a morecomplex habitat layer could be buried beneaththis mound, if we expect that the magnetic expressionsfound depict the presence of dwelling foundations(clusters of rocks), as described by Sugiura (2000) andMcClung and Sugiura (2002).Figure 9B presents the GPR slice of Figure 7A along

with the VMG (horizontal gradient of Figure 5)anomaly map. Projected to the surface, the magneticmodel displayed in Figure 6D is also shown inFigure 9B (hatched square). The group of magneticanomalies observed at the central part of the image iswithin the limits of the circular bordo feature (dashedline), which also includes the rectangular feature(dashed line) detected with the GPR method

Figure 8. Electric tomography (ET) profilesare shownin thewest^east (A) andnorth^south (B) directions.Resistivity valuesbetween 270 ohm-mand 700 ohm-mhavebeenplotted.The bordo limit canbe inferred (dashed line).


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(Figure 7B). The VMG anomaly B is found within anelongated element shown by a dashed line to the left ofFigure 9B. Such a structure suggests the existence of asmall bordo protecting what we believe is a dwellingfoundation. Figure 9C depicts the mound topographysuperimposed with the GPR slice. The bordo limit islocated in the central portion of the topographicalmound, as expected. However, the estimated size ofthis feature is 30m in diameter, much bigger thanthat excavated by McClung and Sugiura (2002), whoreported a 10m in diameter structure on a moundlocated to the south of the studied area, within the lakemargins. Finally, Figure 9D shows the location of theET profiles on top of the VMG anomalies. Thicker lines(in black) represent resistivity values between 650 and750 ohm-m, which correlates with the location of themodelled magnetic body. Grey thick lines representresistivities between 250 ohm-m and 650 ohm-m. Agood correlation exists on the position of the bordolimits and the high resistivity values associated with

the UTP layer. Resistivity anomalies (>700 ohm-m)found in the central portions of profiles ET-1 and ET-4can be correlated with VMG anomaly A (Figure 5),which depict evidence of dwelling foundations.

The combined results obtained helped to define acomplex habitation built on this mound. Size of thebordo suggests either an administrative or religioususe of this site or the emplacement of a wealthysettlement (Sugiura, 2000; McClung and Sugiura,2002). Location and geometries of foundationsdetermined by the VGM and GPR techniques suggestthat this particular mound underwent different activi-ties over time, which makes it very different to itssouthern counterparts.

Conclusions

The most important contribution of this geophysicalstudy was the mapping of a series of geophysical

Figure 9. Theresultsobtainedcanbeintegratedindiagramsdepicting(A) themoundtopographyover theVMGanomalies. (B)TheGPRsliceiscom-pared also with theVMG anomalies. (C) Themound topography is plotted against the GPR slice. (D) TheVMG anomalies are compared with thelocation of themore significant ETanomalies (thicker lines in grayand in black).


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anomalies related to ancient foundations of a complexhabitation space within a lakeshore environment. Thearea studied corresponded to a region not coveredby the lake waters during the rain season duringprehistoric times.Each method applied provided relevant information

about the studied area. The VMG observationsrevealed the position of a series of magnetic anomalies,which are associatedwith dwelling foundations. Thesestructures were made of clusters of basalt rocks, whichhave an important magnetic susceptibility differencewhen compared with the underlying UTP layer. Theestimated area of these foundations range between6 m2 to 12 m2, which is larger than those reported byMcClung and Sugiura (2002) from excavations per-formed in mounds located towards the northern andnorthwestern margins of the lake (south of the studiedarea). The filtering process applied to the magneticdata removed a regional effect, emphasizing theexpression of individual anomalies, and the attenu-ation of the magnetic signature of the UTP layer, aswell as noisy effects. It was also possible tomodel VMGanomaly A (Figure 5) by using three-dimensionalmagnetized blocks (Figure 6D). The GPR interpret-ation depicts an anomaly (Figure 7B, line GPR18, circleA), which coincides with the position at the depth ofthe magnetic anomaly A. It seems that foundationswere set on top of the UTP layer. The GPR studiesimaged the geometry of the UTP layer, which shows anaverage stratigraphic thickness of 1m. Also, thistechnique revealed discontinuities in the UTP layer.A GPR amplitude slice about 1m thick revealed acircular pattern that could be attributed to the limits ofa bordo, a structure built by the ancient settlers toprevent floods from inundating their homes.The resistivity survey confirmed the presence of

features revealed by the magnetic analysis as well asdiscontinuities in the UTP layer, visible in the GPRreflection profiles.The circular pattern of the bordo show some

similarities observed in other pre-Hispanic sites.Dwellings uncovered elsewhere were built around analtar, as in the pre-Hispanic sites of Oztoyohualco, nearTeotihuacan (Manzanilla et al., 1996).The geophysical interpretation obtained suggests that

these features were much more complex than pre-viously understood. Sizes of foundations determinedby the VMG and GPR techniques suggest that thismound was part of a complex habitation related toreligious or administrative services (McClung andSugiura, 2002). However, the possibility exists thatsuch a complex could have been inhabitated bywealthy people (Sugiura, 2000).

In the future, more extensive geophysical work couldbe carried out over the remainder mounds in the area.At least six additional mounds are visible nearby(Figure 2), where the methods presented here could beused. That work could go far toward understandingwhether these mounds were used for administrative,religious or habitation functions. Thus, future geophy-sical study combined with archaeological excavationmay yield a more complete view on the distribution ofthe ancient structures and also reveal ways of life ofthese lakeshore settlers.

Acknowledgements

We would like to express our gratitude toDr Y. Sugiura for attracting our attention to carryout geophysical work at the archaeological site whereshe has been working for the past 20 years. She alsohelped with the logistics; which included the corre-sponding permits by the local authorities. We alsowant to thank Dr L. Barba for his support and valuableunpublished information shared with us. We are dee-ply indebted to Dr P. Lopez, O. DiazM. Elizondo and J.Urbieta for helping uswith the fieldwork. J.Wulsin, A.Chavez and C. Chavez proof read the manuscript. Wedeeply thank Dr L. Conyers for his review and kindcomments and suggestions that fully improved thiswork. This project was partially financed by DGAPA-IN107602, DGAPA-IN104006, DGAPA-IN117408 andIGEF-G113 (2007-2009) projects. Dr R. E. Chavez andDr M. E. Camara were supported by an interchangeprogramme UNAM-IGEF (Mexico) and UPM-ETSII(Spain). Finally D. Argote held a graduate scholarshipawarded by CONACyT-Mexico and DGAPA-UNAMIN104006.

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DOI: 10.1002/arp


geophysical studyof a pre-hispanic lakeshore settlement, chiconahuapan lake, mexico

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