July 2011 GPR Survey Recap

Summer 2011 Guatemala GPR Work

Field research at the site of San Pedro de Aguacatepeque (SPA) has been ongoing since the summer of 2010. As part of this research, considerable effort and expense has been expended on locating the remains of the colonial period Kaqchikel Maya (apprx. AD 1521-1820) occupation of the site. While the colonial period settlement remains the focus of this project, the 2010 excavations have forced us to expand the temporal scope of the project to the earliest observed occupation at the site, which dates to the Late Classic period (apprx. 800-1000AD)  and stretches through the Postclassic (1000-1500AD) and into the Late Colonial Period. The multiple stratified occupations at the site have pushed us to embrace a diachronic framework of analysis (drawing on Lightfoot 1995) for investigating the changes in daily life and material traditions at SPA catalyzed by the challenges and opportunities of the colonial encounter

Common archaeological practice would call for a widespread systematic subsurface survey of the site, wherein test units of 50x50cm would be dug at regular intervals over the area of interest, with the intent of locating buried remains and defining the limits of archaeological deposits at the site. Complicating things at SPA is the most recent use of the land where the site is located, namely as a plantation for high-altitude, primo Guatemalan coffee. An important part of the field work at SPA has centered on a collaborative relationship with multiple stakeholders at the site, namely the landowner, plantation workers that are drawn from local communities, the Instituto de Antropologia e Historia de Guatemala which oversees the management of archaeological resources in Guatemala and Stanford University/my dissertation committee. In 2009 we began consultations with the landowner in the hopes of starting up a project in a region notorious for declining archaeological research on private lands (for a variety of reasons, most prominent of which are safety/security concerns, worries over access to sites and misinformation regarding government land grabs where archaeological resources are encountered). In 2010 we were able to secure permission to undertake preliminary research at the site, on the condition that the coffee plants were not damaged or destroyed in the process. As a result, research carried out over the summer of 2010 thanks to a National Geographic Waitt Grant, centered on geophysical prospecting as a rapid, non-invasive set of techniques for locating subsurface deposits at the site. Electromagnetic Induction, Magnetometry and Ground Penetrating Radar were all used (see Field Work section of the site for more details) to various degrees of success. The latter technique, GPR, yielded the most promising results for the type of silty sand substrate found at SPA.

GPR Basics and 2010 GPR Survey

Ground penetrating radar (GPR) is an active geophysical prospection method that operates by introducing an electromagnetic pulse into the ground, and analyzing the resulting reflections of the wave against buried strata and features.  GPR can provide detailed profiles of the stratigraphy of an area, as well as identify the location and relative depth of archaeological objects and features.  However, while acquiring GPR data is itself a much slower process than acquiring magnetometry data, the processing and interpretation of the data is itself even more time consuming.  As a result, GPR surveys are usually undertaken after initial small scale tests have shown the efficacy of the method in the conditions of the site and region, in order to avoid investing substantial time into the processing of data that may indeed show nothing (if the conditions on the ground are not conducive to the GPR method).  For this reason, the other prospection methods were carried out at the site in 2010 at a greater scale, and the GPR work was undertaken mainly as a small scale preliminary test to evaluate the potential of the method at SPA.

The ground penetrating radar work at the site in 2010 was limited to two 50 meter long transects through parts of the site that with large concentrations of colonial material culture on the surface.  Nigel Crook, of the Stanford Shared Measurement Facility performed the GPR prospection using a Sensors & Software Pulse-Ekko Pro system with 200Mhz antenna.  More GPR transects were planned for the 2010 field season, however

Figure 1: 2010 GPR Survey lines at SPA

issues related to the failure of the GPR batteries as a result of rain and moisture at the site limited the GPR work to two transects (Figure 1). Despite these technical difficulties, the GPR work was quite successful and it was determined that this methods may be the best one for SPA.  The GPR work provided a detailed rendering of stratigraphy to a depth of between 3-4 meters and helped identify certain anomalies in the stratigraphy (i.e. possible archaeological features dug into strata) as well as possible archaeological materials within each strata (Figure 2). An excavation unit was placed on GPR transect 2 in an area that showed a depression in a strata possibly related to cultural activity.  The excavation confirmed the presence of many artifacts as well as confirming that the GPR’s representation of the stratigraphy in the area was indeed

Figure 2: 2010 GPR Survey Profiles

accurate. While no clear archaeological features were detected, GPR survey has great potential to do so when used on a larger scale and may facilitate the rapid evaluation of magnetic and electromagnetic anomalies detected across the site.

These conclusions led to the planning of a large-scale GPR survey in the 2011 field season (which was made possible by a Stanford University Community Engagement Grant) using a mobile GPR system that will allow for faster collection of data and a higher frequency antenna (400Mhz) that will allow for more detailed images of the substrate closer to the surface (1-2meters from surface) where the majority of archaeological material has been encountered.

2011 GPR Survey 

Over the course of the first week of July 2011, I carried out a large scale GPR survey at SPA over the areas of the site containing the remains of the colonial occupation. In total almost 8500 square meters (or .85 hectares) of area were surveyed with a GSSI SIR-3000 using a 400mhz antenna and survey wheel. This setup facilitated the rapid collection of good resolution data on subsurface deposits to a depth of about 150-175cm (more than adequate considering that most archaeological materials were located within 2m of the surface). Conditions were less than ideal for the GPR survey, as the coffee plants restricted the survey to within the coffee rows separating plants. While the rows are fairly regularly spaced; we were unable to very accurately and closely space the GPR transects as is ideal for this type of survey. GPR transects were placed at 1.5m intervals, which is the spacing of the coffee rows. The survey was carried out in several episodes, with survey blocks of different lengths and widths due to the specific obstacles obstructing the GPR in the different areas of the site (tree lines, ledges, rock outcrops, etc). In general, the coffee rows were filled with leaf litter and branches from the coffee plants and their associated shade-giving trees. These rows had to be cleared by hand (thanks to Walter, the plantation worker in charge of the area where the site is located, for helping Luisa and I clear!) before the GPR could pass through, a task complicated by nightly rains that made the entire area quite soggy and difficult to navigate in.

Once the survey was completed, the next task consisted of processing the raw GPR data into a variety of profile and plan images. Dr. Dean Goodman graciously provided the project with a short term license of  GPR-Slice software; a critical contribution as this software allowed us to create plans of the GPR data (at various depths) to then overlay on aerial images of the site. In addition, this software facilitates 3D modeling of the GPR data that provides another perspective on the anomalies detected by the GPR as well as animations showing time slices that roughly correlate to depth.

The GPR data are still being processed, evaluated, and interpreted however numerous subsurface anomalies were detected. In particular, a circular/cone shaped anomaly was found starting at approximately 70 cm below surface and extending to 150cm.in Sector 1 (Figure 3,4,5).


Figure 3: Vertical and Horizontal GPR Slices of Sector 1 2011 Survey at SPA; showing top of circular anomaly.

Feature 4: Horizontal GPR slice of Sector 1 2011 GPR survey at SPA; showing bottom of circular anomaly and modeled anomalies.






Figure 5: Sector 1 2011 GPR Survey, showing circular anomaly; overlaid on site topography.


The anomaly appears to be at least 15-20m in diameter at its top, and narrows to approximately 5m at the bottom. In addition, this anomaly is associated with dense concentrations of colonial period artifacts encountered in the 2010 surface collection (Figure 6), an elevated magnetic anomaly detected by the magnetometer (Figure 7) and near anomalies detected by the EM survey (Figure 7).

Figure 6: Sector 1 2011 GPR Survey showing anomaly overlaid by surface collection points (blue points represent points where colonial period artifacts were found, with point size representing number of colonial artifacts recovered).

Figure 7: Showing Sector 1 of SPA, with overlays of (top to bottom): 1) Topo contours of the site, 2) 2011 GPR survey, 3) EM survey Sensor 1, 4) EM Survey Sensor 2, 5) Magnetometer survey.














These correlations indicate that what the GPR has identified is in fact an anomaly of some sort (likely cultural, due to the association with artifacts) and not an artifact of the survey process or the GPR. Needless to say, we have decided to excavate three test units on or near this anomaly in order to determine what it is (the shape indicates a trash pit of some sort, dug in the early colonial period down into previous occupation strata, but this is only speculation).

A set of linear anomalies were encountered in the southern most portion of the site (Figure 8). These anomalies were also associated with EM and Magnetometer anomalies (Figure 9), as well as some linear features observed on the surface of the site in a high-resolution satellite image taken when the site was cleared of coffee plants. However, the magnetometry in this area did detect large magnetic anomalies likely associated with magnetic volcanic material, and thus excavations are necessary to determine the nature of the anomalies detected in this area. The results of the GPR survey have pushed us to excavate at least two test units on and near these anomalies.

Figure 8: 2011 Sector 2 GPR Survey, with linear anomalies circled.

Figure 9: 2011 Sector 2 GPR Survey, overlaying Magnetometer survey; linear anomalies circled for both GPR and Mag surveys).















In short the GPR work undertaken in 2011 has served to focus the Nov-Dec of 2011 excavation of the site. The location of numerous anomalies, and their observed association with other lines of evidence have allowed us to survey large swaths of the archaeological sites (and its subsurface) and reveal promising areas of the site to excavate, while minimally disturbing the coffee plants on the surface. Excavations have started this week (with 2 units in progress directly over the circular GPR anomaly!) and the final phase of the GPR work -groundtruthing identified anomalies- will commence!

Please check the project website at http://aguacatepeque.com/blog for updates.



Lightfoot, K. G.

1995    Culture Contact Studies: Redefining the Relationship between Prehistoric and Historical Archaeology. American Antiquity 60:199-217.

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