klarich craig saa 2001 poster-libre

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Geophysical Survey in the Lake Titicaca Basin: UncoveringElite Domestic Architecture at Pucará, Peru. 1

Elizabeth Klarich and Nathan CraigUniversity of California, Santa Barbara

Please do no cite in any context without permission of the authors.

IntroductionIt is the aim of PROYECTO de ARQUEOLOGÍA DOMÉSTICA de PUKARA to use data collectedfrom excavations of elite households to determine how aspiring political actors in the LakeTiticaca Basin gained and maintained power during the Upper Formative Period (500 BC- AD400). The site of Pucará (200 BC- AD 400), located in the northwestern Basin in Peru, was one oftwo regional population centers during the Upper Formative and covered an area of at least 1km 2.

1 Following local convention, the spelling “Pucará” refers to the archaeological site and the modern town and thespelling “Pukara” refers to the prehistoric culture. [note: since the presentation of this poster in 2001, we have

shifted to using the spelling “Pukara” to also refer to the archaeological site, following the conventions of the Peruvian National Institute of Culture”]




Monumental architecture, exotic goods, and complex residential areas are evidence for an elitepresence at Pucará, but the foundations of elite power are unclear. Excavation data fromhouseholds, the foci of production, consumption, storage, and distribution activities withinpreindustrial societies (Hirth 1989: 441), will be used to determine what type of economic,

political, and ritual strategies were involved in the development and maintenance of Pukara elites.Excavations by Alfred Kidder II in 1939 exposed a large compound in the central area of the sitewith characteristics typical of an elite household (e.g. large size, quality of construction, centrallocation, both domestic and non-domestic features). In order to develop an excavation strategy ata site that is both large and without surface architecture, a geophysical survey was conductedduring September of 2000.

In this poster results are discussed, including descriptions of significant subsurface featuresencountered, comparisons are made with the results of previous excavations, and futureexcavation strategies are outlined. 2

2 Further information on the project is available at http://titicaca.ucsb.edu/pukara




Background to Pucará

Pucará was one of two major population centers in the Titicaca Basin during the Upper FormativePeriod, contemporaneous with early occupations at Tiwanaku in Bolivia (Stanish n.d.). The site islocated approximately 80 km northwest of the shores of Lake Titicaca at an elevation of 3950masl. The reported size limits of the site vary from 150 ha (Chávez 1992) to 6km 2 (Mujica 1991),illustrating the lack of consensus on the size and scope of Pucará. The site has been characterizedas an early urban center (Mujica 1978; Rowe 1963), a ceremonial center (Chávez 1992: 42), thecenter of a complex chiefdom (Mujica 1991; Stanish et al. 1997), and more generally as thecenter of a ceramic style region (Steadman 1995). Previous archaeological research at Pucará hasclarified the layout of the monumental public architecture and surrounding mound complexes,defined Classic Pukara decorated ceramics and monoliths, and determined two areas of distinctresidential architecture (Chávez 1992; Franquemont 1986; Kidder 1942; Mujica 1991).




Area IV, with its relatively large structure size (compared to the structure by the river),complexity of architecture, “high value” objects, and proximity to monumental architecture fit allof the common criteria used for characterizing elite households (Smith 1987). The nature andfunction of this area, and its relationship to both the area of monumental architecture and non-elite structures by the river, can only be clarified through further excavations.

Rationale for Geophysical Survey at Pucará Due to the nature of building materials utilized in this region, there is little to no visiblearchitecture on the modern ground surface. In Area IV, Kidder noted that he would not haveexcavated in the area if local informants had not reported finding a monolith there (Chávez1992:59). At present, the central area is quite flat, with only a few slight mounds and linearfeatures that may represent subsurface structures or walls. Surface pottery is also scarce andpredominantly dates to the Late Intermediate Period occupation at the site (post- AD 1100, see




Carlevato 1988). In order to determine if additional compounds were located in this central pampa area of the site we conducted a geophysical survey to the north of Kidder’s Area IV. Withgeophysical techniques we hoped to locate areas of architecture, differentiate compounds, anddetect features within those areas.

Geophysical Methods3

Geophysical remote sensing detectors are used here as prospection and discovery tools to locateand identify subsurface anomalies. Interpretation of these sub-surface anomalies is then used todirect future excavations. In general, geophysical surveys are useful when sites are large and thearchaeological features a researcher wishes to investigate are very specific.

It is useful to utilize more than one method of geophysical survey when a site is stratigraphicallycomplex or has multiple periods of occupation (Greenfield 2000: 167), as is the case at Pucará.Use of multiple methods of data collection is also important when trying to target specificarchaeological periods if sites are vertically stratified and if there has been horizontaldisplacement by water movement or by plowing. At Pucará, there have been multiple periods of

occupation and the site has been heavily plowed during historic times.

Two kinds of geophysical remote sensing detectors were used for the survey. These were aGeometrics 858 a cesium magnetometer and Geophysical Survey Systems International (GSSI)SIR-2000 ground penetrating radar (GPR) with a 400 megahertz antenna. To complement thegeophysical research, areas of the site were also mapped using a Leica TCR1105 Total Stationand Trimble Geoexplorer II GPS receiver. Data from the magnetic survey were processed usingGeometrics MagMap2000 software, while GPR data were processed with the RadanNT softwarepackage from GSSI. Surface maps generated by the Leica Total station were imported onto alaptop computer using the Leica Survey Office software, and GPS data were processed andexported using the Trimble Pathfinder Office software package. Output from each of these four

data sources was reassembled in a common map projection and coordinate system using ArcView3.2 GIS software from Environmental Systems Research Institute (ESRI).

The Geophysical Survey: Field MethodsWhen conducting geophysical survey, space is generally partitioned into grids that can besurveyed in regularly spaced transects (Breiner 1999: 12; Conyers and Goodman 1997: 25). Oncerecorded by a detector, these field data can be imported to a computer for further processing andvisualization to delineate anomalous entities within the data. Increasingly, researchers areimporting the results of surveys into GIS systems so that the results of more than one type ofsurvey can be viewed simultaneously. This aids tremendously in the identification of patterns,and frequently leads to the discovery of subtle patterning in multiple data sets that may not be

visible with a single method of remote sensing.

At Pucará, a series of four 50m x 50m grid blocks were laid out using the Leica Total Station.The magnetometer survey was collected along North-South oriented lines and the ends of thetransect lines were marked every meter using wooden stakes. The GPR survey was collectedalong East-West oriented transect lines and the eastern and western ends were marked at one-

3 Special thanks to Mark Aldenderfer of the University of California, Santa Barbara, for the use of his equipment andcrew for the geophysical survey at Pucará.




meter intervals. Additional magnetometer blocks were surveyed in order to determine the shape,orientation, and nature of the magnetic anomalies detected in the first four blocks surveyed. Thearea was expanded with two 20m x 50m blocks to the east and a 30m x 50m block to the south ofthe original survey area. Time did not permit the survey of these additional blocks with the GPR.

In addition to the geophysical survey, the pampa area was mapped in detail using the Leica TotalStation and the GPS. Field walls, both prehistoric and modern, were mapped, as were theboundaries of modern agricultural fields, surface features, and Kidder's backdirt piles andexcavation areas from 1939. Using a map of Kidder's excavation Area IV, we were able todetermine the orientation of the architecture, the size, and which walls were still exposed on thesurface. This guided our survey strategy and helped greatly in interpreting the results from thegeophysical surveys.

Magnetometry

Magnetometers have been used to identify numerous buried anthropological entities including:walls, structures, pottery, bricks, roof tiles, fire pits, buried pathways, tombs, buried entrances,and monuments (Breiner 1999: 45). At Pucará the G-858 was used in a pre-excavation context(Greenfield 2000: 167) primarily as a discovery tool to aid in understanding the location andarrangement of sub-surface deposits. Detection of anomalies in archaeology generally relies onthe fact that the object has stronger magnetic fields than the surrounding matrix.




Heating is known to increase magnetic susceptibility through the production of magnetite(Marmet et al. 1999: 168). Before heating, small regions that are called “domains” located in eachmagnetite crystal are randomly oriented. While hot, particularly at high temperatures, domainsreorient themselves. During cooling, domains tend to align themselves in the general direction ofthe ambient magnetic field and parallel to each other. This realignment and parallel orientation of

domains creates a net magnetization that is fixed with respect to the object (Breiner 1999:10).Some features like walls and tombs have been located because their construction resulted fromthe displacement of uniformly magnetic soil and the features are represented in the magneticsurvey as voids (Breiner 1999: 45). Walls do not always have weak magnetic fields in relation tothe surrounding dirt. Depending on a wall’s construction material and the magnetic properties ofthe surrounding soil, the wall may have higher or lower magnetic properties than the substrate.

Magnetograms, or rasterized output from a magnetometer survey (Neubauer and Eder-Hinterleitner 1997: 179), are becoming an increasingly popular form of representation. A basicunderstanding of magnetics is required to interpret magnetograms. Magnetic fields are polar,having both a positive and negative component. Objects where both the positive and negativepoles can be observed are referred to as di-poles. Adjacent light and dark patches in themagnetograms represent di-polar anomalies that are visible because of the greater intensity of thelocal magnetic field emanating from some object buried beneath the surface.

After the survey was complete the magnetometer data were transferred to the laptop computerwhere they were processed using Magmap2000. The data were plotted in Magmap2000’s 2Dview and displayed with a hillshaded greyscale plot. The plot of each survey was captured to a

jpeg and imported into ArcView GIS where they could be aligned to real world coordinates forcomparison of anomaly patterning in the other data sources.

Ground Penetrating Radar




Ground penetrating radar has also been used extensively as a prospection and discovery tool inarchaeology (Arnold et al. 1997; Clarke et al. 1999; Conyers and Goodman 1997; Savvaidis et al.1999; Tohge et al. 1998). The GPR method relies on the transmission of high frequency radiowaves (Conyers and Goodman 1997: 23; Arnold et al. 1997: 161). Electromagnetic energy ispropagated downward into the ground in a conical shape by an antenna. The two-way travel time

elapsed between transmission, reflection off of buried objects or discontinuities, and reception ofthe wave at the surface by an antenna is measured by the detector. Since the GPR methodmeasures two-way travel time and intensity of energy reflected, it is possible to produce a depthprofile where the horizontal axis represents distance along a transect line and the vertical axisrepresents depth in units of time (see table below).

The depth and resolution of a GPR survey are functions of the wavelength of the antenna usedand the local geologic contexts. Longer wavelength antennas are capable of detecting featuresthat are buried deeper, but with long wavelength antennas smaller features may not be detected.Likewise, shorter wavelength antennas do not provide a picture of great depth, but permit thedetection of much smaller objects. The depth that an antenna can detect buried features is afunction of the soil’s relative dialetric permeability (RDP) (Conyers and Goodman 1997: 53).Increases in soil moisture and salt contents lead to increasing RDP creating greater signalattenuation. Lower RDP permits greater depth penetration.

Since GPR employs measurements of 1) distance along a transect line, 2) time since transmission,and 3) intensity of wavelength reflection, there are many ways that radar data can be visualized.Single transect lines viewed in cross section produce a linescan plot (see figures below).Combining multiple transect lines into a single file and viewing all transect lines as a commontime since transmission produces a time-slice plot (see figures below). Linescan plots offer theadvantage that different depths, measured in time, can be observed. Linescan plots can be used toexamine site stratigraphy over large areas (Baker et al. 1997: 1098; Clarke et al. 1999: 107). Onthe other hand, time-slice plots provide a view with horizontal extent permitting the resolution ofthe shape and extent of subsurface targets (Savvaidis et al. 1999: 72). However, time-slice imagesare limited to a single time interval. Linescan plots can be quite useful when trying to explore orpredict vertical relationships, while time-slice plots provide information on the extent of ananomaly at a given depth.

Block Number Slice Depth (ns) Slice Width (ns)1 4.57 12 8 0.13 3.16 0.84 4.31 0.8

Once the data were collected and transported to the laptop computer they were processed usingRadanNT. All of the radar surveys were assembled into a 3D project where the data were viewedin a linescan plot (figures below). The four surveys were horizontal high pass filtered to removesystem ringing. The filter was set to 1023 scans. Each survey was then transformed with amigration filter to correct for the hyperbolic refraction patterns produced by the conical shape ofthe radar signal transmitted into the ground. Parameters for the migration filter were set usingRadan’s graphical interface. The results of these post-processing steps are shown below. Once




Results and DiscussionGPR and magnetic anomalies were found in each of the survey blocks, including both linearanomalies and anomalies of limited lateral extent (Desvignes 1999: 86). A number of the linearanomalies both extended across more than one survey block and some were perpendicular to theorientation of the transect. This indicates that the anomalies seen do not represent instrument

error or some artifact in the data, but rather supports that the anomalies represent some subsurfacefeature of anthropogenic origin.

Anomalies Visible in the Magnetic Survey Linear anomalies can be seen in the magnetogram by linear arrangements of di-poles (Figure).Patterns of small magnetic anomalies in a linear arrangement may likely be the product of wallbuilding. Kidder (m.s.) reports that sandstone and andesite were both used in the construction ofwalls. Andesitic rocks should produce visible di-polar anomalies. Since they would have beenmoved from their source deposit, we would expect the di-poles to have a random orientation.Ferric sandstones, which are likely to be present since the local sandstones are reddish, may alsoproduce small or weaker anomalies. Like the andesite, these would have random orientations due

to removal from their source deposits.

Large globular anomalies were also seen in the magnetic survey. These globular di-poleanomalies may either represent deep anomalies from a large source, or could possibly beproduced by smaller, shallower objects. These may represent thermal anomalies, such as acooking hearth or an area for firing ceramics. However, since this area is inhabited and plowed bythe local Quechua, there is a possibility that some anomalies are produced by historic or modernpieces of metal. We made a dedicated effort to systematically examine the area of themagnetometer surveys and remove fragments of metal on the surface.

Anomalies Visible in the Ground Penetrating Radar Survey Linear anomalies are seen in the radar data by the patterned differences in reflection intensity at agiven time slice. Numerous linear anomalies were observed in the four GPR survey blocks.Additionally, there are several places with clearly defined corners where roughly perpendicularlinear anomalies meet. Often where these corners meet, one can see either higher or lower radarrefraction patterns within the spaces bounded by linear anomalies.

Linear Anomalies found in both surveysOnce all the data were imported and properly aligned to their real world coordinates in ArcView,it was possible to begin looking for consistent patterns existing in both forms of geophysical

survey. To do this we used the flicker method, which involves turning a layer off and on rapidlyto find anomaly patterns that were congruent in both of the surveys. If the anomaly was distinct inthe radar data and if there was some evident pattern in the magnetometer survey, then theanomaly was digitized. In total, 59 linear anomalies were observable in both surveys. There aremany other anomalies that are observable with only one of the methods and these were notdigitized.




The patterns of lines interpreted from the geophysical survey are quite similar in their size andlayout to the walls that Kidder found in his Area IV excavations. Linear anomalies most likely

represent walls, partitioning space into different activity areas. Aldenderfer defines two broadcategories for interpretation of the use of interior space— “open” and “busy”— based on thepresence or absence of features in the floor (1991). Open spaces, lacking features, would be moreconducive to gatherings of people. In contrast, busy spaces include the presence of pits, hearths orplatforms that most likely reflect room function and would have made the aggregation of peopledifficult (Aldenderfer 1991:244). There are indications from the magnetometer survey thatsimilar variation in the use of space may be present at Pucará. “Busy” areas have multiplemagnetic anomalies that may represent wall fall, but they may also be areas where small fireswere set repeatedly or ceramics or heat-treated lithics were deposited. Conversely, there arespaces that appear “open” and undifferentiated. These areas may be more complex onceexcavated, but at this point there does appear to be variation in the use of space in this area of the

site. Lastly, there do not appear to be consistent patterns either of open or busy spaces in the radarsurvey. Some open spaces appear as voids while others show a certain amount of patterningwithin the bounded space.

Generally, where linear anomalies are encountered in the radar survey the area inside the boundedspace seems to have lower radar refraction patterns. However, this pattern does not always holdtrue, and the variation probably reflects variation in the nature of the construction methods androom contents. One thing seems clear— the area inside these spaces is different than the outside.




The linear anomalies that were interpreted in both geophysical methods appear to be of roughlythe same size as the walls that Kidder found in his Area IV excavations. However, the orientationof the walls appears to be different than those previously excavated. The Area IV walls areoriented along similar lines as the terraces of the Qalasaya monumental architecture. The linear

anomalies found in the 2000 geophysical survey are quite different. It is important to note,however, that the orientation of the linear anomalies is consistent within the two different sets ofsurvey blocks and are congruent between the two different kinds of detectors.

From Survey to Excavation

The ultimate goal of the geophysical survey was to develop an excavation strategy for a largearea (at least 300 m2) within a very large site (at least 1km2). Using the magnetometer and GPR




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