A Ground Penetrating Radar Survey of Forteleza Cerro Colorado, Peru

Bryan S. Haley

Department of Anthropology

Tulane University

Introduction

In August, I performed a ground penetrating radar (GPR) survey of the Forteleza Cerro

Colorado, located in coastal Peru. The goal of the survey was to determine the effectiveness of GPR on

the particular geomorphological setting and with the particular types of features found on this site. The

survey was also intended to augment the research of Allen Rutherford, also of the Tulane Department

of Anthropology, whose ongoing dissertation research is focused on the site. His goals include

exploring the structure of the fort, its defensibility, and other functions that the fort may have played.

Site Description

The site is located in the Huara Valley of the Norte Chico region of Peru, about 180 kilometers

north of Lima, and near the modern city of Huacho. It is about five kilometers from the coast and on a

prominent hilltop in a largely desert region (Rutherford 2010:2). A new settlement, called Asociacion

Seis de Junio, is quickly encroaching on the site.

The site includes three concentric walls and an additional wall section that connects to a

neighboring lower elevation. The surface features of the site were described by Rutherford (2010:2-3)

following his 2010 survey. He determined that fortification walls were constructed of three different

materials: cut rock, adobe, and more expedient angular rocks, and, in many cases, the walls were

modifications of natural features. The outer wall ranges in height from .8 to 5.2 meters, while the

second wall is lower, ranging in height from .8 to 1.6 meters. However, the inner-most wall has been

greatly damaged and, therefore, its height cannot be accurately measured. All of the walls have been

significantly affected by centuries of erosion, dune formation, and looting.

Cerro Colorado was previously dated to either the Early Horizon (or about 900BC to AD200),

the Late Intermediate Period (or AD1000 to 1476), as part of the Chancay culture, or to both (Ruiz

1990; Brown Vega 2008; Brown-Vega 2010). In addition, an Inca period Tambo (or administration

building) is located not far to the southwest of the fortification, although the relationship between the

two is unclear. Based on surface material and wall materials, Rutherford (2010: 4-5) tentatively

assigned the fort exclusively to the Late Intermediate Period.

Methods

GPR operates by propagating a radar pulse into the ground from an antenna. Based on contrasts

in certain electrical properties of the soil and ground targets, the radar wave will reflect back to the

antenna (Weymouth 1986:371; Conyers and Goodman 1997:23). A control unit records the time and

the strength of the reflection.

The travel time of the reflection is related to the material that the radar is propagated through.

In other words, the radar wave travels at different speeds through different media. For example, the

radar energy travels very quickly through sand, but travels very slowly though clay (Conyers and

Goodman 1997: 23, 197-200). Water moisture also slows down the radar wave speed and it also

attenuates the radar energy. As a result of these characteristics, GPR tends to be ineffective in clay, but

works very well in sand (Reynolds 1997: 688). The sandy soil prevalent at Cerro Colorado is ideal for

GPR survey.

An advantage of GPR is that the travel time back and forth to a target can be converted to

actual depth. Therefore the spatial extent of each target can be determined in three dimensions. To do

this, the velocity of the soil must be estimated and this can be easily accomplished using a software

routine (Geophysical Survey Systems Inc. 1999: 83).

Figure 1. A diagram of the collection of a GPR transect and the reflection of GPR energy by a subsurface target

(from Sullivan and Connell 2012).

GPR data is collected by pulling the antenna along a traverse line and placing marks every

meter so that distance can be determined. The lines are collected in a zigzag fashion so that, eventually,

a rectangular grid is covered. Irregularly shaped areas can be surveyed, but for the most efficient

coverage, survey should cover large regular blocks.

GPR allows a real time view of the raw data, which is a profile of the radar reflections along

each transect. This allows you to get a rough idea of the location of key targets as the data is collected.

However, interpreting subtle targets in the raw data is often difficult and additional processing is

usually required to produce a more interpretable representation of the data.

Once the data collection is complete, the raw data is downloaded to a computer and the starting

and ending coordinates of each line are specified. Next the raw data is “sliced” to create a number of

two-dimensional plan-view maps representing specific depths (Goodman et al. 1995: 85). This tends to

be a more productive format for interpreting archaeological features because they often form regular

patterns in plan view.

Ultimately, it is possible to produce a three dimensional volume representation, which allows

the data to be sliced through any axis to determine anomaly shape. Also, the background can be pealed

away, leaving only the 3D shape of the anomaly. This technique is called an isosurface.

As with many types of archaeological data, a geographic information system (or GIS) is often

used to manage GPR results. A GIS basically combines a database with spatial data so that that the

spatial patterning can be analyzed. When used in archaeological research, it can also be used to plan

and optimize excavation strategies.

A few GPR surveys have been conducted in the coastal deserts of northern Peru, including

Sandweiss et al. (2010), who presents a GPR survey of a preceramic monument near the Salinas de

Chao salt flats. However, this research included only the collection and analysis of individual GPR

transects. In contrast, the survey of Cerro Colorado employed a grid collection approach, as well as the

opportunity to assess the utility of GPR for understanding fortifications in the region.

Cerro Colorado presented several unique challenges to GPR survey. One challenge was setting

up an accurate reference grid so that GPR anomalies could later be relocated for excavation. The

project was unable to acquire a total station instrument for survey and we were therefore forced to use

tape measures. We were able to relocate two site datums from the previous season, which were rebar

driven into rock. However, these were located on top of the hill and the extreme topography caused

distortion of the measured distances with the tapes. To solve this, we measured from landmarks closer

to the grid corners whose positions could be determined in the GIS.

Another challenge was related to the close proximity of the site to the encroaching community.

Grid stakes (whether they were wood, plastic, or metal) are often removed by residents of the

settlement each night because they were useful in the building of their structures. After some

experimentation, we found that a cross-shaped pattern made of rock remained undisturbed by local

residents.

A final complication that became evident in the initial test transects at the site was the

undulating bedrock, which occasionally extended above the surface. With respect to GPR

Figure 2. GPR survey areas at Cerro Colorado.

interpretation, this can produce anomalies that are similar to those produced by archaeological features.

However, a key difference is the regular patterning produced by archaeological features compared to

natural ones.

Results

GPR survey was performed on three areas at Cerro Colorado (Figure 2). The first of these was

placed on the apex of the hill and at the heart of the inner-most wall. This area also contained a number

of rises that Rutherford (Personal Communication, August 2012) thought may be related to intact

architecture. On the southern end of Area A, however, bedrock was visible on the surface.

GPR results show a number of strong reflections, indicated by red in the results (Figure 3),

which is a slice representing GPR reflections at a depth of about 32 centimeters, based on velocity

estimates derived from hyperbola fitting. On the south end, irregularly patterned anomalies are almost

certainly related to the dipping bedrock. However, several linear anomalies may represent intact

structural elements. The southern-most of these is located on the highest elevation of the site and may

represent the central structure of the fort. The northern-most anomaly is located downslope and it is

less clear if this target is natural or cultural.

Area B was located to the west and downslope from the apex of the hill. It extended over the

outer wall of the fortification, visible as a rise roughly one meter tall in this area. It also includes a

large flat zone that may contain the remnants of domestic or special purpose architecture.

GPR results show a number of linear features in this area (Figure 4). Again, a number of strong

reflections are present in the same 32 centimeter depth slice presented for Area A. Some of these are

related to the outer wall itself, which is visible as a double line in the radar data – a pattern that may be

related to intact architecture.

Figure 3. GPR results for Area A. Depths is approximately 32 centimeters.

Figure 4. GPR results for Area B. Depth is approximately 32 centimeters.

Other lineaments were more surprising with respect to the surface topography. Two of these

corresponded to subtle depressions – one extending inside the wall and the other extending outside the

wall. Probing of both of these areas revealed a deep deposit of sand overlying bedrock and sharply

upward dipping bedrock to each side. These depressions may represent natural or man-made features

that were used as entrance ways for occupants of the fort. The anomaly to the inside also coincides

with a low area in the wall that Rutherford (Personal Communication, August 2012) had previously

identified as a possible entrance way. Another linear feature coincides with a subtle rise, not previously

identified, which may represent a low wall.

Area C extended about 40 meters to the north of the outer wall. The region seems to contain

sharply dipping bedrock with patches of relatively deep sand, as well as a relatively flat area that may

contain the remnants of structures. GPR results from Area C (Figure 5) contain a number of anomalies,

but most of these are related to undulating bedrock that was partially visible on the ground surface.

The most interesting anomaly is a remarkably regular, rectangular feature in the southwest

corner that may relate to an intact structure. The problem with this interpretation, however, is that the

survey area only captured the edge.

Conclusions

In conclusion, the GPR survey of Cerro Colorado produced a number of anomalies that may

provide insight into the structure of the fort and perhaps indicate entrance ways. Additional processing

may allow more subtle targets, such as quincha architecture to be identified as well. The survey also

revealed the pitfalls involved with using GPR in areas of shallow bedrock. Because it is difficult to

differentiate natural and cultural features, ground truthing is all the more important. Unfortunately, we

were not able to acquire a permit to excavate the GPR anomalies in August and, therefore, the

interpretations presented here are somewhat speculative. Excavations in 2012 will focus on some of

these anomalies and will determine if they are actually related to archaeological features.

Figure 5. GPR results for Area C. Depth is approximately 32 centimeters.

Works Cited

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