Geophysics - Introduction

Introduction

The city of Tiwanaku has attracted antiquarian and archaeological interest since the time of European conquest. Today the site stands with several large monuments in various stages of excavation and reconstruction (Figure 1). In the center lies the Akapana, a large platform mound which likely housed a sunken court at its summit. To the north lies the Kalasasaya, a large platform temple with massive upright stones marking the annual solar cycle, and adjacent Semisubterranean Temple, one of the earliest monumental constructions. To the east lies the scattered ruins of the Kantatayita, the only monument visible on the surface that has not yet been excavated (though excavations are currently underway). Far to the southwest lies the Pumapunku, a large platform mound similar to the Akapana that may have served as a gateway to the city for pilgrims traveling from Lake Titicaca some 10 km to the west (Vranich 2006). For decades these large monumental structures formed the basis for archaeological inquiry and interpretation, leading the long-held belief that Tiwanaku was a vacant ceremonial center (Bennett 1934). Extensive survey and excavations beginning in the 1980s showed that Tiwanaku had a large residential population that covered some six square kilometers at its peak (Kolata 2003b). At least eight residential compounds have been partially excavated—four within the monumental core and four more distant (Kolata 2003a). Yet, large expanses of the site remain unexcavated and are no doubt filled with complex layers of Tiwanaku occupational levels, residential compounds, burials, waterways, revetments, and structures. Large-area geophysical survey provides a means to explore these areas in considerable detail, revealing not only the locations of buried features, but a buried landscape that is otherwise invisible short of complete site-wide excavation. Extensive geophysical surveys allow us to ask and begin to answer new archaeological questions.

Figure 1: Topographic map showing principle Tiwanaku monuments mentioned in the text (contour interval 1 m). Key: 1 Akapana, 2 Kalasasaya, 3 Semisubterranean Temple (Templete), 4 Kantatayita, 5 Pumapunku, 6 Mollo Kontu; 7 Putuni. Digital Elevation Model provided by Adam Barnes (see Barnes and Cothren 2007)

Geophysical investigations have recently taken a prominent role in Tiwanaku Archaeology. Williams, Couture, and Blom (2007) conducted geophysical surveys (magnetometry, resistance, and ground-penetrating radar) in small portions of the Putuni complex, Akapana East 1 residential complex, and a large area at Mollu Kontu to the south. These and other preliminary investigations showed the potential of geophysical methods at Tiwanaku. The current project (Proyecto Arqueologico Pumapunku-Akapana, PAPA) includes five years of geophysical survey over vast portions of the monumental core. Koons (2006) conducted a very large GPR survey directly east of the Akapana and also reinterpreted data collected by Henderson (2004) directly west of the Akapana. To complement and extend these surveys we have used electrical resistance, electromagnetic induction (EMI), and magnetometry over selected areas previously surveyed with GPR. We have also extended the GPR survey to the northeast sector of the monumental core.

The multi-sensor approach is very useful in discovering several subsurface features and distinguishing their depth and nature of construction materials. Ground-penetrating radar has proven to be the most useful and versatile method because it detects the majority of subsurface features and allows their depths to be estimated. Magnetometry provides a powerful complement by distinguishing between igneous rock (e.g. andesite) and other common rocks used in construction (e.g. sandstone and limestone). Magnetic susceptibility and conductivity (both from the EMI instrument) provide additional insight for feature interpretation. The large geophysical dataset provides a database of information that is too detailed to describe here in its entirety, so we focus on selected features located east and northeast of Akapana. These include what we interpret to be two residential compounds, a series of conduits, circular water retention features, revetments, and a square structure. In some cases we have tested these interpretations with excavations, but many remain untested. Our interpretations were made using knowledge from previous excavations and the multiple geophysical datasets available, but their accuracy will no doubt be assessed by future excavations.