Geophysical Prospection

The Monte Albán Geophysical Archaeology Project (MAGAP) focuses on the site’s Main Plaza, which was the civic-ceremonial heart of the ancient capital. The way we view the Main Plaza has far reaching implications for important debates concerning Monte Albán’s initial founding, history of development, political organization, and collapse. Many basic questions concerning the Main Plaza remain unresolved. What kinds of activities or events occurred in the Main Plaza and how did these change over the course of the site’s history? What was the role of the plaza and what did it mean to the people of Monte Albán?

Archaeologists have excavated many of the buildings in and around the Main Plaza, but our project is the first to focus explicitly on the plaza itself. In 2017, we carried out geophysical prospection on the Main Plaza, including ground penetrating radar, gradiometry, and electrical resistance. In addition, we paired this with a detailed study of the plaza surface, aided by UAV (or drone)-mapping and the use of a robotic total station.

Geophysical prospection (GP) is well suited for analysis of very large spaces like the Main Plaza. In contrast to other methods, GP can be carried out across very broad areas in a relatively short period of time to reveal large-scale sub-surface “anomalies” or patterns. Furthermore, these methods are non-destructive and present no risk to archaeological deposits. This makes GP a powerful means of informing and directing preservation efforts as well.

Ground Penetrating Radar (GPR)

Ground-penetrating radar (GPR) works by emitting pulses of radar into the ground, which are reflected, absorbed, or otherwise deflected by buried features.  The return time of these pulses indicate the depth of the anomaly. GPR excels at identifying structure foundations, buried roads/footpaths, and tunnels or other features that trap air.

In 2017, our project utilized a 400 MHz antenna across the entire surface of the Main Plaza and used a 200 MHz antenna in selected areas. Local conditions at Monte Albán (e.g., sediment composition, moisture content) proved favorable and yielded good quality data.

Gradiometry

A gradiometer is a passive sensor that measures changes in magnetic fields. Burning and disturbance both alter the magnetic reading of soil, meaning features such as fire pits, mounds, old excavation units, burials, structures and house floors are detectable using this technology. The gradiometer performed very well in the local conditions and yielded valuable data on subsurface “anomalies” at Monte Albán.

Electrical Resistivity

Electrical resistance measures the resistance of soils and other materials to an electrical current.  Subsurface features with different physical properties have varied levels of resistance to the flow of electricity. Electrical resistivity is useful in identifying buried walls, rubble, surfaces, structures, roads, geological features, ditches, pits, gullies, drains, metal pipes, and graves. Our electrical resistance results were excellent and far exceeded our expectations. Given that we carried out the survey during the rainy season, we think the moist ground conditions improved the quality of our results.